Carb Tuning - Page 3, By Eric Gorr

Eric Gorr

Engine Builder
Jun 29, 1999
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Mechanical Problems

The process of jettingÑchanging air or fuel jets in order to fine-tune engines' performanceÑis very simple. Jetting becomes complicated because mechanical problems sometimes mimic improper jetting. This causes you to waste time and money trying to correct the problem with expensive carburetor jets.

Before you ever attempt to jet a carb, make sure the engine doesn't have any of the problems in the following list. If you are in the process of jetting a carb and you are stumped with a chronic problem, use this section as a guide to enlightenment!

Crankcase air leaksÑAir leaks can occur at the cylinder base, reed valve, or the magneto seal. Air leaks make the throttle response sluggish and may produce a pinging sound. That sound occurs when the air-fuel mixture is too lean.

Crankcase oil leaksÑThe right-side crankcase seal is submerged in the transmission oil. When this seal becomes worn out, oil can leak into the crankcase. The oil is transferred up to the combustion chamber and burned with the air-fuel mixture. The oil causes the spark plug to carbon-foul. This mechanical problem makes the jetting seem to be too rich.

Coolant-system leaksÑCoolant systems leaks commonly occur at the cylinder-head gasket. When the coolant leaks into the combustion chamber, it pollutes the air-fuel mixture and causes a misfire or popping sound at the exhaust pipe. Check the engine's coolant level frequently. Hondas and Kawasakis have characteristic coolant leaks because they use steel head gaskets. Yamahas and Suzukis use O-rings to seal the head and cylinder. Coolant-system leaks lower the engine's peak horsepower. It makes the engine run as if the air-fuel mixture is too rich.

Carbon-seized exhaust valvesÑThe exhaust valves sometimes become carbon-seized in the full-open position. This mechanical problem can make the engine run flat at low rpm and make the slow-speed jetting seem lean. The carbon can be removed from the exhaust valves with oven cleaner. Clean the exhaust valves whenever you replace the piston and rings.

Blown silencerÑWhen the fiberglass packing material blows out of the silencer, excess turbulence forms in the silencer and the turbulence causes a restriction in the exhaust system. This restriction makes the engine run flat at high rpm.

Broken reed-valve petalsÑThe petals of the reed-valve can crack or shatter when the engine is revved too high. This mechanical problem makes the engine difficult to start and can also have a loss of torque. Expert rider should switch to carbon fiber reed petals because they resist breaking at high rpm. Novice riders should use dual-stage fiberglass reeds (Aktive or Boyesen). These types of reed petals provide an increase in torque.

Weak sparkÑWhen the ignition coils deteriorate, the engine performance will become erratic. Normally, the engine will develop a high-rpm misfire problem. Check the condition of the coils with a multimeter.

Clogged carburetor vent hosesÑWhen the carburetor vent hoses get clogged with dirt or pinched closed, the jetting will seem to be too lean, so the engine will run sluggish. Always check the condition of your carburetor vent hoses. Make sure there is no mud in the hoses and that the hoses are not pinched between the suspension linkage.

Carburetor float levelÑWhen the float level is too low, the jetting will seem to be too lean, so the engine performance will be sluggish. When the float level is too high, the jetting will seem to be too rich.

Worn carburetor fuel-inlet needleÑWhen the fuel-inlet needle wears out, excess fuel enters the float bowl and travels up the slow jet and into the engine. This makes the carb jetting seem to be too rich. Replace the fuel-inlet needle and seat every two years.

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