vacuum leak ?

[chainsaw]

Will a vacuum leak cause the engine to idle higher when it gets hot? Its 105 here in Tx. ,just drove around for about 15 minutes,temp gauge was reading 210 -220 around town, lots of stop lights. Truck ran fine the whole time, didnt shoot water out the overflow,didnt bog when leaving stop lights, but was idleing faster ,maybe 100-200rpms faster then when it was cold. Thanks.

[hugger6933]

Any thing is possible on these old trucks and vaccum leaks do funny things did you happen to check the carb to see if the fast idle cam had fallen over?That would do exactly what your saying. Like I said funny thing happen to these oldrides ,it is like they develop their own personality. And some time do stuff just to mess with ya

[11echo]

"Usually" vacuum leaks cause an engine to idle lower and rough. If you think about it your engine uses the vacuum to carry fuel to the combustion chambers, and if there is a leak you lose vacuum and the capacity to carry the fuel that is needed to run efficiently. One way to check for a vacuum leak is to use a can of WD40, and while the engine is idling, spray areas that you think might be leaking. IF the engine surges when you’re spraying you've got the problem. You can check and see what your engine with do with WD40 by spraying the intake of the carb. ...Good Luck!

[Wrenchbender Ret]

Some carbs have what they call a "hot idle compensator". It lets a little more air in to lean out the mixture under real hot conditions. It will make it idle a little faster.

[chainsaw]

Thanks for the replys. Sprayed wd40 around carb and mainfold, no change in engine. It sounds like the carb isnt colsing all the way. Got it hot and disconnected the linkage, still idleing high. manualy opened the secondaries, no difference. Its hot outside,giving up for awhile.

[Rich 5150 69]

What kind of carb are we talking of....?

[chainsaw]

Holly 750 double pumper

[Rich 5150 69]

Is this a really hot build, if not thats way to much carb, vaccum secondaries or mechanicle...? you actually should be around 650 cfm, anyways if this is a used carb it could be wear at the throttle shaft on the primaries allowing the plates to stick in a partly open position or vacuum leak at the shaft itself, your only allternative is to by a new base plate from holley, not sure if they sell a bushing kit for the shafts. Go here for more info. I read that you didnot find any leaks at the base or the manifold, by the way if you check for vacuum leak agian you can use a unlit propane torch, you will not make a mess of your engine, you also might want to check your vaccum advance diaphram and linkage, is it binding up or losing vacuum will also cause the same issue.

http://www.holley.com/applications/C...bSelection.asp