Engine tends to cut when hot

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.

Coil?

Blocked pilot circuit, use the pilot adjustment screw and see if at idle the revs change. If the revs are constant your pilot circuit has a blockage.

Your scenario sounds like a lot of idling. Idling is bad for a Norton - they get way too hot. Tickling a hot engine should not be required so like @kommando said, clogged pilot circuit. Of course if can't tickle, then also are not getting gas to the carb so the tank bag may be the/part of the problem.

Vapor lock?

marshg246 said:

Your scenario sounds like a lot of idling. Idling is bad for a Norton - they get way too hot. Tickling a hot engine should not be required so like @kommando said, clogged pilot circuit. Of course if can't tickle, then also are not getting gas to the carb so the tank bag may be the/part of the problem.

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The tickling bit is just to check for fuel flow, normally not required. It’s a symptom rather than a cause.
I agree that prolonged idling is bad far a Norton (like most air cooled engines), but considering that the Plod were major users of the Commando, it’s not normal behaviour.

SteveBorland said:

but considering that the Plod were major users of the Commando,

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You can usually rely on a national agency to buy the wrong vehicle.

If you only have thin gaskets and O rings, the heat from the head will feed into the carbs causing vapourisation of the fuel. Ideally a minimum of 6mm insulators should be used

Are you using the hard material ("Tufnol"?) Insulators mani to head or just actual fibre gaskets?

Next time try touching the float bowls with bare fingers....ive had them too hot to touch in past begore fitting the thicker insulators. This will boil the fuel in the bowl, making for lean condition initally then stalling out. Loss of tickle flow could be b/c bowls nearly empty of liquid fuel.

I’d pull the fuel taps to insure the filters were clear. I’ve heard that some Indian tanks have liners. Liners can dissolve and create havoc.

Next up, clear the vent in the gas cap.

And yes, Norton’s can Vapor lock.
Confirm proper manifold insulator.
Thicker the better. Thicker also creates clearance issues with the Ham Can.

Andy Higham said:

If you only have thin gaskets and O rings, the heat from the head will feed into the carbs causing vapourisation of the fuel. Ideally a minimum of 6mm insulators should be used

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How do you get 6mm gaskets? Even the thicker ones I have are only perhaps 1mm.....

SteveBorland said:

How do you get 6mm gaskets? Even the thicker ones I have are only perhaps 1mm.....

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AN has:
Thin 06.7842 (30mm)
Thick 06.2700 (32mm), 1/8" (.125", 3.175mm) thick (these will work for 30mm as well)

Amal has:
ABF814 30mm and ABF815 32mm, they are about 1/4" (.25", 6.35mm) thick

These are not gaskets, they are heat insulators.

SteveBorland said:

How do you get 6mm gaskets? Even the thicker ones I have are only perhaps 1mm.....

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If ( if) the problem is due to carb overheatiing in stop and go traffic on a hot day, then it is probably also radiation, and an insulator gasket won't be enough. Maybe a heat shield can help? Normally a Norton can stand the heat pretty well. Today I did 400 km in (partly) over 40 °C ,like riding in a furnace, but the bike behaved perfect. ( open roads)

-When you can prove fuel flow with the tickler, the bike runs, When you can't prove fuel flow with the tickler the bike doesn't..... and that doesn't sound like a fuel delivery problem??

ludwig said:

If ( if) the problem is due to carb overheatiing in stop and go traffic on a hot day, then it is probably also radiation, and an insulator gasket won't be enough. Maybe a heat shield can help? Normally a Norton can stand the heat pretty well. Today I did 400 km in (partly) over 40 °C ,like riding in a furnace, but the bike behaved perfect. ( open roads)

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Ludwig, as far as I recall, you have rubber mounted your carbs? Yet another reason to go this route

o0norton0o said:

-When you can prove fuel flow with the tickler, the bike runs, When you can't prove fuel flow with the tickler the bike doesn't..... and that doesn't sound like a fuel delivery problem??

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Yes, exactly. but I'm rather puzzled as to the why bit. I don't think its related to the fuel cap vent (the Indian cap is rather a poor fit and is probably boosting the income of the oil producing countries by allowing the tank contents to evaporate in the sun!) since opening the cap does not produce a whoosh of air filling a low pressure tank, nor does it solve the delivery problem. rotating the petcock from main to reserve and back again _seems_ to work, bit its not conclusive. Wanted to take the bike to a party today and do some testing, but the bugger refused to behave (fouled plugs, probably from yesterdays experimental starts), so I took the car instead (hangs head in shame, bit a) I was in a hurry and b) it was rather too warm to be buggering about with it).
Tomorrow's scheduled test run might provide an answer.

SteveBorland said:

How do you get 6mm gaskets? Even the thicker ones I have are only perhaps 1mm.....

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If you can't find proper phenolic insulators , go to a proper industrial electrical supply business, and buy that brown insulating electrical board in the thickness required. There is actually two types one is reinforced, I don't think you'd be able to tell the difference. And then , hole saw and hacksaw/file/drill it out . Cheers

SteveBorland said:

Yes, exactly. but I'm rather puzzled as to the why bit. I don't think its related to the fuel cap vent (the Indian cap is rather a poor fit and is probably boosting the income of the oil producing countries by allowing the tank contents to evaporate in the sun!) since opening the cap does not produce a whoosh of air filling a low pressure tank, nor does it solve the delivery problem. rotating the petcock from main to reserve and back again _seems_ to work, bit its not conclusive. Wanted to take the bike to a party today and do some testing, but the bugger refused to behave (fouled plugs, probably from yesterdays experimental starts), so I took the car instead (hangs head in shame, bit a) I was in a hurry and b) it was rather too warm to be buggering about with it).
Tomorrow's scheduled test run might provide an answer.

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You keep mention the fuel tap switching thing, which is odd, doesn’t really make sense.

Maybe faulty taps, swollen internal seals, etc?

Any progress Steve ?

This problem is affecting both cylinders, no report of chugging on one. If the carbs are mounted on rubber manifolds, the conductive heating suggestions are out the window. Tank low on fuel? I've seen Indian tanks with some really weird baffles.