Low compression in 1 cylinder

[oldcobia]

I have a 2001 LX250TXRZ with 120psi on al cylinders except bottom on port side which is 65psi. Motor runs fine. I have never done a leak down test on a 2 stroke. I assume piston needs to be at TDC for the test. Should all other plugs be left in except adjacent plug and how much air pressure should I use? Hopefully I have a bad head gasket and do not need a rebuild.

[99yam40]

I would bore scope to look at top of piston and cylinder wall, or just pull the head to see
if you do get a leak down tester it will have info on PSI of air.

make sure you do not let the device you use to hold flywheel from moving hit you when you put pressure in cylinder

[oldcobia]

Is that advise from personal experience? I could see me doing that. I was thinking if I left most of the plugs in and did not use too much pressure motor would not rotate. I am not looking for a percent of leak down, just hoping I can determine if it is a bad head gasket.

[rodbolt17]

I would just pull the head. you already know something is pooched.

[99yam40]

No not from what I have done, but have read about people getting hit when it moves
how do you plan on putting air into the cylinder?
I guess you have the compression tester with a quick connect you can rig up to air hose.
put some low amount of pressure and listen for the leaks
raise as need

[rodbolt17]

why bother? it is a two smoke. no valves involved with cyl sealing. pull the head,use the peepers and the tool twixt the ears.

[TownsendsFJR1300]

Is that advise from personal experience? I could see me doing that. I was thinking if I left most of the plugs in and did not use too much pressure motor would not rotate. I am not looking for a percent of leak down, just hoping I can determine if it is a bad head gasket.

100 PSI is the general rule. You have to lock up the flywheel on TDC on the bad cylinder OR it will spin over.

Could be a head gasket or a leak in the head (corrosion) itself.

https://www.boats.net/catalog/yamaha...er-crankcase-2

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[oldcobia]

That was my plan. Hope it's the gasket and do not need a rebuild. Thanks

[TownsendsFJR1300]

Please post back what you find... Good luck..

[99yam40]

if this is related to your other thread, did you find a fuel pressure problem causing the drop in RPM?

could be piston went south if running lean

[oldcobia]

Different motor. Found that one with rpm loss had bad connection in harness. Losing power To H.P. pump. The one with low psi is port motor. Pulled head today and found some scoring on wall and small piece piston missing. Getting a rebuild I guess.

[TownsendsFJR1300]

Not good news but it gave ya 17 years service....

Hopefully not too much corrosion inside to re-build

[oldcobia]

It gave 17 years but I’ve only had it 5. Was told it was rebuilt just before I purchased by shop in Wilmington. Have only put about 100 hours on it since then