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Vac Advance to Carb and Adv. Curv. Question

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7.3K views 32 replies 10 participants last post by  68RS-SS  
6
#1 · (Edited)
I just read the sticky on Timing Adv. 101 at the top and found interesting but still uncertain about one point or maybe I misinterpreted the discussion and articles. I would have posted there but those stickies sometimes don't get responded to for months and I really could use some help ASAP.

I decided to re-explore basics and try some new tunings on my ZZ383 w/Quick Fuel 680 cfm vac. sec. carb, 5spd. I have MSD 6AL and MSD dist and get 13inHg vac at 750rpm idle. GM's ZZ Manual instructs to set at 30 total - all in by 3,000 RPM. 30 seems low and maybe must a warranty discalaimer type of approach to their manual - thoughts? So I'd like to try 32 total - seem reasonable or still low? I currently have MSD blue bushing and with initial set at 14 I'm only able to get 28 total out of it and comes all in by 3100 rpm. So going to try the silver (smaller bushing next) and see if that gets me close to 32 total at 3,000rpm.

However, after thinking more and researching, the part I'm really thrown by is in that 101 sticky I interpreted one of the key points being made is that a common misconception exisits that Vac Adv. should be connected to the timed vac. port on the metering block (in case of Holley style carbs). This is always what I've heard people say and see published in MSD, Holley manuals. I called MSD and talked fellas at the local speed shop and they are all baffled at the though of connecting Vac. Adv. to full manifold vac. However, according to this article I gather that Vac Adv. is more beneficial to connect to Full Vac port (below thottle plates in case of holley). So assuming that's true (please tell me if I'm misunderstanding) then would I leave my Vac Adv. connected when setting initial timing and still shoot for around 12-14 deg. initial? Or do I leave it off and plug and then set at 12-14 deg. initial and reconnect when set? I would think that as soon as I connect the vac. adv. line the initial/idle timing would significantly increase?? Is that the intent and a good outcome?

Funny how I thought I knew enough about this topic but now suddenly feel there's much more to know so any help appreciated. :yes:
 
M
#2 ·
You understand a lot more than you give yourself credit for. Yes, unplug the vacuum advance when setting the timing. Shoot for 32-34 deg total and 12-14 deg at idle. Add 12-14 more with vacuum advance for 46-48 at cruise. Use the manifold vacuum. Some guys swear the ported source works better for them but I've never heard a good reason why. It's an emission thing to make the motor less efficient. If the motor feels like it's surging at cruise take away some vacuum advance. The centrifugal advance affects WOT power, the vacuum advance affects mileage and driveability.
 
R
#3 ·
set initial with vacuum advance plugged, then hook to manifold vacuum. Your idle speed will prob increase, then you can turn your idle speed down.
 
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S
#4 · (Edited)
There are several factors that come into play when finding the best total timing so you can't just take the old 34-36 SBC general rule throw it at any setup. Cylinder head efficiency is one of the key ingredients, and modern designs like the ones you have require less total timing than older castings. There is no issue however with experimenting but GM set that engine up on the dyno with a timing number to be both safe and make good power. The numbers in my signature line were obtained on the dyno with only 30 degrees total timing. Not saying don't experiment, but just throwing out something to think about.

I'm using an MSD 8360 with the black bushing for 14 intial and 30 total. The black is the largest bushing and the MSD chart says you should get over 15 degrees with it so it's odd you're only able to get 14 degrees with the blue bushing which is supposed to be good for about 20. You may want to try bypassing the 6AL when setting the timing since the multispark signal can interfere with some timing guns and give you a false read, especially dial back lights. Often you have to use an old style light and timing tape.

Also I'm running the stock can to full manifold vacuum after experimenting with ported vacuum for a while. It has 10 degrees advance all in at about 15". What I found with my throttle body is that at very light throttle (which my car runs at most of the time) the ported vacuum wasn't coming in so I wasn't getting vacuum advance a lot of times. Once I moved it, I was able to lean out my low throttle angle accel enrichments (the newfound extra timing was causing a rich misfire) and also my mpg went up about 2mpg per tank. It also cured an issue that came about with the latest idle control software change made by Retrotek which caused the car to want to stall out when clutching in at low speed.
 
