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Discussion Starter #1
Hello all,

I have a 1968 camaro RS with a 350 I built and have had on the road for about 2 years now. The car used to run like crap, but fixed that issue by increasing the jet size from 65/68 to 69/73. Now the car runs much better, but still has two small problems.
The first is a problem at part throttle under very low load (flat ground, 1800-2400 rpm). The car will "buck" slightly and feels as if it wants more throttle. As soon as I press the pedal and the car accelerates some, it smooths out. Also, at higher speeds when there is more load (65-75 mph, 2300-2600 rpm) there is no bucking to be noticed.
The second problem also occurs under lower load at 2900+ rpm. When engine speed reaches about 2900 rpm, the engine becomes unhappy and the tailpipes light up with loud popping noises. This problem occurs at part throttle either under slight acceleration or when keeping the car at a constant speed over 2900 rpm. Harder acceleration and any deceleration do not cause the stumbling.
Is this a lean condition or timing issue? I have tried different timing settings and advance spring configurations to no avail. Any suggestions as to how to solve this problem would be greatly appreciated.

Specs:
1978 block 350 bored .060 over
holley alum heads 2.02/1.60 valves
9-9.5:1 compression
Holley 670 cfm street avenger carb w/ Vac Sec. and elec. choke 69 pri and 73 sec using 'middle' spring
lunati .530" lift 230-238*@.050" duration 110* lobe separtion hydraulic roller camshaft
0* deg advance of the cam w/ relation to crank
m21 4 spd
long tube headers
3.08 rearend gears
28" rear tires
14* deg initial timing 35* total and Vac. avd. attached to ported vacuum
MSD pro billet dist. with mag pickup
Holley Annihilator electronic ignition system/60000 W coil
The Car weights approx. 3200 lbs
 

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What springs are you running in the distributor? Try a stiffer spring setup.
 

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Plug the vacuum advance port and see what it does.
If it stays the same, then the port might not be getting vacuum and it's possible lean misfire. Moving the vac can to full manifold vac should help.
If it gets better, then it's too possibly too much timing and rich misfire.

Read up on the benefits of running full manifold vac for your timing. The MSD non-HEI distributors typically have 10-12 degrees vac advance which is pretty safe. The HEI ones have 22 or so which are not as easy to work with.
 

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You don't mention what gear yer bucking in but the Bucking issue sounds if ya have the car in to high a gear, yer basically lugging the motor. Try down shifting into a lower gear.
 

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Discussion Starter #5
Thanks. That's a good point with the vacuum advance. I have have thought to pull the hose and try it without the vac. advance before. I will test it soon. As far as running the vacuum advance plugged into manifold vacuum, I have done that before and it quickly over advances the timing quite a bit. It runs about 38 degrees before TDC with the vacuum advance plugged into manifold vacuum. I have read " Timing and Vacuum Advance 101" many times, great article. As far as putting in a stiffer set of springs: I actually went on set lighter not too long ago and it seemed that the car ran better, but I could be wrong. The reason I don't think this is mechanical advance problem is because the only situation in which this car has a problem is under very low loads. At higher loads or at zero load (deceleration) there is no popping out of the exhaust or any problems. The engine is very smooth under higher load and doesn't do anything abnormal ( small popping is present but that is due to a lean condition because the throttle blades are closed.) Thanks again for the help. I will test without vac. advance and get back to y'all ASAP.
 

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So it sounds like there is a problem with the advance canister. The MSD should only have 10 degrees of vacuum advance unless it's the Street Fire HEI. What part number is on it?
 

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Discussion Starter #7
So it sounds like there is a problem with the advance canister. The MSD should only have 10 degrees of vacuum advance unless it's the Street Fire HEI. What part number is on it?
The part number is 85551 on the dist. It is a magnetic pickup distributor.
I unplugged the vac. advance and the car ran much better, much more smooth and I ran increase the engine speed very slowly from 2000 to 5000 withput the exhaust lighting up in sound like I could not before. When the car gets to 180 *F though, it still stumbles and bucks very slightly so I will pull some advance out of the dist via the advance springs and I know the secondary jets are little large so I will step down to 71's from 73. Thanks to all for the help especially Steiner, I will get back asap with results. Any ideas on how to male the vac. adv. useable?
 

