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Sailorwithabike99

1999 CR125R bogs down and won’t start again! HELP!

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Hey everyone!

So I’m stuck in a bit of a dilemma and could really use all your help as I’m kinda new to dirt bikes.

So a few weeks ago I ran the bike too hard without letting it warm up properly and as a result, the piston over-expanded and got damaged. I educated myself and I finally finished rebuilding the engine and made sure everything’s to spec before putting it back together. Here’s where the problems start...

The bike wouldn’t start with the new piston. There is a nice blue spark and the spark plug was getting wet but barely. Slapped a new plug in there after putting a few drops of pre-mixed gas into the spark plug hole and it finally started in 1 kick. I hear cycled it perfectly and it ran very well. All things were going well...until I got to the 3rd gear. As soon as I switched into 3rd the engine bogged and died. At this point I kicked and kicked and it wouldn’t start. It also feels like it lost compression because of this, but after a few slow cranks the compression comes back. How would that even happen?!

I’m really stumped on this because I’ve done everything to spec and heat cycled it properly on a new top end so I really don’t know what could be wrong. Any help will help immensely. Thanks everyone!

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First things first ,,     what is compression  ?   Have you looked into exaust port to inspect for damage ?   What have You done?     We have no easy answers here , just speculation  .....

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6 minutes ago, bweighmaster said:

First things first ,,     what is compression  ?   Have you looked into exaust port to inspect for damage ?   What have You done?     We have no easy answers here , just speculation  .....

I ordered a compression tester and it’s getting here today so I’ll update you on that tonight but it’s a fresh top end and I did it with the proper clearances as instructed by the manual. I have looked into the exhaust port but there’s no visible damages, just some dents here and there on the outside. The only work I’ve done is replace the piston and the base and head gaskets. Just cleaned the carburetor today as well. The floats were waaaay out of wack (way too much gas getting in there) so I fixed that. I’m gonna install it today and see if she starts. Thanks for the answer mate! I’ll update you on compression as soon as I get the tester. 

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I was meaning to inspect piston , through  exaust port ...... move piston up and down to inspect cyl ...

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2 minutes ago, bweighmaster said:

I was meaning to inspect piston , through  exaust port ...... move piston up and down to inspect cyl ...

Ohhhhh ok I’ll do that ASAP. I’ll let you know as soon as I get it done. Thanks!

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Ok so I just checked the exhaust port to see if I can find any damage and I don’t see anything except a shiny new piston. Looks mint to me. Also got the compression tester and tested the compression and it says 30psi after like 10 HARD kicks???!!!! How even?! I’m at a complete loss. 

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OH SHIT! I just removed the cylinder again and noticed the piston is rubbed where the left side of the exhaust is! I’ve attached a picture of the previous piston that initially caused me to replace it in the first place and the run marks are in the exact same place. Any idea what it could be?

C99D65C3-2042-4EDC-8BE4-6227166A3888.jpeg

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Posted (edited)

Thats horrendous .Probably a bent rod if not a bad bore.

Edited by Tony Wyp

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4 hours ago, Tony Wyp said:

Thats horrendous .Probably a bent rod if not a bad bore.

It really is ToT but I figured out so far that it may actually be the power valve. It protrudes into the cylinder a tiny bit when it’s closed and that is most probably what’s causing the rubbing when it expands due to heat. I’m gonna grind down the powervalve a tiny bit to meet the clearance specs and install a new piston. Here we go again😭

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