1989 Honda CR125 Compression #

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4/16/2018 4:34 PM

I've looked around for numbers but have had a hard time finding. My 125 is hard starting and acts like its running too rich.I have 135 psi. Wondering if the compression is the issue.
Anyone willing to share psi compression of their good running late 80's cr125's?

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4/16/2018 6:34 PM

Sounds like it... 135 is kinda low. What is it jetted like?

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4/17/2018 4:39 AM

Funny my brother and I were just talking about my CR125 and how they get hard to start if the compression is down even a bit. I'd get the cylinder off and have a look.

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4/17/2018 4:46 AM

135 is almost too low to start. You should be 165 +. When the compression is low it will show signs of being rich as you are getting incomplete fuel burn.

Paw Paw

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4/17/2018 6:11 AM

Thanks, I recently put this together with a lightly used oversized piston and jug off eBay. It ran rich at first, then I replaced my spark plug and closed the plug gap up to around .005" which made it hit the powerband. I rode the bike for a while like that and had fun with it, then it started getting difficult to start and ran rich again. Maybe this can all be traced back to the lack of compression. The coil is new by the way. Also, I was having a hard time with the float level setting. Gas would pour out of the overflow or I would set the floats lower and I could not get fuel in. It seems tricky to set up properly with the extreme angle that the carb sits at. What compression should this have at best with a fresh piston and ring? This has a iron sleeve it looked like, so I was going to try honing it out and replacing the piston and rings. I would hate to do this work and have the same issue come back!

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4/17/2018 6:39 AM

Before you tear it back down get the carb straightened out. If that doesnt get you running decent i would suspect the used jug and piston / ring...

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4/17/2018 6:53 AM

Honing a worn bore and fitting a new piston will not help your issue. You will just be creating more piston to wall clearance and lower compression even more.

Take the cylinder and piston to a proper machinist for measuring and inspection.

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4/17/2018 7:04 AM

AHRMA361 wrote:

Honing a worn bore and fitting a new piston will not help your issue. You will just be creating more piston to wall clearance and lower compression even more.

Take the cylinder and piston to a proper machinist for measuring and inspection.

Thats a 10/4.

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4/17/2018 7:24 AM

I've had success on snowmobiles with honing an undamaged cylinder, but they are larger ccs and much less sensitive to compression loss, (on a 500 twin 125 psi on both is good) I will look into getting it machined. I noticed that I can get all the way up to 2 mm oversized piston? Any experience on going that big with the stock jug?

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