Just like old times...

[Phil C]

Went out on my 1965 B40F just now. Went about five or six miles into my local town, stopped for a few minutes (oil drip from primary chaincase kind of area persists, sigh), turned round and headed home. A mile or two from home I joined a traffic queue on a narrow road, and very soon became aware the bike was wanting to stall. I kept blipping the throttle to try to keep it alive but after a short time it stalled. Pushed it onto the pavement, and began pushing it up the hill towards a wider bit of path. Just like old times. Pushing a bike uphill reminded me of being eighteen with my Starfire, about 1972, but that used to be because I was out of petrol. I knew this time I had fuel. Anyway, after I'd got my breath back I prodded the kickstart and the engine burst into life again, the traffic lights went green, and I came home, the bike seeming okay. What do you think would be likely causes? A bit of muck in the carb? Loose electrical connection somewhere? Any advice would be much appreciated. Phil

[Phil C]

Oh, btw, I noticed the choke was very slightly closed. Could that be a possible cause?  Phil

[Martin S]

Fuel evaporation maybe, caused by heatsoak from the engine to the carb?

[Phil C]

I wasn't in the queue for more than a few minutes.

[chaz]

coil breaking down , could be an air lock in fuel system, is there a small breather hole in the fuel cap?

[courtney]

Is the plug (the one with all the wires going into it) securely attached to the ignition switch?

[DAVE BRADY]

Hi Phil,

Were you in neutral when the bike wanted to and then did, stall?

Dave.

[Phil C]

Hi Phil,

Were you in neutral when the bike wanted to and then did, stall?

Dave.

Can't remember, Dave.  Why do you ask?   

Btw, Chaz, fuel cap breather holes are clear.

[Phil C]

Is the plug (the one with all the wires going into it) securely attached to the ignition switch?

Haven't checked that, Courtney, but I fitted it about, say, a year ago, so I assume it's okay.   Is that a common issue?   Phil

[DAVE BRADY]

Hi Phil,

If you have the clutch lever held in in gear then the plates driven by the engine are spinning and the others are stationary with the gear box and rear wheel.  Even the best set up clutches in the world will get hot and eventually the plates expand and start to drag.  This creates more heat, expansion etc. until the lever is no longer keeping the plates apart. Eventually the bike wants to go forward but you either do not have enough revs or are holding the brakes and the engine stalls.
A short push and a few minute later all is well again.   

Dave.

[Phil C]

Hmmm, interesting. Thanks Dave. Why do they get hot through turning? I mean, when they're engaged they're turning too.
Can't remember exactly what I was doing, just that I was aware it seemed to want to stall and I was blipping the throttle to try to keep it going. I guess I'll see if it happens again.   Phil

[DAVE BRADY]

Hi Phil,

When engaged the plates are in effect stuck together by the pressure of the springs so they do not (or should not) slip or rub against each other so not generating heat.
If you are in gear and holding the clutch leaver in then the friction plates are being driven by the engine and the plain plates are being kept still by the gear box/rear wheel.  If you are in neutral and the lever is out then the plates turn together as they are not connected to the rear wheel through the gear box.  Next time you have the primary cover off have play and you will see how it all works.

Dave.

[Phil C]

I think I understand roughly the principle of how it works.  I guess the plates, when disconnected from each other, are still close enough to be rubbing (rather than being clear of each other) and so generating heat by friction? Is that it?   Phil

[DAVE BRADY]

Hi,

Yes. In an ideal world they will not touch but also bear in mind that whilst stationary the engine is not getting cooled so everything is heating up.

Dave.

[Phil C]

Thanks very much Dave. Maybe that's what it was - anyway I'll use neutral in future. Thanks also to Chaz, Courtney and Martin S.