A couple of "tune up" questions...

[K2-K6]

To check sparks for each cylinder on these, you may need to flip the induction clamp for the difference in each pair of coil leads from single coil to get signal. They are inverted in comparison to one another as exit each end of that coil, with some clamps directional and need that flip flop to get a read.

I think it's worthwhile at this point to see if you can chase it down. Easily said by me not doing the work  as it's good to have them glitch free, which they should be capable of. Satisfying too, if you get it completely debugged and they are usually very linear in smoothness from their original design intent.

Hopefully you can get a clearer picture as you test the different elements.

[McCabe-Thiele (Ted)]

At the risk of sounding naive you mention it's a low speed stutter - is it possibly a feature of an engine that always needed revs to perform & run smoothly?

Not so much a stutter as low revs death due to a lack of low down torque - as has been said previously opening the throttle wider often makes it seem worse? I guess it might be a question of degree especially if the bike has run without the stutter previously.

[Mikep328]

All the spark/timing tests were good.  I didn't pull the carbs today...just couldn't get up the enthusiasm...maybe tomorrow. 

The stutter/roughness I am speaking of occurs just above idle - 1500-1800 RPM.  It is most apparent if holding a steady throttle in that range - say 1st or 2nd gear in slow-moving traffic or maneuvering in a parking lot. 

[garyjpaterson]

The stutter/roughness I am speaking of occurs just above idle - 1500-1800 RPM.  It is most apparent if holding a steady throttle in that range - say 1st or 2nd gear in slow-moving traffic or maneuvering in a parking lot.

I have it in both my CB350 twin and CB750, I've yet to be able to tune it out. Maybe a little faster, 30mph in 4th gear is the when its most noticeable, just when you crack the throttle it doesn't like staying there. Give it a fistful and away it goes

[Mikep328]

Pretty much the same symptoms!!  I was thinking it was my #1 carb idle circuit since it doesn't respond properly to the movement of the airscrew.  I'm going to try to remove/clean out that jet with the carbs still on the bike.  I squirted carb cleaner through all the jets when I had everything disassembled but I didn't actually check with a wire to be sure the idle jets were totally clear - stupid on my part.  But I'm only checking #1 since the other three react properly to airscrew adjustment.

Removing the carbs/ raising the needles is the next thing I will attempt.

[Mikep328]

Carbs are off, back at the house...  One of the pilot jets was partially clogged so cleaned all with my jet cleaning tool (guitar string). 

One of the pilot jet's emulsion tubes is slightly bent.  Not sure how it could have gotten that way...wondering if that could have any effect on fueling.  Are those jets available individually or only as a carb repair kit?

[McCabe-Thiele (Ted)]

If you need a 400 emulsion tube I have a set of good used ones PM me if I can be of help.

[K2-K6]

Carbs are off, back at the house...  One of the pilot jets was partially clogged so cleaned all with my jet cleaning tool (guitar string). 

One of the pilot jet's emulsion tubes is slightly bent.  Not sure how it could have gotten that way...wondering if that could have any effect on fueling.  Are those jets available individually or only as a carb repair kit?

Definitely....it's fundamental to how the fuel is distributed within the oxygen carrying air volume that's just been ingested from intake stroke. The better (smaller droplets and more even distribution) the fuel charge has, then the more consistently linear the burn characteristics are when the spark happens.

If the emulsion tube has restricted abilities to "atomise" the fuel into droplets that are fine enough to be optimum, then that cylinder has difficulty in completely burning each and every firing stroke.

Obviously you the rider then feel this stumble as that cylinder doesn't contribute regularly to the output, and you either have to close or open the throttle to get it away from that characteristic.

[Mikep328]

Thanks!  Just to clarify since I used the wrong term in my previous post...

The "slow jet" is apparently the correct name for the item I called a "pilot jet."  The tubular portion of the slow jet with the holes in it is the bent part to which I referred. https://www.davidsilverspares.co.uk/CB400F-SUPER-SPORT-FOUR/part_3316/

 

[Oddjob]

Just because DS refers to it by that name doesn't mean it's right. I've always referred to it as either the idle jet or the pilot jet. I think slow jet is more of an American term.

Emulsion tubes are the ones above the main jet, lots of really small holes in them as a rule.

I think I'm right in thinking you mean the emulsion looking type tube on the pilot jet that's bent.

[Mikep328]

Yes.  I tried to take pics of it but it's too small to be really able to see but the bent one has some holes that are not round, they are quite egg-shaped.  it's as if the jet had been screwed in with something stopping the tip from moving forward so it buckled.

[Oddjob]

What size is stamped on it Mike?

[Mikep328]

40 and is identical to the other three...except for being bent!   

[Mikep328]

FWFW - In cleaning all the passages with guitar strings I think I figured out what might have happened to that pilot jet.  There are two fittings on the carb that the pilot jet can screw into.  One is the actual fitting for it, the other has a blanking plug.  Which is which (plug or passage) is determined by whether it's a 1-2 carb or a 3-4 carb.  The blanking plug is far enough in where the jet can start threading in.  I suspect someone did that -  started screwing it in and, when the tube 'bottomed' against the plug, probably thought that the threads were just a bit buggered and continued screwing it in.  Before realizing they had installed it in the wrong fitting, the jet was distorted.  Of course, I can't say for sure that that happened but it would cause exactly the damage that exists and I can't think of any other way the damage could occur.

[K2-K6]

Sounds entirely feasible, agree that it would likely be something of this direction that could compress that section with holes in it.

I'm amazed that for a relatively mundane production item, these carburettor, that they are so finely resolved in design and production tolerance, all made some considerable time ago too.

They really are very very accurate in their manufacture and provide extremely precise metering of fuel for the correct response in running these engines. It's generally when first working on them that a recalibration is needed to just how small a fault can produce errors in their use. I doubt you'll be the last to have to go into them again in a process of getting fully sorted, that's quite usual from experiences on here 

They are quite a marvel really, and hopefully you'll have got to the reason why there's glitches in power delivery.

Quite worthwhile in working through them really, hopefully productive when you next run them as well.