Author Topic: 2 Stroke Leak Testing Courtesy of Nutz4sand  (Read 6844 times)

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Online fabr

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2 Stroke Leak Testing Courtesy of Nutz4sand
« on: June 17, 2010, 08:37:43 PM »
Here yah go. This is part of a teardown and rebuild on any two stroke. It SHOULD be done before you take the worn motor apart and after you got he new motor together to check the work. Anyone who tells you its not needed is gambling.

NEVER ASSUME new seals have sealed and new gaskets are perfect. Often far from it. Especially with stuff being done cheaper and China crap being substituted in!

Any two stroke has seals in the crankcase. If these go bad you usually lean out the motor and fry a piston and can ruin a good cylinder along with that.
To pressure test a two stroke should be done BEFORE you tear it apart for a rebuild.

(If it fails then a top end rebuild alone will fail due to the bottom end is leaking. If the motor fails a crank pressure test and the pistons is bad the bottom end is usually what caused it.)

AFTER you rebuild it to check the seals (and gaskets) you just worked with. And at regular intervals (along with piston compression checks) to monitor your engines internal life.

To do a pressure check you need to remove the pipe and carb and go to a hardware store and get some stuff from the plumbing department to seal both holes. Rubber caps and a hose clamp usually work good on most exhaust pipe snouts and PVC pipe caps can normally be found easy that will fit in the carb boot. You then either need to make or get a fitting that allows you to put an air hose to either one of the caps or into the spark plug hole.

Basically you need to have a way to feed air into or out of the motor.

If you cannot find a way to use the sparkplug hole with a fitting then drill a hole in the PVC pipe cap thats in the carb manifold (Take it out of the manifold first if its already in there! I hope I didn't need to tell you that but just in case! I have seen people drill things where crap from the drill falls into the works. Not good). (My Honda Pilot had a fuel pump hose fitting on the side of the cylinder (some motors have these on the crankcase) for the pulse to make the fuel pump pump fuel. If you have that fitting you can use it (on a single cylinder. In a twin it can only be used for the cylinder its under just so you know)

Once you KNOW the pipe and carb holes are sealed tight (The spark plug too if you have you air fitting there. If the plugs in it should be fine.) make sure the piston is down in the bottom of the cylinder (You don't want it blocking the ports. The very bottom is good. Just clearing the cylinder ports will be fine.) Then you need a vacuum gauge/hand pump ($20 to $30 bucks at any auto store for the hand pump version.) Put about 6 inches of vacuum to the crankcase. If it drops to 4 inches of vacuum in under three minutes the bottom end seals are bad or at least weak. Some say less than an inch of vacuum a minute is fine but as you regularly test your motor if you see it loosing its negative pressure faster then you will know the seals are getting weaker and its a lot cheaper to fix it before it toasts everything. (This requires you to make sure the carb and pipe plugs you made are good so you are only checking motor gaskets/seals.)

Thats a negative pressure test. You can go up a little higher in vacuum pressure. The motor should be able to hold some for over five minutes or the seals are toast. One inch of vacuum a minute lost is starting to get bad.

Many also do a positive pressure test. Same deal as above. I like them better as its easier to see soap bubbles and ID the leak. Seal the motor and get a hose connected with the piston at the bottom of the cylinder like before. In the NEG test above the pump stays in the line and holds the vacuum. For the POS test you will want a little on/off valve in the line. Then once you put pressure to it you can close this valve to hold the pressure in. As long as you have a setup that holds the pressure in at the hose your fine.

HERES THE CATCH. If you put to much POSITIVE pressure to the crack case you can literally Blow the seals out! NEVER JUST PUT A AIRCOMPRESSOR HOSE TO THE SETUP YOU MADE TO PRESSURE TEST IT AND "THINK" YOU CAN HIT IT WITH A LITTLE AIR AND TEST IT THAT WAY. You make one little mistake and its gonna need to be taken apart and repaired. So do this instead. Get one of those small portable air tanks. Put about ten PSI in it with the compressor. Then bleed it down to 6 PSI or maybe 5. This way you cannot put more than that to the crankcase. Better safe than sorry. Put the 5 or 6 PSI to the crankcase and seal the valve. It should not leak more than a pound a minute. Less is better. More is questionable. I will not settle for more than 1 pound every two minutes one my stuff.

You can get a gauge at the auto parts store that reads both POS and NEG on the same gauge. (They are NOT high pressure they are more for car manifold tests) This gauge can do both tests for you. I have in the past used these (I still have mine and use it) AND used a HUGE syringe to provide both the vacuum and the positive pressure. It sometimes takes a few pumps to get the pressures I wished but it can be done.

If that did not make sense GOOGLE pressure testing a crankcase and you will get some more insight. I also test the entire motor and all the seals on my test equipment each time. I have had leaks that formed on the tube joints of my test equipment. The soapy water bubbles led me right to it. I check every seal and gasket line and even around bolt holes.

I HAVE actually discovered air leaking out a cylinder hold down nut like this due to a porous casting flaw from the intake port near the stud the to the engine. I had to go back in and seal it to stop the leak.

Doing this test while you can still see both sides of the main crank seals is best. Easiest to get to to fix too if need be. Cover the entire outside of the motor with soapy water. I have had slow leaks that make the tiniest bubbles that I could not see/locate till nearly 3 minutes into the test. But then noticed.

I won't install a motor that does not pass this test.
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Offline Buggybaz

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Re: 2 Stroke Leak Testing Courtesy of Nutz4sand
« Reply #1 on: April 29, 2011, 07:30:07 AM »
That's a great read.
Can I just add this tho,on the off chance if someone has an early 90's Arctic Cat 550 for test.
You CAN check both cylinders,the whole engine at once,due there being a Labyrinth seal in the crank center.
Just imported a Pilot with AC550 conversion and 6" rear chassis extension,
I recently checked mine and discovered this Labyrinth crank seal....ALSO discovered a major air leak at the PTO side carb boot.
The previous seller had over $2,000 total engine work by so-called pros.
All the jetting was way higher/bigger than need be,was obviously to bandaid/mask/compensate for the undetected leak.
I can't believe a shop would'nt/did'nt do the test before dissasembly and after assembly.
You can also make a hand bellows pump from one of those gas lift chairs.