2001 TM RAcing 125mx Piston meltdown

[thmike]

Just recently rebuilt this bike from the ground up a few weeks ago. Well for the most part. Motor wise I replaced everything it needed. I've been messing with jetting for mid-range since the bike doesn't have the power of my 2006 yz125.  Seemed like I got is pretty good. It doesn't have good bottom-mid but top end is awesome. Typical 125 I guess. Well I felt like I had it running good, so I decided to tell my buddy on his 04 rm125 which is ported to do some drags with me. On the second run, (running the bikes to 6th then slowing down), I did pull from him til I hit 6th and then lost power. Bummer! well no big deal..its only a 2 stroke. Thank god I made the switch back to them!
 
I pulled the piston out and this is what it looked like today. There was aluminum built up on the nikasil at the very top of the cylinder and some on the head. No scaring to the walls, head is all cleaned up now and the aluminum just came right off the nikasil.

Jetting INFO-The bike from the factory comes with 175 main and has a 172 in it. The friend I got the bike off of used to run 50/50 race/pump in the bike at 50:1 and I'm running 32:1 pump gas. So I'm guessing she's lean. Maybe igntion is advanced. No pinging from the motor..that I can tell anyways. Runing Maxima Castor 927 oil.

This was the second time on the bike since build. First was at the track and second was trail riding with my wife and friends. Figured I could get some jetting figured out that I was having problems with at the track.

Here are the pics of the piston

[2T Institute]

That's a lean seizure(pre-ignition), happened when you backed off in 6th.

[scotty dog]

Lucky ya stopped when ya did, that could of been a lot worse, definately lean

[beaner]

175 to 172 won't be the difference between ok and seizure. My guess is either too much timing or compression and not enough octane.

[thmike]

IT didn't happen when I was letting off...soon as I shifted to six and started going through the gear was when it happen. It was a 1 mile hole the bike wide open either. It was get through the gears and slow down. But I didn't chop the throttle.

A guy on thumpertalk said there was a lot a detenation. I know TMs love race gas/pump mix at the minimum but I was hoping to get it right on pump. Does anyone know if running a thicker base gasket along with retarding the timing would prevent this?

[thmike]

175 to 172 won't be the difference between ok and seizure. My guess is either too much timing or compression and not enough octane.

It didn't seize though..thank god. lol. just lost compression.

[SachsGS]

No obvious seizure marks but lots of erosion - must be due to detonation. I would check ignition timing, either lower compression ratio or stick to race/pump gas mix and enrichen fuel mixture slightly to bring down combustion temperatures. 

[beaner]

175 to 172 won't be the difference between ok and seizure. My guess is either too much timing or compression and not enough octane.

It didn't seize though..thank god. lol. just lost compression.

My bad. Should have said "meltdown". Same thing applies.

[thmike]

No obvious seizure marks but lots of erosion - must be due to detonation. I would check ignition timing, either lower compression ratio or stick to race/pump gas mix and enrichen fuel mixture slightly to bring down combustion temperatures. 

So go up on the main jet, take some timing away, and lower the compression. If I do all three I should be able to get rid of this detanation issue.

[Jeram]

if you increase the thickness of the base gasket you will also be increasing  your exhaust duration which means less bottom end and more top end.
proper way to lower the compression is by machining the head.

why dont you just try richening the jetting and take some timing away, it might be enough to take away the detonation.

then just do some test rides to check the jetting (plug chops to make sure your on the money) and at the end of ride inspect piston top and head for detonation....

perhaps go up 2 main jet sizes and see how that goes, but also dont neglect the fact that your needle and idle may or may not be lean too.

[SachsGS]

I think the compression will have to be lowered slightly as well.Why not go the base gasket route as it's cheap,easy and easily reversible (if the motor ends up too pipey).

[2T Institute]

I think the squish band needs attention to, I don't think rasing the cylinder is the answer as it has no mid range now.

[thmike]

I think the squish band needs attention to, I don't think rasing the cylinder is the answer as it has no mid range now.

I've been thinking about the squish...But I know if thats right I will have to run race gas. I'm trying to to go to expensive with this bike. Its a 01 and more of play toy then my race bike. I won't mind a pipey bike. Heck thats fun! But what I'm gathering its 100% detanation.  The motor did feel real hot. Cases wise was a lot hotter then my yz125.

[arnego2]

When you ride track your jetting has to differ between a drag race and a track day specially when the drag race is quite long.
Street or open dirtroad WOT requiere quite a lot on the main, tell me where is your needle at? I run 4 rth on the JD red needle and 3 rd on the OEM in my 250s. 

[2T Institute]

I think the squish band needs attention to, I don't think rasing the cylinder is the answer as it has no mid range now.

I've been thinking about the squish...But I know if thats right I will have to run race gas. I'm trying to to go to expensive with this bike. Its a 01 and more of play toy then my race bike. I won't mind a pipey bike. Heck thats fun! But what I'm gathering its 100% detanation.  The motor did feel real hot. Cases wise was a lot hotter then my yz125.

Was more pre-ignition than detonation. Check your squish gap and width, it maybe set for short bursts of acceleration not long WOT runs as arnego said above.