98 gp1200 fouled rear plug twice

[gp1200]

HI all its been a while since I was on here which is good because my ski was running great. I posted this in old school also but figured you guys my of had this issue years ago.It has fouled the rear plug twice. compression is 110 110 90. What are your thoughts? Rings, gasket, carbs. I rebuilt it about 10yrs ago and never did a thing to it other the change plugs ever couple of years. its premix. Thanks in advance.

[Helsinky Smashrod]

90 is too far off. Something is wrong with that cylinder. Some of the people more experienced may have a better idea but it my be your piston rings. You might try adding some oil through the spark hole then testing compression. If it reads higher you probably need new rings. Either way your going to have to take apart that cylinder.

[birdgod]

the piston in the last cylinder is a goner. This is pretty common as the seal leak the air got in and lean condition occur. Do a leak down test and you will know. I would do that before pulling the head off.