Idle problem

[phase_one]

I have a250 with an offenhausen intake and a holley 390 carburetor. I can crank the engine and it's runs well as long as I rev the engine. When I let off the gas it dies. I have tried different size jets and adjusting the needle valve. Any suggestions?

[LongBox]

Idle speed.]? Easy adjustment
Vacuum leak? Easy to check.
Carb to manifold bolts tightened? Easy to check.
Initial ignition timing. Need a timing light to check.

That should get you started on fixing this.

['68OrangeSunshine]

Sounds like a vacuum leak.
What kind of exhaust do you have? You only mentioned the intake and carb.

[Gromit]

Is there a vacuum assist brake booster on your truck? If so you can try tapping the brake at idle (or best effort at idle) and see if you can sense any change in the motor.. shouldn't be one at idle but if there is it might be a hint that there is a vacuum leak which I agree with 68Orange sounds like a good first guess. I suggest putting right-sized German style flat-wound spring clamps on all vacuum connections - for example the type as shown in attached photo..

[phase_one]

Originally Posted by Gromit

Is there a vacuum assist brake booster on your truck? If so you can try tapping the brake at idle (or best effort at idle) and see if you can sense any change in the motor.. shouldn't be one at idle but if there is it might be a hint that there is a vacuum leak which I agree with 68Orange sounds like a good first guess. I suggest putting right-sized German style flat-wound spring clamps on all vacuum connections - for example the type as shown in attached photo..

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[phase_one]

Originally Posted by phase_one

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I have headers/ dual exhaust
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[LongBox]

Headers and dual exhaust on a 250 six sounds interesting. I'd love to see a picture or two of the engine. Hopped up 6s are a lot of fun, and the exhaust bark is lovely.

[hugger6933]

Take some carb cleaner and spray around the base of your carb while someone is keeping the engine running. Spray along the intake as well and if the spray at some point raises the engines RPM you have not only found your problem but you have found a vacuum leak

['68OrangeSunshine]

Originally Posted by phase_one

I have headers/ dual exhaust
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Since you have headers, we need to know if you still have the manifold alignment pins on the ends of the head?
They work OK for stock exhaust -- allowing for the cast iron manifolds to expand and contract, but with headers, they don't seal well.
If you haven't pulled them, you need to do it soon. The guide pins come out with a good grip with a vicegrips and thread out counter-clockwise. It's 3/8-16UNC. Thread in a stud and nut to secure the header flange ends. Or use a grade 8 bolt. If you have done this, then nevermind.
I have a 292 w/Offy intake, Clifford headers, dual exhaust w/ crossover and an Edelbrock 1404 500 CFM 4 barrel.

[phase_one]

I removed the pins and have the headers bolted. I didn't have time to check for vacuum leaks. Gonna do that tomorrow.

[phase_one]

Originally Posted by phase_one

I removed the pins and have the headers bolted. I didn't have time to check for vacuum leaks. Gonna do that tomorrow.

No vacuum leaks
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