Coughing and popping after 6K RPM

[BLUE1980CB750C]

My 8k miles 1980 750 C is acting up. Any time I rev it over 6k she start popping and coughing and I checked
compression
coils - with others I have -
spark plugs
carbs
ignition timing, - with a spare -
CDI - also with a spare -
battery,
plug cables
rectifier

but I just can't figure out what it is; i added a wire harness ground because the existent one seemed not enough and the bike - seemed - to be ok for a quick ride and then the next day she popped and coughed after 6k again. The idle is fine.

Any suggestions? thanks!

 

[brettp]

It might need a valve job.

 

[BLUE1980CB750C]

thank you brettp; compression is very good and I checked clearance too, all in spec.

I even took the crankshaft and the two cam chains out to see if maybe they had issues; all good.

Plus the bike does not smoke and idles great.

I think it is an electric problem and trying to figure out if there is a way to check the CDI boxes.

I am checking youtube for original honda electrical troubleshooting videos from the '70s..

 

[brettp]

Is it consistently at 6k or will it run fine sometimes then act up?

 

[BLUE1980CB750C]

consistent; but when I added an extra ground to the wiring harness she ran good for about 10min.

that is why I am thinking more electric than mechanical..

 

[brettp]

Ok. I'm inclined to believe that, too.
Have you synced the carbs? Silly things can happen when they are off.

 

[BLUE1980CB750C]

yes, cleaned and synced...yet, I may just do that again..

 

[brettp]

Battery ground free of corrosion?

 

[BLUE1980CB750C]

yep

 

[brettp]

I'm thinking spark units (silver boxes under the seat) or possible corrosion on the coil connections.
After that, I'm running out of ideas

 

[BLUE1980CB750C]

do you know how to check a spark unit?

 

[brettp]

Sorry, wish I did.

 

[BLUE1980CB750C]

I dont think you can.. only way is a switcharoo with another...but the other I have is also 45 years old...

 

[Loose Chain]

I even took the crankshaft and the two cam chains out to see if maybe they had issues; all good.

Are you saying you completely tore the engine down because it is missing? I have never heard of someone taking a crankshaft out of an engine to check it for an engine miss problem. When you say the valve clearances are good, what are your clearance numbers? What do your plugs look like? An engine going lean on the top end can cause the problem you are having. So tell us about the plugs.

 

[BLUE1980CB750C]

no no I meant the camshafts; when i ck'd the valves I did a visual check of the cams and chains.

Valves are in spec at .08 intake and .10 exaust

the plugs do look a bit on the lean side and I did fatten the jets a few years back; maybe re-jeting again?

 

[Loose Chain]

The plugs will help to decide on jetting. If you have made changes to the exhaust or intake like different exhaust system and pod style air cleaners then you will need a jet change. You need to start with a fresh set of plugs and then a hard run up in the range where it is missing, then pull the plugs and see what they look like.

 

[BLUE1980CB750C]

bike is stock; jetted a bit richer and with the adjustable metal/plastic floats (from a goldwing I think) instead of the rigid plastic ones.

how white would u call a lean spark plug on these motors? 'cause they where always on the light gray side.

 

[Loose Chain]

What size jets in it now?

 

[brettp]

That's a great question, Loose Chain. Why didn't I think to ask that?!

 

[BLUE1980CB750C]

that is a great question...I don't remember but I will let you know when I get to it in a few week-ends

thank you for your feedback gents, happy father's day week-end..!