Motorcycle Repair: 1980 Honda CB 900 custom dies while running, honda cb 900, honda cb 900 custom

Question
Hi, I know your specialty is not in cruisers, but I thought I would ask for your help still. I have a 1980 Honda CB 900 Custom. When I have been recently riding, the bike will all of a sudden lose power and start to die. Sometimes I can down shift the bike and bring it back, but today, it just completely died and then I had to start it again. I just today put in a new battery and spark plugs on the bike, so I know that they are the problem, and it has a full tank of gas. Do you have any idea's of what may be causing this problem?

Answer
Aaron, this bike was derived from the street and sports versions, so I make an exception! :>)

Well, there are couple of possibilities:
Charging system failure, causing voltage to drop low enough to have the transistor ignition quit. Usual causes for this are worn down carbon brushes which ride on the rotor and/or the rotor itself, which tends to go into high resistance or open circuit when the part gets hot.
http://www.electrosport.com/electrosport_press4.html

If the battery is staying fully charged, then the next potential is the fuel system. Although it doesn't show on the BikeBandit.com OEM illustrations listings, in 1980 Honda started putting vacuum operated petcocks on many bikes. The diaphragms eventually get a hole/tear in them and then you get a vacuum leak and no fuel flow at the same time. Check your petcock to see if there is just a fuel line coming off the side or if there is another smaller vacuum line attached to it, as well. If there is, either the line is off the carbs or fitting or the diaphragm has failed.

Something that can also happen to many applications back in that era was that there is an anti-backfire valve/diaphragm mounted on the side of the carb body with two small screws. IF that diaphragm dies, the bike will start backfiring due to both the vacuum leak and the lack of control on the fuel system on deceleration.

Finally, there is a screen up inside the fuel tank. The fine mesh can get plugged and severely restrict the fuel flow to the carbs. The gas caps should have a vent system, too, to allow the air to be drawn into the tank, so it doesn't get a vacuum lock inside.

Hope this is helpful...

Bill Silver