VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Bucking or missing at light throttle

Hi all,

My 31 has developed a bucking or missing at light throttle, but isn't apparent at heavier throttle. Idle has some miss that I haven't been able to resolve, but the light throttle issue is more pronounced, and has developed more recently. Timing, spark gap, and points all are set as recommended on the forum.

Thoughts?

Thanks
Steve

not sure but did you check for vacuum/ air leaks around carburetor and manifolds ??

my 1929 was idling rough found out the carb gasket was shot and sucking in air.

if nto maybe the idle adj screw or so on the carb

Sounds like a vacuum leak. Easy to check, spray WD-40 or Staring Fluid around Intake Manifold and at base of Cab. If Engine RPM'S increase then it's a Vacuum leak.

In addition to what BearsFan said, check your vacuum advance (if it has one) to make sure it's holding a vacuum. Simple to do by disconnecting the vacuum line that runs to it, pushing in the advance, placing your thumb over the inlet and then letting go of the advance. If the advance returns, you've god a bad vacuum - should stay pushed in. My engine was acting goofy (popping, backfiring a little at startup and initial throttle) until I put in a new vacuum advance. Made a world of difference. Runs great now.

Thanks all for the input!

No vacuum advance on a ”˜31, but just checked for vacuum leaks. No increase at the intake manafold, but there is at the base of the carb. As luck would have it, I just happen to have a new gasket in the pile of stuff.

While i have the carb off to replace the gasket, I took the bowl off to check for any dirt and confirm the float level. Looking from the bottom of the carb, I noticed that the metering rod fits pretty loosely in the multi-jet tube(?) opening. Is this normal, or how does one tell if the metering rod is worn and need of replacement?

Thanks

That is normal.... I would advise sending the carb. to an expert.

The metering rod must fit loosely in the jet otherwise gasoline will not be able to flow past them. The only way to check the metering rod and multijet is to measure them and compare with original specifications.

Carter started using "metering rod technology" in 1929.

Most of the rods are tapered, giving the effect of having an automatically adjustable jet, improving both drivability and economy.

The effective metering area is the area of the jet less the dynamic area of the rod (the portion or the rod in the jet at any time). The early application were mechanical, that is, the metering rod moved with the movement of the throttle. Later applications (think "modern" Carter AFB 4-barrels) use dynamic vacuum opposing a tensioned spring to control the movement of the rod.

Jon.

Thanks all for the input.

I reinstalled the carb with a new gasket, and while the vacuum leak appears to be resolved, it doesn't appear to run any smoother than before. The bucking issue may be resolved, but won't know until I get a chance to give it a good road test.

I have a few other items on the list for the winter, and may add measuring the metering rod to the list. May have no bearing on how it's running, but I have the original specs so why not.

Everything ignition related has been replaced over the past year, but still haven't been able to dial it in to run smoothly, which is a tad frustrating. Oh well, learning a lot along the way and much of what I enjoy.

Try plugging the hose that goes to the wiper motor. The closer to the engine the better. A leak in that hose will cause poor running.

Thanks Dogbreath - That was actually the first thing I did. Still haven't reconnected, just to keep it eliminated.

If you removed the idle jet (low speed jet), did you slightly enlarge the upper end so it would re-seal when re-installed?

Jon.

I didn’t remove anything beyond the bowl to check the float level.

Road test today. No noticeable improvement, and not able to locate any additional vacuum leaks.

Any other advice on what to look at next?

Thanks

Hello SteveEC,
Work toward finding the cause of the idle miss first. That might clear all of your issues. Double check: Mechanical, Electrical, Fuel and Air in that order to keep from jumping around and becoming confused. Do a compression test. Check rocker arm/valve gap. Check that the distributor is timed correctly to the engine and cap and wires to spark plugs follow the firing order. With engine at idle, pull the plug wires off/on one at a time from the spark plugs to verify that each cylinder is firing. Wear insulated gloves when pulling the wires off. If you find a dead cylinder, swap spark plugs with another cylinder to see if the dead cylinder moved with the plug or stayed indicating a weak plug wire. Points, rotor, condenser, coil and wires could have issues. When all this has been eliminated, suspect vacuum leak, carb air/fuel adjustment. You wouldn't think it's a fuel pump issue if the engine runs OK at higher throttle settings. Keep us informed.

Thanks for the additional thoughts. I've done a compression check (all 75 to 78), timing, points gap, and plug gap all confirmed, all ignition components have been replaced, and believe I've resolved all vacuum leaks, so really stumped. I adjusted the valve lash early on, but will go through that again the next time I have it out for a good drive, and will try pulling a plug wire at a time.

Check your electrolock connection were it attaches to the distributor. It could be grounding out from time to time under light throttle.

I’ll give that a look. Thanks

Great. Let us know how you make out.

Electrolock appears fine with no indication that it’s grounding to the distributor. Next time I’m out with it, I’ll try pulling a plug wire at a time to see if it’s isolated to one cylinder, them warm it up well and check the valve lash.

With the car running did you try to jiggle the electrolock back and forth to see if it makes the engine miss?

Yup. There is an irregular miss already, but didn’t notice any change for the better or worse.

Hi SteveEC

I would love to hear a more definitive explanation of what is really happening, in your post you mentioned a bucking?

Sorry I hit the send button before I was finished, I think I might have the same problem as you with my 31 coupe