1969 GMC 1500 stumbles / stalls from dead stop

[PanhandleShantyman]

Hi all, hoping for some wisdom from the wise men of this forum.

My 1969 GMC 1500 has developed a problem of stumbling really bad whenever it first goes under load. This is mostly a problem going into first gear from a dead stop. I have to rev it a bit and let the clutch in slowly so it starts rolling gradually and really baby it into gear, or it stumbles and stalls. Once it's rolling it stops stumbling. If I go into second or third at relatively low MPH, it similarly stumbles until it gets rolling faster. It's been doing this all the time lately, and is really a problem sometimes at red lights on an uphill grade! The truck idles fine. I can rev it in neutral without issues. It is specifically a problem when just getting it rolling from stopped position.

The truck is an inline 6 250 , 3-speed manual transmission, points distributor, Rochester MV carburetor, all stock, just an old truck with no modifications.

The truck ran pretty bad when I got it a few months ago, but it did not have this problem. This problem first showed up after I replaced the intake/exhaust manifold gasket to fix a vacuum leak, after which I set the timing back to spec +4, adjusted idle speed and mixture for max vac (gauge reads 19 inHg), etc. (The previous owner had advanced the base timing to like +26, I guess to keep it running with the vacuum leak). All the previous issues with high idle, backfiring, dieseling, etc, went away, but the "stumbling into first" issue emerged at that time. I advanced the base timing to +14 and the stumbling issue went away for a while, but now it's back.

I replaced plugs, plug wires, distributor cap, and cleaned all electrical contacts to the coil. On the theory that maybe I had a fuel supply issue, I replaced the fuel filter in the carb and I removed the inline fuel filter that used to be between the gas tank and the fuel pump. I checked fuel pump pressure and it is at spec at 5.75 PSI. The carburetor is a newly rebuilt one I bought from National Carburetors (the carb that was on the truck when I bought it was all jacked up with a stuck throttle plate and the choke held in position with baling wire, so I just replaced the whole thing). I haven't checked the float, since it's so recently rebuilt. I verified that hitting the throttle does generate a spurt of fuel into the carb. I checked the mechanical advance curve and best I can tell, it is more or less at spec at advancing the timing appropriately as I increase RPMS. I do believe the vacuum advance is out, as the mechanism there looks frozen pretty shut--but I'm not convinced it's my problem, because #1, doesn't the vac advance primarily come into play at cruising speed anyway, not at low speed when my problem strikes, and #2 if the issue is just the vacuum advance, wouldn't advancing the base timing like I did put a bandaid on the issue at least? --that was my theory in playing with the timing like I did, anyway.

What do you guys think? Could a stuck vacuum advance be causing the problem, or do you think I've got an issue with the coil, or the float, or something else?

[Steeveedee]

Sure sounds like low float/accelerator pump shot is weak to me. The vacuum advance isn't the issue- some vehicles run without it, no problem.

[PanhandleShantyman]

Steven,

You were right! I pulled the carb off and checked out the float bowl area. The float was set pretty low, like 5/8" below gasket surface instead of the specified 1/4". Also there was a fair amount of fine sediment in the float bowl around the base of the metering rod, where the tiny intake holes are for the idle and power circuits. The accelerator pump seemed to be working fine. I cleaned out the sediment, air-blasted the area, adjusted the float, and after putting it all back together and resetting the carb adjustments, no more stalling! Between the sediment around the power circuit fuel intake hole, and the low float setting, there must not have been enough fuel getting where it needed to go.

Thanks!

-Will

[Steeveedee]

👍 Thanks for providing results!