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Old 03-23-2007, 05:07 PM
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Default wicked backfire on deceleration

I'm still working my way through the new 40 and futsin' around with random stuff. I got the idle down, passed emissions, and lots of other little things.

Next is dealing with the, as they say back East, wicked backfiyah. the Engine compartment looks alot different than my 60 and I'm not sure what the PO did to emissions, etc. OH...backfires through exhaust, not carb - and only on Decel. I can drive it carefully enough (usually) to not backfire.

When I decel 'quickly' I can get it to really backfire, especially between 3rd and 4th if I'm "gettin' on it". (Ask Subzali if his hearing is back after showing him the BF).

I've done the carb cleaner trick around the manifold...no change in rpms.

I fixed the idle, and it passed emissions with flying colors.

The smog pump is hooked up, but there are other things disconnected/plugged - not sure how they all work together. But other than the backfire it runs really strong. I kinda think that's part of the culprit...maybe it's pushing air and it shouldn't??

I'll take some pics this weekend showing what's hookedup and what's plugged. Since I don't know much about this stuff, I'm hesitant to just pull the smog pump and 'see what happens'. Like, should other things be plugged up or plugged inwith the pump off? Or will it negatively effect things.

Help...I'm babbling - don't know what to do, what to check, etc.

Last edited by CardinalFJ60; 03-23-2007 at 05:08 PM. Reason: clarification...
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Old 03-23-2007, 05:21 PM
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A backfiring in the exhaust is usually air getting in to the exhaust somewhere. But a Wicked Back Fiyah, sounds worse than the standard popping.
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Old 03-23-2007, 05:56 PM
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your anti backfire valve is not working and no I am not joking
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Old 03-23-2007, 05:59 PM
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part #5. Follow the big hose from the smog pump to the air rails. In the middle is the abv. It has one hose that hooks up to the carb. This shuts off the smog pump under deceleration. On the 79 40's the valve is controlled by another on the driver's side fender.
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Old 03-23-2007, 07:46 PM
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Does the 40 have the fuel cut solenoid on the carb? Some people modify it to be on whenever the engine is running rather than running the ground through the emissions computer. The computer actually shuts off the solenoid when decelerating - part of the "slow fuel cut" system.
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Old 03-25-2007, 10:20 AM
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Default excellent!

Once again, thanks for the advice!

Can you remind me of the method to check the fuel cut solenoid? It does have the wire all hooked up...I feel like there's a "click" involved.

Also, is there a way to test the air bypass valve? Or not worth the effort?

thanks!

-s
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Old 03-25-2007, 10:44 AM
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Default Anti-backfire Valve

Just thought I'd post this up...did some more digging after TR's post.

http://forum.ih8mud.com/showthread.p...ht=air+by-pass
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Old 03-25-2007, 02:40 PM
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when you turn the key on you should hear a click at the solenoid on the carb. I don't think it's that though...if that's not working you can also get a "dieseling" effect when you turn the key off...
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Old 04-03-2007, 12:42 PM
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Default ABV going in tonight...

Just an update...I'm puttin' in the new (used) anti-backfire valve tonight...

Fingers are crossed.
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Old 04-03-2007, 02:32 PM
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If that dont work , keep looking for an exhaust leak, Thats usually what it is on most autos.
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