I have enjoyed about 700 miles of trouble free riding since finishing the WM20 last Feb. However during a run out of Friday she nipped up on me. I had noticed a bit of loss of power over the previous few miles befoe she started to nip up. I pulled the clutch sharpish and came to a halt. After a few minutes a bit of pressure on the kickstart confirmed the piston had freed up and I was able to start and ride gingerly 7 miles home.
I pulled the plug on getting home and it looked a bit grey rather than brown. I checked the ignition timing and that is fine (marginally over advanced on full advance) and points gap 12 thou. I pulled the carb part and all seemed OK. Tappet clearances spot on.
You don't state at what throttle opening you had when it nipped up. A weak mixture will cause the engine to over heat. Try lifting the jet needle one notch to allow more fuel at middle throttle openings.
Hi Matty
Thansk for the reply. Having checked other likely causes I too was leaning to a weak mixture. As she now has a few miles under her belt I was giving her a bit more welly on my last ride. I did notice that she seems low on power at wider throttle openings and as I said the plug was greyish on returning home.
I'll try your suggestio of raising the neeedle a groove and if this does not sort it I will try a larger main jet.
I am a bit confused as to the needle jet as this does not have a size stamped on it, just 'amal' (it should be a 106 according to Burlen).
If you were at full throttle the size of the main jet controls that and not the needle jet. If you've done 700 miles on mainly half throttle I would expect the needle jet and needle position to be correct. A carb repair kit includes all the jets and needle etc and costs about £20 so just change the lot. I've just bought one and was surprised what's included for that price. Ride the bike at about half throttle, kill the engine and check the plug, then the same at full throttle. Another way to tell if it's running too lean is to ride at a fixed speed and put the choke on, if it goes better you need to richening the mixture for that throttle setting.
Horror, Thanks for the reply. I'll try the main jet and do a plug check.
I've also checked for air leaks but no evidence.
In the meantime I've got the clutch cover off to check an engine/transmission noise source. Jeez, getting the tin clutch cover off is a time waster. Not the source of the noise but took the opportunity to check clutch lift, oil tightness etc. Blimey, what a lot of rust in there after just 8 months!
Managed to pull one of the 6 pressure plate studs out in the process....GRRRR So just brazed it back on and surface filed/grpound flat again. Now need to knock up a new clutch dome gasket. I'll make do with the dents that have developed in the so called hardened pushrod.
Still debating the 4-spring clutch conversion. I get on with these really well but the cost is somewhat painful.
Old stock needle jets were only marked if 105 or 107 as the standard needle jet was 106 and fitted to 90 per cent of bikes so they did not stamp them. Modern needle jets are stamped with the size including 106.
