gene10170
CB750 New member
Hi I’m new to this forum but I have read several of your posts. Well here’s my problem. I have a 1974 cb750K that I have been battling black spark plugs only in number 2&3 cylinders ever sense I bought the bike two years ago. The plugs are black and sooty with no wetness and no smell of fuel. Here’s what I’ve done, Did a compression test warm and every cylinder is 170 across the board, I’ve synchronized the carburetor’s with a set of gauges, I’ve replaced the floats (# 3 was fuel soaked) set them at 26mm I’ve cleaned the jets at least five times, I’ve looked the carburetor’s over at least five times and can’t see anything wrong physically. Next, I ordered a rebuilt set of carburetor’s and still have the same problem. Next, I rechecked the timing and it is right on the (F) and the advance is right in between the two advance marks. I did a voltage drop test on the blue wire from the points to the coil, (it was .32 volts drop), Next I replaced the coil set and I still have 2&3 black plugs. I don’t know of anything else to check.
(Just a note the bike has run absolutely great sense I’ve had it, no missing, no fouled plugs, no performance problems just black plugs in (2&3) any help would be greatly appreciated. Today I retested the the spark out of each plug. just looking at each plug firing they all look the same and great. I then bypassed the bike harness and put a jumper wire from the 2&3 points to the yellow wire at the 2&3 coil. the spark all look the same
(Just a note the bike has run absolutely great sense I’ve had it, no missing, no fouled plugs, no performance problems just black plugs in (2&3) any help would be greatly appreciated. Today I retested the the spark out of each plug. just looking at each plug firing they all look the same and great. I then bypassed the bike harness and put a jumper wire from the 2&3 points to the yellow wire at the 2&3 coil. the spark all look the same
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