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1980 Yamaha MX175 Two stroke issue. #5191120 08/19/19 08:41 AM
Joined: Dec 2006
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Panzerman Offline OP
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I have a old but in really nice shape 1980 Yamaha MX175.
It's the white off road only version of the Yamaha DT line.
I have a problem with it, that's driving me crazy.
It quit running and I couldn't get it restarted.
I was getting eradic spark so I changed the coil and spark plugs wire, still eradic, changed the CDI and I looked into the flywheel which I found out was the wrong flywheel, but this bike was running good on it, it was not tight and slipped off, no puller needed. I replaced it with the correct flywheel.
Good blue spark, still won't start. Tore the top end off.
Had compression but the rings were definitely worn, the Piston had a small chunk out on the exhaust side so I chucked it.
But a Wiseco kit and rebuilt the top end. Honed the cylinder, like new. No grooves, no gouges. Nice, standard bore.
All new gaskets, put it together. Still won't start, nice blue spark. Lots of compression. Tore the carb apart, nice and clean, the pilot jet was dirty but not that crazy. It's getting fuel.
Only thing left is it's out of time. Guess I ll pull the flywheel and check the stator plate to see if it came loose and turned but I am about out of things to look at.
Driving me crazy now. Anything else to look at.
I am a two stroke guy but this bike has be ready to part it out.
I have a Old 1980 DT175 that I still have plates on. I bought this bike for parts but it actually ran better than the DT so I just rode it off road as a fun bike then it just stopped and wouldn't start. These are pretty simple durable machines.
Truly the Apocalypse bikes or so I thought.



2006 Dodge Ram 2500 Hemi 4x4
2016 Dodge Challenger Scat Pack
Re: 1980 Yamaha MX175 Two stroke issue. [Re: Panzerman] #5191207 08/19/19 10:44 AM
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sunruh Offline
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throw the PJ in the trash and put in a new one


motorcycle oil myth buster
Re: 1980 Yamaha MX175 Two stroke issue. [Re: Panzerman] #5191272 08/19/19 12:25 PM
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ZeeOSix Offline
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If you have a timing light, hook it up and have someone kick the bike over while you look at the timing marks on the flywheel to see if the timing is good or not. If so, then squirt a little gas in the carb while someone kicks it over to ensure it's really getting fuel.

Re: 1980 Yamaha MX175 Two stroke issue. [Re: Panzerman] #5191498 08/19/19 04:51 PM
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Cujet Offline
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I always start with the basics. Spark plugs can spark just fine and be useless. Carbs can fail to enrich and cause cold start problems.

Recently had a 2 stroke not start due to the crankcase filling with fuel. Had to drain it out first.

Those things remind me just how important things like crank seals and reeds are or even a piston that fails to seal the exhaust side due to a missing chunk. As these engines can be sensitive.

My guess, your new flywheel changes the spark timing. That could be why the other flywheel was on it,,,???

Last edited by Cujet; 08/19/19 04:52 PM.

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Re: 1980 Yamaha MX175 Two stroke issue. [Re: Panzerman] #5191798 08/20/19 04:58 AM
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Panzerman Offline OP
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I took the flywheel off. It appears the timing is pre set on the backplate with no adjustment. They are simple animals.
I think it may be the coil pulser since it's when it actually tells it when to spark. I ordered a new one on eBay. Try it when it gets here.
I be had the crankcases fill up with fuel before. I had the top end off and cleaned all that out.
Polaris two strokes are nice they have a crankcase drain screw for that. Japanese motorcycles the only way to get them running is tied to the back of a truck and pull them till they fire up.



2006 Dodge Ram 2500 Hemi 4x4
2016 Dodge Challenger Scat Pack
Re: 1980 Yamaha MX175 Two stroke issue. [Re: Panzerman] #5191828 08/20/19 06:18 AM
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Cujet Offline
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Pulling it to jump start it was NY next suggestion.

I have a 1974 Honda xl100, it runs only occasionally and will happily leave me stranded, unwilling to start or run. Despite pulling it. In its case, electrical components are original and prob quit when hot. I purchased a few new things and will prob get a replacement stator and regulator also. Hope to resurrect it and enjoy a vintage ride.


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