Need help with CR250

[flyincat]

I just found this forum and after reading some posts, this is the most knowledgable crowd around so I need your help. To make a long story short, I bought a 98 cr250 and blew it up after about an hour on the first ride. I was told that it was a stock engine but when I tore it down I found a sleeved cylinder and a wiseco piston. Well I got the cylinder bored and all new top end. It fired right up so I thought I was all set. Then the mystery begins....
I heard a knocking/clanking noise in the top of the engine when I let it idle a little to break it in. I'm obviously not an engine expert but I wondered if it could be detonating and it sounded real lean (which could be a leaking seal, another potential problem). I tore the carb apart and cleaned it and found all stock jets in it. So I fired it up again and it was still knocking. I did a compression check with two gauges and both read 240psi!!!! What the heck is going on? I'm almost positive the head wasn't cut at all so I have no idea why the compression is so high. I still haven't got a consistent answer on the compression for the stock bike. I was told anywhere from 130psi to 235psi after a rebuild by dealers and local shops. As far as the knocking, I am starting to believe that the piston might be hitting the head stightly because I can hear a faint sound when I rotate the flywheel. As far as the high compression, the only idea I have left is that the sleeve is not positioned right. Could the Cometic head gasket be the wrong one? Is this compression even too high to run race gas? I was thinking I was going to use high octane pump gas since I thought it was stock. Give me all you guys got because I'm lost.

 

[flyincat]

OOPS Sorry didn't mean to post this here and I don't know how to move it. Check the Mods/Performance forum.

 

[Rich Rohrich]

It belongs in this forum. ;)

 

[PowerJoe]

I think stock compression is around 150psi. If you really have 240psi, something is wrong. Maybe the wrong piston?? The wrong gasket kit won't make that much of a difference in compression.

Use a piece of soldier to check the clearance between the piston and head at TDC. I don't know the exact spuish of where this motor should be at, but my guess is somewhere between .030" and .070" so use soldier thicker than .070".

Good Luck.

 

[Mr. Clean]

A friend of mine had a bike with similar symptoms and it turned that it was overheating. With the radiator fluid low we could see the tops of the tubes were almost closed with mineral deposits. I removed it with a small pick and it ran fine. Moral of the story.....use distilled water.

 

[darringer]

My 99 CR has about 210-215 psi with a fresh top end. 240 sounds a bit high, but not unheard of. The head may have been milled for more compression.

 

[flyincat]

I spoke to Eric Gorr and we concluded that the top of the cylinder must have been milled too far down when someone surfaced it after installing the sleeve. He mentioned that I could stack base gaskets to get some clearance between the head and the piston and get the compression down for now. The other fix would be to get the head modified to make clearence for the piston.
The head was never modified after the cylinder was bored, so is it alright to have the head overhanging the edge of the cylinder a bit? I ask this because I am going to do the base gasket thing for now just so I can ride.

I must say Eric was much more willing to help than the local shops, not what I expected with having his name all over the internet!! Thanks

 

[whyzee]

Your compression reading is not out of range, a strong 250 should read around 210 & up.
125's will fall in around 150 to 190.

As far as the sound you are hearing ... ? I don't know. Maybe detonation, head tap, con rod run out ... lots of possibilities.

Eric is a straight up guy. He did wonders to my 125. Mic your cylinder, send him your head, tell 'em what kinda gas ya want to run ... he'll hook ya up!

 

[jmics19067]

Don't run the engine until you check this out.
If the cylinder has been lined, decked, and overbored take the head off. rotate the crank until you have the piston at top dead center. Visualize that I am concerned about the edges of your overbored piston sticking up out of the decked cylinder and hitting the edges of the un modified head. From what I understand you want .040" clearance between the edges of the piston to the head. Depending on the combustion shape, power characteristics you want , etc etc that .040" should be a certain percentage of the piston area starting from the outside edge. Can't help you too much with deciphering that but, if there is no clearance it is definately wrong and sounds like it would be very expensive to run

Also if you have an overbore piston your exhaust valves might be scraping the side of the piston. rotate the crank so the piston is bottom and make sure they aren't scraped up where they may protrude into the the area the piston will be. also take off your pipe and look thru the exhaust port with a flashlight and slowly rotate the crank making sure that the sides of the piston or the ring doesn't have a scrape mark on it that would match the powervalve.

these things are probably worst case scenerios but will be very expensive to repair if you try and run the engine especially if you run it hard. It might cost a bit to straighten it out but a heck of a lot less than what can happen if what I think is your problem.


P.S.

Eric is a rare example of a long forgotten work ethic in my opinion. Any body who takes his time to explain what he thinks and what he can do for you with honesty and integrety is worth every penny he charges. Mind you he can only go by what you say on the phone but he listens and gives it his best shot from your information.

 

[therocket]

A few years back I did a top end job with a complete piston kit, and had similar sounds to yours. Couldn't believe the racket. Thought I must have really screwed up. Well in the end found out that the piston pin was the wrong size. Changed the pin to the proper one and it ran great.

 

[flyincat]

Well I took the cylinder off to try adding base gaskets. I monkeyed around some more trying to figure out where the noise was coming from and here is what I found. It looks like the clunking is from the gear on the end of the crank. I must have just been transmitting through the entire engine. When I hold tension on the rod and rock the flywheel back and forth, I hear the same noise I heard when I ran it. The same effect happens when I rock the clutch basket. From what I know, gears have to have some backlash in them. Do those gears normally have to be replaced at some point because of this problem? Or is there an underlying problem that causes this, like a bad bottom end?

 

[jmics19067]

That sounds odd to me . usually a bad crank bearing would make a growling noise not a knock. Spin the crank , does it spin smooth and freely? grab the crank and wiggle it up and down It should be tight and would start getting sloppy if have a worn bottom end.

A possibility that comes to me is that rubber bushings inside your clutch hub have given up. Am I understanding that you have your sound when you slightly rotate your crank and you feel slop,hear clunk ? grab a holdof your clutch basket and wiggle it making sure it is tight and only rotates no up and down or sided to side movement. maybe the basket is loose or the bearings worn out. And watch the gears where they mesh when you rotate back and forth. See if the clutch basket gear stops spinning and yet the basket still moves a little till you get the clunk