Equally low compression on all 4 cylinders

[Tanerrx]

I'm getting 60psi (+/- 1psi) across all 4 cylinders. Is there anything that would cause equal low compression besides timing? I've done the timing exactly as the manual shows. There is now slack in the chain in front of the exhaust cam and all the marks are on point even when I crank the motor around a few times.

The bike will consistently start and run for 3 to 8 seconds with the choke fully on. I've completely taken apart the top end and rebuilt it before I started experiencing these problems. Also, I've done a half assed leak down test with my air compressor and when on TDC of 1-4 I get air coming out of the spark plug holes of 2 and 3 when blowing air in 1 and 4. When I do the same to 2 and 3 I don't get air out of 1 and 4. What is this telling me? Bad head gasket?

Could equal low compression be caused by anything else?

 

[lemonshindig]

1) The compression test must be done with the throttle held wide open. Doing it with the throttle closed will result in low values.

2) Valves. More than likely, your valve clearance is zero, which means that the cam lobes are preventing the valves from closing all the way. Adjust the valves.

http://members.dslextreme.com/users/bikecliff/images/gs850valve_adjust.pdf

Using zip ties in place of the special tool:

http://members.dslextreme.com/users/bikecliff/images/valve_tool_zip_tie.pdf

As a side note, spark timing has absolutely nothing to do with compression. Valve timing, however, does. But unless the previous owner has removed the cam chain and put it back on in the wrong spot, or your chain is REALLY stretched (unlikely), then the odds of your valve timing being off are extremely low.

 

[Tanerrx]

1) The compression test must be done with the throttle held wide open. Doing it with the throttle closed will result in low values.

2) Valves. More than likely, your valve clearance is zero, which means that the cam lobes are preventing the valves from closing all the way. Adjust the valves.

As a side note, spark timing has absolutely nothing to do with compression. Valve timing, however, does. But unless the previous owner has removed the cam chain and put it back on in the wrong spot, or your chain is REALLY stretched (unlikely), then the odds of your valve timing being off are extremely low.

1) I held the throttle wide open when doing the tests so it's not that. 2) I've checked all my valve gaps and I did replace some of the shims because 4 of them were too tight. They are all properly set between .03mm and .08mm now. 3) I didn't make this clear before but I was talking about cam timing not spark timing.

Anyone have any advice about why I'm getting air leaking from one cylinder to another? Or why the compression is the same across the board but so low?

 

[Brendan W]

Is it possible that the tensioner is not operating correctly either because the set screw is jamming it or the rod is bent? Make sure that setscrew is 1/4 to 1/2 turn backed out from the rod. Turn the motor over backwards and you should feel the tensioner move if you rotate the knob anticlock slightly. Slack on the front at tdc seems wrong. Could the chain be too long or the guides worn out?
Guessing now but could air be going out exhaust valves and coming back in others via the exhaust.

 

[Guest]

You had the cams out, correct? Have you verified twice that they are in the right position? Just make sure you have the correct amount of pins between the marks on the cam gears...
Curt

 

[Tanerrx]

Is it possible that the tensioner is not operating correctly either because the set screw is jamming it or the rod is bent? Make sure that setscrew is 1/4 to 1/2 turn backed out from the rod. Turn the motor over backwards and you should feel the tensioner move if you rotate the knob anticlock slightly. Slack on the front at tdc seems wrong. Could the chain be too long or the guides worn out?
Guessing now but could air be going out exhaust valves and coming back in others via the exhaust.

I meant to say, "there is NO slack in the chain," in my original post. Not "NOW". Sorry about that. The chain tensioner plunger isn't bent and the screw is backed out about a 1/2 turn.

 

[Tanerrx]

You had the cams out, correct? Have you verified twice that they are in the right position? Just make sure you have the correct amount of pins between the marks on the cam gears...
Curt

I've made sure that the cam rods are in the correct spots and counted 20 pins between the 2 and 3 mark as the manual said to do.

