Rough running CB750K1

[AndyH]

Hi Folks,  I recently acquired a nice American import K1.  However, it was running poorly...idling too high, seeming to miss and carry on under throttle etc.  The bike is completely stock, including the 4 into 4 exhaust.

So an initial check revealed a faulty plug cap on #3 cylinder.  After replacing same, the bike still ran poorly.  So I took off the carbs and gave them a full clean (removed and cleaned all brass, ultrasonic clean of carb bodies, slides.)  Reinstalled brass, set float levels (all were out!) and carried out a bench sync.  I also replaced the rubber "insulators" beween carbs and head.  Then I checked and adjusted tappet clearances (they were close to correct).  Primary and secondary coil resistances were checked and were in spec (4.5Ω/14.5kΩ).
Before vacuum sync, I checked that the points timing was correct...it was.  Carbs then reinstalled along with stock airbox and filter.
I proceeded to vacuum sync, and although the sync tool (Syncroking 4 MkII) indicated full carb balance at idle, it still ran a little "lumpy".

Increasing throttle opening still showed very good vacuum balance but bike still didn't sound or feel right.  I double-checked my procedures and compared the Syncroking with a mercury manometer.  Both indicated the same good result.
So I popped the tank back on and went for a quick ride, and it proved that something was not quite right....it felt lumpy unless throttle was cranked open.  Closing the throttle brought the bike back to normal idle, again a little lumpy however.

Sorry for the long ramble but I'm at a loss as to what could still be causing such "rough" running.
There are many on this forum far more experienced than I so I'm hoping someone may have an aswer for me.
Thanks in advance!

[K2-K6]

Sounds like the work and synchronisation is doing what it should if it runs well with throttle open etc.

Synchronisation is exactly for that, parity as you bring the slides up in throttle range, it's not specifically for slow running (coincidentally it helps it as byproduct) but often wrongly considered as the cause of low speed concern.

The fine low speed adjustment is (with all the other things you've already set correctly first) the idle air adjustment screws. They have the fine response to give this micro adjustment. There's specific reference to proceedure in Honda's manuals.

That procedure brings parity of combustion at idle speed, and so refinement of firing from cylinder to cylinder.

Reality check first though, if when running you turn each idle airscrew in isolation you should get a response from each one, no response suggests there may be residual blockage in that idle circuit (fairly common if stored and not used for a while) with back to cleaning and verification of that carb body and it's critical flow passages the route to check.

[cbxman]

Andy,  I have a K1.  I have found that a small air leak on the exhaust adapters can influence idle.  Like you have been chasing my tail getting smooth idle and sweet acceleration. The PO hadn't used any sealant where the headers fit on to the flanged adapters.  I pulled the pipes and used a small amount black high temperature silicone.  The idle is transformed, but I haven't been out for a test ride yet.

Good luck

Cheers,

Jerry

[AndyH]

Andy,  I have a K1.  I have found that a small air leak on the exhaust adapters can influence idle.  Like you have been chasing my tail getting smooth idle and sweet acceleration. The PO hadn't used any sealant where the headers fit on to the flanged adapters.  I pulled the pipes and used a small amount black high temperature silicone.  The idle is transformed, but I haven't been out for a test ride yet.

Good luck

Cheers,

Jerry

Thanks for the info Jerry!  Have you been able to get out for a ride yet or is it a tad too chilly in Suffolk!
Still high 20s/lo 30s Celsius here in Perth!
I'll be removing the exhausts over the next few days and will have a close look for any leak indications.

Cheers
Andy

[cbxman]

Andy,

I did get out but although the idle was much better, I still have a problem with pick-up hesitation after deceleration and from a standing start.  I have changed the slow jets to genuine Keihin and now waiting for the weather to improve to see if I have sorted it or not.  I has been really chilly today and we are expecting more cold weather for two or three days,

Cheers,

Jerry