QuestionCB750 1991, only #1,#2 cylinders are firing.coils within specs, wiring within specs, fuel ok, did all the normal trouible shooting,ICM ok, battery ok, I'm lost, help!
AnswerPaul, start with the basics.... COMPRESSION, SPARK, FUEL...
Compression should be about 175psi. You can "listen" to the motor, by turning the Kill Switch to OFF and then spinning the engine over with the starter. Listen for an even cadence. If you hear it speeding up on one or two cycles, then the compression is low on those cylinders coming up on the compression stroke.
1-2 firing only is odd, as the coils are 1-4 and 2-3. Coils are double ended so fire both plugs at the same time, when the coils are triggered. Put NEW spark plugs in the plug caps, lay them on the cylinder head and spin the motor over with Kill Switch ON, and check for good spark on all plugs.
Drain all the carbs, to check for water or contamination in the bottoms of the float bowls. Catch the drained fuel for inspection in a small container. If the bike has a vacuum petcock, then refilling the carbs may take some time, unless you can apply an external vacuum source to open the petcock diaphragm to help reload the carbs.
Check all the plug caps for matching resistance values. Should be around 5k ohms.
Note that vacuum petcocks can cause fuel restriction and air leaks when the diaphragm ruptures.
Bill Silver
