GY6 150 failing during post-rebuild break-in By kamchatka (Read 750 times)

[kamchatka]

Hi chums,

Sorry for creating a thread for what I'm sure is a common issue, but I've done my reading and still feel like I need to consult.

Context: tore down the GY6 150 on my 2008 UM Matrix II up to the point of needing to split the crankcase, for practice and education. Put back together; no problems for first 60km of break-in.

At that point, engine began to white smoke and would not start without opening throttle/would not run under 3000RPM. Spark plug was quite wet. Air filter completely clean. Oil somewhat blacker than I would expect after a couple dozen miles of riding.

I think oil is entering the combustion chamber via piston rings or the valve stem oil seals. A reasonable guess? I've got it apart once again.

The piston rings look fine. The piston seems to fit in the cylinder OK. Goes up and down with some resistance. No scoring or scratches on the cylinder that I can see. No deformation obvious to my eye. I believe I should measure gap in the piston rings. Do I do this with:
a) rings on the piston
b) rings off the piston
c) rings off the piston but fit inside the cylinder
a combination of a), b), c)?
Will a digital micrometer from an auto parts store be sufficient to take a measure? Anything else I can do to determine if this is the issue?

I am still working on getting the valve springs off. I have no idea what I need to be looking for once I can actually see the oil seal. Will I need to take the valve guides out of the head to inspect them? Heat up the head and all that?

Thanks

[justbuggin2]

ok did you hone the cylinder before putting it back together are the rings stager and did you reuse the gaskets this info will help with what needs to be done to measure ring gap you place the rings in the cylinder one at a time square it up with the piston and use feeler gauges to fine out what the end gap is

[kamchatka]

Thanks for replying

I don't know what honing the cylinder means...

Staggering rings is same as clocking right? Arranging gaps 120degs apart? I did that.

I replaced these gaskets:
-cylinder gasket (paper one)
-chain tensioner gasket
-exhaust gasket

I reused these gaskets:
-cylinder head gasket (metal one)
-valve cover O-ring
-carburetor O-ring

[kamchatka]

Looked up cylinder honing. I Didn't even know that was a thing!

[justbuggin2]

I suggest you get a new ring set hone the cylinder and replace all the gaskets for the cylinder, head, valve cover and yes stagger the rings is the same as clocking

[alleyoop]

Wait just a minute before you go tearing into something that is not the problem. The BIG question here is the SMOKE coming out of the MUFFLER is it BLUEISH or WHITE. Big difference, if WHITE it is GAS if BLUEISH it is OIL. Very simple to test, start it up and put your hand by the muffler and if gas it will wet your hand with gas spray.
Alleyoop

[kamchatka]

Enthusiasm got the better of me and I tore into it already... I'll try to remember the hand test for gas spray in the future.

Smoke was definitely white. Not to contradict you alleyoop, I know from reading your other posts you've been in this game for awhile, but I was CONVINCED from reading manuals, forum threads, etc that BLUE was gas and WHITE was oil. How did I get that one mixed up... Spark plug was also looking pretty oily as I recall; apparently I cleaned it off between the time the problem arose and I posted about it.

[alleyoop]

Not a problem, all I do is give advice and or suggestions, the person having the problem can take it or leave it or take someone Else's advice and or suggestion.
Alleyoop

[kamchatka]

Gotcha. It's much appreciated. Can you suggest a course of action based on where I'm at?
In brief: white smoke, wet fouled spark plug, engine dying below 3000RPM
justbuggin2 suggests new rings, hone cylinder, and new gaskets... do you concur?

[alleyoop]

First thing I would do is check the valves make sure they are set right and have not tightened up, adjust them both to .004 inchs.
Then see how it Idles.
By the way what size carb is on there, also did you change out any of the jets. It should be a 24mm carb on there and if the motor is stock the Pilot jet should be a #35 and the Main jet anywhere from 103-107. The Fuel Ratio screw should be between 2-3 turns out. Start it up let it warm up at least 5 minutes before messing with the carb. Since the plug is getting wet turn the fuel ratio screw 1/4 at a time CLOCKWISE to take some fuel away. If that is the problem the idle should start to get higher and adjust for the highest idle you can get out of the Fuel Ratio Mixture screw.

If after all that the plug is still getting wet then it is sucking FUEL from some other place. So take the Vacuum line off going to the Fuel pump or petcock if wet replace the Petcock or Fuel pump.
Alleyoop

[kamchatka]

regarding carb: everything is stock
I assume carb size is inner diameter of outlet. Mine is 24.5mm
Main jet is 107
Pilot jet is 35

Have not yet put top end back together. May not get a chance to before having to leave town for work. Strongly considering new piston rings and valve oil seals. Even if they aren't defective, the cost is low and potential benefit is high. Will update as things develop.

[alleyoop]

Just a F.Y.I make sure you have the AIR FILTER on the carb and sealed up with the AIR FILTER itself. Just went through with another guy on another forum with the same kind of problem. I told him if it only stays running at 3000 rpms you must have some kind of air leak. Well after pages and many PMS I asked for pictures of the carb area and hoses because what I had told him should have fixed it right up. Well he had the Air Box cover off and no filter so it was sucking in a lot of air and of course not enough gas. First thing I had him do is adjust the valves then checking for extra fuel being delivered had him adjust the carb and nothing was working.
Alleyoop