I don't know much about carbs and I'm doing a compression check. I've read that I'm supposed to have WOT when checking compression but this can not be done with the grip on an '81 because the carbs are vacuum opperated? If this is correct how does one go about "propping the slides open"?
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Carb/WOT Compression question
- Thread starter Paddy399
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- carburetors engine how to
You want to open the throttle wide, which opens the butterflys in the carbs. That will give an open passageway for the air flow to the cylinders. Not really necessary to open up the slides.
To add to what was said:
It is the petcocks that are vacuum operated, not the carbs. So, twist the throttle sleeve all the way, and proceed with the compression check.
The idea is to let air into the engine while doing the check. That is what twisting the throttle sleeve does, it opens the butterflies which lets unobstructed air in. The slides don't ever completely close, so as stated, you don't really need to prop them open. Fuel and or the petcocks have nothing to do with a compression check, so your vacuum operated petcocks don't affect a compression check.
It is the petcocks that are vacuum operated, not the carbs. So, twist the throttle sleeve all the way, and proceed with the compression check.
The idea is to let air into the engine while doing the check. That is what twisting the throttle sleeve does, it opens the butterflies which lets unobstructed air in. The slides don't ever completely close, so as stated, you don't really need to prop them open. Fuel and or the petcocks have nothing to do with a compression check, so your vacuum operated petcocks don't affect a compression check.
DogBunny, all carbs are vacuum operated; whether the slides are vacuum lifted or mechanically lifted, it's engine vacuum that sucks in the air and sucks up the fuel.
But right, with throttle plate held open, enough air will pass the slide cutaway to yield a compression reading; it just takes a little more kicking/cranking. If you have pods or have the airbox off for service, you can get a reading with less effort if you prop the slides up.
But right, with throttle plate held open, enough air will pass the slide cutaway to yield a compression reading; it just takes a little more kicking/cranking. If you have pods or have the airbox off for service, you can get a reading with less effort if you prop the slides up.
Yes, dumb late night senior moment on my part, thanks...
The motor was warm and the airbox was on with the filters in. Just doing some trouble shooting for a missfire on one cylinder. I can live with the compression the way it is as I plan on rebuilding the top end this winter. I just want to be able to ride the rest of the season.
burns
OWG
Actually they're pressure operated. Vacuum is the absence of something and therefore it can do nothing....
actually they are pressure GRADIENT operated. The descending piston creates a lower air pressure in the intake tract than that of the ambient air pressure. "vacuum" in this context is a vernacular term for that gradient and is non-ambiguous usage with the cohort to whom it is addressed (i.e. it is unlikely that this usage will be confused with "total absense of matter" by anyone reading this thread )
if you want to get picky.
burns
OWG
"venturi effect" would be the term then.
