L36 Surges/Pulses at 2800 RPM

[David J.]

After reaching operating temperature and cruising at a steady 2800 RPM (60MPH), the engine goes through continual, very short surges of a couple 100 RPM's. If the RPM are increased or decreased 300-400 RPM's the surges go away. Other than this issue, the engine is very strong and runs great. Is this some kind of a vacuum problem? Any ideas on correcting the issue? The engine is original with no modifications. Although the Holly is not original, it is a correct, replacement Holly.
Thank you.

[Michael W.]

Re: L36 Surges/Pulses at 2800 RPM

Try disconnecting and plugging the vacuum advance line. If that works, you need an advance can with IIRC a 2" higher pull in setting.

[Michael A.]

Re: L36 Surges/Pulses at 2800 RPM

Hey Michael can you explain your suggestion above in more detail for me as I am not mechanically inclined. I have the same engine and the same problem and have been told the spark plugs are fouling (I've changed them twice) because I apparently drive a lot in the city. Are you suggesting we disconnect the Vacuum Advance Line and drive with it disconnected, or simply disconnect it to check for blockage? And what do you mean when you say "you need an advance can with IIRC a 2" higher pull in setting"?

Thanks

Michael

[Michael W.]

Re: L36 Surges/Pulses at 2800 RPM

Try driving with it disconnected, just for trouble shooting. The purpose of the vacuum advance feature is to increase timing (advance) under conditions of higher intake vacuum, which would be encountered typically at steady state cruise. If an advance can is calibrated to move coincidentally at the same vacuum level created during cruise, the timing might wander back and forth leading to the surge the OP has reported. The cure is to swap the can for a similar one with the pull-in (activation) vacuum set 2" of mercury higher.

Duke Williams has written at length on this subject, must be available in the archives.

[Timothy B.]

Re: L36 Surges/Pulses at 2800 RPM

Another area of concern with a surge is a lean carburetor, at cruise speeds of 2800 rpm the carburetor main circuit along with transfer slots are feeding the engine. How far out are the emulsion screws from seated.

I agree to verify the correct V/A control in the distributor, can you post the #'s on the control.

[David J.]

Re: L36 Surges/Pulses at 2800 RPM

I will get the #'s and post. Thank you for the info guys!

[Michael A.]

Re: L36 Surges/Pulses at 2800 RPM

Given the small price of a new can is it safe to say that moving to a B28 for the L36 is a good starting point?

Michael

[Michael W.]

Re: L36 Surges/Pulses at 2800 RPM

Don't change anything until some trouble shooting is done.