AJS & Matchless Owners Club

My 1968 G15CSR is fitted with twin Mk1 Concentric carbs. The engine has recently been rebuilt following a blow-up where the right hand piston disintegrated. Prior to the blow up the bike ran perfectly but now runs roughly at speeds over 30mph and does not want to rev. I have checked the carb settings and everything is per the book.

I am getting frustrated with the twin carb set up and am thinking of fitting a single carb conversion. I believe Allen Performance do a kit with a single Mikuni carb for the Atlas engine. Does anyone have any experience of this conversion?

Hi Alan I have used this kit on 3 G15s the best money I have ever spent.

Norvil supply a single carb manifold for the 750 twins. You must have one of the last G15CSR street scramblers, as you say it is a 1968 model. So the engine number should start with 123. I also have a late G15CSR street scrambler, and i bought it from Coburn & Hughes back in December 1970. It has always had the twin Amal Concentric carbs on it, and it has alway ran perfectly fine with them fitted. I had the carbs overhualed by Martin Bratby, and they work perfectlyf or me. There is also the JRC carb conversion, which is an updated Carb for use with modern fuel. It is very easy to blame the caburation, but check all the usual suspects. This would be the ignition, and engine settings, and then check the carbs. Do you have a brass filter fitted over the main jets? This can also cause allot of problems. You may also have the original factory supplied float bowls. These were modified so that the jet holes went to the base of the float bowl and not just half way down. You could also have a block idol circuit. This is a very common occurance wit the Amal Concentrics. Give the carbs to Martin Bratby and he can resleave the slides, and clean out all the inner drillings and working of your carbs.

It has taken me a while to get round to fitting the single carb and I have to agree with Roy that it is money well spent. Although it is only early days the bike seems transformed. It runs smoothly and evenly throughout the whole range. The difference between the original Concentrics is remarkable.

Fitting was straight forward (a bit tight getting the air filter on) and it was running well with minimum adjustment.

I would recommend this to any fellow hybrid owners.

I always ran single carbs on my Commandos - much smoother and more flexible, easier to set up, and I didn't see any difference in performance at all.

A little off topic but do you mind me asking just how the blow up happened? I've seen and read of a few examples that just blew up, always had me thinking why this would occur so often on an Atlas engine, is the oil pump good enough for it's duties?

Also do all of you find the bike suffers at the top end due to the single carb setup?

Verbatim wrote:
Also do all of you find the bike suffers at the top end due to the single carb setup?

I didn't on my Commandos but then I rarely used full throttle, which is where any effect would be. Anything up to about 3/4 it makes no difference at all, and at lower revs the faster gas flow makes it more flexible.

I think the blow up was due to old age. The timing side piston top came clean off on the ride back from Founder's Day in 2012. Most of the piston disintegrated but it held on around the gudgeon pin, probably saving the con-rod from flying through the cases. John Bolton at Workshop 14 (who rebuilt the engine) advised that this can be a problem with these pistons. Bizarrely when it happened the engine kept running (it was a little rattley). It even re-started after I initially stopped, ever the optimist I hoped stopping and restarting would cure the rattle.

Obviously I haven't had the engine flat out as I've not done many miles since the rebuild but thus far performance seems on a par with the twin Amals. Perhaps President Roy can advise about the 3 bikes he has the single carb fitted to?

My single Carb bikes are just a joy to start and ride the two with twin Carbs are hard to start, as for the speed matter I would say no real difference but I never ride that quick any way. It may be the twin ones are a bit better from the off.