I'm Stumped

[Edwin Spangler]

Kids minibike started sputtering/backfiring between 15% to 50-60% throttle. Happened suddenly in the middle of him riding it and before, it was running perfectly.

One thing to note: He is NOT nice to it. He doesnt abuse it but 50+% of its runtime has been WOT.

Ive examined everything that I know how. Carb is setup good, runs another engine perfectly. I just dont know how to examine the valves. Thats what I suspect as I had the entire engine tore apart, cleaned, built back fresh. They held gas with the head held inverted. The area in which they seal seems to have good metal to metal contact. Lash set correctly.

Something in the carb that could have let go and went missing? Forgot, carb runs the other engine just fine. I dont see any missing parts. Im lost. Been trying everything I can think of the last couple weeks, nothings getting it done!

212 ghost btw.

 

[Denny]

Maybe it gave up the ghost?

 

[Hellion]

What kind of exhaust or muffler does it have? Is it possible the muffler lost a baffle that rusted and shot out the end?

Mufflers are so rust-prone after several complete heat cycles -- it burns every bit of manufacturing residue, oils and moisture out of the metal pores and rust sets it. I think a lot of the mufflers are press-formed in a mold or stamping die and that die has oil or 'mold release agent' applied to it that helps it pop out and also enhances the molding stamping process by reducing friction.

No baffles means a straight pipe and lots of brappy brapp.

 

[Hellion]

Otherwise, I would say it's running a little richer than normal. Could the air filter be clogged a little more causing a rich A/F mixture? It's dirty out there!

 

[Edwin Spangler]

It has a straight pipe on it. I have 100% ruled out the carb, or any of the 5 I tried. It will sputter on this engine, run the next engine perfectly.

Ive also found marring inside the valve cover. Looks like the rockers smacked the inside of the cover.

This is much more than just a louder exhaust. Its a very distinguished break-up and backfire out of the carb inside the aforementioned RPM range. Perfectly clear at the beginning of the RPM range, breaks up mid-range, then clears up at the top of the RPM and takes off like a bat out of hell, ill be gone gone gone.


When it "breaks-up" as I call it, it sounds like a race car hitting two step.

 

[Edwin Spangler]

Otherwise, I would say it's running a little richer than normal. Could the air filter be clogged a little more causing a rich A/F mixture? It's dirty out there!

I had the entire engine tore down and built back twice. Whatever the problem is, its beyond the basic simple stuff we first look for.

 

[Hellion]

Whatever the problem is...

Where's the video, man? The text is nice and all but tells us gruntin' cavemen basically nothing.

 

[Edwin Spangler]

Lol probably should have started with that huh. Ill get it later or the morning. Maybe I feel like waking the neighbors up tomorrow.

 

[Functional Artist]

Why not wake 'em up now???

They don't need no stinkin' afternoon nap
...not today
...no, no, no

 

[Denny]

May be the coil is starting to fail? Replace spark plug lately? In cars we have a saying “if it seems like a fuel issue it’s in reality probably an ignition issue.”

 

[Edwin Spangler]

Before posting the video (its a hassle, dont ask), im going to swap out the coil. Apparently the coil in the ghost engine is digitally limited. I read that I can swap in a clone coil. Ive also read that you cannot but the one person who says they completed it, says it worked so...

Had a machinist friend check out the valves, he says theyre still true as they should be.

If the valves hit the valve cover hard enough, could it have damaged the valve seat? Remember, they seal well enough to hold gasoline.

What could cause the piston to hit so hard?

 

[Denny]

No, valve covers don’t damage valves. Valves and springs and rockers damage valve covers.

 

[Edwin Spangler]

No, valve covers don’t damage valves. Valves and springs and rockers damage valve covers.

Lol I know that. I must have worded things confusingly.

My worry was the valve stem possibly being bent and putting uneven pressure on the valve seat, over and over.

 

[bob58o]

Are there aftermarket ratio rocker arms? I know certain engines need a spacer or aftermarket valve cover to prevent the interference between the valve cover and the rocker arms.

 

[Hellion]

Where's that video, Ed?

 

[Edwin Spangler]

Yea sorry, I got so fed up with this thing.

I would have put the video up, got suggestions... I would have to go check because "I didnt think of that".

Ultimately, the "throwing it off the cliff" thing would have been the outcome. Only I would probably have been riding it at the time.

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[Sparkwizard]

What does your plug look like? You did not answer the question: Did you replace the plug?
It sounds lean.