Topic: stalling out of nowhere; starts right back up

[joes81cb650]

my brother has been riding my 1981 cb650 so most of the details i can offer you are second hand, but it has developed an ever-worsening problem of stalling out of nowhere and then starting right back up. the only time i have ever seen it do this was today, in my driveway, at idle. it stopped as though someone had shut it off and then it started back up without too much fuss. i have to think it's loss of spark.
the service manual is at his house right now, so i am just looking for some direction.
gas is fresh, carbs cleaned this past winter as well as fresh spark plugs.
what would be a good test to start with ? laying a spark plug on the engine isn't going to be much help because, as i say, it starts right back up, so if it IS a spark issue, it's sudden and short-lived.
any help is appreciated.
thanks
joe.

[scottly]

Try cycling the handle bar kill switch a few times. Just a wild ass guess...

[robvangulik]

Try running it with the gas filler cap open...Why do you HAVE to think its loss of spark?

[BenelliSEI]

What revs is it idling at when it stalls?

[joes81cb650]

thanks for the replies !

my reasoning for thinking it's loss of spark is that if it were fuel, the shut down would be less abrupt and the bowls would take a half-minute or so to fill back up ... maybe i am off base there ?

will try wiggling jiggling the kill switch to see if there's a problem there. i will also investigate the gas cap.

as for what revs it was running at when it stalls ... the only time i witnessed it, it was running right at idle speed ... 1000rpm or so. my brother says that it was stalling while riding/giving it some throttle.

i appreciate the feedback very much, it gives me something to do until i get the service manual back, although todays wrenching may be cancelled due to rain.
thanks again,
joe
 

[TwoTired]

Key switch worn?  What happens if you wiggle the switch while running?
Or turn the bars full left or right, to see if there is an intermittent contact.  In fact wiggle anything electrical on the bike to see if you can make it fail.

Can't fix what ain't broke.  You need to make the failure hard or cause/effect predictable.

As mentioned, the gas cap vent could also be a suspect.  Vacuum gets created in the gas tank if it won't vent, making fuel feed limited.

Cheers,

[BenelliSEI]

I also had a similar problem. Bike ran fine, then suddenly rougher, sometimes stopping and then a few minutes later, started right up. Loose main fuse. It had overheated the plastic part the fuse clips went thru’. When it got hot, looser connection. Replaced it and no more problems. This took me 1/2 a riding season to figure out......

[70CB750]

Seems like electrical problem. 

I had something like that and the main cable was lose at the battery.

[joes81cb650]

i really appreciate all the feedback; thank you !

was out there yesterday wiggling wires, connections, ignition switch, kill switch, turned steering fully right and left, gas cap is venting, checked battery connections, petcock/filter has been removed/examined/cleaned, battery is fully charged and charging system putting out good voltage ... can't MAKE  the bike stall, but once again, it stalled on it's own with me not touching anything. starts right back up everytime it does this. no sputtering or high-revving to indicate a possible fuel problem. just a quick shut-off out of nowhere.

fortunately i have another bike to ride, so i can take my time with this but it would sure be nice if the dang thing would give me a clue. the only clues i have to go on are ...
1) the stall happens quickly;
2) the stall only happens when the engine has thoroughly warmed up;
3) the stall can occur under load or at idle.

will check the physical condition of the fuse as suggested and do some more poking around today.

[seanbarney41]

When it stalls, lights and everything shut off, or just the engine?

[seanbarney41]

Pretty easy to just unplug key switch and hot wire with some jumpers on these models, that would at least eliminate that as a culprit.

[tlbranth]

I'd look for corroded bullet connectors, particularly feeding the coils

[1976cb750f836]

Do every one! Unplug and replug ALL connections! Remove all grounds, sand or file, then, Reattach!
Fixed a ton of wireing problems on old bikes and cars this way?

[joes81cb650]

UPDATE :

got around to checking the coils today. the FSM  doesnt give resistance specs, but for 1 coil i am getting 21k ohms across the secondary windings and on the other coil i am getting a measurement of either milli-ohms or mega-ohms (can't recall whether it was a small "m" or a large "M"), either way, that coil is bad. my question is ... is 21K ohms within spec for the "good" coil ?

thanks
joe

[scottly]

Joe, your coil measurements aren't a smoking gun; if one coil was "bad", the motor would still run, but only on two cylinders.

[joes81cb650]

agreed, scottly. but the question still remains ... do i have one bad coil or two, based on those resistance values ?

thanks
joe

[Don R]

 From my experience it could be a flap of rubber inside a fuel line, loose ignition wire, etc. I had a 79 gl1000 with a loose key switch that I had to be careful of. Are you measuring through the plug caps or are they removed?
 Reading material,
http://hondacb650.com/viewtopic.php?t=488

[gmoneymagna]

I have had a very similar problem before. I would check the super easy stuff first:

- battery terminals. Loose terminal connections have caused this problem for me a few times
- Main fuse. The old fuses for these bikes are weird and sometimes can appear intact at a distance even if they are blown. If it is blown you should investigate why.

edit: I should add a question. Do you lose all power to the lights when the stalling happens?

[TwoTired]

UPDATE :

got around to checking the coils today. the FSM  doesnt give resistance specs, but for 1 coil i am getting 21k ohms across the secondary windings and on the other coil i am getting a measurement of either milli-ohms or mega-ohms (can't recall whether it was a small "m" or a large "M"), either way, that coil is bad. my question is ... is 21K ohms within spec for the "good" coil ?

thanks
joe

Did you measure with or without the spark plug boots?

[joes81cb650]

i am measuring resistance with the spark plug leads completely removed from the coils.
never noticed whether i am losing power to the lights during a stall. that will be the first thing i will check next time around.

[gmoneymagna]

i am measuring resistance with the spark plug leads completely removed from the coils.
never noticed whether i am losing power to the lights during a stall. that will be the first thing i will check next time around.

It it's a lose battery terminal the bike will go completely dark and die right away. In my case I would play with the key, it would light up and power on again only to die again in the same way repeatedly.

[Don R]

 also could be a loose exhaust baffle closing off and when the pressure is gone it falls back open.

[joes81cb650]

after checking/cleaning all connections, grounds, kill switch, ignition switch, etc. i turned my attention to the pulse generators. spec is for 530 ohms +/- 50 ohms @ 20c. 
 
waited for the bike to get hot enough to stall and then checked resistance. yellow wires show 630 ohms when hot, blue wires are showing resistance in the 1 - 2 MEGA ohm range and sometimes even show an open when hot. the manual doesn't show specs for a hot engine but surely the numbers i am getting on the blue wires indicate a problem. the problem is further compounded by the unavailability of these pulse generators. the blue wires are for the coil that sparks the #1 and #4 cylinders. i also discovered that the header pipe for #2 cylinder is noticeably less hot that the other 3. a miss here, along with the total loss of #1 and #4 when hot would explain why the bike wouldn't continue to run, albeit poorly, with the loss of 2 cylinders.

so ... while i try to figure out what is wrong with #2, the hunt is on for a set of pulse generators.