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Discussion Starter #1
I've left off the vac advance on the motor, seems it ran better and was more tunable, but after reading many posts, I've decided to hook it back up. OK, Got it hooked to the ported source on the 750dp. Timed it with it disconnected, the dist. is a billet MSD, 6A box. Runs great WOT, and idles a little high, but at 3000 to 3500, the motor pops at light throttle only, I hit the gas and it stops, I thought it was overadvancing, but the plugs look perfect. Leaning out because of part throttle and light pedal? the dist. has light springs in it and all mechanical in by 2500. Any ideas?
 

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What carb?
Is it popping through the intake or exhaust?

If the intake, either a little lean or maybe the plugs are a step too warm.
I'd try a size bigger on the primary jets.

I had a pair of IH trucks that used to do that. You'd get on the highway and about every 15 to 30 seconds, COUGH!------COUGH! on and on. One size bigger jet and it was fine.
This was a holley two barrel on a 404 V8. By one step bigger, I mean two jet numbers on a holley.
A slightly low float level could cause it too.

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[This message has been edited by davidpozzi (edited 07-12-2000).]
 

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Vacuum advance cannister has way too many degrees of supplemental advance.

Cannister is on wrong port, NEVER use ported vacuum unless you have an EGR valve. Early performance engines don't have EGR valves.

Vacuum advance is only supplement for idle and cruise modes (helps stabilize idle against converter, gives better mid-range fuel economy), should never be used as an added top end timing provider.

EGR engines need the added ported source top end timing advance to build heat in the combustion chamber to properly burn the added recirculated gasses from the EGR system.
 

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Your vaccum can may be adjustable. With some, you can use an allen wrench thru the vaccum hose hole to limit total vaccum advance.

The main benefit of the vaccum advance is keeping the plugs clean and better mileage. As IgnitionMan says, it's supplemental and not power producing.

Many pre-EGR cars came with ported vaccum for the vaccum advance. I have several trucks that start at 1961 and up that are that way.

I believe the ported spark helps the engine idle down faster. With a performance engine with longer duration cam, and especially with an automatic trans, full vaccum advance at idle should help the engine run smoother against the torque converter.

If you tried to provide anywhere near the amount of advance provided by a stock vaccum advance distributor by using only the mechanical advance. You would have trouble starting the engine because it would kick back against the starter.

When you get into an aggressive mech advance curve, then you have to limit the vaccum advance, or you'll have too much. Also a heavier springed vaccum can is needed so when you punch it, it will drop the vaccum advance back quickly.

I would think the MSD distributor would come with the proper vaccum advance amount, and not be as bad as a stock distributor.

It's been a while since I've done a street type curve and maybe the fuels used now are making things a little different. But I think what I'm saying here is pretty close even today.
You can run just mechanical advance on the street, but you need to jet lean, have fairly warm plugs and plenty of mechanical advance that comes on quick, with a lot of inital advance. I think I ran over 20 degrees initial advance on mine.
If you can get the vaccum advance to work, you might have better mileage, and cleaner plugs. The horsepower is in the mechanical advance. But if the mech advance only causes the plugs to foul then you can loose power.
David

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Check my web page for suspension info:
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65 Lola T-70 Can Am Vintage Racer

[This message has been edited by davidpozzi (edited 07-21-2000).]
 

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MSD distributor should have an aftermarket B26 ID vacuum advance, stock GM type, p/n 1973448, starts 5.0/7.0 in/hg vacuum. NAPA list part VC1808 (discontinued), 20 degrees @ crankshaft @ 7.0 in/hg, NON-ADJUSTABLE.

Way too much added crankshaft timing, ported or otherwise.

Crane 99601-1 is same mount up as MSD and stock point GM dist, has adjustable features.

About a year ago, MSD had a big dealer promotion, selling "overstock" B26 cannisters, real reason, way too much crankshaft added timing. Don't knowe what they use now, but if it's as much as the B26, you really won't ever get the thing to work. If I had to use the B26, I'd make a stop for it, reducing the pin travel with a stop, coming from the diaphragm end of the pin, and cut at least half the travel right out of that one, maybe a bit more for a big block.

Stock vacuum cannisters have the traditional cone type end of the cannister, just behind the hose fitting, adjustable ones always have one with a hex shape to allow for the adjuster, looks like the ones found on Ford and Chrysler cannisters, which are adjustable from their factorys.

I'm kinda stuypid on this stuff, I just convert these MSD, Mallory and stock point distributors to HEI or MSD trigger units for a living.

