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 Short vs Long Fulcrum Floats 
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 42
Have read on the web that the short fulcrum float Qjets are not as good as the long floats for performance apps. Any truth to that? I'm looking at using a 73 Olds carb and the Olds didn't switch to the long fulcrum floats till 1976.


Fri Apr 22, 2016 5:33 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1695
Short fulcrum
Image

Long fulcrum
Image

The pictures were handy examples on the web. I don't care much for the brass floats because they are heavier than nitrophyl and have less displacement.

Have you read the section in Doug Roe's Rochester book about QJet floats? The premise is the short fulcrum provides more closing force and better liquid level control in rough terrain.

The short fulcrum will require more liquid level drop for the same amount of valve opening. In other words, the liquid level will drop more as power increases with progression of throttle opening.

The long fulcrum provides more valve opening with the same amount of liquid level drop. Most marine QJets use the long fulcrum, presumably because marine engines operate continuously at higher average power output and fuel flow than highway vehicles.

Obviously, the amount of liquid level drop at part-throttle will affect A/F independent of other metering parameters, jet, rod, spring, bleed, etc. This is (was) probably a consideration in calibrating an emissions compliant engine package, but I can't say I have ever noticed anything that gives me a preference for one over the other.

A large displacement high power output application could benefit from the long fulcrum for more flow with less float drop and also use of a large inlet valve with correct fuel pressure. Very large inlet seats can be used with a 3/16" ball valve instead of the rubber tip.

With all the highfalutin' hypotheticals aside, I have used both and have seen nothing that suggests either length fulcrum is inadequate. Both seem to work OK as a basis for a performance carb.


Sat Apr 23, 2016 7:32 pm
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