I have owned my 1941 M20 for over 40 years, and it has always been reliable - as you'd expect. Until recently when it would stop after 10 or 12 miles each time it was ridden. Not fuel or plug problem. I had the mag rebuilt, but the condenser was not changed as it was the "modern" type.
First ride after refitting the mag and the same thing happened. Let it cool for an hour and it restarts. I suspect the condenser is the culprit, the magneto man says that's very unlikely [ but will change it if I insist]
I know M20s can be poor hot starters, but that's usually fuel evaporation. Any thoughts ?

Classic symptoms of a spark plug of the wrong heat range. Have you changed the plug recently? Second is fuel evaporation or vapor lock but if you have an insulating gasket between the cylinder and the carburetter this shouldn't happen.

email (option): jonny.rudge@verizon.net

I've had experiences on other engines in the past with this sort of thing being caused by insufficient valve-clearances - fine when the engine is cold but as it gets hot they'll close up until the valves arent shutting properly . This is very common to old flat four air-cooled Volkswagen Beetle engines , for some reason - happens periodically even when those engines are well looked after .

email (option): leifevans75@yahoo.co.uk

hi peter,this has been discussed before recently,it seems to be occuring more and more.
i have a friend with a sunbeam s7,his friend has one also.
some time back they both filled up at the same station,since then they have had nowt but trouble.the bikes will run ,then just cut out.
the fuel has been drained/changed ,no difference.the bikes will cruise ont motorway fine,soon as they exit .and join normal traffic ,gone,they have even swapped the carbs with each other and it still occurs.
he is so p****d with it he wants to sell
ive read an article in real classics about ethanol causing the carb to ice up,maybe this has some bearing on the problem.could be your condenser,if they will change it for free get it done ,its one route closed off hope you find the answer cheers rick

email (option): richardholt@rocketmail.com

I would check your spark plug grade etc as recommended by John, Then check the spark when the magneto is cold and again when hot. If you have a fat, blue spark when cold and a skinny, weak and pale spark when hot it will be the condensor. Frankly I am surprised anyone would 'overhaul' a Magneto without replacing the condensor..in 99 cases out of a 100 magneto failure is down to that component.
Also I would discount the effects of Ethanol fuel entirely. We still have only 5% Ethanol, as we have done for some time. with an increase to 10% pending.
Everyone is paranoid about the stuff and blaming it for duff mags, knackered coils, badly rebuilt and set up engines and any other faults they may happen to have currently....'My daughters pregnant...it's that bloody Ethanol fuel' ....
I ride my BSAs (and Harley) as everyday transport (I don't run a car) and at current levels of Ethanol have had no discernable problems whatsoever. I have filled up at stations run by all the major producers in this country and abroad. Joe Rosson (USA) reports no serious problems so far at 15% Ethanol levels, which they have had for a number of years....Ian

talking about ethanol would it cause vapour lock at a lower temprature

Here's a link to an ethanol update from RealClassic, which includes a link also to a technical study of more than you ever wanted to know about ethanol: http://www.realclassic.co.uk/techfiles/ethanol_update_june_2011.html

Have been using 10% ethanol in petrol here for about 20 years, no apparent issues. I plonk about mostly on old English singles, but I do have a couple of tasty Japanese bikes,and they don't have noticeable trouble either. I will say the petrol with ethanol in it smells extra 'orrible after sitting in a can over the winter though - not very nice stuff.

I'd put money on the condensor.

email (option): chris@classicbikebooks.com

Well said Chris...my view exactly. I clearly remember the panic at the introduction of Lead Free petrol, yet everyone still seems to have valve seats and valves. Whilst I accept there may be some issues that appear with increasing levels of Ethanol I do wish people would calm down and apply a degree of objectivity rather than running about like headless chickens every time the subject is mentioned....
Rash statements, unfounded panic and the irrational blaming of ethanol for unrelated problems are no help at all..Technical studies are often to blame for this situation as they tend to list all the theoretically POSSIBLE effects and not necessarily the ACTUAL effects. I would suggest there isn't a problem until you actualy have one that you can conclusively and logically put down to the fuel...It's a bit like when you read the sheet regarding possible side effects that comes with most medicines..you don't generally suffer from 99% of them ...Ian

email (option): ian@wright52.plus.com

I quite agree with you both , Chris and Ian . I've been reading up on Ethanol recently and it seems that not only are there ways around the potential (although as yet not particularly occurring) problems with the use of percentage volumes of ethanol in automotive fuels , but there are actually potential benefits as well - for a start it combusts at a lower temperature than fossil petrol . One of the downsides is that as ethanol is hydroscopic there is likely to be a certain water content . Again , though , not an insurmountable problem , I'm sure .

email (option): leifevans75@yahoo.co.uk

see ethanol is a con we are paying £6 a gallon for water how much extra water can we add (instrutions add water and shake the tank)

Gentlemen,
Thankyou for all the collective wisdom. The plug and fuel seem OK. As I said, I suspect the condenser [I don't understand why it wasn't renewed during overhaul] and will have it changed. It is reassuring to hear your various comments.
Ethanol may not make your daughter pregnant, but the Petseal in my tank has softened and disintegrated in the last year after about 12 years of satisfactory life.
This is a very useful, interesting and informative Forum. How did we manage without the internet ?
All the Best
Peter Smith

Hi Ian

At some future date I shall be able to share the results of research with ethanol blended fuels to include theoretical & the "real effects" regretfully found during this research

Due to sensitivity at this time I cannot post on a public forum, but believe me there are consequences for many of us. I know you Will understand why I can't at this time !!!

Job

email (option): Jonnyob1@googlemail.com