2002 yz250f please help

I recently bought a 2002 yz250f that needed the top end rebuilt but it came with all the parts needed to rebuild the top end, brand new wiseco piston and rings, the cylinder is either a 2002 or 2003 yz250f cylinder(used but in good condition), and the head is also either a 2002 or 2003 and a new timing chain. I owned this bike before back in 2014 i got it for cheap because it dropped a valve but i traded it because i couldn't get the money to fix it. Anyway i bought it back. My buddy and i put it back together. The newer(used) head needed a valve so we took a good valve from the old head to use in it. The bike won't start and while trying to start it the bike lost some compression and oil leaked out from the brand new lower head gasket. This bike hasn't ran since 2014 when it dropped a valve. It does drip oil but not from the new gaskets and it dripped oil before the initial compression loss. I dropped all my money into this bike($1100) and i only bought it for $600. Hopefully this weekend my buddy gets time so we can spend all day trying to figure out this bike and we have already checked the timing but we are not to good with timing so any help with all of this would be greatly appreciated. Also when my buddy and i work on it next we are going to tear down the top end again to fix the new gasket and to check and see if the piston rings set correctly. Again any help would be greatly appreciated this bike is a pain in the ass but i love this bike and as soon as it runs good i'm trading it for a 2 stroke 250. Also if this is posted in the wrong section sorry my bad this is only my second ever post on here and my first one was before i bought this bike.

Checking timing is a simple process. I think you might be in over your head but if you are going to continue, read the manual and do what it says. You really shouldn't pull a valve from one used head and stick it in another. It might work but odds are good it won't seat properly.

Checking timing is a simple process. I think you might be in over your head but if you are going to continue, read the manual and do what it says. You really shouldn't pull a valve from one used head and stick it in another. It might work but odds are good it won't seat properly.

 

 

++^^^

There is a reason for it.  The TI valves have a very thin coating of titanium, and they wear in a tulip shape.  Putting a used valve in a different head is asking for a leak.  If you spent all that $$, why not go ahead and do the head correctly and put new valves in and cut the seats properly.  To do otherwise is creating a failure in the near future.  My dad used to say, "If you don't have time to do it right now, when will you have the time to do it right again??

Checking timing is a simple process. I think you might be in over your head but if you are going to continue, read the manual and do what it says. You really shouldn't pull a valve from one used head and stick it in another. It might work but odds are good it won't seat properly.

The valve that was put into the head from the old head is almost a brand new kibblenwhite valve that the owner from 2014 put into it before a different valve dropped.  During the rebuild we followed the manual. and we checked the valves before the head was put on and they all were seat properly. My buddy and i have been riding and working on bikes for around 10 years and this bike has us stumped. I can time a bike i just suck at it, i'm used to two strokes and honda's 4 stroke where timing never gave me a problem.

 

 And to reply to ronbuell, the newer head came with the bike it's a good head and it had all but one valve. If i were going to do it correctly which would be buy a brand new head that would cost me money i don't have like others do. I bought the bike for $600, the rest of the money went towards small things on the bike like pegs,cables, and grips, etc.

 

We know that the parts we put on(the head and etc) are gone over and have no damage. It's got to be something that we messed up during the rebuild .

Also im typing this while i'm tired so if i misread anything sorry my bad.

Edited by zack226

Original post left out ........"brand new kibblewhite valve".

 

Seat still needs cut to square up the mating surface.  A check would be to flip the head upside down and spray brake cleaner into the port and see if it leaks.  If al valves seal, then you're GTG.

 

You didn't mention re-shimming it or checking valve clearances.  

 

Fuel, air, spark, timing.  Its a mechanical "thing" and if all those are present, ......it will run.

Good luck with it.  And get some rest man, I've been there too.  :thumbsup: 

Sorry I haven't said anything for awhile I've been busy. I found the problem with the bike, it was the intake kibblewhite valve that my buddy and I put in, but it's actually weird(at least to me). The valve seat didn't need cut, the valve head actually had build up under it causing it not to seal fully and it was a pain to clean it when i couldn't find my valve puller. And the other two of the intake valves didn't have shims.

But my question now is since I will most likely have to buy a shim kit if the shims I have aren't the right ones, I have stock cams and the only shim kits I can find are either hot cam shim kits or prox shim kits, will either of those kits still work with stock cams? Again thank you all who help.

Yes. Shims is shims as long as you get the right ones for the bike.

I use a socket and a magnet pen as my puller.

Hot Cams kit has a larger selection in the kit.

Thank you all for the help. When I buy a hot cams shim kit, when I shim my valves I don't know what size shims are in the bike already. All of my valves are off spec by alot, where would i start with the shim sizes? Also when checking the valve specs the feeler gauge should be able to go completely between the cam lobe and valve bucket right? Again thank you all for the help I'm not used to 4 strokes.

Edited by zack226

Be sure you get a tapered feeler gauge made especially for getting into the small area.  Cutting down a standard feeler gauge won't work.

I use one of those magnet on a stick tools to get the shims out.

 

What I do is take the measurement and go from there.  If its tight, I just keep checking with a smaller gauge.  If the smallest gauge won't go, then its time to make some guesses.  Record your measurements because remembering won't cut it.  I get some paper and write it down in a graphic position to each valve.  Intakes on the right, exhaust on the left, center intake in the middle.

 

If its loose, then you compare your shim to spec and just add to come up with the closest shim.  I usually go slightly loose, but not too loose, because they will close up slightly.

 

If its tight, make your guess, install it, take your measurement and record, and repeat the process until you get it in spec.

