Sounds like your aiming for good tune paremeters on your timing and you really need to get that idle circuit dialed in to start cause that also plays a large roll in transitioning to primaries (off idle stuff). Not sure what you mean by "can't get idle down". Are you saying you have the idle screw backed out all the way and still too fast?

I'm not sure if the VS Avenger series carb has a means to adust the secondary throttle plate opening baseline setting or not but if it does and out of wack, this can cause issues with balancing your idle settings.

Your carb - is the elec. choke connected to power source and is the choke working properly?

Is Vac Adv. disconnected and carb port plugged when timing?

Is that the stock jetting for that carb or has it been leaned out some?

 

As for the idle, yes. I have the idle speed screw all of the way out and when the idle mixture screws are adjusted for optimum rpm/vacuum the idle is approx 1600-1700.
Electric choke is connected and working properly (choke blade closes when cold and opens when warm).
Vac. Adv. is disconnected when checking timing and port plugged.
Stock jetting from factory.
To get idle @ 900 rpm, the idle mixture screws are (1/2)-(3/4) from closed.

 

Also, since it sounds like this is a fresh build and your just getting it going thought worth asking about the valve/rocker adjustments. Did you perform those adjustments and do you feel confident they were set properly? This could play into some of the other issues you describe.

 

Yes, I believe they are set properly, but there is always a chance they aren't, although I doubt it.

 

I had a similar issue with poor transition from idle a while back involving a small block Chevy 406 with a Holley 83670 - 670 cfm Street Avenger aluminum 4-bbl carburetor with vacuum secondaries and electric choke. Take a look at that thread and see if it helps you at all - http://www.camaros.net/forums/showthread.php?t=229442

 

Thank you Scott! :thumbsup: This thread has given me some good insight and has confirmed my suspicions that the engine is 'starved' for fuel, so I believe I will change my jet size to approx a 68-69 pri and 70-72 sec. :yes:

Did changing the primary jets fix your problem completely, or did you do anything else subsequently to further tune your carb(change the secondaries as well)? You mentioned the possibility of doing it, but I'm sure if you said you had. Also, given your cam is about the same lift/duration as mine, what timing set-up do have and where do you connect your vac adv as of now?

I apologize for asking so many questions, but I've been tuning this thing for darn near a year and a half , and I'm ready to nip this problem in the bud. Well, mostly at least.

An old man once told me: "They're never done."

 

As like posts above..
It sounds like u have installed the carb and dizzy oput of the box and not dialed in these to9 establish your tuning specs.
The carb.. u need to adjust the sec butterflies so the idle speed and mixture screws are 1 1/2 turns +/- 1/2 turn
How to described in many older posts.
The "pop" also sounds like u have a lean issue, bew it in the accel pump adjustment or jetting.....ahain how to etc in older threads.
Dizzie

14* deg initial timing 35* total and Vac

your total should be ball park starting point around the 34 to 36 deg at around 3000 to 3200 rpms NO VA ATTACHED....
If u are measuring the degs simply by static reving in the driveway with VA attached, there is no load on the engine, therefore the VA is being activated....under reeal roasd test, the engine is loaded and the VA much lower therefore VA is not actuated.
Also not out of the box HEI systems are set up for polluton/ EGR engines which have a much different curve and combination of cent / VA degs, and have VA on ported vaccuum....generally requires putting in much more cent degs and reducing VA.
Notr EGR engines generaqlly have 20 +degs in the cent and 7 to 10 in the VA unless dialed in with data loggers and knock sensors.

How tos etc covered in many older threads

 

Had the same problem on my 67. Make sure your timing is where you want it then take apart your carburetor and clean her up mine had carbon build up and was dirty I cleaned it all up put it back together then made sure all the spark plugs were good. Turned on the car reset the air fuel mixture screws then the idle screw. And bam! All better. Hope this works for you buddy. When I have a problem i can't solve myself I come on here and I like to hear all the different problems and solutions there might be. Good luck!

 

Hey guys, I've been having a problem with my '68 camaro. Basicly, I have limited power, off idle stumble, and a slight pop out of the exhaust when shifting. It takes alot to get the rpms up and seems to lose power about 4500 rpm. When idling, a distinctive stumble is heard and throttle response is not that great. Also, if I adjust mixture needles for optimum vacuum, the idle shoots up and I can't get it to idle down. I built the car and engine so I know a little about this, but as for tuning, I know little as compared to many of you, after all, I'm 19. Thank you for the help!