Dave Ray
DAVE's small-body HEI.s
 

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Discussion Starter #6
Wow, thanks for all the info guys, Yes the vac adv. can is a B26. I set the dist. up to be all in by 3000. not 2500 like I said before.( I rechecked it). Thats where the popping starts. I believe its out the exhaust but can't verify, (only does it when I am on the road at 3000 to 3500 very light throttle) so I believe your both correct. My plugs were a little sooty beffore I hookeed it back up, Now the are nice and clean. The can is advancing the timing past the 50 degree mark, 55. and since I am giving it only light throttle, it would pull alot to vac and over timing would result.which I can fix by puuting larger jets in or just getting a different can. It popps even on the other vac source at the bottom of the Holley 750dp.Which confirms that since I have light throttle popping, it should have hi vaccum at either source. Does that sound like I'm talking out of my **s or what? Let me know if I am, Thanks for replies, this will help greatly!!
 

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Pull the vac can you have off the distributor, drill and tap the mouonting plate, between the cannister and the pull pin, make the hole 8/32. Then, make a bar, .065 steel, any old metal will actually do, slot it down the middle, then install it onto the vacuum advance cannister, and place it so just more than half the pin travel is eliminated. Reinstall the vacuum advance, make sure the screw for the limit bar doesn't lock the pin pull bar down solid. Retime the dist to get about 10 to 12 initial degrees timing at the lowest idle you can get, then reconnect the vacuum advance to full intake manifold vacuum.

What you now have is an easy starting engine, not over-advanced, with about 20 degrees timing at idle (initial and vacuum advances), and a curve that doesn't give 45 to 50 total degrees. The vacuum advance falls off when the throttle is opened during acceleration, the mechanical curve works, and some small amount of vacuum advance timing is readded when the cruise vacuum returns to somewhat normal levels.

Jet the whole thing AFTER getting the timing curves right, or the carb will come out wrong until you do.

Point here is, don't cover up the timing problem with trying to rejet it away, fix the timing, then go for the carb.
 

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Ignition Man,

You're saying not to connect to ported vacuum, so a non EGR motor should be connected to full vacuum? I am just trying to get a good understanding because I have an MSD 6AL and Distributor on my new motor and don't want to run into problems. My motor is a small block. Also how do you know if you have the B26 canister? My cam range is 1500 to 6500, where would I want my timing to come in at?

Sorry to piggy back this post but this interests me.

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[This message has been edited by CA420 (edited 07-14-2000).]

[This message has been edited by CA420 (edited 07-14-2000).]
 

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Discussion Starter #9
THe B26 was stamped on the vac adv can. on the mounting boss. tou can see it even when installed. also you will know when you hook it up and rev the motor up, the timing will advance 15 aditional degrees, which in my case, is too much.
 

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And, 68, if you only get vacuum advance when you rev it up, you have the feeder hose on the wrong sourvce, ported, instead of the correct full mainfold vacuum. You want the supplement of the vacuum advance to be at idle, not at upper-mid and top rpms.
 

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Ignition Man,
Great info,
I'm amazed they would put an advance can with that much advance in a perf distirbutor.

How about putting the stop on the other side of the pin? That way the advance would only come on during higher vaccum?
I think it would help it kick off faster when the throttle is stabbed.
I haven't done any tinkering with street distributors in a long time but it seems most have too soft a spring.
I put a crane adjustable type can on a friends Camaro and while I could shorten the advance, I felt it needed to come in at a higher vaccum too.
David

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Check my web page for suspension info:
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69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer
 

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68 396 ragtop,
Most exhaust popping is due to air getting in the system through a leaky header gasket or such. And the more unburned fuel that is present in the exhaust system, the worse is is.
If the ignition were way advanced during cruse, it might overheat the leanest spark plug and cause a pre ignition. This can happen when the intake valve is open and you will get a backfire that is not as loud as a backfire when the throttle is wide open.
Kind of a sneeze rather than a cough.

Along with that, you would have a bucking or loss of power for an instant.

If it were an exhaust related pop there would be no loss of power.
Good luck, David

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Check my web page for suspension info:
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65 Lola T-70 Can Am Vintage Racer
 

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The Crane 99601-1 adjuctable vacuum advance "kit" comes with a plate that has a series of stepped cut-aways to nake the amount of degrees the cannister will deliver adjustable, instead of a set amount of degrees. This "scroll" design plate is supposed to mount on the end screw, furthest from the pull bar that moves the point (reluctor) plate to move the timing.

If the scroll plate is mounted the way Crane specifies, the amount of pin/spring preload in the diaphragm assembly changes upwards (gets tighter) with every cut away added. This in effect, makes the vacuum pull different for every cut away used.

If the scroll is placed in the pull path of the pull pin, then the diaphragm and spring seat in the cannister is allowed to be adjusted without adverse spring pressure changes from the pion being adjusted from its dead end pow stop. What this does is allow completely independant adjustment of both stop and pull parameters, instead of imposing extra limitations on the adjustments.

