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old yamaha YZ125 won't start

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I recently bought a 1982 YZ125 and have been struggling to get it to start. Bike has very low compression, got a new piston ring but didn't help the issue. Has spark, and when I pull the plug out it's wet so gas is getting to it, but I still don't hear any pop or fire or anything when I try to start it. 

I'm no expert but if the plug was sparking inside the cylinder with gas I should hear something right? So I was thinking maybe the timing's off (although it would have to be really off for it not to even fire) or maybe the plug is sparking outside the cylinder but not inside? Anyways I'm not sure what to do next any suggestions appreciated!!

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Make sure you are using a new plug (and the right one for that bike, whatever that is).  

Has this bike ever started for you?

You stated that the bike has low compression - does it still have low compression after you installed the new ring?  That wasn't clear.  Did you measure before/after in psi or is it just a gut/feel from the kickstart lever?

Timing (as you mentioned) is the next suspect, and certainly it can be far enough off that you don't get teased with a cough.  But without knowing if the bike has ever started for you, hard to say whether to consider timing as a likely cause.

Understood that the plug is wet after trying to start it, but have you checked float height?  A flooded engine that won't start will have a wet plug too.

Edited by Tankslap

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What do the reed valves look like? I second the new spark plug. I have a few IT bikes and even though the spark looks good you would be amazed what a new plug does. In fact grab a dozen extra as you work out the issues. 

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The compression didn't change after installing the ring. It almost seems pointless to do a compression test it's so easy to push down with my hand. 

I've cleaned out the carb so there's no gas in the float. One thing that could be a problem though is I'm not sure if the air/fuel screw is adjusted properly. 

As for the reed valves, they look clean and seem to seal properly when I open one with my hand. Not sure if there's anything else I should be looking for there. I'll grab some new spark plugs too just to be sure. I didn't know it was important to have the exact plug that was meant for the bike. 

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You need 3 things, fuel, compression and spark to fire. Sounds like no compression to me. Did you use new head/jug seals? If not you may be losing compression there but other than that you need to measure the Piston vrs the cylinder to be certain it's the proper size

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Ya I think I might need to get an oversized piston ring in case the cylinder has been bored out. The seals on the head look fine and I'm not sure what jug seals are? Guessing the seals on the bottom?

anyways even if it has low compression if that were the only problem I think it should fire because gas is getting to the plug...

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Gonna need three tools that's pretty cheap really, compression tester and (preferably) digital caliper and a feeler gauge to measure the cylinder,Piston and ring gap

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you could also try a pressure test both wet and dry. Do it dry first and then add a little oil and do the compression test wet. Did it change? If not your rings are probably okay and your gasket may be shot. Did you remove the piston and install it back the correct way?

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True adding oil will change it but after compression is tired of going up it'll go down and out anywhere! Replace all seals and spec the cylinder, right the first time.

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