Engine tends to cut when hot

Fast Eddie · Aug 26, 2025

Yakatak said:
If the carbs are mounted on rubber manifolds…

They are not…

Shelby-Right · Aug 27, 2025

SteveBorland said:
I've been using my Commando to ride to work this week. It's been fairly wam weather (for Denmark at least), around 28 -30 C. The traffic has been horrible, long queues where filtering is not often possible. Twice now the bike has simply cut dead while at walking speed, once on the motorway, which was an interesting experience trying to push over to the very narrow outer edge.

I noticed that when trying to tickle the carbs, there was no fuel flow. Switching from main to reserve produced no change. I was using a tank bag, so next thought was a vent problem so I opened the tank cap. No change in the tickle behaviour. Switching back to main again produced fuel and off I went. Ran perfectly well even when giving it stick, until when very near home, I managed to stall the engine at an uphill stop sign. With the high compression pistons, I simply cannot kick the bike on its wheels, so another few sweaty moments pushing uphill to the side of the road, when the tickle problem repeated itself.

It's an Indian tank and fuel cap which I'm actually quite certain does not seal as well as the original. The petcocks are new BAP type, which flow fine when tested. I have the thin gaskets on the manifold - head and o-rings between the carbs and the manifold.

Since there's no problem when running at speed, I don't think it's a fuel flow problem. I think I have a set of thicker gaskets in the workshop, and I can try gaskets instead of O-rings on the carbs, but otherwise I'm a bit stumped.

Could your float level be too high. So it's OK at speed because its using gas, at idle level gets higher..?

Fast Eddie · Sep 9, 2025

@SteveBorland … any progress or conclusion ?

SteveBorland · Sep 9, 2025

Fast Eddie said:
@SteveBorland … any progress or conclusion ?

I'm afraid not. so far.
A), the weather has cooled off considerably, b) I really don't like riding to work now, due to the horrible traffic so I've not been able to recreate the problem situation again.

Actually, that's quite a relief - having the bike die when in the "fast" lane traffic queue and having to push the dead bike uphill across the "slow" lane to reach an off ramp(*) where it might be possible to fix the bugger is not really high up on my list of "must do again" things

.

The only thing I've found relating to the fuel problem is that the BAP taps I have fitted have 3 lever positions but only one position where it's actually open - the bodies are only drilled for a on/off function rather than on/off/reserve. This leads me to suspect brain fade on my part, since I thought that switching between on and "reserve" made a difference. It doesn't!

(*)Most of the motorway here has an emergency lane, but the Norton has now managed to die twice when on the few bits without any hard shoulder, on both occasions requiring an uphill push! The first time it was an earth lead that had detached it's-self, so no relation to the present situation other than the placement (which to top it all was in a tunnel....)

Engine tends to cut when hot

Fast Eddie

Shelby-Right

Fast Eddie

SteveBorland

Similar threads