Amal Concentric Mk2

Hi Guys - I have a Commando Mk2A 1974 upgraded to a 920 and have a slight problem with warm and hot starting. It has been fitted with a single Amal Mk2 Concentric since prior to being rebuilt as a 920 - it runs well and starts from cold brilliantly (it started today first kick after standing in the cold garage for 4 months) however after running for 3 or 4 minutes I killed the engine and then it would not start unless I applied the cold start (choke) - this is also true if I stop the engine after a good fast run and the engine is fully warmed-up - once started the cold start lever can be switched off immediately and the engine idles fine and runs well. The carb is a 36mm (which I suspect may be too big - everybody seems to favour 34mm) - I've tried #2.5 and #3 slides and also a range of pilot jets from 35 right down to 10 (which is currently fitted) but nothing seems to help much. Do I need to bite the bullet and buy a 34mm unit - note that I have a spare new 36mm complete Mk2 carb - do you think it cost-effective to just buy a new main 34mm carb body and slide and biuld a 'new' carb from the parts as I imagine most other parts (float bowl, float etc. will be 'common') - the INOA technical digest suggests the following setup ( Iassume for a 850) :-

#25 pilot jet
#3-1/2 slide
2A1 needle
#106 needle jet
#260 main jet
34 mm body.

Any ideas greatfully received.

Hello mwoo,

The carb is a 36mm (which I suspect may be too big

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I wouldn't think a 36 is too big for a 920

have a slight problem with warm and hot starting

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Perhaps the float level is a little too low or you are starting with the throttle to far open.

pilot jets from 35 right down to 10

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The correct size of pilot jet is proprotional to the number of turns out of the air screw, that is, roughly one and a half turns out. If you have found you do not need to adjust the air screw to accommodate the various jets you've tried perhaps there's a fault in the carb's primary circuit.

But hey! what's the problem? Hell ! it starts :lol:

I hope this is of help.

Cash

Thanks Cash - I hadn't thought about the float height - you may be onto something - I'll check it.

Cheers,

I have Mark II's on my 78 Bonneville and I generally have to have the choke on to restart the engine, throttle closed. It is easy to flood though. If after a couple of kicks it won't start, take the choke off and open the throttle about 3/4 and try again.