AJS & Matchless Owners Club

Need some advice on misfire that develops usually after about 3/4 hour of running when deaccelerating comng up to a junction or similar.On acceleration from juctions engine misfires for a 10 - 20 seconds then clears and off we go again runs fine with no misfiring until the next junction. I've altered jet needle height making it richer which improved matters immensely doing 50 miles trip with no real issues until nearly home when we had a blip . Checked points and regapped to 10 - 12 thou was set very wide 18 -20 thou checked timing which seemed to be about right. Carb was taken off 2 weeks ago and cleaned throughly tank taps out and tank rinsed. Will be re setting points back to huge gap or ripping mag off for testing.

Any ideas gladly received

A poor mag will break down under load, so its probably not that if the bike runs well and apparently 'clears'. I wouldn't create a too larger points gap as this will only make the mag work too hard. I would suggest that the fault is the carb, possibly the slow running/idle jet. If this is blocked or set too weak then the mixture is too weak or just fumes when accelerating from a stop or very small throttle opening.

As you altered the carb to give a richer mixture and that made a significant improvement it seems that maybe the basic cause of the problem.

Do you have the correct heat spacer between the inlet manifold and the carb?

My thoughts being too much heat is transferring back to the carb and weakening the air fuel mixture by evaporation before it gets into the engine.

Also there could be an air leak at the mating faces of the carb, spacer & manifold made worse when things get really hot.

Thanks will try a spacer as there isn't one fitted however there is an O ring sealing the carb to manifold. The carbs 3 years old.

Advice needed on settiing air screw when ticking over.Do you alter it until the engine runs best and then set the throttle screw ?

Prior to adjusting needle height the engine was getting v hot and it ran like pig after 30 mins or so. Shouldn't be any crap in the carb as it was stripped down and cleaned.

quote:
Advice needed on settiing air screw when ticking over.Do you alter it until the engine runs best and then set the throttle screw ?


Hi John
Try this:

Pilot Jet adjustment.
With engine idling too fast with the twist grip shut off and the throttle shut down on to the throttle adjusting screw, and ignition set for best slow running: (1) Screw out throttle adjusting screw until the engine runs slower and begins to falter, then screw pilot air adjusting screw in or out to make engine run regularly and faster. (2) Now gently lower the throttle adjusting screw until the engine runs slower and just begins to falter, adjust the pilot air adjusting screw to get best slow running: if this 2nd adjustment makes engine run too fast, go over the job again a third time.

Tried adjusting Air screw/throttle which seemed okay once done so off we went for a spin for 30 or 40 mins and all seemed well no misfiring/spluttering at junctions etc. On way home feeling very pleased.Stopped at back gate on prop stand and then it all goes pete tonge !.

Sounded like it was running on one cylinder then it 2 cyl kicks in then out ! out with screwdriver nothing ! Umm is it electrical ?

Your thoughts anyone !

John , you can borrow my carb off my G11 , as it's off the road at the moment with the alternator adaptor sheered off in the crankshaft .
If after that you still have problems , it has to be the mag

give me a ring in the morning if it helps

PeterEdited by - g80csp11 on 22 Apr 2011 07:18:40 AM

quote: Stopped at back gate on prop stand and then it all goes pete tonge

When the machine leans over to the left, the fuel level lowers in the bowl under the jet block and can tend to lean off the mixture.

Paul
G.M.

Pete thanks for the offer will call 2mrw. Started to poke about with mag today and found that one of the pickups isn't good which can't be helping at all. All a bit novicey i know but i'd only just got my head around a single cylinder AJ last year and now i've got a twin. Twice the trouble me thinks least that what me brother says !

Had wondered about fuelling issue when on propstand problem fairly major though so inclin would suggest problem may be elsewhere.

Thanks for the help

Hi John, I have a spare twin mag and new pickups that you are welcome to borrow for process of elimination, or pop over to Haywards Heath and I will help you sort it. And Davis