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curtis.w 06-04-2011 11:26 PM

Rough idle & backfire
Over the past couple of months, my 87 4.2L has developed some symptoms that sound like carb problems to me:

Idling at low RPM's (600-1000) after the choke disengages is really rough. I cranked the idle screw up to about 1500 to make driving around easier while I figure out the problem but it still drops low at times. The most unusual symptom is that it tends to drop and stick around 300-400 RPMs when I'm breaking. It's like the carb isn't compensating fast enough for the extra engine load. After a few seconds around 300 rpm the engine stalls.

The other symptom is backfiring. When coasting above 5 mph (in gear but not accelerating), I get noticeable backfire. It never backfires when accelerating though.

So far I've done some light venturi cleaning and run seafoam through the carb several times. These solutions worked the first few times but don't have an effect anymore. The next step I can think of is to rebuild the carb (Carter BBD. I would just upgrade to a new one but I live in California). Any thoughts?

Garyk 06-05-2011 11:13 AM

Sounds like you are getting raw fuel in exhaust system,

Snuffy115 06-05-2011 11:33 AM

Sounds like a vacuum leak to me. I would start checking vacuum lines

curtis.w 06-06-2011 02:39 AM

Vacuum leaks were another cause I was suspicious of. Does anyone know where I can find a good vacuum diagram for yj's? I've gotten all the info I've ever needed from my Hayne's manual but it doesn't have anything on vacuum lines.

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