The farthest I have seen a Crane adjustable cannister spring start at is 13 in/hg vacuum, and stoppiong down at 2 in/hg (this would be using the full mainfold vacuum pull, not ported). Of course, this gives between 11 and down to no in/hg adjustment. When the pin is preloaded, as in the crane method, the amount of pull is severely dropped as the pin is further preloaded ( I have seen as little as 2 in/hg movement in well mis-adjusted units).
 

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Discussion Starter #14
Ok, an update sort of, I drove the car this weekend, and I left the advance hooked up even tho we realize there is a timing/fuel problem. The car runs, idles, and has plenty of power. But again at 3000 rpm it pops at light pedal. Ok, I took a small piece of metal and soldered it to the vac can to take up 1/2 of the travel in it, retimed it. Going to drive it tomorrow. The popping is definately exhaust, no noticeble power loss.Checked the plugs, they are nice and clean. Also the popping is a "quiet" pop, not a full blown backfire, which I would definately park the car until fixed. What was said about the possibility of having the wrong vac source hooked up, this is possible. Because when I retimed the engine after the mod to the vac can, It still pulled 50 degrees at 3000 rpm, so it would seem that I do have vaccum at mid to high rpms.That will be another thing to look at tomorrow. Thanks alot guys and I'll update tomorrow!!
 

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Still sounds like you have the vacuum advance feed from a ported source, wrong source. Feed hose should have vacuum at idle, not at off idle or mid-range.
 

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Discussion Starter #16
Ok, took the car out today after the said mod. Still popped at 3000 rpm. I checked all the vac ports, the one on the carb bowl, the one on the mounting plate below the bowl and a port directly behind the carb directly on the intake manifold. (plugged otherwise). The vaccum at all three is the same! I have 6inches at idle and goes to 18inches when I rev it up, the only time it dropped is when I stabbed the throttle, when the butterflies where way open. But at all mid to high rpm, it has alot of vaccum. Is this normal? That of course explains the overadvancing, so in ending, I guess I should be looking at a adjustable vac can. So that I only have advance at low rpm. Thanks for all the help!
 

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I-man,

In summary-

Use a Crane adjustable vacuum advance but put the adjustment cam on the pull side of the arm. Adjust cam or slotted adjuster so that 8 degrees additional advance is kicked in at idle.

Set initial timing at 12 deg. with vacuum disconnected. Total approx 36 deg advance in by 3000 RPM. (12 initial + 24 mechanical)

Connect vacuum advance to manifold vacuum port and adjust idle and air speeds.

The one thing I'm unclear about is how to properly determine the adjustment of the vacuum canister. At what point should the vacuum advance start the ignition retard (vacuum drop?) I have no clue.

I've learned a lot about timing in the past few days. Thanks.

Carl



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The 8 degrees is an arbitrary starting point, finalize by adjusting as you go. In your summary, 12 initial + 8 supplemental vacuum @ idle gives 20 idle degrees, engines like this, or someplace near this figure, depending on compression and cam specs. Lower performance/compression will toleraqte more supplemental vacuumm idle timing assistance, higher comp/cam/performance engines will need less and less, as the initial will go up for the configuration, UNTIL the comp/cam/performance level will no longer support vacuum advance of any kind (very radical engine packages).

As far as when the vacuum advance starts to retard out of use, immediately when the vacuum in the intake manifold drops, as in when the carb is opened off-idle. When engine go0es from higher vacuum to lower vacuum as carb is opened, mechanical timing curve starts, and aids acceleration. Cruise intake manifold vacuum levels do come back yup some, and will bring in a bit of tghe vacuum advance timing, to help with engine intercooling and economy.



[This message has been edited by IgnitionMan (edited 07-19-2000).]
 

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Sorry I-man, I may have mis-stated my question or I'm not getting it. The latter is usually the case.

In an earlier post you mentioned that the Crane adjustable advance has maybe between 11 and 0 inHg adjustment. I'm assuming (bad thing to do) that this adjustment is made with the allen key through the vacuum port.

What's the best way to make and tune this adjustment (allen key)?

This adjustable vacuum cannister is all new to me.

Thanks again.

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In the full Crane kits, there is the adjustable vacuum cannister with allen wrench, AND the degree limiter scroll plate.

The scroll sets the total amount of pin movement for total amount of degrees of advance movement. The allen wrenck adjusts the amount of vacuum pull the cannister pulls against the scroll plate.

There are two separate adjustments to be made, amount of degrees, and the rate it is provided with in vacuum inches (in/hg).

If you don't use the scroll to limit the amount of advance, the cannister will close down a few degrees as the tension is raised on the diaphragm inside the cannister, but using and adjusting both the scroll and cannister vacuum pull is what is really needed.
 
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