Please diagnose !!!

[norcalduner]

The bike lost compression and died at WOT in the middle of the desert this weekend. The bike did NOT overheat as I was running only around 150F. Also running 100 octane at an elevation of 3000 ft which I think is overkill as my compression which I checked several times before I left was at 175psi. at almost sea level Here is a pic after I pulled the head off.

[norcalduner]

Here is a pic of the plug. Also performed a leakdown test prior to trip and it passed. It ran great for the most part except for a slight bog off idle and a hard time starting which I think could be corrected by going a size up on the pilot jet.

[trx310R#24]

it looks like 100 oct was a littlie to much... maybe the motor didnt over heat but the rings, piston an spark plug look like they did... well i cant see the rings but my guess is they look the same

[norcalduner]

Heres a pic of the piston :(

[quad9]

just a guess but was your head gasket leaking coolant into the cylinder? the top of that piston is very clean.

[deathman53]

thats bad, I see why you lost all compression. The deep gouges in the piston tell it all. I've had mine running great and do the same thing, scratched cylinder and gouged piston. I don't know why it happened. It happened to me twice, I got rid of the bike soon after the second time. The guy that bought parts of the bike from me(kinda sold him a roller), blew 5 top ends, similar thing. I bought the bike back, rebuilt the motor, and the top end has almost 2 1/2 years on it, no problem at all. I used a different radiator(s), it was in an atc and it was mounted in a trx for almost a year.

[86 Quad R]

hey deathman53 was the engine built each time with the same crank when ya had it and did the guy you sold it to do repeated builds on said crank?

see where i'm headed with this?

[wilkin250r]

Looks to me like a seizure.

You've heard of people relieving the the exhaust bridge on the stock 250r cylinder, right? Because the bridge is directly in the path of the exhaust gasses, it gets hotter than other areas of the cylinder walls, and that excess heat causes expansion, which then expands against the piston, causing a seizure. So that bridge needs to be "relieved", it gets cut back a few thousanths to allow for the extra expansion.

I'm thinking something similar happened to your engine, because it's right at the exhaust port.

How many hours did this top end and piston have since it was last rebuilt? How long was it running before this happened? Who built this engine?

[deathman53]

I kept the same crank(I kept the motor, however later on did damgae to the cases from a stupid mistake). The guy I sold the bike to got a motor for it, he kept blowing them and a very similar pattern. I told him when I sold him the bike about my issues with motors. He bought an 85 atc250r motor. He had the motor rebuilt with a hot rod crank. He blew it up several times, I rebuilt it 3 times(new bore and piston). I did a break-in cycle at his house once and the motor got real hot quick. I told him that the radiator is in kinda sad shape(alot of bent fins and some collapsed in, maybe even clogged). The second time I measured the crank clearance, it was well with-in spec and water pump was rebuilt when the motor was. Blew another, I reminded him about how bad the radiator is and that could be part of the reason.

fast forward 2 years, I bought the bike back. Rebuilt the motor with an oem crank, new pwr radiators(atc) and put on the 310pv I had(from another motor that had tranny issues) and that has almost 3 years on it and no issues. I really think the issue was something with the cooling, as the motor got hot really quick and ran real hot. The same motor with pwr radiators(when on the atc) runs ~180 and when in the quad with afco dual pass radiator runs 160-180. When the other guy had it, it had a stock cylinder(30 over), no port work done, lrd pipe and a 38 pj carb. It could have been lean jetting, but when I had it, it was jetted rich, same problems. 2 different motors with the same motor issues and the only common part was the radiator.

[All250R]

I think it's interesting that its on one side of the exhaust and not symmetrical with gas flow or crank rotation. I know Neil (C-Leigh) has had a few things to say about making sure the bore is cut straight. Maybe he'd have an idea if the bore man might have something to say about it!

[norcalduner]

This is on a brand new esr pro-x 310 pv with about 20 hours. I was actually very patient in breaking in motor as I followed the Duncan website break-in procedure. Only possible causes that I can think of are:

1. Not enough break in (used 2 full desert tanks of gas at 50/50)
2. pilot jet too small
3. Too hot of temp during break-in with temps reaching 220F which caused me to put a bigger afco rad
4. Possible top ring end gap too small (what should it measure?)

BTW, piston is a wiseco. Like I said, after the bigger rad was installed, the temps never climbed above 180 and it died after running at a constant 6th gear WOT for approx 10 min.

[norcalduner]

Just to add, I put this top end together myself and performed several leakdown tests after the build. Does that plug color look lean to you guys coz I normally jet for chocolate brown ?

Also, would you guys replace/rebuild the crank after something like this ?

[norcalduner]

Originally posted by All250R
I think it's interesting that its on one side of the exhaust and not symmetrical with gas flow or crank rotation. I know Neil (C-Leigh) has had a few things to say about making sure the bore is cut straight. Maybe he'd have an idea if the bore man might have something to say about it!

Could the cylinder sleeve warp enough during break-in even though it didn't overheat to possibly cause this.

[250RNUT]

I believe the problem was caused by running WOT for much to long and not giving time enough for the oil to lube the piston.

[norcalduner]

Second look at piston showed seizure marks exactly 180 opposite each other.

[atvmxr]

Id say something wrong with the bore then...

[86 Quad R]

welp! so much for my theory. odd that it's damaged opposite sides as it is. that plug color looks fine to me. :confused:

[C-LEIGH RACING]

What it looks like to me is lean conditions at WOT & idle.
You can seize a 2 stroke from a pilot jet being to small just like you can from to small of a main.

You take a 2 stroke engine, your going to run for extended amounts of time at WOT, that engine has to be set up different than say a drag, MX or TT engine.
The main jet, needs to be big enough that when holding WOT theres enough fuel to kinda hold the engine back to keep it from topping out high rpm.
The pilot jet, what happens when its to lean.
Say your WOT to the top of a hill & then have a long one to come down, engines good & hot & you back off the throttle before you start to come down.
With the engine still revved up, you start your decent & with the throttle just about closed, you are now just running the engine on the pilot jet which if it is to small, the engine has gone from running off the main getting plenty of fuel to a very lean condition.
Crank, rod & piston are still doing what it was at WOT, but now its not getting enough fuel to keep all the moving parts lubed & cool. Ol piston cant take it anymore & starts to melt.

When you look at a piston like yours & you have the dark brown discoloring on the sides, that comes from stuck rings. On each compression stroke after the plug fires, its blowing fire down past the rings & burning in the fuel to the sides.
For some reason those rings stuck down in the grooves & once the blow by started, it just super heated the piston even more.

Now, reason you see so much damage on the left corner of the cylinder, 250R cylinders, be it a stock one or Pro-x, they run hotter on that corner because of how the intake is set up.
The angle of the intake tends to pack more fresh charge into the right side of the engine & in turn causes the left side to run a bit leaner.



:eek: , You remember the stock TRX250R head has the spark plug hole at an angle, thats because when a plug sparks is projects its spark straight out & if the plug is angled to the left it would project to the right creating more heat on the right side.
You'll notice, a CT Racing Pro-x head, the plug hole is angled as well for this same reason.
Neil