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MG MGB Technical - how to adjust the idle ?

hi
79 mgb, zs carbs, recently rebuilt distributor.
it's spring and time for a complete(?) check up.

i thought i was doing as the haynes manual suggested, but i can't bring the idle down below 1000+ rpm.
this has happened to me in previous springs. i need some help/advice.

so, what buttons do i push to lower the idle to at least 850 - 900 ?

please be specific for this old man. pointing to a diagram or such in the haynes manual would be very welcome, or, simply, written instructions.

thanks !

john



john sutter

John

This site may shed some light on your problem

http://www.theautoist.com/zs_water_chokes.htm
Kent
DK McNeill

kent

thanks for the site, but i forgot to mention, i had removed the water choke. much simpler now.

john
john sutter

Something is holding the butterfly open, or the spindle and bushing is so worn fuel/air mixture can bypass the fully closed butterfly. If the butterfly has the poppet valve in it these can cause problems, as can the butterfly being misaligned in the throat, not enough slack in the throttle cable, or anything else stopping the spindle returning all the way.
Paul Hunt

John,

Were you able to sort out your idle problem? There is a by-pass valve on the side of the carb with a diaphram in it. Not sure what happens if the diaphram has a tear in it but can tell you from experience that the spring pressure inside this by-pass valve can be adjusted. If relaxed too much or you have done something to allow engine to breathe easier, like free flow exhaust, you need to increase the spring pressure. By pass valve is only intended for deacceleration but other things will cause it to open partially or never close completely.

HTH

Fred
Fred Wright

thanks for the help, i really appreciate it !

but, what is a poppet ?

and, can i possibly fix the problem without removing the carb ?

john
john sutter

Poppet valve is fitted in the butterfly plate on late SUs - don't think it applies to ZS carbs like yours.

If it is a regular spring thing then it may be the engine is running weak on old fuel or some restriction.

Check the mixture as well - the engine runs slower on a rich mixture than a weak one for a given throttle setting - also check the ignition timing as this also affects idle speed.
Chris at Octarine Services

The fastest idle comes at the ideal mixture, but you should always be able to slow the idle with the idle screw to stalling point, including if you have a vacuum leak (which is a often mistakenly mentioned as a cause of not being able to get the idle down). The engine can only run when it is getting a fuel/air mixture, and if the butterfly fully closes the throat as it should then none can get past - unless possibly the spindle and bushes are so badly worn it can sneak past the closed butterfly. The usual cause is accellerator cable, linkage or butterfly alignment problems. The latter can really only be checked by removing the carb and looking down the throat against a light source, idle screw fully backed off, looking for any light coming through. But always eliminatev cable and linkage probelms first, if necessary by disconnecting the cable from the carb.
Paul Hunt

Oh, and if you still have the water choke that has a fast-idle function which could be holding the butterfly open as well.
Paul Hunt

This thread was discussed between 17/04/2009 and 23/04/2009

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