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Discussion Starter · #1 ·
I am in desperate need of advice. I bought a used 250R and I pulled the top end apart because I had no idea what condition it was in. I brought the jug and piston to a local ATV shop to get their opinion. They said the piston and cylinder looked great and they recommended I replace the rings, bearing, and gaskets and just hone the cylinder. I went with this advice against my better judgement. The guy to a quick look at the piston which is a 1mm over wiseco and he went in the back and grabbed a top end bearing, gaskets, and rings. Here's the problem. All the parts he gave me looked different, 1 way or the other,then what I replaced them with. The top end gasket he gave me was twice as thick as the one I took off. The rings I took off had nothing written on them at all and the ones he gave me had #'s. The package didn't have anything on it to indicate they were 1mm oversized rings. They didn't look like they were the same size but I figured it was because the new ones haven't been compressed yet and so the gap was larger. I put it together with the #'s on the rings facing upwards like he said (Is that right?) and checked the compression cold. It was 160 psi. I wasn't too worried about that at the time but the bike ran extremly rich right away. I tried several carbs and jetted both carbs with jets so small it's ridiculous and yet it still runs so rich it fouls plugs in 10 minutes. I changed the slow jets, main jets, leaner needles, ect. 1 of the carbs was rebuilt but the other was brand new. I refuse to run any leaner then 32:1 premix and I've even tried different oils. At this point I was concerned compression wasn't high enough to burn the fuel. I switched top end gaskets with the thinner one that was on it before I took it apart. Now the compression reads 175 psi cold so at least it's in the 170-200 range the specs call for. It feels a little harder to kick but it runs exactly the same. I'm using BR9ES plugs, 93 octane gas, I replaced the coil. I checked the timing with a gun to make sure the f mark is lined up. I went over it with a fluke meter and the ignition is all in specs. Is it possible the guy gave me all the wrong parts? Is it even possible to install the wrong size rings? If the rings are .020 oversize when I should have .010 would that explain my problem? Thanks for any reasonable thoughts and suggestions -Rob

P.S. The only other thing I can think of is the expansion pie has a decent leak even with a new gasket. If there's not enough back pressure to clear the cylinder will that cause it to run rich? I read that expansion pipes create a vacuum which helps clear the cylinder for more complete combustion. I was tols they all leak and it's normal?
 

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The stock gasket is a metal 3 piece one.You can seperate the pieces and use the thinner ones to raise the compression.The old gasket you have could be an aftermarket one or just a thin piece from a stock one.I dont think.020 rings would fit in a .010 over cylinder,the gap would close before you could fit them in the cylinder.You mention it was at 67mm(1mm over),which would be .040 over.Your compression will raise once the rings seat after a proper break-in.I would guess you would need somewhere around a 150-155 main providing you are still using a Keihin carb.Do a few heat cycles to break it in and then check compression.
 

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Discussion Starter · #3 ·
Sounds about right if I had a 34mm PJ. In 85' they used a PE round slide. The main is a 142 stock and this bike actually hada #!30 it. The other carb was a 35mm PWK a/s. I even talked the guy from Sudco ito giving me jetting specs for it. Stock is a #160 main, #48 slow, DEK needle. 250R specs are #155 main, #60 slow(huge?) , and a DGJ needle which is 1 size richer. The #155 main, #48 slow, and the DEK needle wound up working the best and I tried a #55 & #52 slow jet also. Either way it smokes like a banshee and fouls plugs.

To be honest, the cylinder honing was the part I just didn't want the guy to do and I think it's to blame if the rings are correct. Without getting into detail about auto brake systems. Old school mechanics will tell you to machine rotors every time you replace pads. After much debate, even GM has come around and issued T.S.B.s saying if the rotor has no groves and it's otherwise in specs to just "pad slap" it. The reason is when you machine a rotor you raise the RA value(roughness average) and decrease the coefficient of friction. Less friction, less stopping power. So I'm thinking the same must apply for a ring and cylinder seal. Less friction, less compression? I could be completely wrong about the cylinder but I've been to more brake training seminars then I care to remember and I'm A.S.E. and all that BS.

Has anyone else ever had a problem like this after honing a cylinder? My plugs look really weird too. The white part of the plug gets a black, glazed & burnt looking circle on it wherever it lines up with the exhaust port. The rest of it is pure white and looks brand new. The tip looks new also with no color to it. The burnt, black spot almost looks like the plug melted until you touch it. I even used a cheap non synthetic for the break in so I would get the proper friction, Valvoline I think.

Waht should I do? Buy a new piston this time and start from scratch?
 

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The crank seal wouldnt be a bad thing to check out.I would also check the reeds.If you dont care to tear it down,I would say to check the cylinder for taper.i think most pistons call for .003 cylinder clearance.If it is out of square down the bore,then get the next size piston and bore the cylinder.Your theory about the cross hatch pattern on the cylinder walls is the same way I think.You need some roughness to make the rings work properly.What break-in procedure did you use when you rebuilt it,if you just put it together and went straight to riding it,its possible the rings were damaged.
 

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you typically need to hone the cylinder when you re-ring, to take off the old ring glazing. the cross hatching in the cylinder holds oil which works as a lubricant and a sealer. poor cross hatching would result in lower compression. after break in your rings should have slowly seated to the new cross hatching and your compression can go up.
 
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