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S
#5 ·
I called MSD and talked fellas at the local speed shop and they are all baffled at the though of connecting Vac. Adv. to full manifold vac
Click to expand...
Bit of history lesson /background...
The chsances are if u asked them if they where around in the pre EGR pollution days, and as mentioned in the 101 ...those engines run with manifold vaccuum.
With egr and recycling gases etc mixture timing becomes avery different kettle of fish.

Back in the days of manifold vac VA...and as u allude to, yes there was consevitism re advancing/ permormance... warrantees etc....So quickening up the curve bit, lighter springs and a couple degrees extra total above factory tented to get these factory engines jumping quite nice....
BUT those who neve knew much about timing , whys etc .. would simply up the intial a heap... in effect giving more advance early in the curve (basically same as lighter springs) but wacking in a heap more total...then they got the idea that more advance more power... so advanced engines up till a couple degrees below detonation.
What happens here is as a engine is advanced it sounds nicer, but is lik an optical illussion and power drops off peek very slow.
GM's ZZ Manual instructs to set at 30 total - all in by 3,000 RPM.
Click to expand...
The general princplesof consevitism still apply so adding 2 or 3 degs... even 4 , ans dropping a couple 100 rpms should give u that little extra .....keep in mind 3000 rpms is already reasonble low total for a street car.

With nonm egr manifold vaccuum , intial is kept low to add easy start and take load of start motor armitures..I like below 12 , 14 in my books is starting to push limits..but ok.
The VA is all in at the engine vaccuum , no VA , at idle rpms (requires adjusting speed to establish the engine vac ) or at least no more than 1" above this.
engine fires in intial, the engine pulls a vaccuum , so idle is intial +VA.
Or one establishes the idle advance, and the intial is a matter of juggling the cent and VA to suit.
With ported vaccuum the is no VA at idle... or there is not meant to be as there should not be enough vaccuum above the closed butterflies.....but in saying that some engines have VA canisters that start at very low vaccuums...2.5" where as manifls VA generally start arounf the 7".
 
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#6 ·
Woke up in middle of night and first thing that hits the brain is checking to see if anyone replied to my timing question so good to know that my ramble made some sense and really appreciate all the feedback, tips, history, etc. All this gives me exactly what I need to get back out there after the rooster crows and try again.

Steiner - I also thought was peculiar that I could only achieve high 20s total timing with the blue bushing which doesn't correlate with the MSD charts for using that sized bushing (I believe they tag that bushing at providing 21 deg. mech. adv. as general guideline). MSD tech also mentioned that digital guns can cause issues with readings so I got out the old Craftsman timing gun that my Dad gave me (vintage 1970). It still works and read initial and total adv. consistent with the newer digital gun. I'll use the old gun going forward just in case. With the ZZ383 plastic timing cover, it's nearly impossible to get a direct read on the tiny timing pointer that's cast into the cover so I bent a clothes hanger in fashion that attaches to cylinder head acc. hole and extends down and over and aligns with pointer notch so I can see a reference line across the balancer and #s. This GM balancer has the #s stamped into the shell and is what I'm referencing. I've never used timing tape and not sure if needed with this type balancer? The #s are in increments of 10 with each line in between indicating 2 degree intervals. Does this sound like an appropriate method to obtain total adv?

I'll reconsider my inclination to take into mid-30s total adv. Those are some mighty fine #s you got with 30 deg!! So for now goal is to establish a reading of 30-32 total and undestand better why blue bushing wouldn't provide that (& beyond) with the 14 initial I currently have set at.

Steptoe - thanks for the detailed expl. Checked out your pics - you spare no mercy on that beautiful blue Camaro (parade dutie and towing the ol wagon - good for you) and looks like you are fortunate to live in some very scenic mountain country so you and I may share high altitude carb tuning stories somewhere along the way. I've often wondered if high altitude (less dense O2) also impacts/reduces engines ability to produce vacuum - have you noticed that to be case?

Thanks again to all for the help.
 
