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Discussion Starter #1
Hi guys, I'm pretty new to atv / car repair, and I have a very troublesome 1986 TRX125 and I've been trying to get it to run for a while now (installed new stator, pulse generator, etc). After a lot of different fix attempts, I finally decided to do a compression test and a leak down test. It failed the compression test (it seemed to only barely get off the 0 and not even up to 20 or anything), and when I ran a leak down test at top center, the air is flowing right through and out the exhaust pipe. When it's not at top center, the air runs out the intake in the carburetor. Is this a total rebuild job? Is the exhaust valve messed up? Would I be better off buying a new top end on ebay for $100, or try to fix this one? Ive never opened up an engine outside of auto-shop back in high school haha. Thanks! Reid
 

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To do a leak down test, you have to lock the piston at or very near TDC - at the bottom of the stroke one of the two valves is open slightly - the only time both valves are completely closed is from approximately 35 degrees after BDC of the compression stroke to about 35 degrees after TDC of the combustion stroke.

A typical 4 stroke engine begins to open the intake valve 10 degrees before TDC - the piston passes TDC and sucks in fuel/air and the intake valve closes at about about 35 degrees after BDC - both valves are closed until the exhaust valve opens at about 35 degrees before BDC - the exhaust valve stays open until about 10 degrees after TDC of the exhaust stroke - both valves are open from 10 degrees before TDC to 10 degrees after TDC between the exhaust and intake strokes - this portion of the cycle is referred to as valve overlap - the exhaust valve is closing as the intake valve is opening - so both valves are closed from about 35 degrees before TDC to 35 degrees after TDC. The quoted figures are text book, every engine will have different timings depending on it's design. Different valve timings (and lift) will affect power, economy, torque, RPM limits, smoothness, octane requirement and valve life. Closing a valve too quickly will shorten the life of the valve and can cause bounce at high RPM. Opening a valve too much may fatigue the spring leading to broken or weakened springs. Not opening a valve sufficiently will limit power and speed. Using just valve timing and lift, an engineer may design a gas engine to run at a maximum speed regardless of throttle opening - this is used to design certain 50cc scooters with a maximum top speed of 30 MPH regardless of throttle opening or carb size.
 

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Discussion Starter #3 (Edited)
Yeah I think I was doing the leak down test at the wrong place. It was on the wrong stroke it seems (on the exhaust stroke). I tried letting the piston go around one more full rotation and now it's leaking out the carb intake. But maybe it shouldn't be right on the "T" - but a little before it where it's really hard to turn? - Reid
 

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Best place to have it is right on TDC and lock the crank - remove the alternator cover and use a wrench on the flywheel nut - lodge the wrench under a foot peg with the piston just past TDC when rotating the crank counterclockwise, then apply pressure to the spark plug hole
 

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Discussion Starter #5
Yeah, that's what I've been doing. It's leaking either out the carb or the exhaust. There is no point where it doesn't leak. Does that mean a rebuild?
 

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If you haven't , adjust the valves - if it still has no compression after the valve adjust then it probably needs the head serviced at the least - iyou might find out during the disassembly the cam chain needs replaced and perhaps the cam jumped time. Before you start tearing it down, make a list of parts you think you might need and find out what parts are available and what parts are not available - it may be the difference between fixing and replacing.
 

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Discussion Starter #7
I adjusted the exhaust valve, but the other valve's cover is stripped and I'm having a hard time getting the cover off :(
But I'll try that first. Thanks for the advice. I'll keep you posted.
 

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Discussion Starter #8
Turns out the valves were both a little bent. I took them and cleaned them and it still leaked really bad. So I bought some new valves and installed them and it finally fired right up!
 
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