Accelerator jet symptom

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hans3

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When holding RPM at 2000 rpm in neutral, I depress the gas a touch more and the engine "loses breath," or bogs. It then recovers and gains rpms when I depress the gas pedal even more. It's only a mometary hesitation, but it's repeatable and consistent after holding the throttle steady. Is this a characteristic of weak Zenith accelerator jets? Thx!
 
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Hans,

Yes. The stumble at 2000 rpm is most likely the accelerator pumps. Look down the throat of each carb while operating the pump arm and see what's going on. Get that going first. An owner doesn't have to remove the carb to get to this section. It can be done by removing the top section, then the jet section.

To a lesser degree the mixture could be a little off either at the butterly adjustment screw or idle screw, or combination.

We are of course talking about a warmed engine.
 
What's the butterfly adjustment mixture screw? Is this the thumb wheel that adjusts the rear carb linkage relative to the front?
 
Carbs continued

It's the off vertical screw on the choke side of the carb vs. the horizontal idle mixture screw on the bottom closest to the valve cover.

The adjustment you refer to is part of the throttle rod. Because of geometry of the action of the linkage, the rear portion of the rod tends to react or rise than the front carb. That thumbwheel sets the whole rod to move in unison. best vviewed when the hood is up , pressing the accelerator, and observing the carb connection to make certain both move in unison.
 
Could you perhaps link a picture of this off-vertical screw? This is the first I've heard of it, and I thought I identified all the parts during rebuild. Perhaps my Zeniths don't have them.
 
Courtesy of DeQuincey:

karbus.jpg


green arrows are butterfly screws, red are idle mixture.
 
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