91 VXR 650 Engine Problem

[JutMiller]

Hello.

I apologize for the lengthy post. I just want all the information out there for the best result.

Problem: On cylinder runs rich, the other runs lean.

I recently bought a 1991 VXR 650. I paid 50 dollars for it. That should give you an idea of its original condition. The ski was missing some parts and components which I bought used and replaced (EX. Exhaust, Fuel Filter, Hood Latch). The original owner thought the engine was locked up. After I started to inspect the engine, I found the starter bendix to be bad and replaced. The engine turned over fine, spark plugs ignited well. I cleaned the stator and flywheel before reassembly, (I Did Not Remove The Stator) with non-conductive parts cleaner. I then continued to remove the head and jug to inspect/clean the top end of the motor. The pistons looked fairly new, no pitting or signs of lean/rich conditions. The crank shaft appeared to be ok. There was an oily water mixture in the crankcase which I vacuumed out and the cleaned with parts cleaner. Before reassembly I brushed 2 stroke oil on the bearings and other areas of movement. I removed the old gaskets and replaced with new.

At this point I poured a bit of premix down the carb and tried to start the engine. The engine did run and would rev up nicely and idle until the fuel was used up. I then removed the oil injector pump along with tank and converted over to a 50:1 premix setup. I removed the fuel tank cleaned/drained and put 5 gallons of premix in. I removed the carburetor cleaned all internal parts with cleaner, inspected and put back together. I replaced all fuel lines and filters as well. The ski would start, rev and idle with no problems. *Note* these test where done out of the water and the engine never ran for more than seconds. I performed a compression test and each cylinder showed 142 psi and held the pressure.

Friday came along and I loaded the ski up and headed to the lake to see what the result would be. The ski started, revved and idled great while in the water. I quickly checked if the cooling system was functioning correctly and it was working flawlessly. I took the ski out for a ride. Once warm I started to have some fun, full throttle turns/spins. I would guess I hit 40 mph or so. The ski ran great for about 30 minutes, then Baaaahhhhh... lost power. Would somewhat idle and would not fully rev up with throttle fully pulled. I limped to shore and changed the spark plugs. The ski started right back up and ran great once again. I decided to run it for about a minute at full throttle then kill the engine to check fuel mixture. *Note* this is where my problem is. The rear cylinder plug is black carbon coated. The front cylinder is almost new looking, with the electrode dusted with white buildup. Rich and lean running cylinders.

This ski runs a single carb, so in theory both cylinders should reflect the same changes to the carb settings. I inspected the intake reeds and found 2 flaps on the lean cylinder side slightly open. I bent them back by hand reassembled and then tried again with new plugs. No change. I then switched the reeds to opposite cylinders, changed plugs. No change. It seems even if I richen the low and high speed needles of the carb, the only effect it has is it speeds up the time it takes to fowl the rear spark plug, the front never changes (remains lean). The other item that confuses me, the ski seems to run great until the rear plug fowls. At this rate the lean cylinder's days are numbered.

What is going on? Any help would greatly be appreciated. I am hoping someone has seen this problem. I will provide any other information that I can.

Thanks for the help!