Backfiring 4.0, 91 YJ

[spiritree]

My 91 YJ with 213,000+ miles has started backfiring of late. It does not use oil, starts and runs strong but backfires and rumbles on the down hills and in between agressive shifting. Ideas of what to check first? Thanks in advance. Spiritree

 

[TWDJ]

There are a couple of easy things to check right away ... (1) ignition timing and (2) carb adjustment. Timing too far advanced or a rich fuel/air mixture will both cause or contribute to backfiring.

If you get the kind of backfiring which is an intermittant rumble-pop-rumble while decellerating, think ignition timing. If you get the LOUD BOOM between shifts then check the fuel/air mixture.

 

[spiritree]

Thanks, I will check both of those and let you know if they help. It has run good for so long that I sometimes think I never need to serviced it except for oil and grease and bigger tires! Spiritree

 

[LVRockCrawler]

91... should be fuel injected.... the only "rich" scenario could possibly be a bad injector or 'puter glitch - check the exhaust - if it has a problem it could trap unspent fuel in the exhaust and then "boom" - the rumble on downshifting (reminiscent of a glasspack) makes me suspect an exhaust problem - the timing issue remains and could be a factor w/or w/out the fuel injection... good luck and let us know what you find out! hope it helps!