Help With Carb Adjustments

[The Fast Forester]

I have a 1978 Camaro dirt track stock car with a 350 small block, 4 barrel carb. Last couple times out it has backfired badly on the deceleration into the corners. Checked timing it seemed ok at 30, changed plugs which seemed to help for awhile, but later in the race was back to throwing fire through the exhaust headers. I'm thinking I need to adjust the carb, it might be running too lean? Is there a site out there or anybody here that can describe the tuning process in detail for this carb. It is a Holley, I think 750cfm. Thanks.

[Morley]

If it really did backfire and you don't have the powervalve kit for the carb, then you probably blew out the powervalve.

[hotrod_chevyz]

Dont 90% of stock cars do that because they are over-carbed and have open exhaust? Here at the speedway every time the stocks are entering turn 1 and turn 3 it sounds like a gun fight...

Lose the holley carb

[The Fast Forester]

It's backfiring (igniting gas) in the exhaust headers underneath the car. Not sure what the powervalve is?

[Morley]

Originally Posted by The Fast Forester

It's backfiring (igniting gas) in the exhaust headers underneath the car. Not sure what the powervalve is?

If it is in the exhaust then it is really "after firing". Backfiring is when it fires back up the intake and out the carb.

[The Fast Forester]

Yes, it is afterfiring, and pretty extensive. I looked at the video from the last race and it seemed like I was on fire under the car in Turn 1. So I think it's a bit excessive even for a race car. It seems to only flame up through the carb when I start it up after it sits for awhile.

Today I made some adjustments on the air screws (all four of them). Shut them, then backed them off 1 turn, and tweaked the idle. Seemed to run pretty good but when I cranked up the rpms and let off there was the popping again. So I closed the screws another 1/4 turn, fired it up and let er rip and gas started shooting out of the tube in front of the carb? Am I close, or should I now look at replacing the cap/rotor to try and help. Thanks for all the replies.

[Morley]

Never a good thing to have gas come back up out of the carb.
Check here for how to tune the Holley carb. http://www.mortec.com/carbtip1.htm
They also explain what the power valves are and what they do.

[The Fast Forester]

Thanks for the link Morley

[The Fast Forester]

After some more tweaking, I noticed that wherever the carb is set once the water temp goes over 200 degrees gas starts coming out of the carb vents, floods and kills the engine. Is this a symptom of vapor locking? My outlet radiator hose actually touches the fuel line running up from the fuel pump, I'm thinking that could be part of this problem as well. Think I'll try some heat tape around the fuel line and see if that helps.

[The Fast Forester]

Well the latest opinion I had from an expert suggested I take off the front and back bowls of the carb and check the jet sizes. He suggested it might be flooding at the higher temps because it's running too rich to begin with, then when it gets hotter it's really too rich because of the oversized jets. Took them out tonight, 71 on front and 73 on back jets. Any help with jet size would be appreciated. Thanks.

[The Fast Forester]

And I pulled out the powervalve.......because the more I read on the powervalve I'm thinking it might be shot and causing my problem of running too rich, flooding and backfiring. Does the powervalve kit mentioned above just shut this down so you have to jet up, or does it protect it from backfire somehow? Thanks for the info.

[hotrod_chevyz]

sounds to me like that carb would make a better door stop...

To know what jets ect you need you must know EXACTLY whats done to the motor. Things like intake/heads/cam/exhaust all play a role in the overall efficiency of the engine, wich would then decide what needs to be done to the carb to make it match up. If this engine is in a stock car, im thinking its over carbed.

I have a feeling this is a seperate issue than the flooding, wich from the sounds of it is pretty extreme. It is the job of the needle and seat to shut the fuel off before it overfills the bowl.

BTW 200 temp is low for a stock car engine. The way those things get wound out 200 is chillin.