Motorcycle Repair: lacks power, stock bikes, plug caps

Question
hi bill I have a 1980 Honda cm 400t and it just doesn't seem to have the power it should 60 mph is all I can get out of it I have cleaned the carbs very well adjusted the valves and the timing chain and even replaced the small O-rings for the air mixture screws this thing is driving me nuts do u have any ideas

Answer
Lucas, as long as the intake and exhaust systems are not altered the stock jetting should be fine for your bike. If they have been changed, the main jets need to be increased in size. This is especially true now that we have alcohol fuels to contend with, as they tend to lean out the mixtures. I usually put in new larger main jets on stock bikes, as well as modified ones, now.

If jetting issues are not the problem, then probably the spark advance isn't working in the CDI module. You need to get a dynamic timing light on the engine and check the spark timing at idle and at full advance. The earlier 400T models had issues with the pulser coils and the modules, causing Honda to redesign the system later on.

This all assumes that you have adjusted the valves and the compression is high and even on both sides. You should be seeing about 175 psi.

Other possibilities are weak coil and/or bad spark plug caps which should read out 5k ohms. Make sure your spark plugs are fresh.

Bill Silver