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lta50 (two stroke) leakdown test results

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Old 09-22-2013, 09:31 PM
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Default lta50 (two stroke) leakdown test results

I've been tinkering with this bike for longer than I care to admit. It all started when the bike suddenly shut down and wouldn't restart. Needless to say, my little boy was very unhappy.

I did a compression test and it was only 55psi...that's really bad. There is no way it was running on that low of compression. I bit the bullet and pulled the head and jug. Took them to machine shop and put a new oversized piston in with new rings (since they were all original and cylinder wall looked badly scored).

Got it all back together with new gasket and proper torque and sequencing per manual, and head re-surfaced smooth...still no start. I have fuel getting to the carb, I have good spark, but I have no fuel on the plug (leads to think there is a vacuum leak somewhere). I next bought and installed a carb rebuild kit, since all internals were original and carb may have been plugged up. Inside of carb looks brand new and all jet openings are new and clear. Still no fuel to the plug.

I did another compression test, and now I'm at only 78psi with all new piston and gaskets...something still isn't right with that. I should be at 120+psi. Time to get out the bigger tools causes dad is getting pissed off now.

I stuffed a rag in the intake elbow, and another rag in the exhaust tip. I then hooked up my leak down tester and turned on the air. I sprayed some soapy water on the engine and nothing was bubbling...but I could hear air escaping from somewhere. Low and behold, the air was coming from behind the magneto. I was able to turn the crank a little bit and it stopped from that area and shifted to blowing air to the exhaust like it should.

That leads me to think I have a bad oil seal (crank seal) on the left side behind the magneto, does this sound correct? My searches indicate that particular seal can be changed without removing the engine or splitting the case.

Any idea how to remove the magneto wheel cover without buying the special $35 tool that I will never need to use again?

I look forward to replies. Thanks!
 
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Old 09-23-2013, 07:22 AM
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I agree that you may have a bad crank seal but a crank seal has nothing to do with compression.
Your comp will raise once it's broke in and the rings seat.
IMO, You have a carb problem still.
 
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Old 09-23-2013, 08:11 AM
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Originally Posted by LT80 View Post
I agree that you may have a bad crank seal but a crank seal has nothing to do with compression.
Your comp will raise once it's broke in and the rings seat.
IMO, You have a carb problem still.

Thanks, LT80. I'm really hoping that the vacuum will increase when I change out the seal. My thinking is that the increased vacuum will assist with fuel being pulled into the combustion chamber. It's the only hope I have at this point. If that doesn't do it, I guess I will be looking at a new carb. Will keep you posted.
 
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Old 09-23-2013, 05:06 PM
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I think you're on the right track.
 
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Old 11-20-2018, 06:59 PM
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what was the problem in the end, im in the same boat !
cheers
T
 
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Old 11-20-2018, 09:55 PM
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Originally Posted by Tarmop View Post
what was the problem in the end, im in the same boat !
cheers
T
I had a few issues, and they were all attributed to hardened crank seals that caused vacuum leaks and a very lean running condition. The stock piston and bore were destroyed, so I I bored the jug to the next size and tossed in a Wiseco piston and decked the head .030 to bump up the compression a tad. It ran ok for a few hours, then ruined the new bore and piston. At that point I was really annoyed. I removed the magneto and noticed the left crank seal was barely even in the hole. I replaced it with a new one and a cheapo ebay top end kit. The quad started up good after that, but then started smoking real bad and fouling plugs. I had to replace the right crank seal at that point because it was sucking transmission fluid into the crankcase and burning it. In the end...new crank seals was all it needed. Never had any problems after that. Time takes its toll on rubber seals. I think the problem gets worse for quads that sit for long periods without being used.
 
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Old 11-21-2018, 03:23 PM
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I just blew the crank seal on a 2-stroke blower. I work on a lot of 2-stroke equipment and M/Cs. With equipment, blown seals keep you from using the machine most of the time since your carb needs the pulsation to pump gas. Not so for M/Cs and ATVs. Gotta keep on top of those crank seals.

If I see a scored piston on a 2-stroke, my 1st suspicion is no or bad premix oil. Second is some kind of air leak. 3rd, bad air filtration.
 
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