Author Topic: WTF?? three torched pistons (Read 30497 times)

Daniel370r · « **Reply #30 on:** July 20, 2014, 10:15:38 PM »

I'd say ignition I've never seen a oem one do that. Can you take a pic of your flywheel and pick up coil. Maybe it has a timing advancer on it.

christph · « **Reply #31 on:** July 20, 2014, 10:16:30 PM »

Just by the looks of your spark plug, it looks like a detonation issue to me. I had a detonation issue in the past and it took the tip off the plug. The top of your piston is also speckled, which I believe is another sign of detonation. Detonation can result in significant damage quickly, which comports with the fact it only takes 10 minutes. Also, just to note something that hasn't been mentioned, you might consider your cooling system as a contributing factor, more specifically check that your impeller nut it tight. I've had that happen too. The impeller wasn't spinning, the coolant wasn't flowing, and the engine overheated, which created a detonation problem.

Daniel370r · « **Reply #32 on:** July 20, 2014, 10:19:28 PM »

If it was a fresh bore and was a heat issue wouldn't the sides of the piston be trashed? Wouldn't the piston expand faster than the bore and 4 corner seize?

christph · « **Reply #33 on:** July 20, 2014, 10:35:00 PM »

I was just reading about detonation in Alexander Graham Bell's Two Stroke Tuner's Guide. Here is a direct quote:

"Detonation leaves many tell-tale signs for which the two-stroke tuner should have an ever-wary eye. The most obvious sign is a piston crown peppered around the edge as though it has been sand blasted. Bikes with plated aluminium cylinders will usually show the same sand blasted effect around the top lip of the bore. A cracked (not molten) spark plug insulator also indicates detonation. If kept running, a detonating engine will eventually seize and/or have a hole punched right through the top of the piston. The conditions leading to detonation are high fuel/air mixture density, high compression, high charge temperature and excessive spark advance. A high piston crown or combustion chamber temperature can also lead to this condition. In a racing 13 Two Stroke Performance Tuning two-stroke all of these detonation triggers are virtually unavoidable, with the exception of excessive spark lead."

With detonation you should also be hearing a kind of knocking, i.e., the engine almost working against itself because the fuel is self-igniting too early.

fearlessfred · « **Reply #34 on:** July 20, 2014, 11:26:17 PM »

If all ignition components have been changed out and the timing looks to be most of the problem, the only thing left would be the keyway is indexed wrong on the crank,the plugs color is soot black that would make you think it wasn't lean,but yet the hole appears to be melted from preignition and not detonation, preignition can be started from a lean condition that it doesn't seem to have .Sorry just thinking out loud ,somebody jump in here edit reread post and seen new crank was used , it couldn't be keyway

atvcrazy · « **Reply #35 on:** July 20, 2014, 11:32:23 PM »

I'm sorry in my post I said a dej needle as leaner but its actually richer in the 1/2 throttle position than a dej so just like most have said you have ignition issues or air leaks. The key way on the crank isn't a bad idea to look at either.

fearlessfred · « **Reply #36 on:** July 21, 2014, 02:54:45 AM »

I think we really have preignition and not detonation

fearlessfred · « **Reply #37 on:** July 21, 2014, 03:30:56 AM »

if the carb and cylinder are the only things that haven't been changed out, that would leave the carb as the culprit.I think you have a fuel delivery problem and the bowl is going empty ,you may have good flow at the hose going to carb,but have you checked for good flow though carb.preigniion causes the piston to have a melted look and detonation would have more of a broken look.preignition can cause detonation to occur along with it

christph · « **Reply #38 on:** July 21, 2014, 04:12:43 AM »

If it is pre-ignition then you have to ask, what is the source? My understanding is that pre-ignition results from a hot spot in the cylinder, either built up carbon or a spark plug that is too hot, i.e., the wrong heat range. Pre-ignition is not due to too much advance; it is not due to an electrical spark at all. Since you have a new engine that eliminates carbon build up. Check the heat range of your plug, it should be an 8 or 9 (the higher the number the colder the plug).

fearlessfred · « **Reply #39 on:** July 21, 2014, 11:40:00 AM »

Quote from: christph;39556

If it is pre-ignition then you have to ask, what is the source? My understanding is that pre-ignition results from a hot spot in the cylinder, either built up carbon or a spark plug that is too hot, i.e., the wrong heat range. Pre-ignition is not due to too much advance; it is not due to an electrical spark at all. Since you have a new engine that eliminates carbon build up. Check the heat range of your plug, it should be an 8 or 9 (the higher the number the colder the plug).

your understanding and mine are the same ,a lean condition is the only thing left that could get the piston glowing, most often that results in the exhaust edge being melted,the carb is the only left that has not be changed out and the piston appears to be melted and not broke

Bio86 · « **Reply #40 on:** July 21, 2014, 04:44:55 PM »

Kind of weird, I've accidently ran mine real lean on the bottom end thinking I had it too rich at first and didn't notice any kind of damage. Could it be super retarded and rich? Sometimes that can cause extremely high temps as well.

rjt500r · « **Reply #41 on:** July 21, 2014, 05:04:05 PM »

The carb is not the culprit, it was on my Zilla and worked fine. The CDI is also one item that has been on both engines. Here are some pics of the ignition.

Bio86 · « **Reply #42 on:** July 21, 2014, 05:27:18 PM »

You know what your squish, band width, kicking compression was before you ever even fired it up? Just curious.

fearlessfred · « **Reply #43 on:** July 22, 2014, 12:20:19 AM »

Quote from: rjt500r;39535

I bought the bike in Feb it has never ran longer than ten minutes without burning up the piston. Yes it has been rebuilt, my original post states that. At this point the only things that have not changed are the carb and cylinder.

Im sorry but Im really lost, in this post you say that the only thing that hasn't been changed out is the carb and cylinder .now your saying It cant be carb and the cdi hasn't been changed out. What parts are the common denominator that resulted in the three pistons having holes in them. The CDI ?

fearlessfred · « **Reply #44 on:** July 22, 2014, 12:40:10 AM »

Quote from: Bio86;39591

Kind of weird, I've accidently ran mine real lean on the bottom end thinking I had it too rich at first and didn't notice any kind of damage. Could it be super retarded and rich? Sometimes that can cause extremely high temps as well.

Lean coming off the pilot would cause bog but not damage motor according to Jerry Hall ,but if jetting is correct and motor is making power and it runs out of fuel that could be different.In 27 years the only time I have burnt a hole in piston was running out of fuel on the hill at Dumont.this happened at full throttle and the hole was on edge of exhaust..On car motors I have seen the results of retarded timing and that was the exhaust manifolds glowing from the fuel still burning as it went out the exhaust