1948 Model 18 stop/start problem

Sounds like blocked petrol filler cap breather hole. Try a pin through the tiny hole.

If it re-starts when hot it isn't likely to be the mag.

It sounds like the petrol tank filler cap has either got a blocked airway or if new it may be the cap is one of those without an airhole.

Sounds simple but I had the same issue after a rebuild.

Good luck Iain

Previously carl_sanders wrote:

Previously iain_murdoch wrote:

It sounds like the petrol tank filler cap has either got a blocked airway or if new it may be the cap is one of those without an airhole.

Sounds simple but I had the same issue after a rebuild.

Good luck Iain

Hi,

I have tried that even drilled the hole out to 1mm and run with the fuel cap open but it still does it.

Have you pulled your petcock and cleaned your screens?

Previously carl_sanders wrote:

Previously michael_sullivan wrote:

Previously carl_sanders wrote:

Previously iain_murdoch wrote:

It sounds like the petrol tank filler cap has either got a blocked airway or if new it may be the cap is one of those without an airhole.

Sounds simple but I had the same issue after a rebuild.

Good luck Iain

Hi,

I have tried that even drilled the hole out to 1mm and run with the fuel cap open but it still does it.

Have you pulled your petcock and cleaned your screens?

I have cleaned the main and on the reserve I have removed the screen completely to just try but that did not work either

when I have both open

Is your petcock one of the Euwart (sp?) type or something else? Have you taken the petcock apart to see if there is an obstruction in the internal passages? Also, have you tried draining the fuel into a can 5 or 10 minutes to see if the flow rate changes to a dribble?

I had a similar problem with my rebuild engine. I eventually worked out it was due to Ethanol in the fuelgunging up the carb. The float and needle were both sticking and cutting off the fuel supply. The solution was Ethanol proof carb bits.

But..........don't ignore other possibilities such as sticking valves, partially seizing pistonsor a magneto failing when warm/hot. Some plugs also fail once they get hot. I would be very tempted to put back some of the old ignition gear so as to rule this out. eg a used plug kept as a spare.

OK. I note that you have fitted a new carburettor, I have read about problems with new units having a few problems with sharp edges or burrssnagging float needles, momentarily stopping the fuel flow to the bowl. I know this sometimes happens to the Monobloc series 3 carbys, but I presume you have the correct type 276 fitted. I think to eliminate the carburettor, I would remove the small blanking screw in the bowl banjo and let the petrol run, just to see if there is an uninterrupted flow.

Also, I like the suggestion from Phil (above) that a valve could be a bit tight in the guide, preventing it from fully closing when hot. As this happens, you may hear the valve train getting a bit noisier as the engine stops firing?

Paul

Further to Paul's suggestion - I have just had problems with getting my prewar 16H. It would die on small throttle without full air slide. Pilot air passages were partly blocked - they look so small as to be almost insignificant and adjusting the pilot air screw has very little effect - but cleaning it from debris made a huge difference. So maybe some swarf lying about? I had to dismantle to remove the inner jet block.

It could well be a valve sticking in a guide when warm. I had asimilar problem on an iron-head BSA after a top-end rebuild. The exhaust guide in particular needs a bit of clearance. I gave the BSA 3 thou and it has been fine ever since. Is your compression OK when it stops. It there a click when you kick it over after it stops?

That's .003" clearance between the valve and the guide, not the valve (tappet) clearance. The same thing happened to a friend's Thruxton. Unexpected engine stop when hot, re-started OK. It was a valve too tight in its guide.