AJS & Matchless Owners Club

Hi all,

New problem on my '55 G80CS.
Just out and about showing off my newly painted tank and testing the freshly rebuilt Generator.

Strangly, when pulling up a longish hill merging onto the motorway, the bike began to miss, pop, then come good momentarily, then miss pop, and snuff out, but, could be recovered by backing off the throttle then reapplying.

Carb seems clear, fresh petrol etc. My '56 did exactly this the other weekend, ALSO, when putting the freshly painted tank on and going for a test ride. Mind you, it would actually stall and, happen much more consistantly.

So, my '55 is now playing silly games. On my return trip home, (after exhibiting this issue 5 times in total, all up some sort of hill, under load, but not necessarily labouring badly) the bike behaved perfectly, and no matter how much throttle I gave it, it did not faulter.

Any clues? Could this also be the condensor or something to do with the maggie? Points look good, though I haven't checked the gap. Timing is correct as I could get it prior to going for today's ride.

Any help greatly appreciated.

In addition,... I'd love to see a list of failure symptoms for one of the great mysteries of the world...

The Lucas MAGNETO!

Breather in fuel cap partially blocked?

Stuoyb wrote:Breather in fuel cap partially blocked?

Fuel Lock?? Interesting. I just made a new Tyre Tube seal for the cap before riding it today.
What's the theory?

EDIT: Ten points to you. Thank you. 'Always' something simple. I just opened the tank and it sucked in a gob full of air. Blocked indeed.
Seems Rubber tyre tube does not like petrol vapour and it expands and goes all crinkly, enough to block the breather hole.

Here I was being smart and making a new seal to stop petrol leaking out the top all over my new paint!
I'm guessing I'll have to track down some cork?

Breather hole in fuel cap allows air to be drawn into the tank to prevent an air lock.
Polish/grease/dust can partially block the vent hole restricting fuel flow to carb.
Your new seal around the cap could be the tipping point restricting the flow.

I sprayed a cap silver on one of my little Reliants years ago, I got about 15 miles . At the time about a mile into Hatfield forest. Car stoped. No fuel gauge. I was sure I had plenty of fuel. Took off the cap to try a dip stick in the tank. As soon as I removed the cap I could hear air being sucked in. Yes, paint blocked the cap hole. Your bike will not quickly stop as a warning if your oil cap gets the hole blocked.

Interesting development.

I've just ridden my '56 G80CS, and whilst going up a long hill, it began to 'bog' just like the '55! Now, the Mag was just rebuilt with new EVERYTHING, and no, not NOS. Timed to perfection (well, near enough that I can get it) it goes like a rocket, except for long drag up a hill. Now, this only happened on the way home, and just like the '55, it came good with a back off of throttle, then reapply under less load, and went like a rocket again.

The '55 did it quite badly the other day, to the point of almost grinding to a halt. It did it whilst trying to accellerate up a long but not steep incline, to overtake a car, and it started to slow, and lose power.

Both bikes run a 389 Monobloc, and *seem* to be jetted well. The '56 goes like a rocket in comparison to the '55, (short stroke vs long stroke) but seemingly, have a similar problem!

Until now, I've been thinking the Magneto on the '55 is on the way out, as it feels like dropping power and dieing, under heavy load, but now that the '56 experienced this....

I'm almost beginning to think it's either flooding, or, something to do with not burning all the fuel in the combustion chamber? In both cases, the problem shows up with a constant throttle, under load. ONCE the problem arises, adding throttle makes the problem worse, reducing throttle makes it come back.

My ideas as it stands:
1. Not burning all fuel? - Need hotter plug?
- Lean off main?
2. Flooding? - Float level?

Both bikes start easy, idle well, and run very well when singing along the motorway... just seems strange that the '55 has this issue with hills / load, and now I've experienced it briefly with the '56. I've only ridden the '56 4 times now, so it's behavior is still becoming known to me.

Any ideas?

Could be ignition; not enough voltage for spark to jump gap when under load. It will be fine on level road or downhill.

Rob C

Matchymarty wrote:
it goes like a rocket, except for long drag up a hill............ back off of throttle, then reapply under less load, and went like a rocket again.

It did it whilst trying to accellerate up a long but not steep incline, to overtake a car, and it started to slow, and lose power.....and now I've experienced it briefly with the '56

Any ideas?

Since it has started happening to both bikes at more or less the same time it's likely that it's not ignition related.
Ignoring the remote possibility that both engines are overheating and nipping up...........

Unlikely as it may seem, the symptoms you describe are absolutely typical of carb icing or frosting. The ambient air temp doesn't have to be particularly cold. I have had the problem when under load on full or nearly full open throttle in summer at an air temp of anywhere up to 20 degrees, particularly in early morning or evening mist or daytime damp air. Perhaps the changing season to your winter weather conditions are to blame?

Rob,
I thought of the ignition thing, and was blaming the Mag on the '55. Then, when it occurred on the '56 yesterday, with a freshly professionally rebuilt top to bottom Mag, I figured it can't be that. The mag was rebuilt due to dying on hills, to the point of stalling. Aparently, the condensor was gone, and there was a break in the armature.

Spriddler,
Thanks for the idea, but I live on the Gold coast in OZ, and although we are at the end of Winter, it was 23 deg C, and T-shirt and Leather jacket weather. Also, in all cases with the '55, the problem exists in warm or cold weather - doesn't much matter. I've only had carb icing issues once in my life in OZ... back in 2000, with my ZX6R Kwacka, and it was -5 deg C. hehehehe.

Rode the '56 to work today, 40 miles, ran perfect, except the tail light lens fell off, battery stopped charging and couldn't use headlight after 10 mins on the motorway, and it was 5am, so dark all the way... Damn these old regulators! DVR2 here I come! Now all my old bikes will be converted.

Load is the only issue with this snuffing thing, and seemingly only on long steepish hills.

So you don't think float level could do this?