Engine 'pulls' - Team Camaro Tech
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post #1 of 9 (permalink) Old Dec 16th, 04, 11:07 AM Thread Starter
 
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Hi......

When running in 1st or 2nd gear (4 spd) I notice that my '68 396/325 engine seems to 'pull'. If I maintain a steady speed of say 10-20 mph, the engine feels as though the timing advance is 'hunting'. Probably not very good description but I dont know how else to explain it. I dont seem to notice this when running at highway speed...more pronounced at the lower end. I've pulled a vacuum on the distributor & the vacuum advance unit is functional.

Any ideas would be appreciated!
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post #2 of 9 (permalink) Old Dec 16th, 04, 12:19 PM
 
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The distributor isn't using the port on the carb that has direct manifold vacuum is it? If so the advance will come in waaay early.
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post #3 of 9 (permalink) Old Dec 16th, 04, 01:58 PM
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Quote:
Originally posted by sicsD8:
The distributor isn't using the port on the carb that has direct manifold vacuum is it? If so the advance will come in waaay early.
Once the throttle is cracked the ported and manifold vacuum will be the same.

Sounds like you have a lean surge going at low rpm's. What is the jetting and power valve?

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post #4 of 9 (permalink) Old Dec 16th, 04, 03:28 PM Thread Starter
 
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This is the orignal Quadrajet carb # 7028217EG. To my knowledge nothing has been changed within the carb.
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post #5 of 9 (permalink) Old Dec 17th, 04, 01:54 AM
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onovakind67:
Quote:
Once the throttle is cracked the ported and manifold vacuum will be the same. Sounds like you have a lean surge going at low rpm's.
I'd check for a vacuum leak. At road speed, you're deep into the throttle, thus, overcoming a vacuum leak.

At lower speed/rpm's, the transition circuitry of the carb is trying to supply fuel between idle and off-idle. Either a vacuum leak or low fuel level, or plugged emulsion tubes, those two brass tubes stamped into the carb air horn.

Use your vacuum pump as a vacuum gauge and see what the engine manifold is showing. Its a valuable tool.

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post #6 of 9 (permalink) Old Dec 17th, 04, 07:41 AM Thread Starter
 
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OK...here's what I've found.

The vacuum advance is pulling from the correct port on the carb. Following another suggestion, I reset the timing from 6 BTDC to 10 BTDC & the problem got worse. The engine developed a VERY noticeable miss that I just could not get rid of. I changed the plugs, wires, cap & rotor to no avail. Then out of shear desperation, I retarded the timing to a point where it shows about 6 ATDC with the timing light & bingo, the miss went away & the surge or 'pull' that started all of this cleared up. As a side note, the car now starts better as well. It used to have some resistance to turning over normally & again acted as thought he timing was off. Now it cranks right over as it should.

So....here's the burning question. Why does everything seem to be fine now with the timing at 6ATDC? I cant believe that it's truely running at 6ATDC (because it runs so well) so is it a timing mark/vibration dampner anomoly?

Thanks for all the help!
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post #7 of 9 (permalink) Old Dec 17th, 04, 07:48 AM
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My first guess is the dampner slipped. Get TDC on #1 intake and see if the timing mark lines up with 0. You might need one of those brass piston stops that screw into the spark plug hole.
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post #8 of 9 (permalink) Old Dec 17th, 04, 07:50 AM
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All depends on what RPM you are at. if you are over 2500 to 3000, the power valve is out of the question,my guess is that you need wires or plugs.if it is under 2500rpm take a long thin object like the inside of a big pen or the little tube from a WD40 can (the red one) and take the air cleaner off and stick the tube down the air horn. The air horn is the pipe at the front of the carb that sticks out. try to balance the little tube on top of the power valve and hit the gas pedal at the same time. If the power valve doesnt push the little tube up, then probally the spring is collapsed or broken. Thats all i can think about right now but do tell us at what rpm is your problem at. Nick

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post #9 of 9 (permalink) Old Dec 17th, 04, 10:38 PM
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I agree with deerhunter - verify where your timing mark is ok then check your total timing - At 3000rpm min it should be no more than 34-36 degrees.....

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