How do you do a leak-down test?

[pastorbud]

I kinda been holding off on the torch. One broken bolt is right beside the thermostat, and I’d like to not have to replace that. Not because of thermostat is expensive, but because it is held in with 1/4-20 screws that will likely break off when I try to replace it.

 

[pastorbud]

Tried to remove the four screws holding the thermostat. Two came and two shredded the (straight slot) heads. I’m gonna have to torch it and deal with the consequences.

 

[Bluestream]

When you have a cylinder at 30 psi. What will a leak down test tell you that the compression test didn't. And compression tests are FAR EASIER than a leakdown test.

Okay where did anyone say 30 PSI? if it were that low, a finger blow out the plug hole is all that's needed. Leak down will give you exact condition of the piston/rings and cylinder wall at TDC. If one jug is toast you can read the others to see what condition they are in. My old 9.9 Merc had 4% leak down and that's good for an old engine. Compression showed 100PSI, but the engine has exhaust relief posts for easy pull starting and that distorts the compression readings. Also if your reeds are stuck open a compression test will read low, as the fuel/air charge is pushed back out the intake instead of up into combustion chamber. Worn seals can also lower the readings. Many high performance two-stroke's with high/raised exhaust ports will read low on a compression gauge at hand cranking speed, but a leak down test gives the real condition.

In the OPs case if its more than a head gasket, I doubt that 39 year old engine is worth spending any time and money on given its age. Parts, (if you can find them ) labor, and machine shop costs make in an expensive venture. Ask your local marine guy about that. But it's your call...

 

[pastorbud]

Not trying to pick a fight here, but I mentioned the 30 PSI in post #11 above, which I think you actually quoted in your first response, at #13 above. It was buried at the bottom.

I waste time on old beat up stuff all the time. After all, I’m a Force owner! (And FWIW, a pastor and a prison chaplain. I might have a soft spot for “lost causes.)

I appreciate all the input. Every little bit helps.

 

[flyingscott]

Okay where did anyone say 30 PSI? if it were that low, a finger blow out the plug hole is all that's needed. Leak down will give you exact condition of the piston/rings and cylinder wall at TDC. If one jug is toast you can read the others to see what condition they are in. My old 9.9 Merc had 4% leak down and that's good for an old engine. Compression showed 100PSI, but the engine has exhaust relief posts for easy pull starting and that distorts the compression readings. Also if your reeds are stuck open a compression test will read low, as the fuel/air charge is pushed back out the intake instead of up into combustion chamber. Worn seals can also lower the readings. Many high performance two-stroke's with high/raised exhaust ports will read low on a compression gauge at hand cranking speed, but a leak down test gives the real condition.

In the OPs case if its more than a head gasket, I doubt that 39 year old engine is worth spending any time and money on given its age. Parts, (if you can find them ) labor, and machine shop costs make in an expensive venture. Ask your local marine guy about that. But it's your call...

Huh? A lot wrong here might want to do a google search for how a 2 strk works.

 

[racerone]

Bluestream-----The reed valves have nothing to do with a low value during a compression test !

 

[Bluestream]

Bluestream-----The reed valves have nothing to do with a low value during a compression test !

Yeah, they do. They seal the crankcase when the piston goes down and forces the fuel air charge into combustion chamber. (They arct like intake valves on a 4 stroke.) Without crankcase pressure integrity when the reeds seal, the fuel/air charge will be blow out the reeds/intake and result and result in low compression due to insufficient volume of fuel/air in chamber.

 

[Bluestream]

Huh? A lot wrong here might want to do a google search for how a 2 strk works.

Like what?

 

[Bluestream]

Bluestream-----The reed valves have nothing to do with a low value during a compression test !

Please watch this short video and educate yourself on red valves

https://www.youtube.com/watch?v=FohhtYJpQ7Q

 

[flyingscott]

Reed valves or leaks in the crankcase will not effect compression? Here is why the piston does not care where it gets it's air from. All it does is compress what it gets. Reed valves only keep the fuel/air charge in the crankcase. Sleds are probably some of the higher performance 2 strks you will have and I never had one read low compression. A 2 strk Does not need reeds to function ever hear of a piston port motor.
https://www.youtube.com/watch?v=xNLE8G3pC0k

 

[racerone]

Sorry.------You can throw the reed valves on the shop floor and still do a compression test on an outboard motor !!

 

[jerryjerry05]

Reeds have nothing to do with the comp readings, unless they broken apart and the screw
ended up in the cyl. and poked a hole in the piston.

The removal of the 2 studs should be fairly easy now.
The corrosion is what kept them locked in place.

A propane torch is all the heat you need to remove the studs and the screws on the thermo cover.

I worked about 3 hours on getting 1 thermo screw out.
Heat, WD, PB or Krol spray tap with a drift, repeat.
Then a small pair of vice grips.

You might have to cut the screw head off, remove and then attempt the procedure.
There are a LOT of thermo covers with only 3 screws holding them in place.

 

[pastorbud]

Got the head up about 1/16” up the stub with s torch pry bars, pickle fork (careful there, keep it away from the mating surfaces!) and dead blow hammer. “By perseverance, the snail reached the ark.”

​​​​​​—C. H. Spurgeon

 

[Jiggz]

Yeah, they do. They seal the crankcase when the piston goes down and forces the fuel air charge into combustion chamber. (They arct like intake valves on a 4 stroke.) Without crankcase pressure integrity when the reeds seal, the fuel/air charge will be blow out the reeds/intake and result and result in low compression due to insufficient volume of fuel/air in chamber.

Absolutely not true. Although, you were right to state that positive pressure at the crankcase cavity (plenum) forces the fresh fuel-oil-air mixture into the cylinder (while forcing exhaust gases to the exhaust ports) with the piston coming down, atmospheric pressure will equalize as soon as the piston hits bottom dead center. Reason is because both intake and exhaust ports will be exposed. However, since the fresh fuel oil air mixture is already at the top of the cylinder (caused by the piston crown deflector), combustion can take place again when the piston travels upward to compress the mixture.

This is the very reason why when you have a broken reed petal the compression numbers would not change but will still have a misfire because the fresh fuel oil air mixture instead of being forced to the top of the cylinder exits back to the carb indicated by fuel being spitted out. But atmospheric pressure (exhaust gases) instead will fill the top cylinder. Hence, the same volume of air except it's not fresh fuel-oil-air mixture but exhaust gases. Thus misfiring but compression numbers stays the same.

 

[jerryjerry05]

When I rebuild, I always do a comp test after I get the port covers and head installed.
That's without the reeds and intake installed.
Then after it's run I do another test and have never found any difference.

 

[racerone]

You-tube videos can be entertaining.-----Glad I did not use them to learn about outboard motor basics !!

 

[pastorbud]

[No message]

 

[pastorbud]

I got the head off today. More pics later (head, cylinder walls, etc.), but you can see the gasket failure on #4, just shy of 12 o’clock.

 

[pastorbud]

I may have bent gasket at #3 taking it off, but the #3 cylinder looks okay. More pics tomorrow.

 

[pastorbud]

#1 cylinder