4.2 258 backfire when decelerating - Page 2 - JeepForum.com
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post #16 of 21 Old 06-14-2021, 11:17 AM
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1981 CJ5 
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Had a similar situation on my '81-5 with 258 recently. I switched the distributor vacuum over to manifold, and put vacuum for the backfire device shown to carb vacuum. Every time I would shift gears, truck would backfire. Put a "T" in the line for distributor and this, so both would draw from manifold, and solved the problem.

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post #17 of 21 Old 06-14-2021, 07:11 PM Thread Starter
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I’ve teed the vac advance into the line you said but still no luck. I might go back and check the timing and carb idle mixture as well since ive changed where the vacuum advance is picking up vacuum…
But what rpm does one set the timing at. 1600rpm is where I heard is a good point at 8 degrees. But the original 79 year old guy I bought it off had it set at 12 degrees in his personal maintenance log book with the truck
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post #18 of 21 Old 06-14-2021, 07:38 PM
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The static idle setting (no vacuum) is the position of the distributor, usually reading 8/10/12 degrees depending upon other factors. That setting has nothing to do with setting idle speed. Idle speed is further determined with additional adjustments to the mixture and the idle speed screw(s) depending upon carb make. 1600 RPM MIGHT refer to a fast idle speed during initial warm-up where a carb closes its choke and steps-up an idle to ensure proper air flow and mixture when cold.

Now, that doesn't mean that you can't change idle speed by changing the static setting, but you DON'T set the idle speed by changing the static setting...that's a backwards way to do it.

Warm the motor up...set the distributor to read timing at 8* BTDC with a timing light and have the vacuum line disconnected from the distributor and plugged (do NOT disconnect from the carb end as that lets air into the system and screws you up) and leave it the hell alone. Reconnect the vacuum line and move on to other things to get a proper idle speed.
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post #19 of 21 Old 06-16-2021, 04:35 PM
T Hoff
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1975 CJ5 
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Since you had Carb issues, I'd look harder there. Is float level set too high? Downhill causes rich condition especially on deceleration.

'75 CJ5 ---Chevy HP 350 motor-- All Fiberglas---one piece tilt forward nose
1985 chevy stepside pickup, 300K on the clock: Original Owner
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post #20 of 21 Old 06-18-2021, 04:21 AM Thread Starter
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When I pull the manual choke on it doesn’t do it when warm. I think this tells me there is possibly a vacuum leak or manifold leak somewhere
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post #21 of 21 Old 06-24-2021, 11:56 PM Thread Starter
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So I think I found the problem. There is a vacuum leak around the brake booster hence it stalls when decelerating downhill or coming to an intersection… I plugged the hose going from manifold to the brake booster and instantly idle speed rose. Question around the heat riser valve on the manifold will this increase brake booster vacuum as it opens when engine warms up
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