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It probably has something to do with vacuum, your cam is fairly large, and since you plugged it and it ran better, that makes me think even more that it's vacuum related? What is your vacuum at idle? It would help to get an adjustable vacuum advance can and run full time vacuum and limit the advance. I was at 18* initial, and over 60* total until I decreased the vacuum advance, ran much better. Also, 3.08's and that cam is not the best combo, that's probably part of the problem as well.
 

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Discussion Starter #9 (Edited)
It probably has something to do with vacuum, your cam is fairly large, and since you plugged it and it ran better, that makes me think even more that it's vacuum related? What is your vacuum at idle? It would help to get an adjustable vacuum advance can and run full time vacuum and limit the advance. I was at 18* initial, and over 60* total until I decreased the vacuum advance, ran much better. Also, 3.08's and that cam is not the best combo, that's probably part of the problem as well.
It makes 11" of Hg at idle. This same bucking problem happened with the previous FT cam which made 18" of Hg at idle. I agree this has to do with vacuum, but it would seem that the new cam pulling less vacuum would cause it to run better given obviously the vac. adv is pulling way to many degrees of advance causing the rich misfire. I am not certain why this dist. is so sensitive to vacuum, but it appears it is. Do you know of any adjustable vac. can for this dist that is available? I checked mine and it is not adjustable. Thanks again.
 

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Going by your full manifold experience, it just sounds like the can is either wrong or just bad. It should only provide 10 degrees of advance but your numbers show you were getting 24.

There are a few things you can do in what would be my order if I had to do it.....
(1)-Call MSD and tell them and they may just send you a good one.
(2)-Since they use a standard GM points dizzy can, you can look up that article that lists all the GM vac cans and specs, then buy one with 10-12 degrees advance that will work with your vacuum level at NAPA or somewhere and install it. There are also a couple of places that sell an adjustable can.
(3)-With the engine idling and timing light going, use a hand vacuum pump on the vac advance and note at what vacuum level you get 10-12 degrees advance. Then pop off the cap and rotor, use the pump again on the vac can to apply that previously noted amount of vacuum, and note how far the canisters actuator rod moves. Make a stop to limit the actuator at this point.

Regarding the light stumble you've got now, you can most likely do what you already mentioned, and I'd try either timing or fuel first....keep your current advance, but just swap out one of the springs for the next heavier one to slow the curve down a little bit. You can try this without doing the jets. Less advance will run better with a rich mixture. If you were to change the jets but leave the timing you might find it runs the same since a lean mix needs a little more timing.

***Edit. I found Lars's timing papers and it looks like there is not a GM can that provides only 10-12 degrees advance. I think I remember reading somewhere that MSD just uses on of the GM 16 degree (8 distributor degree) cans and just puts a bushing around the arm so it bottoms out on the bracket.
 

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Discussion Starter #13
I apologize, the part number is 8361. Current spring set is: one light silver and one light blue if I remember correctly (yes I know it is quick). This motor likes advance under acceleration. Current fuel is 93 oct. 100% pump gas (no ethanol).

I read some of that MSD forum thread and I find it funny that msdtech22 said the vac. adv drops out at 2000 rpm when obviously it does not. In fact, vac. adv is used at steady state cruise to help with the leaner mixture then. This was proven by the fact removing the vac. adv hose cause the engine to run more smoothly.

The reason I am removing some jet is because the engine runs somewhat rich on the road. I will change from 73 to 71 secondary jet first and see what that does. If it doesn't completely fix the stumble, I'll change to the next stiffer set of springs which would be two blue springs.
 
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