XR75ko engine HELP!

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#1
We have a 73' XR75 that runs absolutely perfect for about 5 minutes when cold, after it warms up it starts spitting, sputtering and losing power and throttle response. I have changed points, carb and adjusted the valves with no change. It also seems to get really warm, maybe even hot. Any Ideas?
 
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#2
Have you checked the compression? Very worn rings will cause the engine to progressively lose power and run poorly the warmer the engine gets, and the blow-by will cause it to overheat.
 
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#3
Compression is 115psi. What is normal compression for for a small displacement single cylinder?
 
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#4
115 psi is low by any engine standards, although those small engines don't run the high compression of bigger bikes. I would think it should have at least 140-150 psi, but maybe someone else here that owns one can chime in with the specs. Did you test it warm, with the throttle wide open? If so, the next thing I would do is a leak-down test, I think you are going to find worn rings.
 
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#5
Spanky, very good questions. I will warm it up, and check it again as you stated, never thought about checking it when I was actually having the problems. Thanks
 

HiG4s

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#8
They should be .004 intake and .006 exhaust. Small XR/XLs are known for running real poor after heating up if the exhaust is too tight. They heat up, expand and don't fully close. I actually run my son's XR100 at .006 in and .008 ex. They rattle a little until it warms up but then it runs great. I wouldn't reccommend running them that loose unless you plan to check them frequently.
 
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#9
.003" does seem a little tight. Small XRs are known for the exhaust valves tightening up substantially when hot. Try more valve clearance and see how it runs. I'm still sceptical of the compression reading you got, but I don't really know what that engine should read.
 
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#10
Well, the compression remained the same after the engine was warmed to the point where it started running irractically, also pulled the valve cover and there was clearance on the valves. This thing is getting hot real quick, the exhaust pipe is radiating heat like had been run like a dog on a 110* day in only about 3 or 4 minutes. Could this be valve timing or ignition timing?
 
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#12
Have you tried running it with partial choke to see if it is lean? Four-stroke pipes do get very hot after just a couple of minutes of idling. I've seen plenty of YZ-F pipes glow .
 
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#13
Just thinking if it runs good when cold...would it not need less fuel after warm up. I have put it up for the night, but will try any suggestions that are posted. BTW, not sure what the compression reading should be but it starts really easy on one kick when cold. What would be proper procedure for a leak down test?