Hello, I just put a new 350 in my truck and I can not get it to run right. It has a holley 670 avenger on it currently. When im going down the road and I stomp it, it cuts out. Any time I try to give it a decent bit of gas from a low rpm, wants to cut off and I am forced to feather the gas to keep it from dieing. If i get the revs up first and then get on it, it will pull fine. I do not no any thing about tuning carb's. Please help me figure this out, it is driving me crazy! Please pardon me for my ignorance. Thanks in advance.

 

Is it a new carb or used? Sounds like your having some accelerator pump issues.

 

Brand new. How would I go about fixing that?

 

There are a few really good Holley books out there that show you but if its brand new I would call Holley up and talk to them.

Agree on the accelerator.

 

You should be able to see gas squirting out of the accelerator pump shooters when you give it a little throttle, if you're looking into the carb. If they are not working, then you will have a stumble. Most of Holley's info is on the net, you should be able to google it. Bob

 

step #1 (always) : check the fuel level in the bowl.

You probably need to raise the fuel level, it's leaning out.

 

that sounds like you need to adjust the accelerator pump. What it probably happening is the butturflys are opening before any gas is being squirted. Use a feeler gauge between the arm and the lever. you will want about .015-.020 clearance.

 

Timing can be an issue as well.

 

You want .15-.20 clearance left in the pump lever at WFO so you don't hurt the pump diaphragm. The pump should begin to squirt as soon as you touch the gas and there should be no play at the idle setting. An easy check without doing anything is with the engine at warm idle just barely bump the accelerator pump lever and the engine should slow down. If it speeds up turn your idle mixture screws out a little. (not saying you didn't know this i'm just clarifying for the op)

Also verify that your float level is correct. Should be right at the bottom of the window. If it is there already i would suggest raising it a flat or two on the adjuster. This will increase the pressure head on the transition slots and richen them slightly. That should help it some, and depending on how sensitive your idle mixture screws are you can screw them out 1/8 turn or so and it should again improve your problem.

Another thing is if the carb is used or was for some different application it may have too low of a power valve which will delay opening when it begins to use the main circuits making for a lean ratio until it opens. The enrichment added by the power valve is worth about 10 jet sizes when it comes in. Its what allows you to run at a lean ratio for cruising say 14.5 and then a rich ratio for max power somewhere in the 12's. If it comes in late, you will certainly be lean under a moderate power condition and then pop.

Lastly i would look to the accelerator pump. The reason i say last is I hate to tell people to put in larger squirters. Every time you hit the gas you discharge fuel, that is to say if you have a 30cc pump every time you blip the throttle you just burned that much fuel. Obviously this is makes for bad economy. But if you do all the other stuff and its still doing it. Go up 3 squirter sizes if you have a 31 in it put a 34 in it. If that doesn't do it look to a more aggressive pump cam, i like to use the pink ones that come stock in the bg stuff it gives you a very good initial hit and then tapers off.

Also make sure your timing is correct on the engine don't do any carb tuning until the timing it set and locked down. On sbc stuff with mild cams i like to run about 12-16 degrees of timing. I also like my timing all in by 2500 or so, and run a vac advance can thats worth about 10-12 degrees maximum.