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Low compression - what to do?

Chippe

Well-known Member
Joined
Jun 27, 2020
Total Posts
76
Total likes
1
Location
Sweden
The compression is around 150 psi and it's time to do something about it. I know it could be the gaskets, the valves or the piston rings.
How and in what order do you think I should find out why the compression is low?

I've seen people test the valves if they leak with gas or soap water and then blow with air from behind. I have new engine gaskets but I guess once I put them on I don't want to open up the engine again or can I?
I think I will be able to fix the valves but how do I found out if I really need to buy new piston rings?
 
If a customer brought an engine to Me for rebuilding, because it had 150 psi of compression, I would tell him"save your money, there's nothing to be gained by tearing it apart". 150 psi.in the real world, isn't too bad, after you factor in altitude, barometric pressure(weather) and the differences in compression gauges 150 psi is a pretty acceptable number. Does it start well?, does it idle normally?, does it smoke? Does it push a lot of air out the breather hose when the engine is running? Rings that are sealing well enough to make 150 psi of compression, at cranking speed, are going to be sealing near 100 percent with the engine running.
 
If a customer brought an engine to Me for rebuilding, because it had 150 psi of compression, I would tell him"save your money, there's nothing to be gained by tearing it apart". 150 psi.in the real world, isn't too bad, after you factor in altitude, barometric pressure(weather) and the differences in compression gauges 150 psi is a pretty acceptable number. Does it start well?, does it idle normally?, does it smoke? Does it push a lot of air out the breather hose when the engine is running? Rings that are sealing well enough to make 150 psi of compression, at cranking speed, are going to be sealing near 100 percent with the engine running.

Interesting. I guess it runs pretty good then since I think it runs well, I just wanted to get the extra power it may offer with better compression but it can wait.
 
I really doubt getting the cranking speed compression numbers up 10-15 psi is going to make a difference you can actually feel. It's your machine, do whatever you want with it.
 
At 150 you are @80% of stock compression, 70% or @135 is about as low as you can go and still have the engine perform somewhat reasonably.
There a good chance that the pistons and bores will be in spec still and just needing a hone and new rings.
The valves are a replacement only item, the hard facing on the is only .0002" thick so lapping will cut thru that immediately and obviously grinding is worse. The seats can be cut easily to match new valves.
 
Just to be sure, compression test done with engine at operating temperature, choke and throttle wide open.

A leakdown test can give you an idea about air leakage past valves or rings.
 
Too bad, I was almost looking forward to lap the valves :) but hopefully I don't need to get new ones. Ok, then I'll do a leak test on the rings too when it's time but I think I'll ride it as it is for a while and do this work when the bike is ready for a repaint and engine polish.
 
A leakdown test can give you an idea about air leakage past valves or rings.

This is my suggestion as well.

And what type compression tester are you using? An automotive type may not be giving you accurate readings.
 
This is the compression tool that I used. My neighbour had it in his garage but we don't know if it's accurate but it probably is.
IMG_2294.JPG
 
That gauge looks as good as any other. The important things to do are hold the throttle wide open, choke in OFF position, both plugs out and crank the engine until the gauge stops rising.
Once the test is done add 5-10 cc's of oil to each cylinder and do the test again.
If the numbers increase significantly then that says the rings aren't sealing, if they don't then that says the valves aren't sealing.
 
Looking to buy a compression tester, any recommendations?

Go to your auto parts store. Don't buy the cheapest, you get what you pay for.

In the future when you have a question please start your own topic rather than hi-jacking an old one.
 
The compression is around 150 psi and it's time to do something about it. I know it could be the gaskets, the valves or the piston rings.
How and in what order do you think I should find out why the compression is low?

I've seen people test the valves if they leak with gas or soap water and then blow with air from behind. I have new engine gaskets but I guess once I put them on I don't want to open up the engine again or can I?
I think I will be able to fix the valves but how do I found out if I really need to buy new piston rings?

You did check and adjust valves first. Right?
 
Go to your auto parts store. Don't buy the cheapest, you get what you pay for.



Actually, the cheapy is often the one without the hose that you don't need (unless doing hard to reach automotive) and can cause lower readings. My best only has a rubber tip with a schraeder valve it. My old KD gauge and an old no name cheapy gauge both read the same when I switch the tip, but there probably is some real junk out there these days.
 
Last edited by a moderator:
Go to your auto parts store. Don't buy the cheapest, you get what you pay for.

In the future when you have a question please start your own topic rather than hi-jacking an old one.
Ok sorry
 
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