MTFCA Forum

When I am driving down the road and throttle down before coming to a stop, the engine rhythmically surges. Not heavy but noticeable. It doesn't do that when I increase the throttle above an idle position while still slowing down. It also doesn't do that with the car slowing down in neutral. I've noticed this on a couple cars I've owned. What could be the cause? I'm sure I'm not the only one who has experienced this.

Try opening (running too lean) or closing (running too rich) the spray needle and retard the spark some, see if that helps. Question, what carb are you running? Have you checked for vacuum leaks?

They idle better with the spark retarded. Not all the way up but about 3 or 4 notches above where it runs well at cruising speed. It could also be the carb mixture as posted above. Some carburetors have an adjustable idle jet. In that case you can try adjusting it for smoothest idle.

Thanks for the input. I continue to work thru the vaccuum leaks. I am running a swaybuck NH carb allegedly restored but I have my doubts. I do hope it is a vaccuum leak issue and will clear up once I get that resolved. I have tried different needle and spark positions, also adjusted the float to where it starts easiest and runs good above idle.

Re vaccuum leaks, It could well be the steel rings that hold the manifolds to the car are bottoming out before fully compressing the copper washers. The mating surface of the block was surfaced to remove pitting and the lips inside the ports were recessed to restore the original distance, but Ive heard some reproduction rings were made too thick. I may try running the car with the fiber gaskets that come with the gasket sets you buy not using rings to see if that solves the problem. If so I'll figure out how to reduce the thickness of the rings before switching back to the copper washers.

Try fitting the rings with out the coppers and see how it fits.

I still think that your timing is too far advanced and what you are experiencing is pretty much to be expected. A vacuum leak should have little effect on an unloaded engine decelerating, and for sure not enough to be able to feel it from the driver's seat. Surging at cruising speed will occur from either too lean a mixture or too far advanced timing but the surge you're describing has to come from timing.

Gene, there's no law against adding a little hi-temp RTV to those copper rings. I do that because my block hasn't been surfaced to remove the pits.