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#7 · (Edited)
NEW DEVELOPMENT: This morning I took off the dist. cap and inspected internals, cleaning and was checking if the mech. adv. mechanism was possibly binding. It does move but maybe only 1/4" out at best. However, in the process I found 1/2 of a broken snap ring (about the same diameter of a penny) laying along side one of the elec. comonents inside the dist. Just by chance the light hit it at the righ angle as my finger moved it to notice it. Possibly the other 1/2 is binding somewhere and causing mech. advance to be limited.

The larger sized snap ring toward bottom (about diameter of quarter) is in place. I have to assume this other snap ring served a purpose in the dist but don't see it referenced or depicted in the MSD manuals. When I pull up on the rotor assembly I get approx. 1/16th - 3/32 up/down movement. I noticed when up it moves up the rotor position moves slightly a couple degrees. Is this movement/play normal? I see a spot where this snap wring possibly could be installed and would take up that slop? I need to pull the dist and inspect further after I bring Wife her Sat. morning coffee :) In meatime, anyobody have feedback on this new finding - possibly an image that shows a 2nd snap ring that the MSD photos don't identify?

I think it's the wring shown on page 15 of this all incompassing MSD product manual. I should probably just get a new dist. as this one is about 15 years old and concerned about what may have happened to it as a result of this snap ring breaking. I never removed it myself so possibly it got stretched too far when originally assembled. Don't see how the normal day to day operations of the dist. could cause it to brake - interesting.

http://www.msdignition.com/instructions/Products/Tech_Info/WDTN.PDF?terms=WDTN
 
B
#9 ·
68RS-SS said:
I should probably just get a new dist. as this one is about 15 years old and concerned about what may have happened to it as a result of this snap ring breaking.
Click to expand...
I would probably check with MSD first and see if they have a rebuilding service. I think they do. That would be less expensive than a new one.

Check here at this link.
http://www.msdignition.com/page.aspx?id=3366
 
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S
#10 ·
I must explain somwething further about ported and manifold and Stiener, who also knows how things work... And stiener is working on ported.
Never loose site of the reason for advance....
To put the max piont of explosion at a given rpms, engine load, and other varibles like compression rear axle ratio, at a given point between 12 and 18 degs for any particular engine.
Much of it is juggling amounts in cent and VA...
Since most of my dialing in has been on a wide varitey of engine , and fuel types, trying to get ported reasonably accurate between idle and total rpms ....getting the end pionts of idle and total is easy....and the bits between cruise loads, with ported is usually a mission, if not impossible.
But going with the traditional method old car manfactures used , damn near world wide...setting the intial , idle , and the total....the bits in between....and all in at cruise ends up damn close very easy.
I also note....
If I dial in an engine one day... then say do the same engine a few months later ....the chances are the amount of degs in the cent and VA, intial, all up (intial+cent+VA) will be slightly different, but the total , idle will be damn near the same, withing a deg.

you spare no mercy on that beautiful blue Camaro (parade dutie and towing the ol wagon - good for you)
Click to expand...
No.. it has been the daily driver, go to work tow the firewood, vintage cars, do the shopping for 32 yrs...and at the moment (a thread last week) is off the road for its 12yr extenal body rebuild....counting on about 9 weeks.

I've often wondered if high altitude (less dense O2) also impacts/reduces engines ability to produce vacuum - have you noticed that to be case?
Click to expand...
Well to get to alpine type altude is about 400 miles away, hills inbetween not an issue, and my tuning is done at an altitude of about 30'
From your comment u have a in cab vaccuum meter... good man...very important to watch and know the vaccuum charactorists of the engine to be able to dial in the specs.
Altitude... yes, high altitude means less O2, less Os means lean mixture, I not only loose a little vaccuum (basically the engine tune now out of spec... mixtures and timing for those mixtures) but power, and its the only time the engine runs warmer, above 5 deg of the thermostat rating...when high enough to be close to summer snow line in the central platuo of the Nth Island of NZ around Mt ruapahu... Even in winter in the snow at those levels get warm lol...hit 195 to 200 degs water temp.
 
