Noticed a fuel leak on back of 4779 750 holley carb, turns out inlet threads gone on big inlet nut. Swap out rear bowl, has new needle and seat and notched float. Now car is a tenth slow. Did leak some gas down intake during swap because float stuck, so changed oil and cleaned plugs. Can`t figure it out. will try new plugs. Any Ideas?

measure and set the float in the replacement bowl exactly the same as the float in the old bowl and confirm it has the same size N/S.

That said, how much time past...hours, days, weeks, between swapping bowls and making the tenth slower pass?

Thanks 1320, it was the same day then week later and then another week later. looking at weather and conditions and comparing to other cars it has been about a tenth off each day. For some reason float adjustment is as low as possible but it still seems high at sight hole. Thanks again

its the year 2017, we haven't adjusted floats using the sight glass since the 80s.

bowls have to be off the carb to baseline set them.

Sounds like the float is heavy. I would change it out. set the float level with a 3/8 drill bit before you put it on and if you have to adjust it it much after that you probably have a heavy float, leaking needle and seat or too much fuel pressure.

Do as Ed says to set the float, many bowls have the sight plugs at different hieghts cant rely on it.

Will do, it is a new holley notched float, the only other thing I changed was jet extensions, I used to have the push on type, switched to the screw in type you screw into metering block then screw jets in to them

The jet extension shouldn't be an issue on a 750 gas carb, but worth swapping to try. On a larger carb and high HP engine the thread on extension can be an issue, I've found in a few where the jet was bigger than the extension....

Rule of thumb ideally is to have any passage after the jet double the diameter to prevent a restriction in flow. Realistically the extension should be drilled or reamed to the .150 range for most, when you get over 900 HP on gas or less with oxygenated fuels use a slip on or one like the Holley 122-5002 where the jet slips in and threads against a shoulder.

This message has been edited. Last edited by: jmarkaudio,

Would agree, toss the screw in extension and go back on a slip on. Also best to set floats using a float drop method rather than site glass method. AS suggested easiest way id off the car float upside down insert a 3/8" to start drill bit between the float and bowl and adjust the needle and seat so it just touched the drill bit.

I would do a compression test to verify you do not have a bent rod from a hydraulic situation. Don't ask how I know.

Just read a friend's facebook post where he changed converters multiple times looking for et loss. problem was carb linkage not opening secondaries all of the way!
Basics! Ck everything for a bind in the linkage and have someone step on the gas pedal while you look down the carn to see if it's opening all of the way. Secondary bowl is near the dist on a Chevy, easy to bump cap or possibly move dist.

Just a follow up, reset float and back to Slip on jet extensions, seems car is back to normal, thanks all for help!

Fast Frank you have a PM from me.

Is it possible that the screw-in extensions are a bit longer, and the float was stuck under the extension and not allowed to rise with the fuel?

I typically only have the bowls off when rebuilding in the offseason, but always install the secondary bowl with the carburetor upside-down for this reason.

what length extensions were u using , and were they plastic???