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General Compression question

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Just bought a bike, told it had "new top end". Just bought a compression gauge. The price was set for a new top end.

2003 CR 125

Set by step how to do correct compression test?

this is what i did

1. pull plug, screw compression test in, keep line straight

2. gas off, hold kill switch

3. kick bike over 2-3 times.

4. read maximum numbers.

Now on both my bikes they are reading very low! Am I doing something wrong?

I read 50psi on my Kx 100 and 90 on the CR 125. They bikes are running strong... I'm thinking its a testing error?

:busted:

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Still only went to 90. He said he only replaced the piston. I'm guessing the gaskets, rings are worn along with probably not being torqued right... oh man here we go again. oh well, still a good deal on the big. :busted: Thanks ya'll

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It would be very easy to pull the top end off and check the piston and rings if you are worried. I doubt he changed a piston without replacing the rings at the same time (rings come with the piston usually). Also, gaskets leaking will cause a water leak so I doubt that is an issue. It can be difficult to get an accurate compression reading on a 2 stroke so I would not assume it is correct.

CoKTM

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The KX, nor any other bike for that matter, would start at 50 psi. I doubt the 125 would start or if it did it wouldn't run very well at all.

Have somebody check your tester against a known value.

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Thanks for all the responses! from what I can find + the responses it seems that different pressure gauges are different.

What should I be looking for in the top end to know it's time.(scarring, what should the rings look like?) I know for sure the KX needs one. But the Cr is a different story.

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Also you can easily check the rings. Pour a little oil down the spark plug hole. If it jumps way up the rings are shot.

THIS! Try this and see if the compression is higher.

There are a lot of good advice already in this thread.

#1 keep going until the compression max's out, dont just do 2-3 kicks.

#2 make sure engines are warmed up

#3 could be a bad gauge

#4 try a teaspoon of oil down the spark plug hole then do the test

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The KX, nor any other bike for that matter, would start at 50 psi. I doubt the 125 would start or if it did it wouldn't run very well at all.

Have somebody check your tester against a known value.

I bought a yz125 a couple years ago. It started, although not very easily. Had no low end power, but ripped up top. Compression test showed 45 psi... lol.

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I bought a yz125 a couple years ago. It started, although not very easily. Had no low end power, but ripped up top. Compression test showed 45 psi... lol.

That's the lowest I've ever heard of on a 2T and it still ran. :busted:

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That's the lowest I've ever heard of on a 2T and it still ran. :busted:

Definitely was the lowest I'd seen in a running 125+ 2 stroke. I was really surprised it was THAT low, because it did seem to run pretty well once you got into the upper RPM's.

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Ya, I mean the 125 hauls some serious butt, it runs strong. starts first kick always. the kx not so much. but still I think something must be wrong with the compressor.:busted:

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Thanks for all the responses! from what I can find + the responses it seems that different pressure gauges are different.

What should I be looking for in the top end to know it's time.(scarring, what should the rings look like?) I know for sure the KX needs one. But the Cr is a different story.

If you want to do this right, GET A MANUAL.

It will tell you the service limits of all the parts on the bike. You will need a micrometer and feeler gauges.

You can't check compression WITH A COLD ENGINE either.

It has to be hot.

The piston is oval shaped, the bore is round.

The piston expands side-to-side when it gets hot, and will push the rings correctly.

If you don't want to do it correctly, then, use the 5w oil method. Pout 25cc of 5wt oil into the spark plug hole of a hot motor after first taking a compression measurement.

If the compression measurement goes up more than 10 lbs, you need a new top end.

Remember, you can only get so many 'top end jobs' out of a motor before the crank is worn out, usually the bottom end. Plus, if you can ride a two stroke a looooong time with a worn top end, but it is hard on the crank and crank seals.

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