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I don't think those in the UK do much riding in temperatures over 100 degrees (38 degrees C) and don't have any big hills or mountains to climb on their M20 when it is this hot.
My M21 will on a very hot day be very difficult to start in these temperatures unless the carb has cooled down. The bike will run fine but once parked the carb will boil over in a few minutes, squirting fuel out the float bowl vents on top until it has no fuel left. If you turn the fuel tap back on, it boils out of the carb also.
I rode my M21 last year in a temperature of 118 degrees up a small mountain to 4000 feet. The bike ran perfect but once I parked it, the carb immediately boiled over. I measured the cylinder head temp at 580 degrees. After lunch and a few pints of beer it had cooled down enough to start on the first kick. Fitting insulating gaskets to the carb helps a little but not a lot when it is this hot.
In the winter, no problems at all.
By the way has any one else noticed that other than the cylinder and head, the rest of a M motor runs much cooler than and overhead valve bike. Even after long rides the oil tank never gets all that hot. My pre-unit Triumphs in the summer, you can't touch the oil tank.
Prompted by John Parker's request for problem solvers, this is my solution for hot carbys. I machined a shallow groove on each side of the insulating spacer to take a 1/8inch viton O-ring sitting slightly proud of the spacer to be compressed/squeezed into the groove & against the barrel on one side & the carby flange on the other side to give a air tight seal. No other gasket is required. You do of course have to make longer studs to suit. My bike is 600cc & running Goldie touring cams & in Queensland (OZ) with summer temps of 30plus I have had no problems over a couple of years. I pulled it down recently & could have used the O-rings again but hang the expense, new ones went in.
Good to see some one contributing as to what solved their problem. Cheers. Bonza.
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These BSA singles, both SV and OHV are very 'cool running' in terms of heating up the oil...OHV models in particular are known for it...Ian
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Bruce, do not drink and drive. 🥴
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