Hello new here but not new to vintage Honda's. I always seem to figure out my own issues but I seem to have hit a dead end here with this one. Its an 82 cb750c that's all original. The carbs are clean and functioning perfectly so I'm going to rule that out for everyone. The problem is somewhere in the electrical side of things. When I got the bike non running I put new plug wires and ngk caps on it. After starting it it seemed sluggish on revv up. Felt the head pipes and 1 was cold. Pulled the ug on it and it had spark to it....weird. while checking for spark the bike started on 3 cylinders and for kicks I revved the engine while grounding the plug....the spark disappeared. It returned when idling. I'm running ngk plugs. After more tinkering and checking grounds and connections theproblem seems to jump around between cylinders. Had it on 1-2-3-4 act up. Both coils check at 11k ohms on secondary and 2.6 on the primary. Is this within specs?? I get about 11.xx volts feeding the primary side on both coils....guessing there's a but if resistance somewhere in the system. Battery is new. Any help is appreciated!! Thanks.
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82 cb750c coil issues??
- Thread starter sks72107
- Start date
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- Tags
- dohc electrical
amc49
CB750 Guru
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Last edited:
amc49
CB750 Guru
Kill switch being old often lowers available spark voltage.
Yank each plug/wire and ohm them out, anything over 5000 ohm is a problem. Don't use resistor wires AND plugs, too much. The coils are not that powerful even when new. Weak grounding can make your spark/no spark thing and even your hand grounding of plugs can be at fault. Common. The system is a waste spark one and anything wrong with one cylinder as far as spark then affects its' twin, or 1 and 4, 2 and 3. One plug of each pair fires backwards, yeah, you read that right.
You are about the 5000th person to insist that the carbs are clean when they may not be, these do that to EVERYBODY. No insult intended at all, just the reality of the thing. There are places in those carbs where you absolutely cannot tell whether they are clean or not.
The service manual gives no spec on the coil ohms, only that it must make a 1/4" long spark.
How about the pickups under left side cover? Spec air gap there is .016"-.027" and closer is better. I use closer to like .012" but obviously they can never touch or damage. You get a stronger ignition signal to fire the coil with closer.
Yank each plug/wire and ohm them out, anything over 5000 ohm is a problem. Don't use resistor wires AND plugs, too much. The coils are not that powerful even when new. Weak grounding can make your spark/no spark thing and even your hand grounding of plugs can be at fault. Common. The system is a waste spark one and anything wrong with one cylinder as far as spark then affects its' twin, or 1 and 4, 2 and 3. One plug of each pair fires backwards, yeah, you read that right.
You are about the 5000th person to insist that the carbs are clean when they may not be, these do that to EVERYBODY. No insult intended at all, just the reality of the thing. There are places in those carbs where you absolutely cannot tell whether they are clean or not.
The service manual gives no spec on the coil ohms, only that it must make a 1/4" long spark.
How about the pickups under left side cover? Spec air gap there is .016"-.027" and closer is better. I use closer to like .012" but obviously they can never touch or damage. You get a stronger ignition signal to fire the coil with closer.
Thanks for the info! I redid the carbs on an 81 cb750 and boy was that fun. I learned my lesson on these carbs. From now on the welch plugs come out and all holes are wired so on so forth. Anyway so i went out and checked the air gap on the pick up coils. They were on the wide side so i narrowed them down to about 0.013". They were out to about 0.025". So i also checked the resistance of the new wires and caps. The caps are ngk rated at 5k ohm. 3 out of the 4 measured 4.7 from wire end to plug connection. But one measured 5.7! When i started the bike 3 cylinders were running good but the one with the higher resistance was not firing right. When it cools down im going to swap that wire out with the other similar wire and see if the problem follows. Any other pointers anyone?? Thanks.
amc49
CB750 Guru
There can be defective resistors in the plug caps. I personally prefer no resistor in either cap or wire and then bump plug gap open a bit more to make up for it.
amc49
CB750 Guru
The R means resistor FYI.