Specs:
m21 4 spd
3.08 gears
28" rear tires

This sounds like you have a vacuum leak somewhere. You have to back out the idle screws all the way to get enough fuel to stabilize the idle. Plus the engine won't idle at a slow speed. This sounds like it's too lean because you have an air leak somewhere ; possibly a vacuum hose, carb gasket or intake gasket.

Your combo has a very "tall" gear for acceleration. 3.08 gears with a 28" tall rear tires and a 2.2 first gear = slow acceleration.

For good acceleration you want a 1st gear X rear axle ratio of close to 10.

You have 2.2 X 3.08 = 6.776.

A 2.52 M22 trans with a 3.73 gear would accelerate much faster
2.52 X 3.73 = 9.4

 

Tough to do without removing the carburetor and flipping it upside down on the bench. I also machined on my bench grinder a custom screw bit from a broken #2 screw gun bit that fits in a 1/4" box end wrench and allows me to adjust the secondary idle speed screw (the pain in the A$$ screw on the underside of the throttle plate near the vacuum canister for the secondaries)

As mentioned in many other threads...the soln is simple
take a rivet, flatten the end with a hammer into a small screw driver blade... then bend the shaft to near on 90 degs...u now have a tool to get to the adjustment stop screw under the bottom edge of the carb...while the engine is running...
DONOT close them up so the butterflies sit in the bores... this causes binding and damage to the butterfly alignments and huge tuning headaches

This sounds like you have a vacuum leak somewhere. You have to back out the idle screws all the way to get enough fuel to stabilize the idle. Plus the engine won't idle at a slow speed. This sounds like it's too lean because you have an air leak somewhere ; possibly a vacuum hose, carb gasket or intake gasket.

yes...the "pop" also look to a faulty/ dirty/ or PCV valve incorrect calibration.

 

A step up of 2 jet sizes on the primaries only will fix a lean condition as u describe.
If no ther leaks secondary issue and adjustment now correct...ie idle speed and mixture screws are 1 1/2 turns out +/- 1/2 turn.
1st check the PCV valve is correct rating and clean.
It only be open under light loads to WOT and closed under higher vaccuums like idle and deceleration, lift off throttle.

Basic rule: NEVER make more than 1 change at a time.

 

I will also play with othe advance curve/stop bushing/initial timing set-ups to see what's best.

If u play you will end up going in cicles, with the end result odf a dizzie with a patched / compromised tunining rather than correct.
THE ONLY way to dial the timing is 1st set the dizzie to ball park specs....
THEN dial the carb circuits (idle, power and cruise) individually to correct AFRs
THEN establish independently your dizzie tuning specs (idle advance, total advance, VA degs, VA vaccuum start and finish pionts) locking the the cent weights.
THEN once those dialed in specs are known, then make the internal adjustments to the dizzie....
Then re check your AFRs which if done right will be as close as u can get without getting into laboratory type dial in equipment.
I have been doing this sort of stuff for over 30yrs, on CNG, LPG, non egr, and vintage pre war engines to get them to run as well if not better than they originally did from factory on modern fuels.

 

Well I got the 67's in the primary side, and the #35 squirter. Runs way better. Off idle stumble is gone, idle is good, at 950 rpm, and sounds great. Two things i found as well: first I had a burnt plug wire on the #2 cylinder and an exhaust leak. Both fixed now.

The only thing now is: If I accelerate anywhere above 2500 rpm and let off the throttle to maintain speed(no more acceleration), I get the same "backfire" popping from the exhaust. It also occurs if I accelerate at a slower rate and exceed 2500 rpm.

Is this a lean condition issue? It's the same popping as described in previous posts.
If it is a lean condition, do I need to move up jet sizes in the primary again to say, 69.
And if so, do I need to change the secondaries to two sizes greater? (71)
Current jetting: 67 pri. 68 sec. #35 shooter #65 power valve silver spring in the vac. sec.

 

So it appears going richer is heading in the right direction. Now the task is to find when the primary and secondary circuits are each rich enough. When you are getting the popping out the exhaust, are you still in just the primary circuit or are the secondaries opening up?

To determine this you can temporarily wire the secondaries closed and see if anything changes. What you describe sounds like a lean condition to me. Have you pulled the plugs to see what they show as fas as mixture goes (both before and after re-jetting)?

