Compression & Valves

T

treitz

XS400 Member
Messages
18
Reaction score
0
Points
1
Location
Boise Idaho
Hello group. I've introduced myself, but this is my first question thread. As mentioned in my into, the bike I am working on reportedly had low compression, but ran. Here are my initial compression test results:

Left: 110
Right: 145

NOTE: It took about 20 kicks to get the compression to the point that the needle stopped moving at these numbers. That seams excessive. Normal, or something I should be concerned about?

It sounds like 110 is definitely on the low side. Apparently you can get away with 120, but 110 is pretty low.

Based on everything I have read here I should start with valve clearance before jumping to any conclusions about piston rings or gaskets, so I'll do that. I also need to run the compression test after adding some oil still. Next step would be a leak down test.

There is some oil on the floor of the garage, so some gasket replacement may be in my future anyway.

QUESTION: Based on the manual, setting the valve clearance requires the engine be at TDC, which can only be set by removing the rotor cover. However, with oil in the engine, I don't see how you could do this without draining the oil. Unless leaning the bike to the right will keep the oil from draining out? I don't have a problem draining the oil, but it sure makes it hard to test the compression again immediately after adjusting the valves.

Any input appreciated.
 
Last edited:
In my experience only a tiny amount of oil will come out of the rotor cover if the bike is on a centre stand. Like two tablespoons worth. But I am no expert. I just put a small container under to catch any drips and go on with adjusting.
 
jthiessen said:
In my experience only a tiny amount of oil will come out of the rotor cover if the bike is on a centre stand. Like two tablespoons worth. But I am no expert. I just put a small container under to catch any drips and go on with adjusting.
Click to expand...

That was sort of my assumption. Unfortunately the center stand was removed from the bike (great)... but I'll just put some boards under the side stand to bring it close to level. I'll end up changing the oil anyway, so I'm not too worried about losing oil. Just don't want to lose it all, then have to refill it just to test the compression again.
 
You shouldn't need to put oil back in to test compression. You're not starting it up, just turn it over a few times and test it. There will be no problem doing that with the oil low or drained.
 
rshutchinson said:
You shouldn't need to put oil back in to test compression. You're not starting it up, just turn it over a few times and test it. There will be no problem doing that with the oil low or drained.
Click to expand...

Really?? You're talking about turning the motor over with little to no lubrication... sounds risky to me.
 
Oil will still be clinging to things that need it. For the 5 or so kicks you are going to give it it will be fine.

You do what you feel comfortable doing, though.
 
A some point find a center stand. You will need it to get an accurate reading on the oil level in the motor. Did you open the throttle fully while kicking? This will give you a more accurate reading. There are a bunch of good threads in the forum on valve setting, look them up:wink2:. With the four bolt access cover off and the bike mostly level only a few drips should be back there.
 
xschris said:
A some point find a center stand. You will need it to get an accurate reading on the oil level in the motor. Did you open the throttle fully while kicking? This will give you a more accurate reading. There are a bunch of good threads in the forum on valve setting, look them up:wink2:. With the four bolt access cover off and the bike mostly level only a few drips should be back there.
Click to expand...

Yes, full throttle on the compression test.

I looked them up and have read through them, don't worry :) However none of them specifically mention the oil/draining the oil which I thought was interesting. However with a center stand I can see how it wouldn't be an issue.
 
The first compression tests I ever did on my bike came in at 90 for each cylinder. Haven't replaced a thing but I've done plenty of cleaning, polishing, lubricating, and derusting.

Reads closer to 130psi now and runs pretty well.

Comparing readings from dry compression tests to wet compression tests helped during my diagnosis.

A wet test involves putting a bit of oil into the cylinders before a compression test. A couple of tablespoons will do.
It seals up around the piston rings, meaning any leaks evident in the wet test are likely happening at the top of the cylinder, the valves.
If the wet test gets 150psi, compared to the dry test's 110psi, your piston rings/cylinder are the biggest cause of the low compression. If the wet test reads much below 150 your problem may be leaky valves.

It's not an absolute diagnosis but it helps.
Leak down tests have also been recommended, though I've never done one.
 
My compression was 30 psi on both cylinders. I'm currently rebuilding it. Head and valves were filthy and all rings are completely seized. No getting the rings out.
 
BBS360 said:
The first compression tests I ever did on my bike came in at 90 for each cylinder. Haven't replaced a thing but I've done plenty of cleaning, polishing, lubricating, and derusting.

Reads closer to 130psi now and runs pretty well.

Comparing readings from dry compression tests to wet compression tests helped during my diagnosis.

A wet test involves putting a bit of oil into the cylinders before a compression test. A couple of tablespoons will do.
It seals up around the piston rings, meaning any leaks evident in the wet test are likely happening at the top of the cylinder, the valves.
If the wet test gets 150psi, compared to the dry test's 110psi, your piston rings/cylinder are the biggest cause of the low compression. If the wet test reads much below 150 your problem may be leaky valves.

It's not an absolute diagnosis but it helps.
Leak down tests have also been recommended, though I've never done one.
Click to expand...

Wet test and leak down are both on my to-do list. Definitely not jumping to any conclusions yet.
 
Adjusted the left exhaust valve and the compression is up to 145. So I have 145 on both cylinders. Guess a rebuild isn't in my future after all.
 
Speaking of saving money...

For those of us who do not have the equipment necessary to check our own compression, how much on average should one expect to shell out for a shop to check a wet and dry compression?
 
Its cheaper to get a compression gauge of your own. I got one at napa for under $30:) It has paid for itself many times:D
 
Glad to hear the compression is back up.

Definitely cheaper to buy a gauge. It's the labour that'll kill your wallet.
 
Hey guys. I'm back. Quick question about compression tests on these kick only bikes...

For a kick start only bike, how many times should you kick it over for a compression test?

1) First article I read said that you should only have to kick it 3 - 5 times to reach maximum compression.

2) Second article said kick it until it stops going up.

So here are my results... I can get both cylinders to read 145lb's if I kick it 10 - 15 times. Question is, if the first article is correct, the compression is bad. If the second article is right, the compression is good.

Thoughts?
 

Similar threads

Zippo
Backfire after carb sync
Replies
4
Views
477
Zippo
Zippo
pulmenti
What am I Missing? (timing?)
Replies
6
Views
825
pulmenti
pulmenti
G
Carburetors shooting flames!
Replies
14
Views
801
OldDirtyHippy
OldDirtyHippy
E
78 XS 400 2E won't start!
Replies
6
Views
518
Ed Dunham
E
Ben Thompson
82 XS400J/Maxim - Oil Problem - Top end not getting oil
Replies
9
Views
1K
tstidham
tstidham
Back
Top