Timing is good but still backfiring. - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 5 (permalink) Old Jul 5th, 03, 05:48 AM Thread Starter
 
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I think i still need to adjust the mixture screws but i'm not sure how to do that. I have heard that you turn down one at a time untill it starts to stall and them back it off 1-2 turns. cna you help me out?
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post #2 of 5 (permalink) Old Jul 5th, 03, 06:11 AM
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If its holley, back the idle screws 1.5 turns (common starting point and probably on the rich side). Then, screw inward each screw .25 turns. Keep screwing inward till idle becomes reduced or rough or increased (sooner or later one of the above will ocurr). Back the screws off .25 and you should be done.

Verify the idle speed adjustment is correct. If you don't get any responce from mixture screw adjustment, the idle speed adjustment is probably set too high. Get the Idle speed screw adjusted correctly before mixture adjustment!

good luck
joe c

69 X-44 base coupe, ZZ4, 16lb nodular flywheel, Center force DF clutch + PP, Holley 600DP, Hooker super comp 1 3/4 ceramics, 2.5" dual exhaust w/ X-pipe, flowmaster 40 series mufflers.
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post #3 of 5 (permalink) Old Jul 5th, 03, 10:25 AM
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Is it only back firing when you step down on the peddle? If so, when you feel like you've got your idle issues under control take a look at the accellerator pump system. Might need a different cam or larger squirters. Back-fire is caused by a lean condition.

When your adjusting those idle mixture screws, do they seem to have an impact on the idle, or do you seem to be able to turn them a half a turn or more with no effect on idle? If the effect is not up to par and you're dealing with a holley, you might need to crack open the secondaries a little so you can drop the primary blades closed a bit more and get into the idle circuit better. You'll find several posts in the archives on this.

-dnult

-dnult

Dave
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post #4 of 5 (permalink) Old Jul 6th, 03, 07:57 AM Thread Starter
 
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Thanks guys i will give it a shot and see what happens. Thanks for all your help.
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post #5 of 5 (permalink) Old Jul 8th, 03, 05:31 AM
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There are a couple ways I've done the mixture screws. One is to use a vacum gage and set the idle down as low as possible. Turn one misture screw at a time until the highest possible vacum is achieved, then I slowly turn them in, there will be some hysterysis, until the vacum just starts to drop. You want maximum vacum with the screws turned out as little as possible. You can also use a good low rpm tachometer, don't try to use the one that came in the car, it isn't accurate enough, it will get you in the ballpark, but that's it. Set as described above only you're shooting for highest rpm instead of vacum.

What maintenance or component replacement if any was done prior to the backfiring starting? If you're sure the carb is set right and it's still backfiring I'd do dynamic and static compression checks. The static check will only tell you the cylinders sealing ability, this is the one everyone does. The dynamic and snap throttle tests will tell you if you have restricted cylinder or valvetrain problems, it actually measures each individual cylinders breathing ability. If it's backfiring under a load only and idles fine you really should do the dynamic and snap throttle tests.

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