Technical Q and A for classic motorcycle maintenance and repair.
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1973 Kawasaki H2 750 Is Running Lean
Q: I bought a 1973 Kawasaki H2 750 2-stroke triple. It was supposed to have been rebuilt, and the guy did a compression check to show me. It had about 160psi for all three cylinders. It has oil injection, and he said it was using Bel-Ray Si-7. I filled it with premium gas and put some city miles on it, and then went on the freeway. After only a few miles at 70-80mph, it lost power and smoked a lot from the left-hand cylinder. I pulled the plugs and the left-hand one was silver. The others had a light tan color. Any idea what happened and what I should do? - Richard Langley/via email
A: It sounds to me like the left carburetor was running lean. The ideal air to fuel ratio for combustion — the stoichiometric mixture — is about 14:1. Add more air to that ratio and you get a lean mixture. Running a slightly lean mixture is important for emissions and gas mileage. Running lean also results in higher combustion chamber temperatures and pre-ignition. When an engine pre-ignites, it fires before the spark is triggered for that cylinder. This is usually caused by a carbon deposit in the cylinder head glowing hot enough to act as the spark plug. Pre-ignition and detonation are particularly hard on engines because the explosive pressure of the burning mixture is meeting the piston before it reaches top dead center. You can hear this as knocking, sometimes characterized as the sound of marbles rattling in a can. Running a richer mixture can help tame pre-ignition via the cooling effect of the gasoline as it vaporizes in the cylinder, drawing off heat.
Silver on a spark plug usually means trouble, in the form of melted alloy from the piston being deposited on the ceramic insulator. The worst case scenario is you’ve melted a hole through the piston crown and spattered hot metal into the crankcase. If you’re lucky, you’ve only melted a bit of the crown and not gone all the way through. You should repeat the compression test. If you find reduced compression, you’ll have to pull the head off to see how much damage there is. If you’ve melted the piston clear through, you’ll have to completely rebuild the engine to ensure no metal debris finds its way into the engine bearings. If you’ve only started to melt a piston you’ll possibly need to replace that piston. You’ll definitely need to find the cause of the problem, starting with the easiest solution and moving to more complex solutions. Be certain the left carburetor is mounted correctly, as air leaks will make a cylinder run lean. Make sure the left carburetor jets are the correct size and they are clean. Check the float bowl level to be certain it holds enough gas. Once you’ve done that, move on to timing. Your Kawasaki H2 750 should have a CDI electronic ignition. Those seldom go out of time, so I’m more inclined to focus on carburetion. Still, check the timing. MC