Motorcycle Repair: engine, air fuel mixture, piston crowns

Question
 i was having performance problems on my 95 suzuki rf900r. i thought it may have been a carburator problem because it seemed like it wasnt getting enough fuel. i had a spare set of carbs around so installed them and changed the spark plugs. when i fired the bike it was like starting a rocket. i was suprised at the power i had. the problem is the bike ran fine for about 30 miles then started acting up. it seemes once again like i wasnt getting enough fuel. i pulled the carbs cleaned them and checked for any build up. all was good. fuel pump also seems to be fine. on a whim i pulled the new plugs.i was suprised they looked burnt. when i shined a flashlight down inside the plug sleeve to look at the top of the piston i saw what looked to be black burnt oil. i stuck a long q tip in and gently rubbed the top of the cylinder to see what it was. it looks and feels like black dirty oil. also the bike it puffing black smoke when revved high rpms. i could pdrobably explain it better on the phone. if that is possible email me a number and ill call you on my dime/.

thanks
don
thecrow_32724@yahoo.com

Answer
Hi Don,

Valve clearances and cylinder compression must be within service limits before the carburetors will work properly.

Check valve clearances and cylinder compression. Follow-up with a cylinder leak-down test. These tests will determine the sealing properties of the combustion chamber (condition of the engine). If oil is entering the cylinders, a leak-down test will find the source; piston rings or valve guide seals.

Check fuel tank, petcock, hoses, and other fule related components for contamination that may restrict fuel flow.

Check air filter and air passageways for restrictions.

The black coating on the piston crowns is likely carbon combustion deposits from an overly rich air-fuel mixture. There may be a bit of oil included.

The black smoke indicates an excessively rich air-fuel mixture, not oil burning. The carbs may need tuned to lean the A/F mix.

Respectfully,
Mark Shively