QuestionQUESTION: Hello. I just completed a stock rebuild of my 68 Skylark's 350 engine. It starts and idles just fine and the car is driveable, but when you hold a steady RPM at anything above an idle you can feel the engine running roughly and even hear what sounds like a slight miss (or stumble) in the exhaust. This (stumble) is random and lessons slightly after the engine is hot.
I tried replacing the points system with an electronic one (Magnetic Switch/Magnet Ring type), but the condition worsened to a level where you could feel and hear it even when the car was in motion. I then re-installed the points and began an extensive check of the entire ignition system.
The only odd thing I found is when checking the + side of the Coil while the engine is running. At idle I show 9 volts, but as I increase RPM up to highway speeds, the voltage at the + side of the Coil goes up to 13 volts.
With engine off and points closed, the + side of the Coil shows 5.5 volts. Points open - 12 volts. (These fall right into specs according to the 68 Buick Shop Manual).
My question: Is the voltage increase on the + side of Coil (as RPM is increased), normal? If not, what could be causing this condition?
If it IS normal, what could explain the roughness (stumble) above idle?
I have tried everything I can think of: new plug wires, plugs, distributor cap, coil, voltage regulator, altenator, but no improvement. The only change in behavior was the condition worsened when I replaced the points with electronic.
Thanks for your time.
- Jay
ANSWER: You might want to check the distributor shaft for wobble caused by worn bushings in the distributor. The shaft should have just about zero wobble. Check and let me know and we will go to step two.
Brad
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QUESTION: No wobble in dist shaft that I can detect.
I take it then that the answer to my question about voltage increase (with RPM increase) at the battery side of the Coil is...yes (normal)?
If no, is it possible the Coil is somehow leaking secondary energy back into the primary winding, thus causing the increase?
ANSWER: The coil voltages sound fine. I still lean towards the distributor for some reason. Try an experiment to humor an old man. Disconnect the vacuum line to the distributor and bring the engine up to speed. Listen to the sound of the engine. Then with the engine running at speed plug the vacuum line back onto the distributor and listen. let me know what you hear.
Brad
---------- FOLLOW-UP ----------
QUESTION: Only a increase in engine RPM when the vacuum line is plugged back in. The stuttering is present either way.
AnswerOk, that tells me that the vacuum advance is working and that the breaker plate in the distributor is not worn and loose and wobbling. Do you have a welding torch set up or a propane torch? If so remove the air cleaner and add propane or acetylene slowly into the carburetor intake when the engine is at speed. If the engine speeds up and smoothes out the carburetor mixture is too lean or there is a vacuum leak. If the engine immediately slows down the mixture is a bit rich. To confirm this remove the plugs and keep them in order. Read the plugs for color and condition. they should be a light tan if all is well and all be the same. If they are bone white then the mixture is too lean and dark brown/black and puffy dry soot indicates rich mixture. Wet black indicates oil. Let me know.
Brad
PS: Using a propane torch you might have to take the restrictor jet out of the torch to get enough propane to flow.