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#11 ·
Update: Pulled my dist. and took it town and compared to new one at the Albuquerque speed shop (Yearwood). Found the other 1/2 of snap ring stuck to magnet. The mech adv. hardware was binding, wouldn't fully extend and once it was out, it stuck there and wouldn't return w/o force applied. This unit is about 15 yrs. old - the new version (same part # 8361) doesn't use this snap ring.

Thanks for the tip Bryon and I'd send this one to MSD but only a few weeks left in the driving season where I'm at and trying to get her ready for the last shin-dig at the Alb. dragway this coming wknd - the Hot Rod Rumble - only pre-73 cars allowed and is a great event - girls come dressed up in high heel rebel themed 50s outfits along w/the hair-dos and lots of rods and customs and show cars attend. The track allows special times for those running their virgin show cars down the track (last year there were a couple decked out twin turblo 1st gens). Possibly members here....

Anyway, I put the new dist. in and also got some new wires polished aluminum wire looms that encapsulate the wires so hidden as they travel along the valve covers - really trick pieces that look awesome on there. Course they don't come with brackets that work well so had to make 4 brackets and with running new wires and all, time & energy ran out before I could get her restarted, timed and tuned again. Also put some larger-sized air bleeds in the carb to help me be able to turn out my idle adju. screws a little more (currenly 1/2 to 3/4 out) and assist with a lean mixture I've been getting when transitioning off of tip in off of cruise (which could have also been as a result of dist. not working properly).

So Steiner, you were right on about noticing that total timing should have been higher and I'm relieved to have found the reason why.

Looks like 65 and sunnny today so looking forward to wrappin this up and gettin a nice cruise in.

Steptoe - yes, as of late I've been discovering benefits of using the vac. gauge - finding it to be very helpful data. Always somethin new to learn. The other day I had 2 timing lights connected (new on for RPM only), vac. gauge, tubes running all over, etc. and for a moment or 2 felt like a surgeon performing an operation checking all the vital signs, etc - in many ways mechanics and surgeons have a lot in common. Sounds like your car has definitely earned the attention you give it. Have fun with the 9-wk project.
 
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#12 · (Edited)
With new dist. and timing set at 13 + 18 and total of 31 at the balancer looks like my timing is working properly. I hooked up vac. adv. to full manifold vac and had to lower idle by about 200rpm after doing that. Took out for test drive. Stumble/lean cond. from light cruise to tip in is cured.

However, when I mashed the pedal I got a 4-5 sec. delay/stumble/lean cond. Pulled over and backed out the vac. sec. screw 1 full turn (new Quickfuel carb that I'm still tweaking on - their design uses a screw instead of changing springs like on Holleys). That helped a lot so backed it out another 1/4 turn. Still have a 1-2 sec. lean cond where A/F pegs on the lean end prior to engine picking up and from there pulls well and stays in good A/F range -12.5 - 13.8 for as long as I have it floored.

So possibly I need to back vac. sec. screw out a little more but wanted to check opinions if this seems like right thing to do. Car is running good at idle, cruise, tip-in, etc. but just this hesitation on full throttle seems longer than ideal - it doesn't run lean or rich in normal driving. Want to make sure I'm not masking another tuning issue by backing out the vac sec. screw too much. I suppose that would eventually show up as rich condition - sound right??

Also - I changed idle bleeds form 70s to 73s on all 4 corners thinking this would help me be able to back out my idle mixture screws a little more (currently approx. 3/4 turn out from bottom) but that didn't seem to help much - possibly too small of a change to make a diff. My thought was by adding more air to idle circuit, I could backout mixture screws to be richer. Maybe I don't need to be concerned but have read posts that seem to indicate that 1.5 turns out is ideal range - not sure if that really matters??
 
#13 ·
With a 4 corner idle carb, I usually end up at 3/4 to 1 turn out on the idle mixture screws. The secondary setting with the quick fuel piece, is the way to go, I have 3 of them and would never go back to springs. The factory setting is 1 1/2 out, I am now at 2. Make a move at half a turn at a time to get it close. keep opening it up until you get a hesitation, you may be already out to far. Use the orange cam in position 1 as a start, nothing bigger than .031 squirters.
 
