It still sounds that the carburetors are basically sound internally... but with external adjustable slow speed needle valves, yes, as Crosbyman states in not so many words, that could, and usually does have a negative affect as time passes.
In adjusting the individual slow speed needle valves, you do not need to adjust anything else, excepting possible the threaded trunion on the throttle cable where it is clamped to the engine. The needle valve adjustment may cause the idle to increase or decrease rpms as the valve adjustment would be changing the fuel/air mixture and that trunion would need adjusting to have the idle at the desired rpms..
Before touching those needle valves, be sure to make a note of where the valves are set at (the slot angle and how many turns in it takes to seat them).
HOWEVER... Before trying that adjustment procedure.... you stated that pumping the fuel primer bulb makes no difference... BUT... are you having someone constantly pump the fuel bulb BEFORE that problem surfaces so as to avoid the problem altogether... OR... are you waiting for the problem to surface, THEN someone starts to pump that bulb? There is a difference.
The past practice of adjusting the slow speed needle valves are as follows... however, the adjusting start scenario (how many turns out to begin) may be different on the 1994 model. If so, I'm sure another member will jump in here with the correction.
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(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
Note 1: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.
Note 2: If the engine should be a three (3) cylinder engine with three (3) carburetors, start the adjustment sequence with the center carburetor.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
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