 

Once you get the primary jets sized correctly you can work on the secondaries. Read the plugs and see what they show and proceed from there.

 

Well I changed the power valve to a 4.5 from the 6.5 and the engine performed about the same but idled to high. I tested vacuum reading at idle (1000rpm) and I have 15-16 in Hg. So I went back to the 6.5. This coming weekend I will try 69 pri and 70 sec jets and see where that gets me.

Will the increase in jet size also increase idle speed and change my idle mixture adjustment?

Once again, thanks for all of the help guys. I really appreciate it.

 

Will the increase in jet size also increase idle speed and change my idle mixture adjustment?

In therory no.. in practice maybe... and most likely due to having the carb apart , very minor changes in gaskets etc...
u have to think of carbs like this
u have 3 separate carbs
1 for idle... the mixture screws
1 for cruise, engine brake.. the primary jets
1 for full on high rpm loads power... the secondary circuits/ powervalves

And a accelerator pump to transition from one to the other
EG if u richen mixture screws up then the amount of mixture change is insignificant to make any significant and in most cases un readable changes to the mixtures once over about 1000/1200 rpms.
Any change in jet sizes , will not change idle mixtures any significant numbers as the amount of air that passes them in the venturi is not enough to create enough suction

With modern unleaded fuels , reading plugs is a very hit and miss unreliable method of reading mixtures, as timing and other things influence greatly also.....change a mixture u change the efficiency of the burn and lead time to max explosion... cyclinder pressure.
Also if have header, u can get scavenging and other effects taking place outside the rpm ranges.
If u want to dial in your tuning specs, the only real way is a Data logger at least and min a AFR gauge... digital one... anolog take too long to respond.

 

In addition to the good advice offered, a couple of things -

A trick I learned when working on Holleys and trying to figure out if it's the primary or seconday side that is causing the problem is to disconnect the secondaries and run it through it's paces. Easy to do on a double pumper, should be easy on a vac sec carb as well - just disconnect the rod from the vac pot on the pass side. If the problem disappears it is likely on the secondary side and if it stays likely on the primary.

Check your spark plugs for a cracked ceramic insulator and inspect your cap and rotor for cracks as well as crud buildup. A check of the plug wires with an ohm meter wouldn't hurt either.

 

Check your float level also. Chased a stumble (off idle and at cruise)in mine for months. Brought the float level from bottom of site glasses to midway up and stumble was gone.

 

Check your float level also. Chased a stumble (off idle and at cruise)in mine for months. Brought the float level from bottom of site glasses to midway up and stumble was gone.

Bazaar. My AED Holley is spec'd the same. The world is flat.
Of course that can't be done without clear sight glasses.

 

14* deg initial timing 35* total and Vac. avd. attached to ported vacuum

Why ported and have you tried a full time source? Changing the power valve didn't change the idle speed. With 15" idle vacuum you can use an 8.5 or 10.5 unit. Popping from the exhaust during deceleration is usually from a slight exhaust leak. Check your header gaskets.

 

Yes I have, pulls way too much advance at idle. Also MSD recommends it, so I followed for now. I'm looking into another can for use with full vacuum. There's a lot of ambiguity as to which to use. I read "Timing and Vacuum Advance 101" awhile back, but until i put a vacuum pump on the advance can, I'm not changing it. I've got 15-16" Hg at 950 rpm.

 

Never been a fan of clear sight glasses..Pretty thu....only real way to adjust level is lower the level below the plugs, pull the plugs and then slowly increase height till when u grab the fender give the car a good shake , u can slosh a bit out.

 

Steps, do you think that such a high float level is common with sight glasses???? After all, 'level' is a relative thing.

 

Nunno Fred... dont really take much notice as to what was before... one tends to simply go in and do set the basics.
, cleck float valve is clean THEN ..set up float level so slooshes.. stuff what the pretty clear window 'says'
set the accel pump clearance..
check how much play in the butterfly bushes
set the butterfly bushes
set idle speed and mixture screws to the appropriate idle advance for the engine.
10 mins.. stand back and ask , do u like that now?
not rocket science stuff really

 

Until you go want more power and go blow thru lol

 

There's a lot of ambiguity as to which to use. I read "Timing and Vacuum Advance 101"

How to set up and mod VA cans, the difference between non egr AND egr dizzie setups and why... that off the shelf "ready to run " dizzies is BS..and modern off the shelf dizzies are ball park set up for EGR engines.
Thread after thread for several years now.

