Quadrajet Off Idle Stumble

[Shady Tree Garage]

Just finished a rebuild on 17080213 carb on my 1984 K2500 350.  Clean as a whistle now thanks to Ethan's suggestion to soda blast.  Jetted 72/50 per oem spec, apt measured and set with caliper and seated at idle, manifold vac approx. 17-18 with a "8" spring under power piston, starts to pull up around 6-7 inches actually.  Set accel pump rod height according to spec sheet (9/32), but I notice that the pump rod doesn't even start to move until throttle plates have opened a few degrees.  Coinciding with this, as I come off idle, at first smooth then (I believe just as pump begins to actuate) stumbles, worse when cold.  Once engine is warm, if I "feather" the accelerator, I can get past the stumble, but it seems like the pump shot causes the stumble!?  Mixture screws have little affect except on vacuum and even turned all the way in don't kill the engine.  Checked and re-checked my adjustments on most everything I can think of.  Idle and cruise are smooth and beautiful otherwise.  Big question bugging me is should there be movement of the throttle plates of a few degrees before the movement of the accel pump rod and linkage? Thanks in advance for your thoughts!   

[429bbf]

shady something doesn't sound right . you should have to push the acc. pump down a little to get the pin in on the pump lever . there should be no gap . do you have the right return spring under the acc.pump?did you test for leakage if the ball was seating properly?what you are explaining would cause your problem no added fuel from the acc.pump when you start to open the throttle plates will cause a big stumble. fwiw

[Shady Tree Garage]

Thanks for the info 429bbf.  There is no gap between the pump lever and the piston and the ball appears to be seating with no leaking.. I get a good pump shot when the linkage pushes up but the throttle linkage opens a bit before the "cam" that is attached to the pump linkage begins to move and push the linkage up.  The throttle and the "cam" that pushes the pump linkage up are on the same shaft (throttle shaft), but they move separately with the cam delayed by a few degrees and I can't see where the mechanism intersects at the point of movement.  The choke apparatus blocks my view of the throttle on the passenger side.  Do you know if there is a tang (perhaps on the passenger side out of sight) that needs adjustment so both move simultaneously?  Hoping to avoid removing carb again unless I know what I am looking for.  Thanks again for the help!

[429bbf]

there can only be a couple of problem. ether the tang (the piece that the acc rod attaches to is loose on the throttle shaft)or your holes are worn out,or your rod ends are worn out. the piece on the throttle shaft should be solid . no moment. where are you hooked to the bell crank ?inside or out . inside gives more pump shot.fwiw

[Shady Tree Garage]

Thanks again!  That sounds like the likely problem then.  The rod is connected to the inside hole.  If the tang that the lower end of the rod is attached to is supposed to be solid with the throttle shaft, that is where the problem lies, because it is not.  Not sure what the fix is, but now I know where to work.  Thank you very much.... I will report back when I figure out a solution.

[429bbf]

some times you can cheat and just peen the end of the shaft where all that garbage is .all the pieces slide over the shaft and then they peen it so it won't come off.that new of carb should not be loose . have you always owned the carb?

[Shady Tree Garage]

No, it is not the original carb.  It was a re-build done by guaranteed carburetors in FL in 2012.  It has always had this stumble problem to some degree and it has been tough to diagnose until I noticed this delay in the pump action.  I just finished an engine swap and while the carb was off (it was really carbonned up and filthy, float setting was too high, APT was way rich and the power piston didn't need flicking as it was already up but not sure it didn't pop out when I removed the air horn), I rebuilt and cleaned it again using a factory manual for specs, Doug Roe's book, a white paper from Lars and info from Cliff and the forum.  I think I have it set up and adjusted sufficiently.  Thank you so much for taking the time to help me out with this, I'll let you know what I find.

[Shady Tree Garage]

OK 429bbf!  Success!  Boy feeling a little stupid now ... decided I would pull the throttle linkage off the shaft and try to secure the tang better and I am embarrassed to say I found the throttle linkage screw slightly loose!  Tightened it and voila, the two work simultaneously now.  After re-adjusting idle screws to best vacuum, timing and idle speed (because I had screwed around with them all to remedy the situation), the truck has barely any hesitation if at all.  I may play a little with the timing because the note on the air cleaner says 4 degrees and the manual and specs (as well as the new engine's info sheet) specify 10 degrees btdc.  I set it at 10.  I did just get a slight dieseling when I shut it down, but couldn't reproduce it when I tried.  If it keeps up, I may tweek a little, but thank you again for the guidance and time helping me out!