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#14 ·
1971ls6 said:
With a 4 corner idle carb, I usually end up at 3/4 to 1 turn out on the idle mixture screws. The secondary setting with the quick fuel piece, is the way to go, I have 3 of them and would never go back to springs. The factory setting is 1 1/2 out, I am now at 2. Make a move at half a turn at a time to get it close. keep opening it up until you get a hesitation, you may be already out to far. Use the orange cam in position 1 as a start, nothing bigger than .031 squirters.
Click to expand...
Thanks for sharing that info. I'm sorta confused about htis cause I started with a huge hesitation and as I'm backin out th adj. screw and it's getting better but still have hesitation and my A/F is showing very lean during that period. Do you think that the standard method where backing out until you get bog may be relative to a rich condition vs. what I'm getting?
 
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#15 ·
Called Quickfuel and ran it past a tech and now more confused. He says put in larger squirter. Then he says put 4.5 PV and then later changed his mind to 8.5 PV. Then said scrap all that and put in 2 sizes larger primary jets. I guess I'll just have to keep trying different things until I get it right.

1971ls6- why do you recommend not going w/larger than 31 squirter? 31 is stock in this carb.
 
#16 ·
If you are mashing the pedal WOT, power valve number won't matter, vacuum will respond quickly and it will be all in. If you are truly going lean then getting to your desired WOT AFR, sounds like a pump shot problem. Squirters determine the rate of delivery of the pump shot, the cam determines rate and total volume. Which pump cam and which hole? Even more basic, do you have any slop in the arm actuator? It doesn't take much slop to cause a lean spike.
 
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#20 ·
r_w said:
If you are mashing the pedal WOT, power valve number won't matter, vacuum will respond quickly and it will be all in. If you are truly going lean then getting to your desired WOT AFR, sounds like a pump shot problem. Squirters determine the rate of delivery of the pump shot, the cam determines rate and total volume. Which pump cam and which hole? Even more basic, do you have any slop in the arm actuator? It doesn't take much slop to cause a lean spike.
Click to expand...

Thanks for the response. It is most definitley going lean cause the A/F gauge spikes above 17 and corresponds with the stumble/hesitation.

I visibly checked pump arm linkage and observed that as soon as throttle is pulled the pump arm is moving down w/o any delay/gap. I imagine you don't want to preload the arm/plunger but let me know if I should.

There isn't any visble gap between the plunger arm and spring loaded mechanism that pushes it. I see where your comin from though and will check all this again. I left in the stock cam and hole position but not sure which they are on a Quick Fuel. Will check that tonight.

I have to conclude that the Vac Sec setting was nearly all the way in from factory cause I backed it out 1.5 turns the other night from original setting and checked it last night and was only 1.75 turns out which means it was only 1/4 turn out when I was getting the 4 sec delay/bog/stumble conditon which didn't help matters. At 1.75 turns it's still an issue though. From my Holley experience in changing springs I would not experience such a lean/hesitation issue when swapping springs which makes me wonder if is this issue is being caused by someting else (such as the cam/hole position or linkage adustment that you mentioned). But this is somewhat apples to oranges as relateds to vac sec engagment and I don't know where Quick Fuels bottomed out positon compares to Holley's stiffest spring.
 
E
#17 ·
What color bushing are you using in the new distributor and which springs?

The amount of initial advance can have a big impact on how the car responds to quick throttle stabs. If you don't have enough initial timing and/or if the curve is too slow you can definitely experience hesitations at no fault of the carb.

Generally speaking, the larger your camshaft (from stock) the more initial timing you will want to have. Personally, I've always had good luck with the black bushing which provides about 18* initial timing when total timing is set at 36*.

I also like using springs that allow base timing at idle and rapidly adds timing starting about 1200-1400 RPM and has full advance by about 2500 RPM.

I also prefer to use ported vacuum advance -- I personally believe using manifold vacuum advance causes too unstable of an idle.

Once you have the distributor set up (as mentioned above) if you still have a hesitation I would go smaller with the low speed air bleed. Try a .006 smaller bleed than what you have in there now.
 
