AJS & Matchless Owners Club

On my 63 engine I am setting up a new monobloc, unfortunately it is 1 3/16" not 1 1/8".

It starts and runs well until 3/4 throttle at which point the engine dies until the throttle is returned below 3/4.

What I have read suggests this would be an incorrect main jet and the way the engine cuts out without black smoke and chugging I have taken it to be a lean fuel problem. But none of the larger jets helped and running with no main jet also produced the same results.

Other things I tried, smaller main jets and various needle jets have no affect above 3/4 throttle. Everything in the ignition (distributor) swapped to eliminate a problem there (hopefully) including fitting a wasted spark coil.

The only other thing I can think of is a richer throttle slide, but the tuning guides I have read say the cut away affects mixture at lower throttle settings.

What am I over looking? All suggestions welcome

Is there a “step” between the carb bore and the manifold bore? If so, it'll never run right...

Thanks Paul, I did match the manifold spacer and carb, they have been on and off a few times so worth a look. Took the lot off as one piece and ensured to line everything up, unfortunately it has made no difference.

Ah good, just thought I'd check as you mentioned it was a 1 3/16" Monobloc...

Black smoke / 4 stroking = way too rich.

If it dies on open throttle, check the needle is on the right notch (and that the main jet is seated correctly).
You can always try lowering the needle a notch to reduce the richness at open throttle, though do this one step at a time as you don't want it going lean!

Stupid thought: is the choke fully off?

Hi

A 'hard' shut down sounds more like an ignition fault to me.

Does it always shut down at the same revs or can you vary the point by sneaking up or racing up to it?

If it varies I'd look to carburation, if it's constant I'd look to ignition.

For ignition I'd look to vibration causing grounding within the CB assembly, (or maybe a duff capacitor).

Regards Mick

Choke removed and blanked. Have the twist grip marked in quarters, always occurs at 3/4 mark, no tacho may have rig one to confirm if it is always same revs. Have some condensers from a different source which I will fit and try on Sunday.

Thanks for bouncing ideas

Did it run O.K. before you changed to the 'oversize' 1 3/16" carb?
Does it also die in 2nd and 3rd gear at above 3/4 throttle opening?
You mentioned in a previous post that you have removed the choke slide and blanked off the choke cable entry in the carb top. I just wonder whether the void in the throttle slide (that used to be occupied by the choke slide) is causing some turbulence in the airflow.......? I'm merely guessing here as I've never run a Monobloc without a choke slide but perhaps it's worth refitting the choke slide.

I did match the manifold spacer and carb, they have been on and off a few times so worth a look. Took the lot off as one piece and ensured to line everything up, unfortunately it has made no difference.

I may be getting confused here, but if the spacer and particularly the port in the cylinder head casting were originally for a 1 1/8" carb how can their diameters match your current 1 3/16" carb without a 'step'?

A Monobloc works just fine without the choke slider (as long as the cable hole is blanked)

I'm still a bit worried about the manifold “step”. It's super-important that the carb-manifold path is smooth, otherwise you get fuel droplets and it'll run awful...

clanger9 wrote:A Monobloc works just fine without the choke slider (as long as the cable hole is blanked)

Useful to know, ta. It's not the choke then.

I'm still a bit worried about the manifold “step”.

Yes, even though it's only a step of 1/16" which doesn't sound much (or 1.59mm which sounds more) it would be an obstruction all the way around the inlet orifice.
It could still be ignition though - maybe the points spring is weak permitting 'points bounce' at higher revs - (if there is such a thing ) Or an electrical short to earth due to vibration as Mick D surmised..........(although it seems to die rather than misfire).

Here's a great explanation of why manifold steps matter:
http://archives.jampot.dk/technical/Car ... mplin).pdf