SOHC.co.uk Forums > CB750

throttle hangs

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Martin6:
Have you completely ruled out an air leak? A check of how well the rubbers are seated on the carbs and head. Check the washers are all present in the vacuum take offs and screwed in tight. Maybe also check the rubber seals under the cap at the top of the carb slide, if you haven't already.

Sesman:
Just a couple of pointers based on my own, limited experience. The bench sync needs to ensure that the throttle slide gaps are set with the main idle adjuster just holding the slides open. In other words, if after setting the slider gaps it’s not possible to close them completely with the master idle screw, the gaps are too large. The large gaps lead to excessive vacuum when adjusting the idle.

Basically, as Ted has alluded, if the vacuum is set too high at idle, a hanging throttle will be created due to the idle circuit/ slider height relationship and the vacuum created when closing the throttle.

A alternative way to adjust the throttle sliders is with carb vacuum gauges attached, ensuring that the master or reference cylinder (say cylinder 1) does not pull an excessive vacuum, then matching the other vacuums as prescribed by the manual. The manual helps by providing an approximate value of vacuum.

This of course assumes all other contributory factors have been eliminated (air leaks, jetting, cleaning, tappet clearances, timing etc).
 

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