The Old Lawnmower Club
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Throttle wear / idle question zenith 13 tca2

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goodgrass Sat, 06/08/2016

What degree of movement between a worn butterfly spindle and body is acceptable, what is the amount of radial and axial play in those areas when slow idle begins to be a problem?

I believe air getting sucked past a sloppy fit at that point can cause erratic idling

Are they repairable ?

Forums

wristpin Sat, 06/08/2016

Axial movement is controlled by the fit of the butterfly in the venturi so as long as it fills the venturi  in the closed position it will control the end float. Radial wear in either the spindle or the carb body is undesirable but inevitable in an old machine. Can it be repaired? - well everything is possible at a price , the carb can be bushed and a new spindle made  but there are plenty of used carbs around, just a question of finding a good one.

As for the amount of  permissible wear, a totally unscientific yard stick is that if the hole in the carb body is obviously oval, look for a better one ; and if the spindle has an obvious  step in it, the same applies. No need to get the vernier out!

Coincidentally I was finishing off one of my Suffolks yesterday and couldn't get the revs down to a satisfactory idle speed, but slackening the screw holding the butterfly to the spindle and then holding it tight shut to centre it, while tightening the screw  cured the problem.