Hello last summer I rood my MC28 and rev up fast 2 times to almost top speed at a length less then a mile. Sudenly I felt that there was something wrong so I shutted down the throtle. Result a molten piston at the exaust side of the lower barrel. Could someone tell me what is wrong in my setup?

I have Jollymoto pipes, VHM cill heads with the standard squis, HRC030 card, standard airbox with the restriction cut off, Tyga mainfolds, MJ sizes *** for the upper and *** for the lower. And I run premix 1:30







ADMIN: NO jetting discussion in the free area.

I would double check the top of the cylinder to see if it is 100% flat . I think that a water leak may have contributed to this seize .

It looks like detonation on the edge of the piston, I would guess lean mix at full throttle?

Water/coolant injected in to the cylinder would prevent detonation, it's been used for that for many decades going back to early days of aviation.

I ran this one past a fairly expert two stroke boffin and he said something like this:

Can be caused by three things:

1) The combination of (A) pipe (stinger size), (B) cylinder head (combustion chamber volume of the head [what cc?]) and (C) main jet, has created too much heat in the pipe, thus burning the front side of the piston.
Going up a couple of main jet sizes on that cylinder would stop it - but using fuel to "fix" an issue will lose power.

2) But can also be caused by a "one point" water injection scenario such as a pinched O-ring (where an O-ring is used instead of a head gasket) - has been seen several times before.
Saying that squirting water in would prevent detonation is to much of a generalisation and the specifics of the individual case need to be looked into. It would work, and does if it is part of a homogenous mixture, but having it at one point, and have steam generated at that point only,... eats the piston.

3) One other scenario that has been seen is a broken muffler tube - it can drop down if the packing isnt holding it in place and create too much back pressure - like a too small stinger.


Last edited by TuckerBag on Tue Jan 31, 2012 8:50 pm; edited 2 times in total

I also think that the stinger size of the pipe is one part of the problem. I also see when I ride pretty fast is that the temprature of the engine goes way past the 70 Celcius. I also took the head of the top barrel and the patern of the piston looked perfect en the inner chamber was perfectly clean.

And the spark plug of the lower barrel looked also pretty dark brown so I dont think ist a jetting problem.

But I find it pretty strange that the problem is only on the lower barrel. I realy want to solve the problem before the nice wheater begins.

I edited the above post to include a third scenario

I dont think the mufler is the problem there jollymoto carbon

Crank pins could be out of phase a tough giving too much advance. Looks like jetting though.....

Could probably be solved with a 0.6mm HRC base gasket on that cylinder. You need to measure the combustion chamber volumes. That's a classic example of detonation when two top heads are used on some MC21's and MC28's.
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show us a picture of the underside of the piston crown .

That is a seriously damaged piston my dutch friend. I would like to second the suggestion of a substantial water leak. I had a similar failure on a piston with my custom 0-ring heads. Water was leaking into the combustion chamber and you could see the water path along the cylinder head and cylinder leading towards the eroded piston top.

I would also carefully check the exhaust bridge of that cylinder for any cracking.