Occasional stall it idle...

[Mikep328]

Once in a while my 400F will be idling smoothly at 1100-1200 RPM and suddenly quit.  On pressing the start button it will restart instantly and idle smoothly once again.  Some days it will do the "quit while idling" and other days it won't.  When it occurs, it might do it after idling for 10 seconds or it might do it after idling for a minute or two . Or, more often, it never does it at all on a ride.  It's not a problem since the bike restarts before I can even remove my finger from the start button.  But I'm a bit anal that things should work as they are supposed to work!  I can't figure out what would cause this, especially the very erratic nature of the occurrence...

[K2-K6]

To get a complete and instant stop, that suggests power supply as reasonable suspect. Do the lights etc go out too when it stops.

Obvious things like security and tightness of battery connection along with other terminals generally around the bike. Sometimes something may have been victim to general vibration.

I had similar recently on my 80s Honda, with ignition barrel position being sensitive to any slight misalignment from exactly centre. Felt like it was "hanging " slightly on detent positioning. I used a product I've got from Land Rover to spray into the lock barrel via key slot, which was very effective in making it wok smoothly with more free engagement of detent hold.
The product is something similar to a very thin chain lube, some light grease, ptfe, alcohol to propell into the nooks and crannies, usual type of mix that should permeate and then stay put after application.

The type of contact blocks on the back of these ignition switch can also get a bit gummed over time, that to provide patchy inconsistent contact too.

[Mikep328]

THANKS!!!  Hadn't thought about the ignition switch.  I'll check that out tomorrow. 

[Oddjob]

In my experience it’s normally a carb problem. The tickover is too high btw, 1000rpm is the norm and at that speed it will happen more often, which is why it’s set at that speed. It highlights any problems. Might be a partial blockage in the idle circuit, might be a carb balance problem or might even be a mixture problem or float height.

[Mikep328]

Interesting re idle speed. On page 6 of the CB 350/400F service manual it states idle is set to 1200RPM.  Was there a bulletin or something that revised it to 1000 RPM or have I missed something elsewhere in the service manual re idle speed (that's certainly quite possible!!!)?

[exvalvesetdabbler]

1200 rpm for the 400F idle.

That's assuming the tacho is spot on at that reading. Could be 200 out either way.

If it's a bad connection it might get around it without stalling at normal riding rpm, if the idle is set too low it might just trip it up

Wiring seems more likely or grubby contacts breakers.

Regards
Dave

[exvalvesetdabbler]

Worth noting that the tacho I am currently restoring was reading 500 high at 5000 rpm so if you have one like that it could easily screw things up.

I used to have a timing trobe that could measure rpm but haven't seen one like that recently. 

Regards
Dave

[Oddjob]

Idle speed as set by Honda is 1050 + or - 50. It's best at 1000rpm in my experience. It shows up problems much better rather than disguise them by a fast idle. If the engine is in really good condition you can get one down to 600rpm easily. At 500rpm they do start hunting a little and making a bit of noise as things start to lash about inside. I'm not saying run at those speeds, just that it's an indication that all is well if the engine can do them. At 600rpm my 500-4 was still silent, at 1000rpm you couldn't hear it from 6ft away.

Could also be a small air leak, check the carb clamps. 

[K2-K6]

Worth noting that the tacho I am currently restoring was reading 500 high at 5000 rpm so if you have one like that it could easily screw things up.

I used to have a timing trobe that could measure rpm but haven't seen one like that recently. 

Regards
Dave

Some of the relatively cheap DVM have a frequency read, across one set of points will count crankshaft revolution on these.

[Mikep328]

Good info guys, thanks!!  I'll check out the carbs AND the actual RPM!! 

Am I correct that reading "frequency" across one set of points would indicate 1/2 of engine RPM?  IOW, double that reading would be actual engine RPM?

[K2-K6]

Good info guys, thanks!!  I'll check out the carbs AND the actual RPM!! 

Am I correct that reading "frequency" across one set of points would indicate 1/2 of engine RPM?  IOW, double that reading would be actual engine RPM?

It should be direct as the points cam lobe is not geared to crankshaft (as in a car distributor @ 1/2 speed, and needing correction) in these engine either of the points directly represents actual crankshaft movement.

[Mikep328]

Thanks!  I was never good at math anyway!!! 

[K2-K6]

Thanks!  I was never good at math anyway!!! 

You were right though, in a way. Old car type multifunction meters for dwell, tacho, voltage etc  you'd need to set to 8 cylinder to read from these engine in many cases.

These Honda are effectively two twin cylinder engine "flying" in close company  two 360 degree twins 1&4  2&3.

[Mikep328]

So if my (weak) math is correct, and the "distributor" cam is spinning at engine RPM, at red line (10,000RPM) those points are opening/closing more than 166 times per second.  Holy moly!!!  Is that even possible or have I grossly mis-figured?

[McCabe-Thiele (Ted)]

So if my (weak) math is correct, and the "distributor" cam is spinning at engine RPM, at red line (10,000RPM) those points are opening/closing more than 166 times per second.  Holy moly!!!  Is that even possible or have I grossly mis-figured?

Yep 166.66 times per second for each set of points.