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#21 ·
Eric68 said:
What color bushing are you using in the new distributor and which springs?

The amount of initial advance can have a big impact on how the car responds to quick throttle stabs. If you don't have enough initial timing and/or if the curve is too slow you can definitely experience hesitations at no fault of the carb.

Generally speaking, the larger your camshaft (from stock) the more initial timing you will want to have. Personally, I've always had good luck with the black bushing which provides about 18* initial timing when total timing is set at 36*.

I also like using springs that allow base timing at idle and rapidly adds timing starting about 1200-1400 RPM and has full advance by about 2500 RPM.

I also prefer to use ported vacuum advance -- I personally believe using manifold vacuum advance causes too unstable of an idle.

Once you have the distributor set up (as mentioned above) if you still have a hesitation I would go smaller with the low speed air bleed. Try a .006 smaller bleed than what you have in there now.
Click to expand...
New Dist has the black bushing (18 deg) with one stiff silver and one light silver. Initial is about 13 and total is about 31. According to MSD chart it should be all in by 3000 with the springs I've chosen and need to confirm that. I do know I was getting 31 but not sure exactly which RPM it all came in by. The vac. adv. is currently going to full manifold. I can try swapping back to ported which is where I've always had it before but my idle has no problems maintaining.

I was told by Quick Fuel tech that low speed only impacted idle circuit - do you not agree?
 
S
#18 ·
Just to make it more fun, a rich misfire will sometimes show up as a lean condition on the O2 sensor. Try running with the vac advance line disconnected and the carb port you pulled it off of plugged. If the stumble goes away, then it might be an issue with the transition of fuel shot plus vac advance timing not dropping out fast enough causing a rich misfire.
 
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#22 ·
Steiner said:
Just to make it more fun, a rich misfire will sometimes show up as a lean condition on the O2 sensor. Try running with the vac advance line disconnected and the carb port you pulled it off of plugged. If the stumble goes away, then it might be an issue with the transition of fuel shot plus vac advance timing not dropping out fast enough causing a rich misfire.
Click to expand...

Wow - that sure does make it "more fun." Now I've got to deploy reverse phsychology on that unit. Good to know all the possibilities - thanks. I don't get an actual backfire during this and I was wondering how an O2 sensor would read unburned fuel in the mix.
 
S
#19 ·
Any mis fire will come up as lean on O2 sensor...
2 to 3 sec hesitation is huge...
Is it only at very low rpms , below about 2500 or when stomp on it above this .
What is your total advance intil + cent and at what rpms is it all in.
 
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#23 · (Edited)
Steptoe said:
Any mis fire will come up as lean on O2 sensor...
2 to 3 sec hesitation is huge...
Is it only at very low rpms , below about 2500 or when stomp on it above this .
What is your total advance intil + cent and at what rpms is it all in.
Click to expand...

I tried stomping it coming off of various RPMs ranging from 2,000 to 4,000 and getting it failry consistenly across the board. After backing out the vac sec. screw 1.5 turns from factory setting (.25 in my case - qaulity control missed setting to 1 or 1.5 as I understand is prescirbed by Quick fuel) and it drastically improved and was not so noticeable when doing a few 1/4 mile type runs from a stop but still too much hesitation when stomping off of mid-to-higher rpm range.

Disclaimer - all this test driving was all done on a desolate hiway road w/o any cars in either lane within a 1/2 mile of me. That's how it is out here - a person can find plenty of open roads w/o traffic which is nice. :yes:
 
#24 ·
Ok, try the .035 squirter. ONLY MAKE 1 CHANGE AT A TIME. then keep turning out the vacuum secondary screw 1/4 at a time. I have not gone past 2. I do all my testing for carb setup at 2000 rpm in first and mat it and see how quick second gear comes and then how long it spins the tires in 2nd. I have always used full manifold vacuum for the distributor. I like 18-20 initial and 38 total for big blocks on the street, all in by 2800, but not starting before 1200. Limit vacuum can to 10 degrees. For me the 2000 rpm number is the crucial rpm, as if too much timing is in at my cruise rpm, it starts surging and hunting. one change at a time
 
