Low leak down AND low compression?

[kawie_guy]

Had 75 psi of compression in my vtwin before I decided to put new valves in it. Now it has 50 psi but shows 3% leak down loss. How's that even possible?
My compression tester has been babied and has been reliable in the past. I don't get it. I've even got 11.5 to 1 compression pistons.

 

[ripcord]

Valve lash too tight? Valve timing off?

 

[Cdn17Sport6MT]

I have an FAA type leakdown tester... (it has the FAA spec 0.040" dia orifice, 0.250" long, with 60 degree approach, departure). I have an unabused conventional compression tester. When, a number of years ago, I tested my Plymouth Reliant 2.5 - I got a lousy indication of leakdown, yet I got a great indication of a compression test.

My point of mentioning this is that one is a dynamic test, and it involves testing whether the cam lobes are worn, as well as testing in essence the whole length of stroke (by way of cylinder wall)... and the other one is a static test... that allows a person to isolate what is bad, albeit just at TDC. They both are useful in diagnosis.

The other comment, re the conventional compression test, is that how quickly and energetically that needle jumps up... with the first compression hit... also tells you something. A lazy approach to a final compression value signals a weak cylinder, too.

 

[Trav]

Had 75 psi of compression in my vtwin before I decided to put new valves in it. Now it has 50 psi but shows 3% leak down loss. How's that even possible?
My compression tester has been babied and has been reliable in the past. I don't get it. I've even got 11.5 to 1 compression pistons.

3% is great 50 psi is not, using a non OE head gasket (thicker than stock) can also cause this, forgetting to open the throttle WOT are two other reasons I can think of.

 

[wpod]

V twin what? What is the compression etc on the other cyl.?

 

[mattd]

Only thing I can think of with those leakdown numbers is that you used a thicker head gasket . Or you had throttle blade closed during the test as Trav mentioned

 

[Jetronic]

or valve timing is off

 

[Trav]

Yep that too.

 

[kawie_guy]

Valve lash too tight? Valve timing off?

No. Quadruple checked.

 

[kawie_guy]

3% is great 50 psi is not, using a non OE head gasket (thicker than stock) can also cause this, forgetting to open the throttle WOT are two other reasons I can think of.

This bike has the auto decomp pins in the cams.
Service manual says anything from 43 to 75 is acceptable.

 

[kawie_guy]

V twin what? What is the compression etc on the other cyl.?

48

 

[kawie_guy]

Only thing I can think of with those leakdown numbers is that you used a thicker head gasket . Or you had throttle blade closed during the test as Trav mentioned

I have only ever used the OEM gaskets. They only come in one thickness.
Also had the throttle opened. Actually held it open with a bungee cord so I could carefully handle the gauge.

 

[Trav]

This bike has the auto decomp pins in the cams.
Service manual says anything from 43 to 75 is acceptable.

In that case its probably all good. Run it and see how it goes.

 

[jdr7]

A weak battery/slow cranking might show a lower psi?

 

[kawie_guy]

A weak battery/slow cranking might show a lower psi?

I think you might be on to something. I just bought a new Die Hard battery that seems a bit sluggish to start the engine compared to some of the Yuasa batteries I've had. All the compression tests I've run have been with this Die Hard.

 

[Superflan]

Have you proceeded to bedding the valve into the seats?

 

[kawie_guy]

Have you proceeded to bedding the valve into the seats?

I've run it about 150 miles since the last test. My battery crapped out in me a couple days ago, so I've got a new battery coming and will do another compression test when it gets here.

 

[Superflan]

I've run it about 150 miles since the last test. My battery crapped out in me a couple days ago, so I've got a new battery coming and will do another compression test when it gets here.

Yep but I was referring to this:

 

[kawie_guy]

Yep but I was referring to this:

Machine shop said they lapped in the valves.

 

[Cujet]

Yep but I was referring to this:

Just an FYI, there is no need to hand-lap valves anymore.

Use a battery drill, some rubber hose on the valve stem, and go both directions. Lift the valve regularly to spread the lapping compound.