I did take the valves out and lightly cleaned them when I had the top end taken off. Could they not be seating properly now? Do valves need to be lapped every time they are taken out and put back in?

 

[tkent02]

Leaking from one cylinder to another can only be a bad head gasket. Low compression after the valves were too tight likely means burnt valves, but it's odd for them all to be even.
How does it run?

 

[Tanerrx]

Leaking from one cylinder to another can only be a bad head gasket. Low compression after the valves were too tight likely means burnt valves, but it's odd for them all to be even.
How does it run?

Ok. I just wanted to get another opinion about the head gasket. I figured that was the only thing that could cause it.

It is super weird that they are all the same low compression. The gasket is one of the only common threads between all the cylinders though so that might have something to do with it.

It doesn't run. It'll start with the choke on and die after 3 to 5 seconds. It will start again and again if you let it rest for 5 seconds in between

 

[mvalenti]

Dumb question... Could the compression tester be faulty?

 

[Brendan W]

Dumb question... Could the compression tester be faulty?

Not so dumb at all.
Op has half the normal pressure and the variation across all four is 5% of the variation the manufacturer specifies.
Either the tester is sticking at sixty or all pots are connected by a secret tunnel.
Even dumber question .... is the head torqued down?

 

[rustybronco]

Anyone have any advice about why I'm getting air leaking from one cylinder to another? Or why the compression is the same across the board but so low?

How did you verify the air leakage from one cylinder to another? Did you put air in one cylinder and the pressure gauge in the adjacent cylinder?

 

[jsandidge]

same thing

same thing

Dumb question... Could the compression tester be faulty?

I've been wondering the same thing about mine. When I first got the bike, it was not running, sat for 14 years. I did a cold compression check. 90 psi across all 4 with throttle wide open. Thought it was strange, but figured I would check again once I got the bike running. Months later, after a good ride, I checked again. 90 psi across all 4. Stranger still, I thought. Figuring compression would probably come up after putting some miles on the bike, I check again. Even after 600 miles and checking after a ride, I still get 90 psi across all 4. Always checked with throttle wide open. Now, I'm convinced it's my tester.

 

[Tanerrx]

Not so dumb at all.
Op has half the normal pressure and the variation across all four is 5% of the variation the manufacturer specifies.
Either the tester is sticking at sixty or all pots are connected by a secret tunnel.
Even dumber question .... is the head torqued down?

Yes. The head is torqued down as tight as I can comfortably get it. I don't have a torque wrench but it's on there good. I'm going to try my compression gauge in one or two other bikes today and just make sure it's working properly.

 

[tkent02]

Anyone have any advice about why I'm getting air leaking from one cylinder to another? Or why the compression is the same across the board but so low?

Can you describe this air leaking better? Can't imagine how you would see this doing a simple compression test.

Also a leakdown test will tell you a lot more.

 

[Tanerrx]

How did you verify the air leakage from one cylinder to another? Did you put air in one cylinder and the pressure gauge in the adjacent cylinder?

I put cylinders 1-4 on TDC and put my air compressor in 4 and I could feel air coming out of 3. Same with 1 and 2 respectively. No gauge was needed, it was leaking out that much.

 

[rustybronco]

I put cylinders 1-4 on TDC and put my air compressor in 4 and I could feel air coming out of 3. Same with 1 and 2 respectively. No gauge was needed, it was leaking out that much.

TDC with both valves closed (compression stroke)?

 

[Tanerrx]

TDC with both valves closed (compression stroke)?

I don't understand your question. What is compression stroke? And wouldn't both valves be closed automatically if it were at TDC?

 

[tkent02]

Suck squeeze bang and blow.
It's what makes the world go.

 

[rustybronco]

And wouldn't both valves be closed automatically if it were at TDC?

No. It takes two revolutions of the crankshaft to complete all four cycles necessary for the engine to function. One, TDC both valves are closed and on the other the exhaust valve is open.

Suction
Compression
Power stroke
Cylinder evacuation.