Hi Neil. As long as you have good compression. Poor starting when hot is a very good indication that your mag is weak. It's just not worth guessing with! A proper rewind and mag overhaul is one of the best investments you could make. Ron
Hi Neil,
Sounds like a duff mag as Ron says. I have had this experience with my M20, runs and starts fine when cold and it will keep going forever as long as you don't stop it. Calling in to fill up with petrol only 5 mins and it won't start a good bump is the only way and you have to get the engine spinnig fast to start. When it does it splutters for a while then ok till you stop again. Sounds like you have checked everything else so it points to the mag, if you take the plug out when worm it will have a very feeble spark if one at all, a new fresh cold plug may produce a spark. All this points to the condenser which is inside the armature and not easy to change. There are ways of cutting this out and fitting one under the points but you have to know what you are doing to do this. For piece of mind it may be best to send the armature for a full rewind and new condenser, if you just send the armature make sure the rest is serviceable that the bearings are sound and the body has good magnetism, also put a steel keeper in the body when you remove the armature for a time, a piece of steel pipe near the same size as the armature will do for this.
Hope this helps.
brightspark do the condensor alternative, a chap named Bill Brown who resides in Normandy was one of the instigators for this, he posts as Groily on the AMC club site which you can join as a guest for a month maybe three for nothing, may be an idea to join and contact Bill to speak about this, he is a very affable man and would be quite willing to give advice, the part is only around£15 I have no personal knowledge of its profficiency, others say it works so it may be worthwhile to investigate. kind regards
One of my M21s back in the day had a weak mag. To get it started when hot I'd unscrew the plug close it to a tiny gap and then spin the engine up like a top with the valve lifter pulled in. Once really spinning I'd release the lifter and it would fire. Then I'd ride it to a hill, regap the plug and bump it down a slope. Ran it like that for two or three years till I got chased [in vain] by The Filth and sold it to a friend before they caught me on it. I'd registered it to an ex girlfriends address in another county so I was safe from a visit.
