QuestionIf you can help that would be great. I just bought a 1981 honda custom 400. There are appears to be a billion of these on the road. And well, it is my first bike and it has been running well. But it just developed a problem and changing the plug didn't fix it.
It is a two cylinder and when I disconnect the 'fouled' plug it runs just on one just like it had the two plugs connected. It seems to run good in high RPM's like both cylinders were working fine. But in low RPM's it runs on just the one.
When I am in neutral it runs hard and as a increase throttle it goes to about 3500 RPM's drops and sits a 3000 for a bit then rev's great up to whatever. But in low RPM's for any gear it runs on one cylinder.
Do I need to do something with the carboratuer or is something else the matter with it?
AnswerBill, These bikes have a 360 degree crankshaft, so both plugs fire at the same time every stroke. One fires compression stroke and the other one is a "wasted" spark on the end of the exhaust stroke.
Spark timing is controlled by the CDI control module and it is supposed to advance the spark at 1600 rpm and then finishes around 4500-5200 rpm.
If you put a fresh plug in place of the fouled one and it still doesn't run on that side, then more than likely it is the carburetor idle circuits being blocked. Many of these bikes have been sitting for years and the gasoline congeals into thick goo that will plug up the tiny idle jets and circuits.
One other thing to check is the spark plug cap, itself. They should have about 5k ohm resistance in them. You will have to check them with an ohm meter or you can swap the caps from side to side and see if the problem changes sides with it. That would indicate a bad plug cap, if you don't have a test meter.
A good carb cleaning and synchronization should get the bike running well again, if the problem is not electrical.
After 24 years the carbs need new o-rings and float valves, etc. There are aftermarket repair kits out there on Ebay and elsewhere.
Bill Silver
