Jump to content
Sign in to follow this  

'01 YZ250F - 50% Leak down test results

Recommended Posts

A short history. Bike supposed to be low on hours in 2001 through 2004. 2005 raced in the vet class. We bought it 01/01/06.

Trail rode about 60 miles 07/30/06 My son has always complained about the low end bog, everything else good! Replaced leak jet with smaller. Pulled main and pilot jets to confirm sizes and replace. Pulled spark plug for inspection. Tried to do a test ride with new leak jet 08/05/06. Little compression.

Did a compression test - 85psi. Did a leak down test - 50%, leaking past the rings, intake valves, and exhaust valves. Just checked the valves one month ago, exhaust good, intake had to re-shim all three. Shim sizes were all still like 1.70 or thicker to get the intake into spec. Rechecking the valve gaps and still where they were when I re-shimmed.

If this is how the engine is going to go, then maybe I should feel lucky it did not explode. Any one care to offer their thoughts to a 4 stroke newbie??

Thanks :thumbsup:

Share this post


Link to post
Share on other sites

new piston, rings, cam chain, valve seals, base gasket, head gasket.

pull the valves and check them real close. maybe re-cut the seats.

decarbon the ports and head.

check the coolant passageways for calcium buildup. CLR/vinegar it out.

when changing the timing chain, triple check the teeth on the crank. they may be shot.

Share this post


Link to post
Share on other sites

I have the engine all torn down now. Nothing really looks terrible. On the teeth on the crank, what exactly am I looking for here? They appear to have a different shape than the cam teeth. They are not hooking or anything say like a front sprocket may look like. All the teeth have the same width (or thickness) while looking at them from the outside the bike. They do have somewhat of a sharper feel on the fingers than the cam gears. Thanks for your input.

Share this post


Link to post
Share on other sites

I would say the bike wasn't as "low hours" as you were lead to believe. Do a complete overhaul (be sure to replace the valve springs, seals, guides), get a good valve seat re-surface job, stay on top of the maintenance, and you'll be happy with the service the bike will give.

Share this post


Link to post
Share on other sites
They do have somewhat of a sharper feel on the fingers than the cam gears.

They should have a sharper feel than the cam teeth, as the cam teeth are more squared off. As long as there are no broken, cracked, or bent teeth, I'd say it's good to go.

Share this post


Link to post
Share on other sites

OK, will borrow a camera and get a couple of pics. Hopefully post them Monday evening. Thanks.

Share this post


Link to post
Share on other sites

I have a good picture of the teeth on the crank, however did not realize that I couldn't just attach it to the reply. I guess that's why so many of you have links to your pictures. Any help here would be appreciated. Removed the valves. The seats and valve faces (contact patch) look pretty consistant everywhere. Lots of carbon inside the head and on the exhaust valves. Even carbon on the intake stems (mostly left and right, middle intake valve very clean). Will keep cleaning until new parts arrive. Thanks.

Share this post


Link to post
Share on other sites

Prior to disassembling the head, I did spray WD 40 on the top side of all the valves and none would leak through. One intake (right) would be moist around the valve face but definitely no dripping. Although, could I speculate that at times maybe chunks of carbon would hold open a valve a couple of rpm's and then fall through and then out the exhaust? I have probably removed a 1/3 of a thimble full of carbon. Thanks.

Share this post


Link to post
Share on other sites

I am putting the engine back together with new piston/rings, new valve springs, valve seals, spring seats to do a new compression/leak down test. I bought new valve cotters, but should I use the old ones? Would the valves and cotters be worn in together? should I keep them a match set? Thanks.

Share this post


Link to post
Share on other sites
I bought new valve cotters, but should I use the old ones? Would the valves and cotters be worn in together?
Use the new cotters. You should never ever re-use cotters. Yes, they do "break in" to fit the valve, but the new ones will also break in to the valve they are fitted to.
One intake (right) would be moist around the valve face but definitely no dripping

That's all it takes. That was a clear indicator that at least that valve isn't sealing properly, and it's a sure bet that if one valve seat/face is worn, they all are. If that were my engine, It would have gotten new valves, guides, and seats while it was apart. Do it right so you don't have to do it twice.

