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gene10170

CB750 New member
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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
 
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Sounds electrical

Trim your plug wires and reinstall your caps tightly; have you replaced plugs? Might want to just to see if your symptoms change.




 
Sorry i failed to post that i have installed new plugs twice and replaced both coils and wires i still have black 2&3 plugs. 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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You DO have fouled plugs if they are black, they just haven't misfired yet. 2 & 3 likely means ignition problem, you just haven't found it yet. Swap the coils to see if the problem jumps to the other pair of plugs.

You haven't described the other two plugs, if they are close as in dark brown leaning toward black they are doing the same. If using resistor wire, AND plugs, AND caps, well of course the plugs will be much harder to fire.
 
thanks for the input. 1&4 are completely clean, I have replaced the coils, wires, and ends, with new ones last week with no good results 2&3 still black. I put in new plugs and test drove it and in 20 miles they were black already. P.S. Can you tell me it the (black and white wire from the coils is a ground or hot, because on this bike the black and white wire in the harness becomes hot when i turn the key on. I thought it should have been a ground. I also have a 1975 Honda cb550, I have gone as far as to swap out the breaker plates and still got the same thing. I realize this shouldn't be rocket science but i just cant find the one thing that is only affecting 2&3 that i haven't addressed. I could send you some pic of the plugs if you would like. (If i can figure out how to do it) Thanks
 
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