Please help with my 300ex thanks

[Steven3472]

I started with what i thought was carb problems i completly rebuilt carb and still had same problems. I then checked compression and only had 85PSI so i took the head off and it was gunked full of carbon so i removed the carbon re-assembled and then rechecked to only get 35PSI.

Attached are pics of what marks i found on head and piston. the piston is the stock 74mm

[Here 2 help]

Well, I usually do my diagnosis with the engine assembled. First question is do you know what caused the damage to the head and piston? Second question is, why is the one valve open with the head off?

[Steven3472]

ok the valve was open cause i took off the springs but i since re-assembled spings, put the plug in and put fluid and its been 45 minutes and nothing has leaked out, also i dont know what caused the damage as there isnt any other damage to anything else, its also the stock piston

[Here 2 help]

You should be checking compression with the throttle held wide open or the carb off. If not, the engine will draw vaccuum on the intake and you will get a low reading, or you will have to crank for an extended period to get an accurate reading.

If you have determined that the compression is actually low then you need to find out where it is going. Lock the crankshaft from turning with the piston at TDC (a penney between the clutch gear and the crank pinion gear is a good way) and pressurize the cylinder (a rubber tipped blow gun on a compressed air hose is how I do it). You should then be able to determine if the combustion chamber is holding pressure or if it is getting past the valves or the rings. Once you know where the pressure is going, then just figure out why.

[Steven3472]

Ok I checked the top no leaks so off to the stealership to see how the jug and piston measure up