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Leak down test on 2 strokes

N

newyota

Regular Contributor
Is it possible to do a test with power head still on leg and cowling.I realize blocking off the exhaust might be hard.I have been looking for info on this and only saw 4 strokes being tested.
 
The piston has to be at TDC to close the ports off no need to plug the exhaust or intake. You also need a long breaker bar to put on the flywheel to hold the piston in place while you are doing the test. On a 2 strk it's the absolute last test to be done and is 99% of the time unnecessary. What problem are you having with the motor. The motor can be left together to do the test.
 
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flyingscott said:
The piston has to be at TDC to close the ports off no need to plug the exhaust or intake. You also need a long breaker bar to put on the flywheel to hold the piston in place while you are doing the test. On a 2 strk it's the absolute last test to be done and is 99% of the time unnecessary. What problem are you having with the motor. The motor can be left together to do the test.
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How do you check for leaky crank seals with the ports closed by the piston?
 
To do a correct 2 stroke leak down test one needs to block intake and exhaust. Dirt bikes easy...outboard not that easy with the powerhead installed. And if one has to pull powerhead might as well to just change seals.

Usually a leakdown test on a 4 stroke is to see of the low compression is from rings or valves.
 
newyota said:
How do you check for leaky crank seals with the ports closed by the piston?
Click to expand...

You asked how to do a leak down test that is how you do it ,a leak down test is done on the pistons. A crankcase pressure test is something else entirely. Like what was said before if you have to do a crankcase pressure test replace the seals and be done with it.
 
ok-thanks.Guess I had heard people refer to the pressure test also as a leak down test, thus my confusion. I appreciate all the info.Glad I asked and got some good feed back.
 
I'll bite...

Did the last one back in 81' on a can am 175. Block off intake and exhaust, plug installed. If fuel pumped one can use that port for charging. Or one needs to put air valve on either blocking cover. Charge with 10psi. Shouldn't bleed down faster than 1psi per minute. Use bubble solution on seals if accessible.

The figure, 40 years ago, used to be 10% bleed down and no more. I figured that ment 1 psi /minute.
 
Boobie said:
Sunk, we used to do this same test on snowmobiles years ago when we had idle probs.
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BINGO! Hearth, Sacks, Rotax, Tohatsu, yea...used to stink gas as the motors were in our lap. Add a lowered piston port and it would spit back bad at low rpms. So we would raise clutch engagement to lower spit by grinding the clutch weights...I still have my snowmobile suit with hole where the carb would flame back sometimes...back in 1975 or so...memories.
 
My memories go back to 1966. I liked the '66 Evinrude with the 14 HP opposed firing motor. I threw my snowmobile suit away when I moved to FL.:cool:
 
OK, so on an Evinrude two stroke, you would have to fabricate a plate to bolt to the intake where the carb would normally bolt. The plate would have an air fitting suitable for hooking up the leakdown tester.
Then putting the piston in BDC, you block off exhaust port from the crankcase.
But, since piston is at BDC, the intake ports/bypass passages are open to crankcase. The air pressure would bypass the piston and go out the open exhaust port that is above the dome of the piston in the cyl wall.

How to block off the required passages?
 
You asked about a 2 stroke...not a 2 stroke ob. I knew where you were going so I didn't go there.. cause if one needs to pull block to check crank seal integrity with a test just replace the seals and be done with it.

I could do one on my 15hp small block if I could block off the water exit along with a big plate to seal off exhaust..the water exit is in the exhaust chest..

The piston position is irrelevant as one is just testing the whole assembly at once for air leaks. One can commonly block off both exhausts..they can be connected for the test. Need the plugs in also. One wants the air pressure to equalize through out the motor to find all leaks to atmosphere.

Come to think of it I just need a plate the shape of the tuner flange...water exit is on the outside..
 
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Hmmm...I wonder how block re manufactures test their stuff without a running test...I really don't know but I suppose they could block off, charge with a detectable gas and look for gas with a spectrometer of sorts on the outside to look for leaks. At least thats what i'd do..
 
The piston skirts are what create the pulse for crankcase pressure. Pull the exhaust cover and inspect the pistons for scoring. I suppose you could rig a vacuum/pressure tester with a check valve and test crankcase pressure similar to a DVA adaptor it would measure peak pulse pressure in the crankcase. It is low pressure high volume normally if the crankcase is leaking out the seals you have oil/fuel present hard to see with lower seal/s. The reeds can also effect crankcase pressure, if out of spec you will normally see fuel spitting back out the carbs when running. A leakdown on a two stroke is a waste of time in my opinion. If it has good compression the skirts are good then it is a matter of seals or reeds IMO. Very rarely do the main bearings cause crankcase pressure issues between chambers in the crankcase, only when the engine was run low on oil and the bearings got cooked.
 
