25HP Briggs & Stratton Engine Surging, Backfiring. Please help!

Engine surging and Backfiring thru Exhaust. Changed fuel, fuel filter, spark plugs. Adjusted valves, not far out to begin with. Checked and adjusted magnetos. Checked for spark, lots of spark. Carburetor is clean, no varnish. Removed flywheel bolt to check key for timing. Key is undamaged. Don't know what else to look at or do. Please Help!

[@merylpelosi8485]

I watch this video when I can't get to sleep. Works every time.

[@trainstractorscarsandtruck7362]

Very happy to help you get your required sleep. I have a few other suggestions to help you get some sleep as well if you would like.

[@George-py6ij]

I've had a similar issue with a Briggs 25hp bought from Princess Auto. One diode was blown in the coil wiring harness, there was also a blown coil that failed at higher revs. The coils kept failing after a few weeks to a month. I'd put a new coil on, and everything would be good again for a while. I finally tracked the problem down to inductive spiking from the PTO clutch taking out the diodes and/or coils. The PTO was an aftermarket XTEN. A 6amp diode across the PTO wires to suppress spiking has fixed it so far... I ended up figuring out that the coils blew when the key switch is turned off (or bum off seat switch) while the PTO is engaged. This grounds the coils at the same time the magnets in the clutch de-energise. Because the clutch doesn't have a clamping diode, an inductive spike takes out the diodes / coils. Kohler has a builtin on the issue and sells a diode to install across the PTO leads..

[@trainstractorscarsandtruck7362]

Thx although mine is manual PTO engagement. Thx for your comments and for watching

[@bargainbin22]

I would have liked to hear it run. Check fire to both plugs while running and see if it is normal then check compression on both cylinders. Sounds like a burnt valve or a loose valve set.

[@trainstractorscarsandtruck7362]

I have it running on a short video if you care to listen. Valves are set, spark is good. Didn't check compression but starts right up so I'm guessing it is good.

[@rickroberts4831]

Had similar problem. Found the flywheel key had broken. Allowing the flywheel to slide on the crank causing ignition timing issues

[@trainstractorscarsandtruck7362]

Yes checked it looks fine. Thx for the comments and the watch. Think it's the coils being to rusty. Will give them a good blasting. Thx

[@MikeHurd-ee2lh]

check the valve guides they kile to move up and hold the valve open

[@trainstractorscarsandtruck7362]

Valve guides are still where they need to be

[@incognito5986]

Just me, but the flywheel needs a good wire brushing as does the magneto 🧲. Also check keyway on flywheel and see if its sheared or loose letting flywheel move, and creating firing issues (out of time/sync) with engine. Take carb off, count screws in required to seat mixture/idle screws in, then back them out completely and take carb apart recording everything so you can reverse assemble. But when apart, put carb in an ultrasonic cleaner and reposition parts several times to help debris become dislodged if thats the problem. Many small orifices/passages can become clogged by very small debris. If as you said valves are adjusted properly, and timing is dead on, then look at air, fuel, spark. Air obviously is associated with the carburetor, as well as FUEL FLOW. Lastly the plastic vane thing along side of flywheel magnets, can inadvertently cause surging if the spring is broken or weak. It relies on the wind generated by the flywheel rotation blowing on it, to automatically regulate the engine speed in sync with your throttle setting. If the paddle fluctuates, the engine surges in time with the fluctuations of the paddle (I don’t know the name of it) its like a governor. If the mower meets resistance to blade rotation, the flywheel also slows down, that decreases the air created by the flywheel-blowing on the paddle, causing it to move, and in turn it automatically increases the throttle, to try to maintain where you have the throttle set to. I’m personally thinking carb problems with fuel flow internally. And specifically to restricted but not entirely blocked. Thats why it runs, but also surges at higher throttle speeds and backfiring could be sporadic fuel flow that fuel suddenly surges past partial blockage snd because engine is running slower than the amount of fuel required at the slower speed-all of a sudden has too much for that speed that its at and when THAT amount fires off in combustion, the sudden surge in speed, flywheel blows harder on governor paddle and adds to the surging. I could be way off on my analogy, but at the minimum start with the carb. And check paddle for smooth consistent travel. No sticky or too weak of a “return “ when you physically move it and observe it returning to original position. It should move freely but under constant pressure from a spring. In your video, with engine off whatever position its in, is its rest position. It should be able to move under some tension in the opposite direction when physically moved by you, and return from where you moved it to, to where it originally was before you moved it,when you let it go…from where you manually moved it to.

[@trainstractorscarsandtruck7362]

Wow, much appreciated that is an extremely thoughtful and thorough explanation. Keyway is perfectly square in Grove. I cleaned up the flywheel around the magnet and cleaned the magnetos. Didn't do a complete cleaning of the carb but took top gasket and bowl off and sprayed some carb clean. Carb had no varnish in bowl and looked crystal clean. Valves were out a bit but I adjusted them earlier with no change. Took covers off again to confirm the sleeves didn't move. They are fine. Spark also seems good before I cleaned magneto. Just waiting for valve cover gasket to cure and I will test engine again. Automatic choke wind sail is working as intended at least with cover off. Spring is there and it rotates smoothly. If engine still gives me issues I'm going to bring it to a dealer. It only has 60 hours on the engine and I need it fixed ASAP. Grass is getting out of control and I'm only home for a day or two every week. Thanks again for your amazing response, appreciate you

[@incognito5986]

@@trainstractorscarsandtruck7362 Just thought of something else. And when off, you can either sandpaper or wire wheel/brush the magneto itself that rests near the flywheel. Heres the video on how. Possible your gap has moved slightly on one side or the other. Oh and lastly do a compression test. Cant hurt to check. kzbin.info/www/bejne/a6GciISKnd-DaNUsi=Tiw3DLpy25IOQKqD

[@trainstractorscarsandtruck7362]

@incognito5986 Have the flywheel and magneto almost free of rust. Have regapped them both. Compression test is a possibility but I think it's good. Engine starts up really quick so that tells me Compression is good I think.

[@user-dd6pd3bw4d]

Coils look corroded and flywheel rust buildup, Makes me wonder if the gap is correct and that spark timing is off due to coil failure.

[@trainstractorscarsandtruck7362]

I buffed rust off coils and magnet and set the gap. Have a spark tester and it appears to have lots of spark.

[@trainstractorscarsandtruck7362]

Will be cleaning them more I think

[@michaelspeakman8924]

Did you check the pushrods sometimes I end up bent somebody decided that they needed to make a weak exhaust pushrod

[@trainstractorscarsandtruck7362]

Yes thanks for the comment and for watching. Pushrods are straight.

[@safiyanislammuddassir1896]

Your videos are not SEO well done so people are not finding your videos. That's why your videos are not being viewed, views are low.This reason the views are not increasing, the subscribers are not increasing. If you want to see, I can show you

[@trainstractorscarsandtruck7362]

I don't have the time to do proper editing, I understand why my channel isn't what it could be. I do have more views and Subscribers than you though.

[@andybooher9367]

Piece of crap. Not very smart. Never buy this brand.

[@trainstractorscarsandtruck7362]

First one lasted 13 years of summer and winter working. This one has 2 easy years on it besides the major snow storm a few months ago. Personally I feel they are all starting to be made cheaply but we will see about this one. I think my next one if I replace this engine will be a Kawasaki. For the price though I think this is my first and last Yard Tractor. Sub Compact Tractor next time I believe.