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Briggs V-Twin backfiring missing on 1 cyl

2295 Views 4 Replies 3 Participants Last post by  deckape
I have a 25hp OHV V-twin model 445777 that is running rough, and seems to be running on the left cylinder only. Exhaust temp with laser theromometer is out of range at the exhaust pipe on the left side, and around 150-190 on the right.

I've checked and replaced both plugs, and adjusted the valves (.005 In, .008 Ex) These settings are .001 higher than specs, but I was given wrong info from a local shop, should be .004/.007 respectively. This surely isn't bad enough to keep it from running smooth, is it?

I also checked the spark, getting a good blue spark up to about 1" from the head when running @ half throttle.

It's backfiring thru the carb, but not consistantly. At idle, the miss is really noticable, as I speed the engine up, the backfiring starts.

I don't use it as a mower, just a yard dog to move trailers and stuff around the yard (got it for free, so I'm not out much in that area, but I have put new tires and some other parts to make it more reliable)

Any suggestions?
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Remove the VC of the cylinder that's not running and look for an exhaust valve that's not opening all the way. Also, you can remove and ground the plug wire for the dead cylinder and if that gets rid of the backfire, that would confirm an exhaust valve not opening.
I've checked the pushrods and the valves appear to be functioning properly; I'll give our suggestion a shot to see what it will tell me.

If the valves are too loose (by .001") that surely wouldn't cause this much grief, would it?
This engine has a coil for each cylinder. The "Kill Wire" system is quirky including a harness with diodes that are supposed to keep the coils from grounding each other out. I would disconnect the kill wire of the questionable cylinder where it connects to the coil, or better yet disconnect both and see if that helps. Keep in mind that you will have to have an alternate means of stopping the engine. Shut off gas, short plug, etc.

Walt Conner
Ignition wiring isn't the problem; the offending cylinder is capable of throwing a spark about 1" long when the engine is running.

I will check compression this weekend, and also check my valve operation, including pushrod travel, valve travel, and valve clearances on all valves.
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