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Compression tester

By RMmatt,
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RMmatt

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I'm buying a compression tester because I feel like my 04 CR250 is to easy to kick over. Now what size is the spark plug hole on the head. I wanna make shure I get a kit with the right attachment.. The one I'm lookin at fits 14mm and 18mm hole.. Also if I have lost compression should I just change the rings? Or do a hole top end rebuild? I have no clue how many hrs are on this bike

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Grand Master

LovingOffroadPain

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I'm buying a compression tester because I feel like my 04 CR250 is to easy to kick over. Now what size is the spark plug hole on the head. I wanna make shure I get a kit with the right attachment.. The one I'm lookin at fits 14mm and 18mm hole.. Also if I have lost compression should I just change the rings? Or do a hole top end rebuild? I have no clue how many hrs are on this bike

 

If you have no clue how many hours are on the bike, just replace the whole top end: piston, wrist pin bearing, rings, circlips, gaskets, etc.  Test the compression with the new top end, and when the compression is 10 lower than when it was new, it's time for at least new rings!

 

I just did a bottom end on my bike, but my piston was perfectly fine with 75 hours on it.  I replaced the rings, the bottom end, but I'll run the piston another 50+ hours before replacing it.  But I only did that because I know the hours on that piston.  Otherwise I woulda replaced it for sure.

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Grand Master

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Your bike should use a BR8EG spark plug, which has a 14mm thread.

I had a friend use my compression tester that I bought down at the auto parts store on his 1970s Johnson 2T boat motor. He complained that the piston might have got damaged because it bashed against the thread of the tester. The thread looked pretty hammered too, lol.

I doubt that would happen with your 2T engine though. If you want to check just hold the fitting beside the spark plug and if it's not going to go in further than the spark plug then you're a-ok.

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Fatt_Tones

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I'm buying a compression tester because I feel like my 04 CR250 is to easy to kick over. Now what size is the spark plug hole on the head. I wanna make shure I get a kit with the right attachment.. The one I'm lookin at fits 14mm and 18mm hole.. Also if I have lost compression should I just change the rings? Or do a hole top end rebuild? I have no clue how many hrs are on this bike

Compression numbers are very relative, and subjective. There are listed specs but they essentially all go out the window.... Compression readings will vary greatly based on your elevation, environment, etc.....even between two individual bikes of same make/model.

The best way to use a compression tester is to use it to a relative benchmark that YOU have established. Here's what I mean. After you do a top end and break it in, take a compression reading and write it down. After your new top end has reached the end of it's service life take another reading and write it down. You now have parameters within which to stay.... This, in my experience has been the most useful way to practically apply a compression tester.... All those numbers that come from others sources are useless....too many variables ?

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Grand Master

The Spanky

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If you think it's "easy to kick over" then it is probably way past time for a rebuild. A fresh 250T is NOT easy to kick.

 

Most people just don't notice the gradual loss of compression and performance because it is so gradual over such a long period of time. You'll probably be schocked at how much better it runs after a rebuild, and wonder why you waited so long.

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RMmatt

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You can also use a leak down gauge and see where the air is leaking from. That will tell you if you have a ring issue or possibly valves.

do u know how many psi to pump up 2 when doinga leak down test? I watched a u tube video and the guy said 9 psi but I believe it was a bigger motor
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Fatt_Tones

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do u know how many psi to pump up 2 when doinga leak down test? I watched a u tube video and the guy said 9 psi but I believe it was a bigger motor

I go to 6psi. 6psi for 6 minutes is the general rule of thumb. I heard if you go past 8 you risk blowing out the seals.....but I have read where guys put like 60 psi on a motor....yikes.

Some PV designs will never seal 100%, thus making some engines incapable of holding static pressure. Therefore a more effective method of leak down test is to just run a steady purge of air into the engine @ 6 psi, then spray soapy water around all potential leak areas.

My homemade tester.....I've since changed the fittings to hook up directly to my compressor:

LeakdownTester2_zps8d905704.jpg

Edited by Fattonz
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Grand Master

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I go to 6psi. 6psi for 6 minutes is the general rule of thumb. I heard if you go past 8 you risk blowing out the seals.....but I have read where guys put like 60 psi on a motor....yikes.

Some PV designs will never seal 100%, thus making some engines incapable of holding static pressure. Therefore a more effective method of leak down test is to just run a steady purge of air into the engine @ 6 psi, then spray soapy water around all potential leak areas.

 

Yeah, that just reminded me of something. I have a couple of 2T twins that have a labyrinth seal on the crank so there is some slow leakage into the opposite cylinder when you test. 

Motion Pro have some instructions for their tester here, which you can print if you want to put them with your tester: 

https://9007c0109a70412e8f84-17cbb53619a640a777e85fbe5231c890.ssl.cf1.rackcdn.com/articles/2502_08-0071.pdf

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RMmatt

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I go to 6psi. 6psi for 6 minutes is the general rule of thumb. I heard if you go past 8 you risk blowing out the seals.....but I have read where guys put like 60 psi on a motor....yikes.

Some PV designs will never seal 100%, thus making some engines incapable of holding static pressure. Therefore a more effective method of leak down test is to just run a steady purge of air into the engine @ 6 psi, then spray soapy water around all potential leak areas.

My homemade tester.....I've since changed the fittings to hook up directly to my compressor:

LeakdownTester2_zps8d905704.jpg

could u tell me the material and fittings I will need to make my own homemade tester like urs.. I'm starting to buy all of the tools I'm going to need to do my own rebuilds, eventually I wanna learn how to split cases and custom build my engine from the bottom up:)
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Fatt_Tones

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could u tell me the material and fittings I will need to make my own homemade tester like urs.. I'm starting to buy all of the tools I'm going to need to do my own rebuilds, eventually I wanna learn how to split cases and custom build my engine from the bottom up:)

You'll need a tee and a 15(ish) psi gauge. Then I suggest you take your carb boot along with you to Home Depot or wherever. You will need a fitting that fits snugly into the carb boot so that it will seal. From there you will just use a series of adapters/fittings so that you can get air from your compressor into the engine. Some people use a scrader valve, but I like to just hook up the compressor and throttle the air supply with the manual valve until the purge is set at 6psi. Some people set the supply pressure at the compressor's regulator, but I find doing it my way is easier since I use a 1/4 turn ball ball. Plus, I don't have to mess with my compressor.....

Last I checked a leakdown tester was close to $300 and I built mine for around $40. Good luck ?

Edited by Fattonz
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RMmatt

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You'll need a tee and a 15(ish) psi gauge. Then I suggest you take your carb boot along with you to Home Depot or wherever. You will need a fitting that fits snugly into the carb boot so that it will seal. From there you will just use a series of adapters/fittings so that you can get air from your compressor into the engine. Some people use a scrader valve, but I like to just hook up the compressor and throttle the air supply with the manual valve until the purge is set at 6psi. Some people set the supply pressure at the compressor's regulator, but I find doing it my way is easier since I use a 1/4 turn ball ball. Plus, I don't have to mess with my compressor.....

Last I checked a leakdown tester was close to $300 and I built mine for around $40. Good luck ?

yeah I'm doing this for shure! Thanks for the info
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