Motorcycle Repair: 1975 cb500t

Question
Hi Bill, my 1975 cb500t is acting as if it's running out of gas. I just rebuilt and tuned the carbs and I know the proper amount of fuel is being recieved by each cylinder. It seems as if the left cylinder is firing fine but the right seems to be misfiring. After I changed my oil the other day I started my bike and let it warm up, it was Idleing great and revved extremely strong. I shut the bike off to go put my gear on for a ride, when I started it back up and went to drive off is when I noticed the problem. When I let off the clutch to take off the bike bogged and had virtually no power.  Can you give me an idea of what the problem could be? Thanks!

Answer
Caleb, There are a few things to check when this happens. First, when the bike appears to be dying off on one side, shut off the petcock and drop the float bowls to see how much gas is in each one.

I'm glad you cleaned the carbs, but did you clean out the petcock passages? The main tube and reserve systems all need to be in good order and clean and clear, all the way out to the 4 hole gasket. If you have any rust or other contaminants inside the gas tank, you may have grit and debris going down into the back side of the float valves and/or into the bowls and getting sucked up into the jets.

If you determine that the fuel flow to the carbs is good and clean, then you can look at the sparkplugs to see if they are both getting a good spark to each one. Spark plug caps can go bad.. resistor plug caps should have 5k ohms in each one. Clean the points and check the gap... should be about .014"-.016" wide, then move the plate and point bases around so they open at the T and LT timing marks on the rotor face.

Check the gas cap gasket. I have seen them deteriorate and cause the fuel tank not to vent properly. When that happens the fuel flow to the carbs is restricted and usually the carb that is further away from the petcock runs out of fuel first.

Make sure you have a good condenser (watch for over-arcing points) and that it is tightly bolted to the coils/frame mount.

I have seen failing coils on the 350-450s recently, due to deterioration of the plug wires where they exit the coil body and/or the primary resistance no longer measures out to about 4.5 ohms.

The CB500T carbs have air cut valves which have rubber diaphragms that can fail, as well.
http://www.cmsnl.com/honda-cb500t-500-twin-dohc-k0-1975-usa_model463/partslist/E #9

Assuming that you have good compression, a strong 12v battery, good spark and clean fuel system, the bike should be up and running fine again after you check the above items. Be mindful of the condition of the intake manifolds, too. They will crack and create air leaks.

Bill Silver