possible Big Bore....PSI <40lbs

A bit of help...advice here.
I've been battling a drowning bike...Symptoms:
Sputtering...loss of power
Stalls when not on gas
Then wouldn't start

Has plenty of Spark.... Fuel pump works and can smell gas... 40psi on compression test. So I'd assume I need new rings..valves or something that keeps compression up.

So if I am going in to do all this work....research... does it make sense to go to Athena 290 or Thumpers 280cc?

I used to rip small engines apart and put them back together. Watched a few videos on the HOW TO and seems challenging but not impossible.

So any suggestions? if so Where should I by my big bore kit from?

Thanks for your input!

If you're going to be tearing the motor apart anyways, and you want more power, that would be the ideal time to make the improvement.

I say DO IT.

When checking the compression are you holding it at full throttle while cranking the bike over with the starter? This makes a big difference with the compression test. Also how many miles on the bike? Could be tight valves too.

I would do the big bore..

Try finding it on this site.. This is where I found mine.. Paid around $480 new from a fellow member..


The added torque is niiice!

You'll never get a real compression reading, unless you disable the auto-decompression feature on the exhaust valve.

What happened before it started running like crap?

skierd wrote:

You'll never get a real compression reading, unless you disable the auto-decompression feature on the exhaust valve.

What happened before it started running like crap?

True but if we can get someone with a service manual to chime in it should have the compression value without disabling anything. I guess another way to kinda diagnose it would be if its burning oil and does your oil smell like fuel? If the valves check out ok I would just go ahead and do a big bore kit, and since at that point your stock cylinder is no good you might as well do the thumper racing 280 kit since they bore your stock cylinder and install a ductile iron sleeve and they give you a very nice forged cp piston which has 2 compression rings like the stock piston. The Athena kit only has 1 compression ring and the oil rings like all the modern mx 4 strokes these arent meant to last as long and have more blow by which dirties oil quicker.

skierd wrote:

You'll never get a real compression reading, unless you disable the auto-decompression feature on the exhaust valve.

What happened before it started running like crap?

I took the bike in the dunes for a day... Then let it sit a few weeks. This past weekend we had great weather so I took it out. It was just running crappy like it had water in the fuel or an ethanol snafu. Drained all the gas and refilled. Still no start.

How might I disable the auto decompression feature?????

[/quote]
True but if we can get someone with a service manual to chime in it should have the compression value without disabling anything. I guess another way to kinda diagnose it would be if its burning oil and does your oil smell like fuel? If the valves check out ok I would just go ahead and do a big bore kit, and since at that point your stock cylinder is no good you might as well do the thumper racing 280 kit since they bore your stock cylinder and install a ductile iron sleeve and they give you a very nice forged cp piston which has 2 compression rings like the stock piston. The Athena kit only has 1 compression ring and the oil rings like all the modern mx 4 strokes these arent meant to last as long and have more blow by which dirties oil quicker.[quote]

Doesn't burn oil...or atleast didn't when it ran. You bring up good notes about Thumper racing. I am weighing out the options they offer. Install in engine or just send sleeve.

Did you check the air filter? Is the battery fully charged and in good shape? Unless you put it under water or were running it without an air filter, one day in the dunes shouldn't kill the motor like that.

There's no mention in the manual for disabling the auto-decomp, but I believe its the weight on the exhaust cam. You might be able to zip tie it so that it doesn't open, but I don't know for sure. They only true way to test the cylinder's health is do a leak down test. Air leaking out the exhaust means bad valves, air in the oil means bad rings, air bubbling in the coolant means bad head gasket.

skierd wrote:

Did you check the air filter? Is the battery fully charged and in good shape? Unless you put it under water or were running it without an air filter, one day in the dunes shouldn't kill the motor like that.

Air filter is K&N and fine. Battery wasn't fully charged but now is and same problem. THis makes me want to

K&n filters aren't the best for desert or offroad ridding as they allow more fine particles to pass than a foam filter. This could be what did the top end in on your wr. It's strange it won't even fire though although if your compression really is only 40psi then it probobly would be really hard to get going. Might just have to tear into it and take a look at the bore and piston for scoring and look at the ring end gap.

So I guess K&N might be the culprit. Stopped by Yamaha today and they said if the filter is clogged it might be subject to not starting. I've cleaned the filter and will dry it out good for a few days then reoil slightly to try to start again.
wish me luck!

so what came of this issue? did the air filter cleaning fix it? did you go with a new BBK?