AMAL Concentric Mk 1 carb tuning

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Andy51
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AMAL Concentric Mk 1 carb tuning

Post by Andy51 »

Hi All,

My '54 18S is fitted with an AMAL Mk 1 carb. Thus is one of the earlier versions with a fixed pilot jet - later ones have a larger, changeable one. The bike pops a bit on the over-run, not enough to cause a problem but it annoys me. Surrey Cycles told me this is a known problem and the jet can be drilled out to 19 thou to fix. However, I cannot see an easy way to do this, so I screwed the air screw in 1/2 turn to enrich the mixture. This brings me to the problem - I set out on a run and the bike ran terribly, misfiring and jerking badly at low speeds. I stopped and put the setting back to previous, smooth running setting; things improved but were still bad. Subsequent fiddling did not make it better, so I returned home to look at the manual. I looked in the tank and it was nearly empty, over a gallon had been used on a 20 mile round trip - surely a maladjusted pilot jet would not cause consumption this bad? Over the next few days I am going to dismantle the carb, clean and reassemble and retune. Has anyone else had this sort of problem and have any suggestions for what I should be looking for? Thanks, Andy
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spookefoote1956
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Re: AMAL Concentric Mk 1 carb tuning

Post by spookefoote1956 »

I ran with a concentric for years, back in the day when monoblocs were only available at a premium from auto jumbles. Since putting a new monobloc on all the problems I had with the concentric disappeared. The concentric would never run right for me.
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Mick D
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Re: AMAL Concentric Mk 1 carb tuning

Post by Mick D »

Hi

First stop for me would be the float assy - sounds as if it could be sticking and causing the carb to flood, another possibility is the float needle valve not sealing correctly with the same effect.

Regards Mick
Dave T_LAPSED
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Location: CALVADOS FRANCE

Re: AMAL Concentric Mk 1 carb tuning

Post by Dave T_LAPSED »

Just a couple 'o thoughts; do eccentrics have air slides? If so where is yours? Also could the main jet have come unscrewed, dropped out?
Good luck, I have a horror of carb problems!
Dave.
Dixter
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Re: AMAL Concentric Mk 1 carb tuning

Post by Dixter »

Andy51 wrote:Hi All,

Has anyone else had this sort of problem and have any suggestions for what I should be looking for? Thanks, Andy
Hi Andy: If the carb is old and worn out, you would do best to buy a new one. Best indicator of "worn out" is loose slide in barrel. If the slide has more than 0.003 / 0.004" (three or four thou) clearance... it is shot. A sloppy slide will pass too much air and you will never be happy.

I'd engourage you to take the time to mark the hand throttle barrel with bits of tape to indicate the following four throttle positions.
1/8 throttle
1/4 throttle
3/4 throttle
full throttle

Obtain an AMAL tuning guide and study the throttle positions to help you better identify which carburetor component is "at play" for each throttle position. For example if you find that you are not getting a crisp throttle response as you move beyond 1/4 throttle, the problem is most likely needle position problem.

Using a systematic approach, using a note pad and a pencil to record each change, and testing after every "single" adjustment pays handsome rewards. In tuning, one should never make two adjustments at the same time. If you do, and the problem is altered, you will not know which of the two adjustments is at play.
T1.jpg
T2.jpg
Patience is rewarded

Ciao, DC
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Dick Casey
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56G80S
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Re: AMAL Concentric Mk 1 carb tuning

Post by 56G80S »

I used a Concentric for a very long time (mid 1970's to 2017) and it was worn out but still provided acceptable consumption and better acceleration and top speed than the new Monobloc which starts and idles well; but with the worn out Concentric wouldn't start well or idle at all.

Dick's advice reads good to me, particulaarly regarding imiting change and noting effect.

By the by, I marked up the throttle the same way he recommends when I was running in the re-lined barrel and re-worked head. I know the object is not to have the engine labour but also wanted to keep throttle opening down.

Johnny B
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