LeakDown test vs Compression Test. Again!?

[zenoahphobic]

If I couple 200psi with valve leak. Not only could there be not be enough valvestem clearance or jamming, but could there actually be significant valve timing difference?

[rodbolt17]

either the valve faces are not correct, rthe seat is not correct or the adjustment is off,way off.
the 30% past the ring on a hone with a used piston. meeeehhhh.
the valve issue is a MUST fix due to hot combustion gasses will smoke a valve fairly rapidly.
if they don't seat they don't seal or cool well.
and that is why I don't bother with a compression test on a 4 stroke.
it to fast and easy and way more accurate than a compression test.
you rotate it to #1 TDC and hit it,if it shows 90% rotate it 360 and hit it again, from there on the inline 4 it is #3,#4,#2 and your done.

[almetelo]

Hi guys!

Thanks for your answers.
Yes, I’ve been busy mokaning which is something I like to do as a hobbie and have been learning a lot but I belive I’m done on power heads for now. Just will keep the F100 and the F60 motors
On my previous rebuilds always tested the valve sealing with gas on bores to check any leak from the valves and had good results before. Using that method I’m sure sealing/lapping was good but if that is not the best method to check sealing of valves than I have something more to learn on this but like I’ve told, worked before with good results.
In fact most of leakage came from the intake ports and exhaust
LDT was made at 7 bar /100 psi like always did before. Maybe that pressure is not enough to seal this valves?
The valves springs could hold the gasoline when checked the sealing and used gas, not water because gas is less dense than water.
Yes, cylinder hone can make the bore looser but is recomended I belive and that was the reason I checked the piston rings clearance on the cylinders before fit them on the pistons and the gap were all withing specification of SM.
I belive 200 psi pressure is not an abnormal value for these motors. I can check the values of my F100 motor as this is a “spare” power head as I bought it cheap. Even with so much leakage on LDT it can makes good pressure on CT. Strange? Maybe not.
After the rebuild, when on the water I always run the motors at diferent speeds/revs.

Something is just not like my previous experience and by theory LDT should be fine as was on previous rebuilds.
If that can be a problem? IDK!

[rodbolt17]

like I said and I will repeat myself at the risk of being cruel.
if AIR blows past the face/seat just think what hot erosive combustion gas will do rushing past at around 3000 PSI or so not to mention the face is not being cooled properly due to a lack of a good seat on the cooling surface.
so run it like you stole it then do another valve job or correct it now, break it in and run it many years.

[99yam40]

OK let me ask how these valves are adjusted?
shims maybe?
with the valve laping (any other grinding of valves or seats?)and other head work, is it possible they are no set right now

[zenoahphobic]

like I said and I will repeat myself at the risk of being cruel.
if AIR blows past the face/seat just think what hot erosive combustion gas will do rushing past at around 3000 PSI or so not to mention the face is not being cooled properly due to a lack of a good seat on the cooling surface.
so run it like you stole it then do another valve job or correct it now, break it in and run it many years.

Your message was clear, leaking valves have a greater propensity to burn damage, but to be fair valves are of greater heat resistance today than of old, and the exhaust glows red hot anyway, the valve just doesn't magically step outside the combustion chamber from the pressure and heat. On the other, an inlet valve leak in this regard will in all likelihood show up as something obvious and debilitating such as backfiring up through the inlet.
Not knowing the specifics of this engine, Yamaha has reliable log living motorcycles that go hard, where the valves and seats last "forever" and without requiring adjustment for honks. My WR250R has servicing intervals almost as far apart as the motorcycle's expected life. (Other major failures occur before valves go and for any clearance adjustments are required)!

[TownsendsFJR1300]

I gather the cam timing marks and the crankshaft are dead nuts on..

Also, did you double check the valve clearance to make sure their fully closed when they should be?

You don't need compression to close valves, valve springs do that.

As for break in, varying RPM's is critical in seating the rings.

30% at 100PSI is EXTREMLY LOW.

**Also possible (not likely), is your flywheel key is off /damaged. That would throw all the cam / crankshaft timing off and cause leak-down issues..



I did have one 10 HP B&S engine (generator) that was almost that low (but had not been run in years). After cleaning the carb, etc, got it running and later did a leak down test. It was now well over 95% with me touching NOTHING internally.. Engine ran great after a "going thru"




**A complete model and year would help us to determine how the valves are adjusted (parts look up)...



.

[rodbolt17]

typically the valve is sealed at the moment of ign.
contact between the valve face and the seat aids in cooling the face.
back when I ran a valve seat grinder and a facing machine we could play with the seat angles to raise or lower the seat. the valve not so much and the width from the valve head to the ground angle has to be 1/32" min.
but if the valve fails to seal it wont last long in operation due to the explosion is now uncontained a a jet of super heated still burning very lean very hot flame shoots past the valve.
it actually gets about as hot as an oxy/actylyne rig.