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#25 ·
OK ..
I suggest start at the begining .. factory settings..
u have no egr therefore u need a ball park dizzy timing curve.
Which is about 10 to 12 intial, an idle around the 16 to 18 , higher if have a real wild cam
The VA all in no more than 1" above at idle rpm, mixture and idle speed screws temp adjusted at intial advance engine vaccuum....usually between 7 and 12 "
The VA makes up the difference between intial and idle advance ...
The cent all in around the 3000/ 3200 rpms .. and total, intial+ cent ball park 32 to 36 degrees.
The carb...idle mixture and idle speed screws adjusted for stable idle at idle advance 1.5 turns out +/- 1/2 turn... if they are outside this adjust the secondaries so they.
With these basics set...
The idle AFR should be around mid 14s to mid 15s depending on how much cam in the engine....then look at setting jet AFRs around the low to mid 15s.
knowing ALL that is correct... and the above can cause hestitions if not...and u still get hestation , then and only then do u start to look at messing with accel pump adjustments.
then set carb jets for cruise around the mid 15s AFR
 
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#27 ·
Steptoe said:
OK ..
I suggest start at the begining .. factory settings..
u have no egr therefore u need a ball park dizzy timing curve.
Which is about 10 to 12 intial, an idle around the 16 to 18 , higher if have a real wild cam
The VA all in no more than 1" above at idle rpm, mixture and idle speed screws temp adjusted at intial advance engine vaccuum....usually between 7 and 12 "
The VA makes up the difference between intial and idle advance ...
The cent all in around the 3000/ 3200 rpms .. and total, intial+ cent ball park 32 to 36 degrees.
The carb...idle mixture and idle speed screws adjusted for stable idle at idle advance 1.5 turns out +/- 1/2 turn... if they are outside this adjust the secondaries so they.
With these basics set...
The idle AFR should be around mid 14s to mid 15s depending on how much cam in the engine....then look at setting jet AFRs around the low to mid 15s.
knowing ALL that is correct... and the above can cause hestitions if not...and u still get hestation , then and only then do u start to look at messing with accel pump adjustments.
then set carb jets for cruise around the mid 15s AFR
Click to expand...
Thanks Steptoe - I'm uncertain what you mean by Idle and Initial Adv. Are you saying that Intitial would be reading when Vac. Adv. is disconnected and plugged and Idle would be when is conntected to FULL Manifold Vac and Idle is reset (since connecting to full raises idle?
 
S
#28 ·
I'm uncertain what you mean by Idle and Initial Adv.
Click to expand...
the 1st thing... a street car requires VA.. not only for cruise economy but also for idle advance..
With manifold pre EGR advance curves...
intial is what the engine fires up on.. the engine starts, pulls a vaccuum and the VA is actuated giving intial +VA for the idle advance.
read the old maintace tuning manuals... note how they say "disconnect the VA to set/check the intial" and "disconnect the VA to set/check the total (intial+cent)"
The reason being at BOTH idle and free reving there is enough engine vaccuum to pull the VA fully activated.
 
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#29 · (Edited)
Thanks for all the recent replies/help. This has been highly challenging to smooth out and mainly as a result of the distributor going bad during the same timeframe that I got a new carb. Also - I need to take everyone's advice on only making 1 change at a time.

So yesterday I was retracing steps and rationalized that the car drove well the day I first put the new carb on after I simply jetted it 3 sizes down for high altitude (I live at 7k ft. but the town and drag strip are at 5.5k ft, (which in itself is pretty big diff in alt. so need to find a good tuning compromise) . I think then coincidentally the distrib adv. started to intermittenly acting up, worsening and finally almost completely stuck. W/o knowing that until more recetnly, I was battling timing issues w/carb adjustments.

Then I was battling what I thought was a lean condition due to my A/F gauge going way lean but and then just learned from ya'll that any misfire can show lean on the A/F. I see that first hand now.

All that said, last night I decided to return the carb to setting that it worked well at when first installed. This meant: changed the primary squirter from 35 back to 31. Changed the idle air bleeds from 73s back to 70s. Put the vac. adv. from full back to ported. The only change is I put the vac. sec. screw at 1.5 turns out vs. the original setting of 1/4 turn out (rationalized that only being 1/4 out was a factory mistake).

