Problems with bike seizing

[socalwr]

2004 YZ250
this weekend at my race i had my bike seize up for the fourth time in the last six months. first time it was minor, had the cylinder wall honed out and installed a new piston. i had a ring break the second t time which gouged the cylinder wall, so i had it resleeved. third time the piston exploded into pieces. i pulled apart the top and bottom end, we found a broken gasket (called the gasket, air cleaner cover on the parts fiche). coolant seemed to be leaking at this point and i thought that this was causing the bike to overheat and possibly leaning out fuel mixture by allowing air into the bottom end (not sure if this is possible). i am not a huge fan of resleeving, because i have heard that the piston runs a little hotter and i thought this was a factor in my problem. so i ponied up the pennies and bought a new cylinder and cylinder head and had the whole motor reassembled, new gaskets and all. I did one race, which was a wide-open sandy gp race w/ a lot of fast fifth gear sections. rode the bike hard, no problems. this last weekend had another gp and on the first practice lap about 3-4 minutes in the bike just died, wouldn't start back up and had very little compression. we took it back to the truck pulled off the head, saw what i thought was a mild seize mark, but nothing major. we ended up putting some premix on the cylinder wall to help seat the rings, put it back together, and the bike fired up with compression close to normal. so in the race the last lap bike seizes up in a wide open sections (complete seize kick starter doesn't move) the gp had some pavement sections that were wide open, but i wasn't holding it wide open the whole time and i was hitting the kill switch repeatedly.

sorry for the long post, but i'm just frustrated and can't see what is causing the problem. any thoughts or suggestions would be appreciated.

Gabe

 

[magneto]

Too lean on the main jet???

 

[hndarida4life]

learntoputapistonin.com

 

[socalwr]

spark plug didn't seem to be too lean, had a brown color, i think it is too lean when the plug is white.

i won't even respond to the other post

 

[socalwr]

btw i had a mechanic assemble the bike this last time to try and omit any errors on my part. also running fmf fatty and vforce reed cage if that means anything

 

[hndarida4life]

nope should still be good....wut year?

 

[socalwr]

2004 yz250

 

[ellandoh]

you have good break-in practices i assume, also warm up before racing , not start it and rip .......right?

 

[magneto]

Did you check for proper ring end gap when you installed the new parts? I had to file the ends on the last Wiseco rings that I put in my CR.

 

[Cr_125]

What does hitting the kill switch repeatedly do?Sorry to ask the question here but you said it up there,when i was selling my lod bike when the lad was test driveing it he kept doing that???What does that do,again sorry for hi jacking you post.

 

[robwbright]

These might help:

http://www.eric-gorr.com/tech/pistonguide.html

Or the same with different formatting:

http://www.eric-gorr.com/twostktech/pistondiaguide_2t.html

 

[robwbright]

BTW, what pre-mix oil and ratio are you using?

 

[socalwr]

yamalube at 40:1

 

[dirtbikegeek]

That is way too lean try 32:1 Bel-Ray

 

[2-strokes-4-life]

you can run 40:1 on a 250 without any probs,but with all the wot riding you seem to do,switching to 32:1 and rejetting may be a good idea. if nothing else for some added insurance

 

[darringer]

Sounds too lean on the main to me also. Have you checked the ignition side crank seal? Could be drawing in air at high rpm and leaning the mixture. Or even a leak at the reed block will do it. 40:1 is acceptable for oil with a 250.

 

[socalwr]

i will probably get a leak down test done rule out these potential problems first. when i get it back together i will go up one on the main

 

[WhiskythrottleKeit]

What size is your main jet?

 

[robwbright]

Lots of guys are running 40:1 on 250s. . .

 

[KDX CRAZY]

With all the WOT sections you seem to race you should be running slightly rich , as the extra fuel helps to cool the engine .

 

[hndarida4life]

idk wut it does never just kept hitting the kill switch lol

 

[holeshot]

2004 YZ250
. so in the race the last lap bike seizes up in a wide open sections (complete seize kick starter doesn't move) the gp had some pavement sections that were wide open....

Gabe

Was this at the Shamrocks GP at LACR? If it was, then a slightly lean two stroke would probably sieze in a race like that. I hit the rev limiter on the four stroke and held it there about 20 or 30 times during the race - definately an engine killer (especially the straight that was real long) :whoa:

If I ever go there again, I'll gear a little higher to keep from screaming it so much. Everything held together this time though.

Sounds like your smoker was too lean (could be an air leak, or it could be as simple as "too lean" on the main jet).

 

[oldfrt613]

Do you run pump fuel or race fuel ( if for no other reason, race gas will provide more consistant jetting ) ? Yes 40:1 should be OK, but 32:1 would be better. If you spend a lot of time WOT you will want to be a little rich ( just check with the desert guys ). I would also look at the cooling system - I love Water Wetter. It is cheap and really seems to help my cooling. It allows the system to hold more water than a 50/50 mix of antifreeze. More water = more cooling. Have you removed your front radiator shrouds ? is the radiator cap sealing good, or is it not holding pressure. I would find it hard to believe the bike was put together wrong 4 times. It's either too lean or the cooling system is screwed up somehow. You may even want to consider a higher pressure water cap to keep from loosing coolant.

 

[socalwr]

Was this at the Shamrocks GP at LACR? If it was, then a slightly lean two stroke would probably sieze in a race like that. I hit the rev limiter on the four stroke and held it there about 20 or 30 times during the race - definately an engine killer (especially the straight that was real long) :whoa:

If I ever go there again, I'll gear a little higher to keep from screaming it so much. Everything held together this time though.

Sounds like your smoker was too lean (could be an air leak, or it could be as simple as "too lean" on the main jet).

this was at shamrocks. all the 4 bangers passed me on the asphalt. the kicker is that i raced at 29 palms and the bike ran solid the entire race. i will go up one on the main jet and hope this helps. once i tear down the bike and look at the piston and cylinder it might give some more clues to what exactly happened. appreciate all the help

 

[E-man811]

1 Inspect pistons to diagnose previous problem, probably lean but could be something else.

2 Leak down test first to try to find the source of lean condition.

3 By now your crank is toast if you've had 3,4 or whatever siezures. Reassemble with new crank, bearings and seals. Leak down test to confirm seal.

4 Carefully inspect the air boot, box, filter seal for any signs of dirt being able to be introduced to the intake charge.

5 Check your intake tract for any possible air leaks post carb. i.e. carb to boot, boot to reed block, reed block to cylinder/cases.

6 Return to base jetting, go one up on main

7 Ride the snot out of it and hope she don't squeak again.

Ride safe
Ian