Crazy misfire

heftynoma

heftynoma

Member
I have a lifan 125 with 2400 miles. I went for a ride the other day and a super crazy misfire appeared out of no where. The speed was limited to 40 and it was shooting flames the whole way home. I only changed my cdi when I got home and it fixed everything.

Today I went to go for a ride and the bike will idle but has super erratic throttle response. You can go to wide open and it will only hit 4k for 5 seconds then randomly revs up to 8k. Then it back fires through the carb.
I've cleaned the carb, changed the gas, checked valve lash, checked timing, checked the crankshaft key, and added a engine to frame ground wire. I've changed my cdi box, coil and spark plug with no change.My stator lost spark at about 1500 miles and I replaced it with no issues sense. The bike will idle for 20 seconds with no throttle input, the. Backfires through the carb.
Is it possible my stator took a crap again. Any help is appreciated thank you.
 
I took my flywheel off and it has very faint scratches. I wiped it off and put it back on. I swapped my wiring harness to the one that comes with the engine. Now the bike will idle all day long but still has intermittent misfire. Sometimes it will rev to 8k no problem and others it won't get past 4k cause of the misfire. I have 4 cdi boxes and it does this with all of them. Why would the wiring harness lessen the misfire but not eliminate them?
 

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Too big of a plug gap will cause higher rpm misfire issues. I chased that problem with a 190cc. Thought it was too rich of a main jet. Pull the spark plug and gap to.028 no bigger.
 
Plug gap was checked, I also tested it with 3 plugs. I bought a new stator and trigger coil and the engine is still doing the same thing. I took a video to show. Starts first kick every time, but has no throttle response and when you hold it at 1/4 throttle it'll sit there and sputter then all of a sudden it'll rev up then it falls on its face again. It never use to shoot flames either. This was a fresh tank of gas as well. I know the gas is good, I ran it in my mower.
 
For what it’s worth: flames in the carburetor indicate to me that the intake valve is open at the wrong time or the ignition is occurring prematurely, a common male problem.
 
The other day I pulled the head off. Lapped the valves. And then i advanced the timing 1 tooth on the cam. Still starts first kick but It did the same thing, then i put it back to stock timing and still runs the same. Just wanted to eliminate that being a possible problem.



So I've changed coil, cdi, plug, stator, trigger coil, wiring harness, carberator, checked for vaccum leaks, messed with timing, and completely disassembled and reassembled the head. Cam chain tensioner is good and working.

All parts were tested on my wifes bike other than the cylinder head. It sounds like the bike is having spark issues. Im at a loss. This engine has always been severely underpowered when compared to my wifes bike.
 
Does anyone have any insight. I pulled the jug and stator off yesterday and inspected the timing chain, its all good. I even set the timing blind. Just using the position of the cam lobes and piston. Both timing marks line up when I do that. I retarded the timing 1 tooth and same thing happens, I put it back to stock timing.

The bike seems like it looses spark above 3 to 4k rpm. This is driving me crazy. I have a whole different Ignition system on it.Every piece of the ignition system was tested on my wifes bike and they all work. 2 stators, 3 pick up coils, 3 plugs, 3 coils, 4 cdi boxes, 2 differnt carbs and a differnt wiring harness. I only found one reddit post of this same problem and I think the guy gave up. He did everything I did with zero luck.
 
I would still pull the harness for a up close and personal inspection. Harness green ground, not grounded is another thought. Sounds like you spent damn near what a new engine costs already.
 
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