Expert: Brad Sears - 1/20/2008

Question
QUESTION: Brad,

 I have a 1956 cadillac with a 365 4bbl.  I recently replaced the plugs, wires, distributor, cap, rotor, and points.  The dwell should be at 26-27 but won't go above 24.  The first time I tuned it, it seemed to hold at 26 dwell and 400-500 rpms in drive.  Suddenly it shifted to 22  dwell and the rpms dropped like it came off fast idle.  I do have the vaccuum adcance off and plugged.  Now it doesn't change on me but the dwell stays at 24 even when I rev the rpms higher.  Any suggestions?

ANSWER: I have a coupe of questions. You mentioned replacing the distributor. Was it new, used, or rebuilt? Dwell on an eight cylinder engine should be 28 to 32 degrees. Have you checked the accuracy of the dwell meter on an other car? If the dwell stays steady with the vacuum advance that tells me that the breaker plate in the distributor is worn and changing positions. Let me know.
Brad

---------- FOLLOW-UP ----------

QUESTION: The distributor is new / rebuilt with all new parts and bench-tested.  The rebuilt one I had in there before did the same thing.  I thought that the slight shaft wobble was causing it so I replaced the distributor.  My mechanic had been able to get it to 28 degrees dwell before with a pretty high rpm last year.

ANSWER: I repeat my question about the accuracy of the dwell meter? Has it been checked and does your mechanic know how to use it? After all, we have not used dwell meters in the average garage in close to 30 years. The last time that I used a dwell meter in modern car repair and diagnosis was  checking carburetor computer controlled solenoids in the mid 80's. Please check your dwell meter.
Brad

---------- FOLLOW-UP ----------

QUESTION: Yes, I am certain that one doesn't certify one's speedometer every year either.  No my meter is probably 20 years old.  It does however work consistently well on my 1970 Buick, showing a solid 30 degrees of dwell on the 455 cu in engine.  

Answer
Sorry about the insistence of verifying the meter but old habits developed as training manager for Sun Electric, makers of testing equipment, die hard. Now that we know the meter is correct lets look at why dwell could be lower than normal. Again I might be asking that you double check some things but, here we go. The point gap as measured with a feeler gage or a dial indicator set up should be .019" right on the money. Now if everything is right the dwell should be 30 degrees plus or minus 2 degrees. If this does not happen then perhaps the cam lobes on the distributor shaft are worn. With the engine running and the dwell meter connected hook a vacuum pump up to the vacuum advance. Add vacuum while watching the dwell. If the dwell changes more than 2 degrees the breaker plate is worn and needs to be replaced. Make sure that the voltage to ignition coil is correct with the engine idling, no lower than 7.5 volts at the plus side of the coil. Check the ground wire in the distributor between the breaker plate and the distributor body. Make sure that the distributor body is grounded to the block. Please keep me posted.
Brad