After making these adjustments and setting idle mixture (using vac. gauge and rpm gauge, I noticed that the front pass. side idle mixture screw could be screwed all the way in and eng. still wouldn't bog. The other 3 ended up at 1/2 to 3/4 turn out for best idle/vac. So I'm thinking could be that the larger air bleeds did help lean out the idle circuit and may end up trying them again. I'll also play with switching vac adv. back to full once I feel things have stabilized.

Drove the car this morning and I thnk we're making good progress and almost there. My A/F still goes high (meaing lean toward 17+) when stomping but not quite as high and engine recovers much faster - about a 1/2 sec. delay. So I'm not sure that this means I'm still rich or possibly lean during that period. Could it be that the 31 squireter is too large? Or maybe I should try messing with the the vac. sec. screw again. I noticed that if I lightly or graudually and even moderatly push toward fulll throttle there is not stumble/hesitation - only when mashing it. Do others get a lean reading on vac. gauges when you stomp to full throttle off of a 2,500 rpm cruise w/gearing ready to pounce? Want to make sure I'm not trying to fight a war I can't 'win...

Also - the pump cam is pinkish in color and screw is in the top hole. Pump linkage is set properly.

Timing was confirmed again - about 12 at idle w/vac disconnected/plugged. I got 30 all in at about 3,000 rpm.
 
S
#30 ·
The chances are u need more advance...
think like this...the advance is to drop the max pressure of expolsion between 12 and 18 degs ATDC...this DOSENT mean u have a 6 deg 'tolerance' every engine has its own very narrow piont with in that 6 deg.
if u have a short block in front of u...piston tdc and push down (explosion) it will not move.. turn a couple degs ATDC, bloody hard, turn further gets easier, the 'explosion' force down has less resistance....BUT the lower the piston is in the cylinder at max explosion the lower the compression it explodes at...so it is a very delicate balance between the 2 variables... there are a heap of others to... fuel type, cam etc...so bit over simplfied to give a picture of what u are doing with timing..
So get the total up around 36....then work backwards till u have a power drop off withing a couple degrees, then advance up a couple...that will put u damn near bang on.
Till u have the bog sorted it is no good setting total accurately, so in the mean time it is better to be a little over advanced, around the 34/36 degs for now.

My A/F still goes high (meaing lean toward 17+) when stomping......I'm not sure that this means I'm still rich or possibly lean during that period.
Click to expand...
1st off lets assume that u dont have a mis fire and timing is in ball park....if u have a very rich mixture u will have partial burn of mixture... including only part burn of the O2 avalible which will give a high O2 (AFR) reading....same as with a ign misfire....this only happens at extreme rich....
If the shot is lean, again a high O2 .....for the same reason.
So I think u may have to go both ways to find out.
A engine should NEVER go lean under load....lower performance engines damage slow... very high performance blow the sump/ block up at 1/2 to 3/4 track with spectacual results.

Another curly.. Im not a fan of sorting these sort of things out with a AFR meter, they tens to have too short sample intervals, if a anolog respond way to slow for accurate readings....they are ok for general use, but to realy get down to it a data logger shows so much more.... espec if hook up imap, throttle postion, rpm and knock . then read back on computer ... even reading off the data logger with lap top beside doesnt compare with raw graphed data
 
6
#33 ·
The car ran great at the drags this last wknd. Better than ever (outside of a couple cool night temp. runs). One of the most important things I've learned in this process is that a rich condition can read as lean on A/F gauge (my gauge is digital and samples/displays at rapid rate). This can be very deceiving to somoene who doesn't know that.

I still get a lean reading when stomping but doesn't last but 1/2 sec. I think this still may be due to a rich condition and need to work that out. It's possible that a smaller squirter may help. Jets seem good as A/F is stable once full throttle is going and also at cruise and moderate acceleration. Idle A/F still rich at 12.5 and will try and lean that with putting the larger idle bleeds back in. 2nd Lesson learned: ONE THING AT A TIME going forward.

Thanks for all the help!!!
 
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