I'm relatively new to classic British motorcycles and am having a tremendous amount of hassle starting my 1948 m20,
I get a rumble every 40 kicks or so but with no joy of maintaining it! The timing seems to be okay, I'm using new fuel and have a new spark plug, I'm also getting a spark when holding the plug on the end of the lead. Easy start through the carb will still not make it have it? I'm well past the extent of my knowledge now and am really stuck with it! Any suggestions or information will be appreciated.
If you have got fuel and a correctly timed spark then in theory (everything else being correct) it should fire. Test the easy bits first.
Is it a big fat healthy spark? Even though the plug is new, as has been commented on here before, it could be a rogue one. A weedy spark might completely fail under compression. That is the easiest check, put another plug in it, even an old one might work better. Have you checked the tech section for the timing tips? Have you timed it with the cable set correctly?
If you have fuel is it getting through to the carb? Is the fuel cap hole that prevents a vacuum building up in the tank blocked? Just loosen the cap to test it. Is fuel getting through to the carb? I.e. coming through when you 'tickle'/flood it for starting? If not then check for a blocked pipe or blocked tap. If it is getting to the float chamber, is it getting through the jets?
If you have fuel to the jets and a correctly timed fat spark then you need to consider other things. Is the valve lifter keeping the exhaust valve open? Are the tappets too tight and not letting the valves seat? Rings knackered and no compression? Do you have good compression? I.e. good resistance at tdc on compression stroke or does it kick over really easy?
Hi,
if BTB's advise doesnt help, you better check the ignition timing once more.
See the technical pages and use the small bolt in the cyl head.
A healthy M20 should start in one or only a few kicks....
Check you timing as I have the same issue with my 3HW after a rebuild and I had set the timing wrong
Also check the passion of your points open at as it should be around 10 oclock I believe
