Compression numbers.

8 replies to this topic
  • Ymmr

Posted April 10, 2004 - 10:18 AM


So based on the replies to my question about buring oil, I purcahsed a compression tester. Just what kind of comression numbers should I be looking for in PSI?
Should I open the carberator needle up, or is it good enough to just give it full throttle?

  • TonyDeCoster

Posted April 10, 2004 - 10:46 AM


You'll need to open the throttle all the way to get an accurate reading. The compression will be lower on the 450s and 426s with the cam mod.

  • Ymmr

Posted April 10, 2004 - 03:36 PM


That answers part of the question.
Any takers on some actual compression numbers they had, or even better, anyone know the specs. I couldn't find any numbers in the manual.
I guess if it's bad rings, I can get a reading, add oil to the cylinder and try again and if the compression goes up that should indicate rings??
I would like some numbers though if anyone has seen any. It would be nice to know how out of spec. it is, if it is. I can push through TDC if I stand on the kick start for a few seconds.
Thanks again.

  • Eddie Sisneros

Posted April 10, 2004 - 03:37 PM


you need to do a leak down test to get a accurate idea of whats going on.

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  • yz007f

Posted April 10, 2004 - 10:43 PM


When you say oil how much you putting in the cylinder? Over a few cc's of oil can make a difference in volume in the cylinder too. Which would mean more compression. Also you running these tests with a cold engine? If so most of your oil will be out of the cylinder at that time. Best would be to run it for a few mins and then check it again.
Just my 2 cents

  • sirthumpalot

Posted April 11, 2004 - 05:40 AM


The oil in the cylinder method does work. Warm the engine, then check the compression. Add a teaspoon or so of oil, kick it over slowly a few times and check again, if it's higher than it's likely the rings that are worn.

What I have never understood is HOW you get a compression reading with one of these engines? I can stand on the kick starter with all of my weight and it takes a good number of seconds to get through the compression stroke. Not easy to kick through! For that reason, I too would vote leak down tester. The results are more accurate, you can listen to where the air is leaking to tell where the leak is, and most importantly I have no idea how you would compression test one of these things if you have to kick it through to get a reading. Just food for thought, good luck!

  • FFRacing79

Posted April 11, 2004 - 08:29 AM


Like burned and sir have stated, the leakdown is the most accurate way to measure top end wear.

  • Ymmr

Posted April 12, 2004 - 04:07 AM


I don't disagree, I think a leak down test would be the best as well. But the equipment runs about $80-$100, plus I would need a compressor $$. I picked up a compression guage for just over $20. I hope to get a chance to see what kind of numbers I can get today.

  • TonyDeCoster

Posted April 12, 2004 - 04:57 PM


If you strip the bike down, a dealer shouldn't charge you too much to pull a leakdown. As for checking the compression, just get the piston past TDC like you normally would and give it a few good boots...

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