Motorcycle Repair: CB350G at high rpm, jumper wire, air leaks

Question
Hi Bill
My CB350 has had problems with running at high rpms for awhile now. It will rev up to about 6500 rpms with no problems, but at that point it starts to run unevenly. This happens in all gears, but is most pronounced in 5th gear at around 65mph, which is the most it will do.   The points and condensors were replaced recently, the points gap has been set, the timing has been checked and rechecked, the valve clearances set, and the carburetors have been syncronized.  It has 9000 miles and the compression is good.
I'm wondering if there is anything I can do to make it run more evenly at high rpms.
Thank you for your help.

Answer
Jeremy, this is part of a service bulletin that may or may not apply:

CB/CL-350 Mid-Range Misfire

There have been several cases where the CB/CL-350 developed an engine misfire that is most noticeable at “mid-range.” Generally, this misfire was assumed to be caused by carburetor adjustment but closer examination revealed that the misfire was the result of inadequate grounding of the condenser.

The condenser is grounded to the frame through the coil mounting bracket. If this misfire occurs in a machine, the condenser ground can be checked by connecting a small temporary jumper wire, from the condenser body to an exposed metal portion of the frame. After connecting the jumper wire, test-ride the motorcycle to determine the existence of a misfire. If the misfire does not occur, remove the temporary jumper wire and scrape some paint from the coil bracket mounting surface on the frame or install a ground wire (18 gauge or larger) from the mounting bracket to an engine mounting bolt.

Couple of other things to check: Carburetor diaphragms for pinholes/tears. No holes allowed here....

Air leaks at the intake manifolds, either at the clamps to the carb boots or manifold to cylinder head.

Watch the ignition points with the cover off and see if either set of points is arcing excessively. Random sparks, here and there are okay/normal, but consistent arcing is sign of a bad condenser or the lead has come loose up by the coils. Check all your wire connections for cleanliness and tight fitting terminals.

Carbs were fairly sorted out by the 1973G edition, so if they are clean and set to specs, then that probably isn't the source.

Make sure that the battery is fully charged and is being charged by the charging system. Low volts = weak spark.

Something that is seldom checked is the mechanical spark advancer, behind the point plate. They can stick, seize up and you don't get full spark advance when you need it, at mid-range and high rpms. Point cam should twist against the spring pressure and rebound to retarded position again.
Mark the point cam, if you have to remove it from the assembly base... you CAN put it on 180 degrees out and it won't run at all afterwards.

Check resistance in the spark plug caps...should be around 5K ohms. Unscrew it from the coil wire and check with a meter. Try NEW spark plugs. Fuel/oil fouled plugs, caused from poor running will cause high speed misfires.

Make sure that the gas cap vent is clear and the petcock passages are all open, screen clean and no rust/water in the fuel tank or in the float bowls!

Bill Silver