Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: Taco on May 23, 2011, 11:18:42 AM
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So I have a 1993 CR250R and went riding yesterday, start of the day the bike did great, idled perfectly. However, After some hours and the engine got warmed up, the idle started acting funny. I'm not sure how to describe it but it pulsates. It wasn't a steady rpm idle, it would rise up, and low, rise up and low and so on. I tried lower and rising the idle but it kept pulsating. it got rather annoying when riding since the pulsing actually pulled the bike a bit. Any ideas? I'll post a video today if possible. I tried to look up videos and I couldn't find anything.
The only way I can describe is that a nice idle would be a constant "vrrooooooooooooooom" and my idle is something like "VROOoommm VROOoommmm VROOOommmm"
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float level is low, trash in the filter,water in the bowl, choke not closing all the way. air leak somewhere in the intake, could be a number of things
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I agree -- My first through was 'lean condition' coming from somewhere. The pilot circuit, carb boot, countershaft side crankcase seal, air screw worked loose, etc....
I hear a very worn top end will simulate a lean condition too. I personally dont know where in the RPM range that would show up. Perhaps you might want to check the compression just for the sake of it.
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Yeah I forgot to mention worn rings will do that too. pinched vent hose too
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I'd check your crankshaft seals and cylinder base gasket. ;D
Sometimes Bing equipped bikes will do this - worn carb slide.
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Alright, my filter is brand new and clean. Getting the carb cleaned tomorrow. What PSI should my compression be?
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I have. KTM which is the jetting very sensitive. I have to adjust for temps all the time. Try the simple things 1st, let's adjust your air mixture screw. Start by counting the number of turns until it bottoms out (so you can return it to it's current setting if hue adjustment does not work) return the screw to where it was and then turn it out an additional 1/2 turn. Continue to adjust until your idle is smooth.
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I've decided to simply buy a whole new top end while I'm at it. I've ordered Wiseco piston kit along with a gasket and bearing and other small misc things. I'm not riding this weekend so it gives me time to work on it. While I wait for that I'm installing new forks. Are wiseco pistons good? I've seen a lot of Wiseco and Namura pistons, and a few PRO X ones too. My friend recommended Wiseco.
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(http://img191.imageshack.us/img191/7852/20110525151853272.jpg)
Here's a pic when cleaning the engine, before taking the top end apart.
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I've decided to simply buy a whole new top end while I'm at it. I've ordered Wiseco piston kit along with a gasket and bearing and other small misc things. I'm not riding this weekend so it gives me time to work on it. While I wait for that I'm installing new forks. Are wiseco pistons good? I've seen a lot of Wiseco and Namura pistons, and a few PRO X ones too. My friend recommended Wiseco.
I use wiseco ultralites in my CR and highly recommend them.
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Wiseco, did you leak test it before you tore it down? Those crank seals could be going too.
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No I didn't leak test it. Where exactly are the crank seals?
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on the crank, duh :P
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Haha yeah, duh. I'll check for leaks when the top end is done. I had a bit of lack of luck today since the forks I bought didn't fit my tree. Anyway, I've been reading my service manual alot and one of the things I noticed is that the clips look difficult to take out, is it? And should I take out my power valve cover?
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So I finished installing the top end today. When I pump the kick start it's very tough and firm. No leaks as far as I can tell as of now. My friend noted me on something though, I never filed the piston rings, I wasn't even aware of such practice. Should I take the jug out and do them? I'd think they would come ready for installation since it all came together.
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So I finished installing the top end today. When I pump the kick start it's very tough and firm. No leaks as far as I can tell as of now. My friend noted me on something though, I never filed the piston rings, I wasn't even aware of such practice. Should I take the jug out and do them? I'd think they would come ready for installation since it all came together.
Just did my top end on my 06' KX 250. Stock bore nothing trick. I have 5hrs on it and its fine. Never filed the piston rings.
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That's a relief to hear. What kind of piston did you use?
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I have NEVER heard of filing piston rings. Where exactly would one apply the file without ruining them? Strange -- I would really check out that advice...
For a leak test, I believe the other member meant to check for intake/crankcase leaks. Moving the kickstarter and feeling the compression is not that test. There are two ways to check for leaks:
1) remove pipe and carb and plug everything up solid somehow. Put piston at BDC so the transfers are open and remove the spark plug and put a small amount of air pressure into the bore (you would need a spark plug hole tool that would include a pressure gauge And shrader valve on it. You put the pressure into the system and then see how fast it leaks out. Somewhere there should be specification for this leak rate -- I don't know what it is. This is a pretty big deal.
2) Instead of all that, button the whole bike up and start it. Set the idle best you can. Get a can of silicon spray and shoot around the connections between the carb and the engine, around the base gasket and under the mag cover (to check for a leak on that side main seal). If there is a leak the engine tone/speed will change while you are spraying in that area -- or the engine may just die. If there is no reaction to the spray then you are good to go. This should be done at idle when there is lots of vacuum downstream of the carb. Much easier than the above method.
So I finished installing the top end today. When I pump the kick start it's very tough and firm. No leaks as far as I can tell as of now. My friend noted me on something though, I never filed the piston rings, I wasn't even aware of such practice. Should I take the jug out and do them? I'd think they would come ready for installation since it all came together.
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That's a relief to hear. What kind of piston did you use?
Wiseco!
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I have NEVER heard of filing piston rings. Where exactly would one apply the file without ruining them? Strange -- I would really check out that advice...
For a leak test, I believe the other member meant to check for intake/crankcase leaks. Moving the kickstarter and feeling the compression is not that test. There are two ways to check for leaks:
1) remove pipe and carb and plug everything up solid somehow. Put piston at BDC so the transfers are open and remove the spark plug and put a small amount of air pressure into the bore (you would need a spark plug hole tool that would include a pressure gauge And shrader valve on it. You put the pressure into the system and then see how fast it leaks out. Somewhere there should be specification for this leak rate -- I don't know what it is. This is a pretty big deal.
2) Instead of all that, button the whole bike up and start it. Set the idle best you can. Get a can of silicon spray and shoot around the connections between the carb and the engine, around the base gasket and under the mag cover (to check for a leak on that side main seal). If there is a leak the engine tone/speed will change while you are spraying in that area -- or the engine may just die. If there is no reaction to the spray then you are good to go. This should be done at idle when there is lots of vacuum downstream of the carb. Much easier than the above method.
So I finished installing the top end today. When I pump the kick start it's very tough and firm. No leaks as far as I can tell as of now. My friend noted me on something though, I never filed the piston rings, I wasn't even aware of such practice. Should I take the jug out and do them? I'd think they would come ready for installation since it all came together.
sometimes if your bore is oversized or for some reason you need a custom piston you need to check your ring end gap and if necessary file to get it in spec. With standard bore sizes and any after market or oem piston i've ever used this hasn't been necessary though.