Share this post


Link to post
Share on other sites

New piston/rings on, cylinder on, cleaned head and valves on. Valves holding fluid on the top side of the valves. No cams installed, just thought I would get some feel for the compression with the new piston/rings before I continued. Very little compression. Did not do a compression test. Is this a fair test as I am reassembling the engine? Do the rings need to be seated before I would be able to get good numbers for a compression/leak down test? Or is there something else going on. Cylinder looked good, still had lots of cross hatching and no vertical lines from the piston anywhere. Measurements taken on the cylinder were within spec (however measured these with a dial caliper). Thanks for any advice. :thumbsup:

Share this post


Link to post
Share on other sites

Here are my numbers for my last reply. Compression - flickers to maybe 20 lbs, but reads 0 when done kicking. Leak down test @ BDC and almost TDC - 93-94% holding (6-7% leaking). Anyone have any thoughts on why I can get good numbers for leak down and then nearly no compression???? :applause: Could the rings be seated at those places for the leak down test but going through the whole stroke for a compression test maybe then somehow allowing the psi to escape??? Could the cylinder be egged shaped??? It had no vertical lines (scratches) on the cylinder wall, in fact it still had a fair amount of the original cross hatching. Please help!!! Thanks. :thumbsup:

Share this post


Link to post
Share on other sites

take the big plug off the left side flywheel cover. try not to let too much oil drain out.

pull in the decomp lever.

put an air wrench on the flywheel bolt and spin it 100 times or so. 200 would not hurt.

put the plug back in.

start it and warm it up.

as soon as it is warm, 1/2 throttle runs up a slight grade, u turn, coast back down the hill using engine braking. do this 1 dozen times.

cool motor.

do again at 3/4 throttle. cool motor.

change oil, go race

Share this post


Link to post
Share on other sites
Compression - flickers to maybe 20 lbs, but reads 0 when done kicking. Leak down test @ BDC and almost TDC - 93-94% holding (6-7% leaking). Anyone have any thoughts on why I can get good numbers for leak down and then nearly no compression????
Cam timing. If your exhaust cam is one tooth retarded, you'll get that very result.

Another thing bothers me:

Did a leak down test - 50%, leaking past the rings, intake valves, and exhaust valves. Just checked the valves one month ago, exhaust good, intake had to re-shim all three.
If you had one or more leaking intakes, and they required a reshim to a 1.70 from what was probably once a 185, that's a lot of wear for a Ti intake valve, and the intakes are most likely not going to seal well, or will quickly wear further. Take a look at This Page for a look at this type of wear. The thing is that once a Ti valve begins to wear at all significantly, it means the hard coating is gone, and the longer you run and reshim them, the greater risk there is of dropping a valve head due to stem fatigue.

If I missed where you replaced the valves, please excuse the oversight.

Share this post


Link to post
Share on other sites

but gray you don't know where his valves started at.

my 06 had 178's and a 176 in it from the factory.

however, the 01's "typically" did have 180+'s in them from the factory. but you can't be certain. 170 is not that much from 185. now if he was in the 165 or less range that would be different.

Share this post


Link to post
Share on other sites

Gray,

Right now, I do not have the cams in the bike, or the cam chain. Just piston, cylinder, and head (w/old valves, but new springs). The head will pass the WD 40 test (sprayed on top of the valves and holds) Thanks.

Share this post


Link to post
Share on other sites
but gray you don't know where his valves started at.

my 06 had 178's and a 176 in it from the factory.

however, the 01's "typically" did have 180+'s in them from the factory. but you can't be certain. 170 is not that much from 185. now if he was in the 165 or less range that would be different.

I agree with SUnruh, without knowing the baseline valve clearance measurements when the valves were new and what shims were in there at that time, it's hard to tell how much they have worn down... :thumbsup:

When in doubt, it might be best to replace the valves, especially on a very old bike and start fresh with new valves, shims, measurements, etc. This is why I don't like buying used, I don't know the conditions of everything...

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×