AJS & Matchless Owners Club

I have recently bought a nicely restored 18S. The revs are fine thru' the first 3 gears, but when I change into 4th, the revs falter.....stop & go. Any ideas on where to look for a solution?

It suggests a problem with the carburation. What carb have you got? If monobloc look under the mainjet to see if there is a bit of crap blocking the jet. Check on jet size and needle position, assuming you are using more than 3/4 throttle then the needle and jet are the control. According to Roy Bacon book a 1956 18 should have a 389 monobloc with 1 5/32 bore, 260 or 250 mainjet, 30 pilot, 3 1/2 slide, needle position 3 and needle jet .106. Check all that out and for air leaks (does it bang on the overrun if so definitely air leaks) and if it is all ok you probably need to think about the mag but not sure why it would rev in other gears if this is the problem. Good luck. Clive