Motorcycle Repair/75 CB500T
QUESTION: I recently acquired a 75 CB500T. I rebuilt the carburetors, cleaned the gas tank and installed a new fuel filter, new points, adjusted the valves, and repaired the electric start. My issue is that The bike runs well on the left cylinder but does not seem to run well on the right. If i manually work the left carb the engine responds well. If I manually work the right carb there is no response. I rechecked points, timing, valve adjustment and still no improvement. Compression is 150 on both cylinders.Fuel is also being supplied to the carburetors. Could it be something inside the right carb? I also set the float level at 20mm. What could be wrong? Your insight would be greatly appreciated?
ANSWER: Juan, if you correctly cleaned and rebuilt the carbs, so they are metering correctly at all throttle settings, then the most likely issues with one side not running are either the condenser or the ignition coil (and/or plug cap).
20mm is the correct float level. I actually have a spare set of parts carbs for that bike, if you want more spares to keep handy.
Isolate the ignition coil primary winding wiring (the small wires) and check the resistance values with an accurate ohms meter. Honda coils for the twins using points and condensers is about 4.5 ohms. Check both coils and compare. Likely the right side is off from the left. If both are the same and around 4.5 ohms, check the spark plug caps to see if they have about 5k ohms resistance readings. With the caps off check the resistance of the spark plug wiring to the coil metal mounting frame. This will be a high reading... around 12-20k, perhaps. Again, see if both sides measure the same.
There was a service bulletin for 350-450s which had misfiring issues that were tracked back to poorly grounded condensers where they bolt to the frame.
Run the engine with the points cover off and watch the points operate. If one side has excessive arcing across the points all the time, then the condenser is failing on that side. The condensers are doubled up, so if one fails you have to buy a pair. Always use OEM Honda parts for the ignition if possible.
Based upon some failed coils on 350-450s which I have seen in the past few years, I would bet on a bad coil, but do your troubleshooting and see what the problem actually is, before you run out and buy parts unnecessarily.
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QUESTION: Bill, I rechecked timing, rechecked the carbs, and tested the coils they read 4.5. How do i check the plug caps? I have a digital multimeter, where would I put the red and black leads to test the plug caps? The condenser and points are brand new and I can visually see the spark firing at the plugs. I also went ahead and rechecked the valve clearances, I set them at .005 mm. the only difference this time is I cant fit my .005 gauge on the exhaust right side no matter how much I adjust it. Could it be stuck? All the other ones were good including the intake right side. Could this be causing the bike to not work properly on the right cylinder? I even checked the float valves in the carb by blowing in the gas inlet hose and manually opening and closing the float. It worked fine. Air in when float down, no air when float up. The more the fix evades me, the more I want to figure out what it is! thank you again. Juan
Juan, if you can't adjust the exhaust valve on the right side, that pretty much points to a problem there. It sounds like the valve has sunk into the valve seat. I hope you know that the valve adjuster shafts should be oriented so that the little index marks on the ends of the shafts are facing away from each other. Somewhat like this: (/)0(\)
If you can spin the adjuster shaft all the way around and not get clearance, then there is a problem with that valve/valve seat. What does the cam lobe look like? If it is scored deeply, then you have a big problem. If not, then there are valve seating issues to address. Those compression readings are about 20 psi too low, as well, unless you checked compression without holding the throttle open. Not a lot of air can flow through the motor when the slides are down and the throttle plates are closed, too.
There were service bulletins on the oiling problems for both 350 and 450 models. Both the clutch cover and dyno cover gaskets must be replaced with updated items. The original gaskets can fail with pieces breaking loose and clogging the oil passages to the top end. That results in a fried cam/cam follower and cam bearing scenario.
In regards to the spark plug caps, just unscrew them from the wire ends and test like a resistor with leads on the plug end and wire ends. There is no polarity so either way will measure the resistance value. They should read out at about 5k ohms.
It sounds like you have a mechanical problem, at this point.