jpage Posted October 28, 2013 Share Posted October 28, 2013 A freind of mine has a '50 GMC 11/2 ton truck with a 6 cyl engine. He has had the carb rebuilt and the truck idles well but upon acceleration it hesitates, coughs and then it will run. Can anyone give some reasons for this. I think maybe a bad accelerator pump. It makes a low geared truck very hard to drive as you loose to many rpms waiting for the carb to kick in between shifts. Also ,there is a heavy rattling noise coming from the bell housing whenever the clutch is depressed. The noise stops when the clutch is let out. Could that be the thowout bearing going? Thanks in advance Jim Link to comment Share on other sites More sharing options...
Owen_Dyneto Posted October 28, 2013 Share Posted October 28, 2013 Among the things to consider are (1) carburetor accelerator pump, (2) leaking vacuum advance diaphragm on the distributor, (3) vacuum leak somewhere, intake manifold gaskets, wiper motor, etc., (4) ignition timing. Or some combination of those. First thing I'd do after checking the ignition timing and the vacuum advance diaphragm is to put a vacuum gauge on and check manifold vacuum. Very easy to check the accelerator pump, just hold the choke valve open and stroke the carb linkage a few times. Link to comment Share on other sites More sharing options...
joe_padavano Posted October 28, 2013 Share Posted October 28, 2013 The noise from the bellhousing sounds like a classic throwout bearing problem. The carb hesitation sounds like a classic accelerator pump problem. That's usually an adjustment that's required after rebuilding. Also, does this truck have a governor? That adds another variable. Link to comment Share on other sites More sharing options...
carbking Posted October 28, 2013 Share Posted October 28, 2013 I'll leave the noise to others.Often, a hesitation from idle is caused by setting the idle mixture way too rich (setting for the highest vacuum), and then diagnosed as a defective accelerator pump.Start the engineStop the engineRemove the air cleanerHold the choke butterfly openWhile looking down the throat of the carb (engine off!), quickly work the throttle from closed to open Good pump - good squirt of fuel Bad pump - weak or no squirt of fuelTesting is always superior to guessing.Assuming the pump is good, Owen_Dyneto's list is very good.Jon. Link to comment Share on other sites More sharing options...
jpage Posted October 28, 2013 Author Share Posted October 28, 2013 Thanks for the hints fellas, we'll be checking it out! Jim Link to comment Share on other sites More sharing options...
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