Someone please explain to me how an Autolite 4100 power valve works?

[rnch]

And what do the different numbers on the power valves indicate?

Thanks!

[miker98038]

Power valves in general (or step rods in an AFB, or etc) are sized by the vacumn at which they open, i.e. 8.5 for 8.5 inches, 2.5 for 2.5 inches (Big cam). Old rule of thumb. If the motor idles at 15 inches, the pv would be half that, 7.5. Usually stamped on the valve. The idea is that when you "get on it" you need additional fuel for power. You don't want a pv that opens at high cruise, like on the freeway.

The real experts will chime in with more info. To do a little reading, start here, and then click Q&Answers.

http://www.carburetor-parts.com/Moto...ve_p_1163.html

[rnch]

Would a higher number power valve have any effect on mitigating (or eliminating) an off idle stumble than cannot be eliminated by any accelerator pump linkage adjustment?

[miker98038]

Probably not. It's more of an enrichment at or near WOT. An off idle stumble, if it's fuel, is something in the transition slots or whatever the 4100 uses. My experience with other carbs is its a lean condition, most likely the idle adjustment or the primary jets. Or something causing the idle setting-mixture, butterflies,,etc being out of whack. But you need an expert for better advice, like Sal or others on this site. Someone with more experience than I have.

[rnch]

Originally Posted by miker98038

Probably not. It's more of an enrichment at or near WOT. An off idle stumble, if it's fuel, is something in the transition slots or whatever the 4100 uses. My experience with other carbs is its a lean condition, most likely the idle adjustment or the primary jets. Or something causing the idle setting-mixture, butterflies,,etc being out of whack. But you need an expert for better advice, like Sal or others on this site. Someone with more experience than I have.

going up to size 49 main jets did lessen the off idle stumble ... some.

[miker98038]

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Up to 49 from what size?

[rnch]

Originally Posted by miker98038

Up to 49 from what size?

The carb had 47F main jets in it.

I tired 51's; but seemed to be too rich, would occasionally "blurp" at curb idle. ALso the 51's made the off idle hesitation/bog worse.

Haven't tried 49's yet.

[miker98038]

Here's a long post back some years on the 4100. Some jet sizes given well into the post,and a couple links. Since your not getting a lot of help here, you might try posting over there, or just run a search for "4100"

http://forums.y-blocksforever.com/Topic107281-1.aspx

[scicala]

Rnch,

Is you're 4100 Autolite carb on the size and model year engine it was originally intended for ? Off idle stumbles are most commonly caused by an accelerator pump that is not squirting as soon as the throttle is just slightly opened. If it's not, make sure the linkage for the pump has no slop, or is out of adjustment. Also make sure the idle mixture needles have been set for the best warm idle (highest vacuum) to give yourself a fighting chance. The carb should operate acceptable with the original jets it was made with, unless it's on an engine not even close to what it was intended for.

If all of this is checked and found to be OK, then it may be time to take apart the carb and look for an issue with dirt or water.

Also possible for a stumble from incorrect ignition advance.

Sal