If it has good compression the skirts are good
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That's wrong as a engine can have good compression and be bad. A compression test only tells if a cylinder can support the combustion process, it tells nothing about piston condition. You can have the whole side of a piston scuff and still have good compression....Example is the 71/72 50hp Johnrude, the skirts wear out and wont idle or troll but will have 125-135 psi compression test
 
Do all omc's pistons thrust on the exhaust side or do some thrust on the intake side like Merc's?

2 stroke motors that have trannys or other complications are better candidates for crankcase leak tests. A lot of times the old dirt bikes had seals that one had to split the tranny case to change seals. I'd rather test those first before splitting the case. Outboards different animal..
 
I guess a real possibility for crankcase pressure testing is to hook up a vac/pressure gauge to the intake on the crank side of the reeds.
You would have to modify the intake by drilling and tapping a suitable port. The hole could then be plugged with a pipe plug when finished.

The pressures could be monitored when running and cyls compared. Even if you do that, I am not sure what GOOD it would do you.....
If it had any value, I would suspect OMC would have gone ahead and manufactured the engines with the appropriate port already built in.

But that said, I have always wanted to have a way of checking the crankcase sealing rings on the crank.....
 
Exact same thing except you need to pull the powerhead to confirm the lower crankcase seal is leaking. The upper seal you just add some air pressure and soap test. As far as main bearings or journals there is no way to test you need to split the crankcase and do a visual.
 
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Well you can test in some cases, but I am not sold yet on the reliability or value of this test yet. Seems logical for it to work:

Run engine, and using adjacent cyls, you pull the plug wire off one of the two cyls. The other cyl is running as normal.
Then spray some kind of extremely combustible spray...like starting fluid that is safe for 2 strokes (oil added)....or ether....something hotter than gas
into the carb of the cyl that has no ignition.


If the engine picks up, then that starting fluid / ether/ whatever is leaking past the crankshaft sealing rings.

Swap the setup, and do the opposite cyls back the other way for verification.

Valid?
 
The only issue with checking between cylinders is segregating each cylinder. If one carb per cylinder one can come up with a test like yours but if common intake manifold that complicates things.

There is a net pressure in the crankcase so the test fluid might just blow back with some fuel from the test cylinder. I never really knew if a lean cylinder caused by a leaking seal was from air entering or from a reduced signal at the venturi of the carb from reduced crankcase pumping efficiency. Sometimes a worn seal will act like a check valve, letting in air but back sealing upon pressure. Not sure if sealing rings will check or not...
 
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If on a v6, you could equate pressure to the position of the pistons....you would see that when #1 is approaching BDC, the pressure is building in the crankcase, until the intake ports are bypassed.
Think about the next cyl to fire...#2. That cyl will be 60* behind #1 with regards to crankcase pressures.
So there will be a positive pressure in #2 when #1 is expelling gas and oil into the cylinder.

I wonder if there would be enough pressure would force fuel thru from #2 to #1?
 
In the past 40+ years working on engines at /for dealers I have only seen about ten V-4/6 motors with crankcase seal ring problems. These problems are most noticeable at 2000rpm and below. The 3 cyl Johnrude that use labyrinth seals in block are bad about getting hammered and cylinders "cross talking".
 
daselbee said:
If on a v6, you could equate pressure to the position of the pistons....you would see that when #1 is approaching BDC, the pressure is building in the crankcase, until the intake ports are bypassed.
Think about the next cyl to fire...#2. That cyl will be 60* behind #1 with regards to crankcase pressures.
So there will be a positive pressure in #2 when #1 is expelling gas and oil into the cylinder.

I wonder if there would be enough pressure would force fuel thru from #2 to #1?
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Well..lets say one can measure all this...How much is too much or too little? One would have to take a perfect motor, create a known leak, retest for output, measure and plot. Create more leak, test and plot. We then might be able to take our test results from the reference motor and results of the test motor to compare. Kinda like looking for a piece of fly poop in a pound of pepper..
 
^^^^^
That's what I am talking about. Not much benefit, if at all. NO reference pressure avail.
If mfg thought it was important to diagnose, they would have engineered in a way.
I did have a 25hp 3 cyl ...that weird one....that would not idle. Had to advance the timing to like 6BTDC before it even tried to idle.
Turned out to be way way worn out. It wasn't generating a legit crankcase pressure to idle.
 
I am glad I asked about this subject.I had seen where someone had a older Johnson like mine and claimed they had a bad lower crank seal causing issues and could not figure how they found a bad lower crank seal and no explanation was givein.It was/is an eduction on the subject.thanks.
 
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