Not hard, just take your time and don't rush it.

 

Do NOT let the shim get sideways in the pocket!!!  They are hard to remove, and you will get a tight valve measurement with a thin shim that way..... not good.  Do NOT over tighten the cam caps.  You will seize the cam if you do that.  

 

Keep the area clean.  Wash the bike with the tank off first so no dirt drops into the head.  Let it dry or blow dry it with the leaf blower, or wipe it dry.

Clean the valve cover gasket, and you can re-use it.  Rip it, and it's done, get a new one.

 

If you remove the cams, then you will be messing with the tension adjuster.  Just make sure you unscrew the tension and let it snap back... don't bolt it back in fully extended.

Good luck.

Thank you ronbuell for telling me to get a tapered feeler gauge that saved me alot of work, I had 4 other feeler gauges but they were not tapered. I ordered a tapered feeler gauge on the tt store and found that only three of my valves are out of spec.Both exhaust and the middle intake are out of spec. My question is that the intake are supposed to be 0.10mm-0.15mm, and currently my middle intake the first feeler gauge size that will fit is 0.08mm, would I go with a smaller shim? Also my exhaust clearance is supposed to be 0.17mm-0.22mm and currently my right exhaust the first feeler gauge that will fit is 0.13mm and the left exhaust the first feeler gauge to fit is 0.15mm, would I also go for a smaller shim for more clearance? I'm pretty sure I need to go with a smaller shim but I just want to be sure. And I'm still waiting for my shim kit to ship so I'm in no rush( come on amazon i ordered it sunday ship already, I'm waiting. has it shipped yet)

 

Also another question I have is that I noticed that when pushing the kicker of the bike down air comes out from where the header pipe goes on to the slip on pipe that is for me by the kicker. I have a white brothers r4 exhaust. But would air coming out of the pipe there cause a small loss in compression? Again thank you all for the help I really appreciate it.

Air out the pipe joint has no baring on compression but it will cause it to pop and rumble on deceleration due to oxygen entering the exhaust stream and causing the end gas to burn.

I usually buy a good exhaust clamp then drill a small hole, 1/16th diameter, in the pipe back from the pipe edge about the thickness of a the clamp so the clamp covers it and then cut a slot in the pipe from the hole to the edge so it will clamp closed.  I did this on my 2003 yz250f.

I got a update on the bike and I wish it was a good update. I shimmed the valves and they are all in spec now but the bike still won't run. It won't even fire. It's in time, it has spark, fuel, and a new trick, it's now leaking antiefreeze from the base cylinder gasket. Even since it ruined the gasket wouldn't it at least fire, make a noise other than air coming out of the exhaust or something. I put all of my money into this bike and I don't know what else to do because I can't get another bike until this one runs so I can trade it. Please any help is very appreciated.

Have you eliminated old fuel as the problem? Spray starter fluid down the carb and see if you get a pop

There is no old fuel this bike hasn't had fuel in it for over a year until i bought it and bought fuel after the rebuild.

A leak down test would tell if you are loosing compression and where.  You can get one online for about 80$.  Its a good investment and will tell you definitively where the leak/leaks are or if it is leaking.  The auto compression release

makes it hard to get an accurate compression test and does not indicate where the loss is.

Edited by Jrchxr

I know it does have a compression leak because after the inital rebuild it blew the base cylinder gasket so I do have a new gasket on the way. Also my bike has stock cams so there is no auto decompression. But even though the base cylinder gasket blew, wouldn't the bike at least try to fire or pop or make some noise?

There is no old fuel this bike hasn't had fuel in it for over a year until i bought it and bought fuel after the rebuild.

 

Yeah but, some fuel will still be in the carb, and the pilot jet and main jet could be plugged up.  You need to clean out the carb.  That's one of the last things I do before i do a restart after a rebuild.

 

I know it does have a compression leak because after the inital rebuild it blew the base cylinder gasket so I do have a new gasket on the way. Also my bike has stock cams so there is no auto decompression. But even though the base cylinder gasket blew, wouldn't the bike at least try to fire or pop or make some noise?

Ahhh, last time I checked, this is a 4-stroke, not a 2-stroke.  So there's no compression in the base.  Now, there will be some blow-by, that's why there's a vent.  But it should not be serious enough to blow the base gasket.

Yeah but, some fuel will still be in the carb, and the pilot jet and main jet could be plugged up.  You need to clean out the carb.  That's one of the last things I do before i do a restart after a rebuild.

 

Ahhh, last time I checked, this is a 4-stroke, not a 2-stroke.  So there's no compression in the base.  Now, there will be some blow-by, that's why there's a vent.  But it should not be serious enough to blow the base gasket.

When i take the top end off again to replace the gasket I'll check the gasket area on the motor for wear and clean the carb. The gasket has got worse since it first blew because it started to leak antifreeze. But what you are saying ronbuell is that base cylinder gasket(and to make sure i'm calling it the right gasket, it's the gasket under the cylinder) being blown shouldn't cause a loss in compression? 

Zack,

Correct, the BASE gasket is at the Base of the Cylinder.  Now, the HEAD gasket is the one between the head and the cylinder.

 

The base gasket will usually be made of fiber material, but the head gasket is usually a cometic or multilayered metal design.

The base gasket holds back oil and coolant, while the head gasket holds back coolant and compression pressure.

 

The proper way to clean the mating surfaces on the engine case, cylinder, and head is to use a 1/4" 90 degree angle grinder with a scotch brite pad on it.  Baring that, the hand method works, but no scratches.  And the sealing area on the base are much narrower, so pay special attention there.

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