LeakDown test vs Compression Test. Again!?

[almetelo]

Here we go again…
I know this thread was been debated to death here but I would like to hear your opinion about my experience
I know what Yamaha thinks about this (CT vs LDT) ( Yamaha recomends compression test to check how “healthy” is the motor and the leakdown test “just” to confirm from were the leak is coming)
Just rebulit a F100 power head, the cylinder head has pressure tested and resurfaced professinally, cylinder walls hone, new piston rings, new head gasket and valves lapped and test with gasoline for the leakage, some valves lapped twice until no leaks from valves.
Assembled the power head and did a leak down test and had leakage of +/- 30% ??? on all cylinders!!!
Something seems not to be right and in fact I can hear and feel the air coming from the exhaust/intake ports on some cylinders and in one of the cylinders the leakage is coming from the crankcase (piston rings)
I know piston rings and valves may need the motor to work for a while for coat/seat better but on previous rebuilds I did on F40, F50 and F60 after the same procedure I had 10% max of leakage and now 30% ???
Than made pressure test and pressure test and had 200 psi on cylinders #1, 2, 4 and 180 psi on #3 which is good as Yamaha recomends minimum pressure 135 psi
Than made a leak down test again still have the same leakage as before.
What is going on here?
I’m chasing a ghost here?
On my previous rebuils always had good sealing on the LDT after the rebuild.

[walleye1]

Take it out and get it broke in [I'd run it like I stold it] get the rings seated then do a leak test again. Of corse rod will have something
wrong with what I say.

[boscoe99]

Take it out and get it broke in [I'd run it like I stold it] get the rings seated then do a leak test again. Of corse rod will have something
wrong with what I say.

Something about teats on boar hogs I suspect.

[boscoe99]

I second taking it out and running it at high power settings for several hours. Then redo the leak down test. I suspect that it will get better.

If the motor starts, idles and runs fine I would not get hung up on leak down values. A valve may not completely seat with just 80 psi pressure pushing on it whereas it might if 180 psi or 1000 psi is pushing on it. Pushing in the direction to make it seat better.

[boscoe99]

How have you been by the way? You seem to have been busy mekaniking.

[dray0151]

I second taking it out and running it at high power settings for several hours. Then redo the leak down test. I suspect that it will get better.

If the motor starts, idles and runs fine I would not get hung up on leak down values. A valve may not completely seat with just 80 psi pressure pushing on it whereas it might if 180 psi or 1000 psi is pushing on it. Pushing in the direction to make it seat better.

But what about that beak in period??????

[boscoe99]

What about it?

Is a beak in period anything similar to a beak down test?

[dray0151]

Well it is a "just rebuilt F100" so I guess if you want to seat the rings real fast you can just run it off the dock "like you stole it", and maybe you will have a break down test before you get out of sight of land! Not an explanation of what the problem might be but no mention was made of any break in at all and the OP speaks of a break in to seat the rings etc.

[pstephens46]

Well it is a "just rebuilt F100" so I guess if you want to seat the rings real fast you can just run it off the dock "like you stole it", and maybe you will have a break down test before you get out of sight of land! Not an explanation of what the problem might be but no mention was made of any break in at all and the OP speaks of a break in to seat the rings etc.

No reason he can't poot around a bit as the manual states. But then vary rpm, including runs to WOT. Should not be any different that any other break in procedure.

[dray0151]

Agreed but " take it out and run it at high power settings for several hours" does not qualify as varying speeds to the naked eye that might read the thread. The OP I'm sure knows about a break in since he mentioned seating the rings but others read these also. Just saying.

[zenoahphobic]

It is not a new engine, so break in is less.
No new piston, cylinder hone makes bore looser and no new bearings etc.
Ha, you found the problem in leakdown test!,
It is not a dynamic test.
Leakdown tests do not replicate operating conditions. This example, and perhaps Yamahas stand, suggests that a compression test is closer to a meaningful test as it involves cranking replicating the engine functioning at low revs at least.
In a compression test the valves are opening and shutting (technically averaging a leak)
Running in is necessary. Would not go as hard as suggested but long as rings won't be seated. Valve seating may or may not improve and I suspect lapping is incomplete. I'd also question 200psi, would be a bit high, so hope head hasn't been over surfaced, might just be excess oil from assembly.

[99yam40]

Having to re lap the valves to keep them from leaking sounds strange
and now they still show signs of leaking.(air out of intake and exhaust)
are you sure you have them adjusted correctly?

[boscoe99]

Agreed but " take it out and run it at high power settings for several hours" does not qualify as varying speeds to the naked eye that might read the thread. The OP I'm sure knows about a break in since he mentioned seating the rings but others read these also. Just saying.

I get what you mean now.

Different folks have different ideas as to how to break in a piston engine. I am for lots of high power usage. Running continuously at one speed or varying the speeds. As one wishes.

[dray0151]

I wonder why the engine was rebuilt if only the heads were machined and new rings added? That brings out another sore issue in my book not replacing bearings, I am one of the ones that believe you need bearings replaced rather than going less $$$$ and using old parts like bearings. Would be interested in why engine had to be rebuilt?

[zenoahphobic]

Having to re lap the valves to keep them from leaking sounds strange
and now they still show signs of leaking.(air out of intake and exhaust)
are you sure you have them adjusted correctly?

Possible the valve stems jamming a little as any work on the valve faces will make them seat further up, This could be that new never worn upper part of the stem now sitting in the valve guide that is larger, maybe also deposits have not been completely removed. They will also appear longer, compounded with resurfacing, necessitating perhaps a significant readjustment or position or function of the "tappers" / follower? on top of the valve stems.