Topic: 1978 CB750K / lean - popping

[mrmach5]

Just purchased an awesome 78 CB750k in exceptional condition and totally unmolested. The guy I purchased it from mentioned in his description that it was hard starting but once warmed up runs good. When it was delivered yesterday I tried to start it and after much coaxing I was finally able to keep it running. Some lean popping at idle and down low. I rode it a few miles and it ran fine in mid range. Pulled some plugs and they are a bit on the light tan-white range. The pipes are blued a bit as well, I'm sure the lean condition is the culprit. All intake boots look to be fine. My first guess is clogged pilot jets,
but I'm open to other possibilities.

[TwoTired]

Stock exhaust and air filter?

Pilot jets are certainly a possibility, as is faulty accelerator pump function.


Cheers,

[mrmach5]

Yes, Stock airbox, stock exhaust, 10k miles

[TwoTired]

Before you go tweaking on carbs,  Do the complete routine 3000 mile tune -up.  Establish that baseline first.

The spark plugs *should* be light tan.   But, you are assessing mixtures for all throttle settings.    What if mid range rich, WOT lean and idle lean.
If you are going to do anything other than cleaning jets, "read" plugs after running under load at a specific throttle setting.  You have mechanically actuated slides, not CV or computer fuel injection.

Another thing to check is if the choke knob really fully closes the choke plates.  Starts better if they do.


Please explain "lean popping".

[mrmach5]

I will start with a baseline tune, before going down any rabbit holes. When I first tried to start it required several attempts to keep it running and quite sometime before I could get it to settle into an idle, then some random pops and misfiring that I attributed to an over lean idle, as well as the discolored pipes.

[TwoTired]

Because your bike has an accelerator pump, you can enrich starting by twisting the throttle one or more times...if the accelerator pump is working, of course.

Cheers,

[mrmach5]

Yes,
Pretty sure its working, as that was the only way I could get it started. I would have to twist the throttle several times then hit the starter button, it would catch and I would have to twist more to keep it running and get the RPM's up, took it a while before I could get it to settle into an high idle.