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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 25 Feb 2008
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Help please- XT500 running rough

I have had a '78 XT500 stripped down mostly to a TT which has been loads of fun. I recently got my hands on an all original '77 XT500 from the original owner with ~10k miles on the clock. I got it with the carb jelled and the tank rusty but apart from that looking fine. Now that I have cleaned out the carb and hooked up my other tank for a temporary setup, I have not been able to get it running well. Fiddling with the carb, I can get the engine to either race at idle or smoke a lot and not really just get it running smooth. The spark was good with a new plug, the air filter was a brand new Uni and the fuel was fresh and stabilized. I then checked the compression on it and found out that it only had ~80 psi. I did not test compression immediately because it felt fine with the kick to me but I have not played with any other XTs than my two. I then did a leakdown test and was not able to find any leaks anywhere. I can hear the air rushing through the tube and into the cylinder, but cannot find out where it goes so fast. The exhaust is quiete, as is the carb, so it is not the valves. I sprayed soap water all around the cylinder and the gaskets seem fine. The crankcase vent did pressurize slightly if I held my finger over it for a while, but I thought this was normal cylinder blow-by. At this point I am just at the end of my rope and looking at taking the whole thing out of the frame and rebuilding it but it seems like such a waste for an engine with so few miles. The PO says it ran fine before it sat and I trust him. Any ideas or things to test for before I just tear it down? Any help would be appreciated. Also, if this post looks familiar, I posted over in the vintage section of thumpertalk... only getting a response to check this place out. So here I am to check out this forum. Thanks in advance.
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Old 25 Feb 2008
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3 things

make an engine run:
1 - ignition
2 - fuel system
3 - mechanical condition of the engine

1 - Ignition: did you check that it is set correctly; is the advance mechanism working as it should?
2 - Carb: you cleaned, but the symptoms you describe do indicate a carb problem. Check the idle circuit, check the choke system, check that the needle(s) are indeed in the correct positions (washers, clips etc). All little air or fuel channels should be clinically clean. The slide returns to idle position smoothly?
3 - Consider a sticking valve; open the valve clearnce adj covers and check that there are no valves that have excessive play

Good luck!
Auke
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Old 26 Feb 2008
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Ignition... I will go over it again to double check. The carb, it seemed to me like the choke was to be blamed but there was a noticable difference between choke lever on/off.. when it was running. Also things like that would not explain loss of compression

I will check the valves because that would explain poor compression, but did not hear/feel anything out the exhaust or the intake with the cylinder pressurized with the leakdown tester.

Thanks for the advice.
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Old 26 Feb 2008
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Oh and about the carb, yeah everything was cleaned quite well and operates very smoothly. I will likely tear it all down and make sure anyways. Where can I find what the stock carb settings should be?
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