The Stovebolt Site Forums The Shop Area The HiPo Shop holley 600cfm jet question

I'm plowing new ground, so please bear with me. SBC 350, 290hp, Chevy crate motor with 600cfm Holley. Smells rich. Can anyone who has this set up give me the correct jet sizes and the color of the pump cam for good fuel mileage and pep? My mpg is not what I had hoped for either. The rear end is 3.23 or .24 with a turbo 350. Thanks in advance.

try #65 jets in the front. make shur the power valve is not blown. the fuel pump cam i use is the white one in the first hole. also check to see if it has been messed with like holes drilled where they don't belong or screws where they don't belong. also it should be a vactum secondry carb for use on a auto trans.

Thanks Bob308, I know may cam is not white. I purchased a varity pack of them last year for my old 390-cfm which is now gone. I will check the jets and power valve next monday, got a major car show the next three days if it doesn't rain us out. I purchased this carb from National Carbs out of Florida. While not new, they said it was like new with blowout protection. But upon starting the motor for the first time, gas was squirting 1" above the carb. It was so bad to begin with, gas was dripping out the tail pipe. Found the floats were not correctly adjusted and were hitting the top before closing the fuel inlet. Hmmmm.

Baseline the carb to stock specs and it should be close, within 5 jet sizes of optimal. Only so much air can flow through the venturis, with appropriate fuel to match. Soulnds like you have the typical Holley problem, too rich at idle. For some reason they make their idle feed restrictors too large by nature. This makes it so that they run rich and stinky. Even with the mixture screws in only 1/2 turn out it's still too much. I've worked over a few holleys and it's been a lot of trial and error.

Devin

Go to the library and pick up the book Super Tuning and Modifying Holley Carburetors.

First and foremost, each engine is different, so there is no canned answer.

Here is my process

Make sure that you do not have any vacuum leaks

Is the carb vacuum or mechanical secondaries? If vacuum, you may need to tune the spring rate. If it is mechanical, then you may be feeding it more than needed for a street engine.

Check vacuum at idle. Then size the power valve accordingly. Generally take the vacuum reading at idle, and divide by 2. This will get you close to the right number. usually a 7.5 or 8.5 for the street (engine dependent). Do this first.

With the engine timing set properly proceed to the following

With the air cleaner on, and choke fully open, lean out the idle mixture screws until the RPM starts to drop, then open just slightly, then using a propane torch, introduce a bit of propane to the air filter. adjust the screws until the RPM's start to drop a bit with the you are looking for no more than a 25 rpm drop with the propane. This will get your idle mixture close to stoich

for adjusting vacuum secondaries, use a paperclip on the actuator rod located at the top. take the vehicle for a ride and do a full load hole shot. Measure the spring movement by seeing how far the paperclip moved. if the secondaries are not opening all the way, then you need to adjust the spring.

For jets, you will need to play with them to get proper sizing. I have had two nearly identical installations, where I had 64's on the primary of one, and 61's on the other. You are looking for good throttle response. I would start with a set of 58's and go 2 sizes larger each change until it starts to stumble off idle. Then drop one size. Repeat for the secondaries, however that is usually at wide open throttle.

beyond that, you will have to dig more into the book.