Carb hesitation

[Cav Trooper]

I know this has probably been gone over more times than can be counted but, here goes. I had a problem with my M37 top end speed. I couldn't get over 35 to 40 mph. I checked timing and corrected then checked rpm and got 2500 max. I then bumped up the governer to 3200 and test drove. The truck would accel great but begin surging/bucking. I readjusted the gov. to the point that there was no adjustment left on the spanner nut. I then decided that I needed to remove the carb and check out the gov for basic adjustment and make sure nothing was broken etc. The carb had been rebuilt before I got the truck and the truck started and ran ok other than the lack of top end. I reset the gov. to basic spec. and while removing the carb I found a piece of black rubber stuck in the carb fuel inlet, evidently from when the PO installed an electric fuel pump and ran fuel rated hose to the carb and I think a small piece of the trimming from the hose washed into the carb inlet. I think that may have been the problem with the surge as the carb may have been starved for fuel at top end. If I let off the throttle and let it idle for a couple of seconds, I could get good accel to top end then the surging would start again. When I removed the carb, I removed the intake/exhaust manifolds and cleaned every thing up and prettied the manifolds up with high temp paint, new gaskets etc. I rebuilt the heat valve and locked it in summer mode. The truck starts ok. I did not open up the carb, just adjusted the gov. When driving, in 2nd gear, when I accel to max, the truck acts like there is no throttle then revs up then drops off again, it doesn't quit. Once I get into 3rd and 4th gears, the throttle seems to work ok on max accel. Where do I start? I have not checked the rpm yet for the gov. setting. I need another person to help with that. Mean time do I need to check elsewhere, linkage, etc. The truck was completely warmed up and at operating temp when all of this took place. I am going to check the manifold bolts tomorrow to make sure nothing is leaking, all gaskets were replaced with new of course. Also replaced the local muffler shop muffler and pipes with repro from Mid West. The timing is good and the points and condenser are new.
Thanks,

CT

[Lifer]

It sounds like you have a fuel-flow restriction in your primary (idle) system. From your description, your secondary (acceleration) circuit is just fine. It could be nothing more than dirt in the primary circuit of the carb. A good cleaning could take care of it.

[MSeriesRebuild]

If the surge is taking place at max RPM; you don't have the governor trimmed out correctly. I noticed you said no adjustment was left at the spanner nut, (your words), not sure what you are calling the spanner nut; however if you are referring to the nut the governor adjustment stud threads into, you are misinformed or there is a problem of some sort. Reason being, that nut has no limitations, it should turn to infinity either direction until you run out of threads on the stud. That would be WAY farther than is needed for correct trim adjustment. The object is to get very robust acceleration up to 3200 RPM. Best to set it until the surge occurs at the top end, then trim just enough to eliminate the surge. That is where it will run the strongest.

[Lifer]

I knew there might be another "something" wrong, but I can't seem to think real well just now. Thanks for the voice of experience, Charles.

[Cav Trooper]

Lifer, Charles, thanks. I didn't do follow through with diagnostics before I posted before. I had a lot of stuff I was working on at the same time and just got twisted up in the job. Here's what I have gotten done. I first needed a second person to help which wasn't available, can't count on the wife.. I drove the truck without checking the basics after reassembly so, I went back and began step by step. I reved the engine in neutral and found it surged, now I knew the governer needed adjusted. I adjusted it according to the artical that Charles has posted here, very good write up. I got the engine up to about 3200 and no surge. I next drove the truck and it ran well, I do get a surge when I let off the throttle and at cruise ie; slight throttle at 40-45 mph. When the surge happens, if I let off the gas, it stops and I can accelerate again without the surge as long as I don't hold a constant speed at the higher rpm. I don't which way to adjust the gov. from here. What should I do next? The truck will run up to about 50 now, only about 35 to 40 before I began all of the adjustments. I think it probably needs a little tweeking yet but it is much better.

[MSeriesRebuild]

Based on your info; I'm still thinking the governor adjustment is not trimmed exactly. My guess is you are very close to having it perfect, sometimes 2 or 3 tweaking sessions is what it takes. Turn one way, test, if the surge gets worse, then go the other way, I'd suggest quarter turn segments once you have determined which direction is correct. When the surge is gone, then recheck the RPM to be sure you have not increased or decreased away from the 3200 cut off setting. Correct that if necessary by turning the adjustment stud slightly in the appropriate direction.