It is clear that probably both the butterfly valves in my two zenith stromberg carbs do not fully close. The idle adjustment screws are fully unscrewed and yet the engine runs at about 1100 rpm. This is an engine in otherwise very good condition- compression, timing etc.
When I cleaned these carbs, I am sure the butterfly’s were installed correctly with the “dot” indicator on the correct side
My question is there anything I can do to get the butterflies to fully close without taking the carbs off the engine?
i have all the tune info for the carbs in my library for free, covers all models> try the comprehensive section, covers bot manual & auto chokes
Nope… you’ll need to pull them.
Is the butterfly jamming in the throat?
Or is it just that the throttle linkages are incorrectly set? Or the choke mechanism isn’t completely off.
Release the throttle links from both carbs and see if you can get them to close.
If not as Paul says you’ll likely have to take the carbs off to rectify the problem.
Or you have a vacuum leak, and/or your timing is off. I’d start there before pulling the carbs.
Before you remove them I would try this
First I would disconnect the throttles from each other. This will allow the throttles to operate independently. They should return to their closed position with their own springs. Listen to the hiss of air from each. They should be the same, if they aren’t then the loudest one is the one that’s staying open.
With the engine OFF You “might” be able to loosen the screws holding the throttle plates to the shaft. They are at an angle so I say “might”. If you can you could try opening and closing the throttle on that carb quickly to see if you can nudge it into better alignment.
The aforementioned is kinda clutching at straws really but you could try it.
If you try this don’t forget to TIGHTEN THE SCREWS.