Oh and that most VA are all in at below your engine...and that will also be part of your idle timing if put on manifold vaccuum.....and that for non EGR u will need over 20 degs in the cent.
And that u need to have the carb AFRs in ball park BEFORE setting up timing.
Doing it around the wrong way is like drinking a glass of beer before u have even opened the bottle

So what is ambiguous?

 

I may have figured out why my idle speed surges when coming to a stop. When I am slowing down for a red light, stop sign, etc and I push the clutch, the idle surges up to 1500 or so and sometimes as high as 2000 rpm. If i give the throttle a hard smack, it goes back down to 900 rpm. Sometimes I have to hit it a few times to get the idle back to normal. To me, it seems like the fuel pressure is too high during cruise and is blowing the needle seat seals and filling the bowls up above normal operating level-at the bottom of the sight windows.

Proposed fix: Use a fuel pressure regulator set at 5 psi given holleys don't really like fuel pressure over 5.5 psi.

Does this seem like it would fix the surging problem. I had a friend with the same problem who did this and completely fixed his problem.

 

All else4 good either u have some crap in the float valves and/or too high pressure, and waaay too high.
And also have a very goods look at your throttle linkages, binding somewhere....fix the bind do not simply add a bigger return spring, u will screw the butterfly bushes and have a similar issue for a different reason.

 

Well, I put the fuel pressure regulator on and set it to 5.5 psi, and the same thing is still happening. I haven't adjusted the carburetor at all since the install though. I will do so soon.
My friend's '99 lumina blew an intake gasket Saturday and he needed the car to get to work Monday morning. So that's where my weekend went. The oil was like milk.
Back to the relevant issue: Should the addition of the fuel pressure regulator instantly fix the problem, or are adjustments needed to make everything right?

 

Well, I put the fuel pressure regulator on and set it to 5.5 psi, and the same thing is still happening.

All else4 good either u have some crap in the float valves and/or...

A pressure regulator doesnt fix crap in the float valves.

Another old school side of the road trick that usually clears jets etc,,, remove the air filter, while running take rpms up a bit and put your hand over the carb almost choking it engine,very high vaccuums..then suddenly remove your hand... repeat 3 to 5 times.

And look for binding in linkages and butterfly bushes , misaligned butterflies etc.

 

I don't think I have trash in the float valves/needle seats, but it is a possibility.
As far as I can tell, no binding of the linkage.
Why would a binding linkage exhibit symptoms like I have (high idle when the clutch is pushed in, then after hitting the throttle hard, idle goes back to normal)?

 

I don't think I have trash in the float

I really despise the "I dont think" phrase... sry m8 dont mean to sound gruff....
but its not about thinking or a55 u me... its all about knowing 100%
crap in valves / carbs is about 95% certain to have after a car resto that has taken a while...a while off the road....deposits flake off in fuel lines... gum forms inj tanks, espec with modern fuels... and to put in perspective .. it was a common issue back in the days of leaded fuels to.
One can add filters, flush fuel lines and still happens and often a couple times over a few weeks.

 

I understand where you are coming form, but this is a brand new car almost with new stainless fuel lines and new stainless tank. The car has been on the road since June 2012 and the car has had this high idle problem ever since it first started. I had many big problems before, but now those "big" things are fixed. Just trying to iron out the kinks. Its always an option that there's trash in the fuel lines, but I've cleaned out the carb numerous times changing jets, power valves, etc and the same high idle problem comes back.

 

Two thoughts/possibilities;

Are you sure your choke isn't coming back in a small amount causing the first step of the fast idle cam to increase your idle rpm's, with the cooler weather moving in this could possibly be happening.

Lean idle mix and/or vacuum leak causing higher idle rpm?

 

Nay on the vacuum leak and most likely a no to the choke coming in because the car was doing it in the summer. Then again that may have been for another reason. worth looking into.

Once I pulled the car off of the road after driving wile it was doing the high idle thing and took off the breather. I didn't notice the choke blade being closed/semi-closed but that doesn't mean it wasn't. I opened the throttle very rapidly with my hand, and right back to 900 rpm. I didn't check the float levels at that time though (like and idiot) :sad: