engine sounds like popcorn over 5000 RPM

[miatawnt2b]

Have a strange issue. One of my three TAOTAO 125d's sounds very strange when you get it above 5000 RPM (WOT as it doesn't go much faster). It runs great otherwise, pulls hard no hesitation. However when it gets over 5000 RPM it starts to sound like popcorn popping under the tank. My other two 125d's that are set up identically do not do this. Strangely enough, this is the one motor I had to put a new head on because it bent the exhaust valve. I have double checked valve lash, swapped the CDI with one that runs fine, and put in a much larger main jet just to see if the problem would change or go away. It didn't. Still sounds like popping corn above 5000RPM.

I'm kinda at a loss on this one. All 3 quads seem to max out about 5200RPM, and all seem to run the same, but something is up with this one.

Any ideas of what to try/look for? It's not bogging or hesitation. WOT, it pulls hard to 5k.

[JerryHawk250]

You're either hitting the rev limiter or floating the valves. You might have weaker valve springs on that one engine.

[miatawnt2b]

I thought the same thing about valve float, but doesn't 5k RPM seem awfully low for that to happen?

[TxTaoRider]

I'm a fan of pulling out the spark plug and looking for signs of something going on inside the combustion chamber. If it come type of ignition or fuel issue (or debris), the plug will sometimes show it.
Besides it only takes a couple minutes to pull...heck you might find it's something simple like its not tight.

[miatawnt2b]

Originally Posted by TxTaoRider

I'm a fan of pulling out the spark plug and looking for signs of something going on inside the combustion chamber. If it come type of ignition or fuel issue (or debris), the plug will sometimes show it.
Besides it only takes a couple minutes to pull...heck you might find it's something simple like its not tight.

Yep, done that. Plug looks perfect.

[franque]

It might be helpful if you made a video... It could be a bunch of things, but without hearing it, we can't be of too much help.

[FLASHLIGHTBOY]

check to see if the pop is from the carb.... if not..it might be a head gasket...they will do that...remove the exhaust...if the muffler will come off ..do that....might be back pressure...if with the muffler off.. or the exhaust pipe off....and it is better...then it is back pressure...only going by what you say....timing is good...valves are set right...sparkplug ..good...you changed the CDI...try the coil../... if everything is good and it still does this.....I would change the carb....if you change the carb,,,,use the throttle body from the same setup ..that you take the carb from....do not mix and match...as WHY you have the bigger jet...??? this is the problem alot of time...use factory stuff...if you have problems...go to factory setting...
Start from square one..

[TxTaoRider]

Might also check for a bad exhaust gasket.

[FLASHLIGHTBOY]

is this when riding full out?? that you know at 5000 rpm??? or just standing reving??? please check the wire connections to the coil... puch on connections... loose... not held secure.... like they are floating??? I had them just loosly fitting and when standing... usually ok.. riding ( just taking off) .. popping.. as they were loose... and cutting the spark off... usually a minor thing... just do the easy stuff first... was it any good before you changed the jet???