Compression numbers.

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?


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.

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.

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

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

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!

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

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.

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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