carb tuning question

[sodak69c10]

hello i'm new here but i was hoping that some one might have an idea on what is wrong with my carb, the carb is a 575cfm speed demon VE that i purchased a year and a half ago and put on my 283 in 69 stepside, the carb ran great until about a 3 months ago now when you start the engine it will idle fine until you step on the throttle ,then it pops back through the throttle body and then idles up to about 3,000 rpm and stays there i tried some carb cleaner to no avale, now i got a full rebuild kit need to find time to rebuild it. just wondering if anyone know the cause of this.

[mnunn454]

Sounds to me like the accelerator pump isn't working for some reason (linkage fall off perhaps?) causing the leanout backfire you described. In the process, the resultant backfire blew off a vacuum hose causing it to idle way too fast. In addition, check to ensure the fast idle cam associated with the automatic choke isn't propping the throttle open any when the choke butterfly is wide open. I'd start by looking around for missing or loose springs and linkages. Let us know what you find.

[sodak69c10]

i checked the accelerator pump and it working fine. i'm a mechanic and a tech student one of my instructors said maybe powervalve but i not a carb specialist thanks for the tip on the electric choke linkage now that i think about it i did have some trouble geting it set when i first got it

[Tech @ BG]

If the carburetor worked properly before it’s not going to just change how it’s performing without something else changing. The first thing to do would be to make sure your float bowl screws are tight. As carburetors go through heat cycles the gaskets expand and contract, and will need to be retighten (like you would a header). If that doesn’t do it, spray out the air bleeds in the top of the carburetor with some carburetor cleaner, and compressed air.

A blown Power Valve would actually cause the engine to be a little rich, which intern usually causes an afterfire or backfire out of the exhaust. A backfire through the carburetor is usually an indication of being too lean. Combine that with your high idle situation, and it is most likely a vacuum leak from a loose float bowl.

[sodak69c10]

FIGURED IT OUT!! thanks for the advice i rebuilot the carb and found sound stuff that shouldn't be in there, but that wasn't the only problem i found that my vacuum advance diaphram had a leak (lean backfire condition), as well as my vaccuum secondary linkage was bent just a little so that at idle it would pull the secondary butterfly open enough to cause my high idle it runs good again i retuned every thing and took a test drive, and it still spins cookies as good as it did with the big block 402 it had before, thanx for the advice to

[mnunn454]

Sounds like another sweet success story with a happy ending.

We're happy for you!