Topic: CB750K Ignition Coil and Cap Tests

[mikehelvey]

I am working on a 76 CB750 that fires up okay then starts missing on #2 a lot.  The others may be missing from time to time too.  I tested the coils and caps and found:

Coil 1 (1/4):  Primary 5.5 ohms
                   Secondary: 15.66 (set on 2K)

Coil 2 (2/3):  Primary 5.0 ohms
                    Secondary 15.28

Cap 1: 10.5
Cap 2: open circuit
Cap 3: open circuit
Cap 4: 11.5

I'm uncertain whether the coils are within spec. I believe the primary should be between 4 and 5 ohms and secondary ~15K.  I'm thinking I should replace them both but wanted to hear from the internet experts!

Appreciate the help if you know whether these coils are still within specks.

Mike

[dave500]

replace all the caps for a start,also the plugs as may be fouled from not firing?

[scottly]

I wouldn't worry about the coils. What do the spark plugs look like?

[HondaMan]

Just for your reference: the OEM sparkplug caps made before the 750K4 (1973) were 7.5k ohms. The ones after that (on all SOHC4) were 10k ohms.
The 7.5k were considered 'burned out' at 9k ohms or so, while the 10k versions are still workable up to 12k ohms.

Over time without use, the metal strips that press into the resistor chip (carbon) inside the plug cap can tarnish. When this happens they will read "open circuit" with any digital ohmmeter, while an analog meter will short some sort of reading that usually fluctuates up & down erratically. These caps can be saved by running them for a while: the use will burn away the tarnish coating after about 1000 miles or so, restoring good spark. I've seen this dozens of times when the bikes were parked for a long while: even my own 750 did it after it was parked for 5 years after I had cancer (and all the recovery from it, didn't ride then). I just rode it anyway, and after about a month it 'fixed' the plug caps for another 8k miles or so before I did a full-bike rebuild. I still have those caps as my 'backup' parts.

[mikehelvey]

I wouldn't worry about the coils. What do the spark plugs look like?

New plugs. Rebuilt the top end because the valves were no stating. Right now they are looking a little black.

[mikehelvey]

replace all the caps for a start,also the plugs as may be fouled from not firing?

Caps ordered, I wasn't sure whether the coils are good.  Will trim the leads and see how it goes.
Thanks.

[scottly]

I wouldn't worry about the coils. What do the spark plugs look like?

New plugs. Rebuilt the top end because the valves were no stating. Right now they are looking a little black.

Black spark plugs imply a carburetor problem: too much fuel or too little air. If it was a lack of spark, there would be no fire and no black carbon.

[mikehelvey]

I wouldn't worry about the coils. What do the spark plugs look like?

New plugs. Rebuilt the top end because the valves were no stating. Right now they are looking a little black.

Black spark plugs imply a carburetor problem: too much fuel or too little air. If it was a lack of spark, there would be no fire and no black carbon.

Probably too much fuel.  Just put it back together after a top end and carb rebuild.  Haven't gotten to tuning them in yet.  Running velocity stacks and straight exhaust on a hardtail chopper, so it may be a bit until I get it running smooth.  First stop, getting the ignition reliable! 

[HondaMan]

Umm...did you retain the OEM brass parts in the carbs (needles, jets, emulsifier tubes)?
There are NO aftermarket carb kits with the correct brass in them now.