Roadholder 370 - October 2018

20 concentric experiences Andy Heathwood F or those of us who persevere with the standard AMAL Concentric carburettors, there are a few traps and pitfalls out there. However, when tuned correctly and with components in good condition, they still perform well. There is much useful guidance on the web and on AMAL's own website: www.amalcarb.co.uk . www.jba.bc.ca/Bushmans%20Carb%20 Tuning.html www.princeton.edu/ssp/65-cub-data/ library/amalbritbike.pdf www.jonsnell.co.uk/images/tuning_ a n d _ u s i n g _ ama l _mk 1 _ c o n c e n t r i c _ carburetters_.pdf In my opinion the following site has the clearest diagrams explaining carb operation: www.vintagebikemagazine.com/technical- articles/amal/ AMAL (now owned by Burlen Ltd.) continue to improve on the standard design and produce replacement parts. The improved ‘Premier’ Concentric carburettors (based on the Mk.1) are widely available. This article summarises recent experiences in reconditioning AMAL Concentric Mk.1 carburettors fitted to Norton Commandos and also provides guidance on aspects of tuning and operation. Throttle slides - too loose then too tight. The throttle slides in the well used 30mm Concentrics fitted to my 750 Roadster were very loose in the bodies and were causing poor running. I had the slides re-sleeved in brass and the bodies bored by a specialist. The slides were now a tight and even fit. This led to a subsequent problem when both slides became stuck in the carb bodies! Fortunately this was found whilst testing the bike in the garage and not out riding. I put this down to temperature changes and differences in expansion rates of the carb body and slide, since all had been OK when first fitted. From reading the forums, it looks like there are reconditioners out there who are boring carb bodies and sleeving slides to tolerances which are much too tight. The throttle slides on my carbs had less than one thou clearance. John Healy, in his guide ( www.princeton. edu/ssp/65-cub-data/library/amalbritbike.pdf ), indicates 3.5 to 4 thou slide clearance and on the Britbike forum, 'AMAL has compensated for the air that goes around the slide in how they prepared the parts that control the mixture. Barry Johnston and Jeff Binks, the two engineers that designed the Concentric, did understand how this carburettor worked when they specified the .0035" to .004" clearance. They also knew the consequences of using less. In my opinion you run an AMAL slide at less than 0.0035" clearance at your own risk.' I used fine emery cloth and metal polish to work on the brass slide covers to increase the clearance. All is OK now, but a cautionary tale. Tight banjos Whilst removing and replacing the fuel lines and float bowls, I was concerned that I might cross-thread the bolts joining the fuel line banjos to the float bowls as the bolt fit was very tight and difficult to engage. I did some research and found that the bolt thread is size 3/8" 26tpi BSCy. I happened to already have a tap and die for this thread size available to check. The two bolts were fine with the die running easily down both but the bowl threads were a different story. In both new bowls the tap was a very tight fit. I re-tapped the threads ensuring that all swarf was removed. I also found that the original float bowls benefited from re-tapping. The bolts now fit well with no concerns about cross-threading. Two recent purchases have been fine, so this may be an old problem now solved by the manufacturer. The needle moves! In use, I have now twice had the throttle slide needle and clip migrate up the throttle valve spring. This results in a rich mixture causing misfiring and a soot-black spark plug. The