Anyone using an Amal Concentric MK 1 ?

Trev Sellars
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Anyone using an Amal Concentric MK 1 ?

Post by Trev Sellars »

Well, having recently acquired a 1961 Arrow, it came fitted with an Amal 600 22mm dia series Concentric.
After trying to sort out the poor performance issues, I found out it indeed was a 4 stroke carb! So, I replaced the needle jet, main jet and also the
needle jet spray tube, to a sloped one, being correct for the 2 stroke application.

The idle jet is fixed with a tube insert, as found on 4 stroke carbs and is equivalent to a size 20 idle jet. I could drift this out and fit a 30 idle jet but the lean mix problem is 1/2 to 3/4 throttle, this would indicate the needle needs lifting or the needle jet needs enlarging. I changed the needle jet from 105 to 106 and tried a 107 but this was too much.

The engine is standard and is fitted with H.C. heads. Crank seals and gaskets good, no leaks. Compression cold 125PSI both barrels, standard gearing.

I have now got it as best as I can using the following;
Needle jet 106, main jet 150, middle groove on needle. If I lower the the needle clip to the bottom groove, the bike runs rich. Slide is No 3
Using 35 to 1 oil-petrol mix. Air filter Amal gauze type, nice and clean.

The lean mix is most apparent on going up hills, under load. I open the throttle and the bike runs lean, the engine slows and I have to change down, to third or sometimes 2nd and rev the engine hard. Top end is lovely and the spark plug checks, indicate a good burning process.

I think it may be down to the gearing of the bike, and could be a characteristic of the design. Can anyone verify this please?

It is different to any other 2 stroke I have tuned or ridden. It just does not like a quick throttle responses in the mid range.

I mounted a very cheap tacho which detects ignition pulses, this shows 6500rpm revs at Wide Open Throttle, Pic attached.

Any ideas would be welcome.
Thanks
tacho.jpg
Trev Sellars
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Re: Anyone using an Amal Concentric MK 1 ?

Post by Trev Sellars »

Someone has suggested fitting an Ariel Arrow 200cc rear sprocket, This has 49 Teeth compared to the 47 on the 250cc machine.
will_curry
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Re: Anyone using an Amal Concentric MK 1 ?

Post by will_curry »

The only - legal - issue with that is that the speedo drives off the gearbox
and altering the back sprocket thus alters the speedo's accuracy.

Ariel 200s had a different layshaft and speedo drive gear.
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Re: Anyone using an Amal Concentric MK 1 ?

Post by bh.sprint »

Hi, I've no experience of using Concentric carbs, just Amal and Mikuni. Is there anyone near to you who could lend you a working carb to try in place of the Concentric.

The heavier Leader pulls well up most hills even with camping equipment ,using standard gearing, although this bike does have a 376 carb.

Brian H.
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JohnnyBeckett
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Re: Anyone using an Amal Concentric MK 1 ?

Post by JohnnyBeckett »

ih you can use a GPS speedo or a sat nav if you are worried about what speed you are doing and i would talk to surrey cycles he is very good with Amal Concentric carbs he has does my carbs jetting and problands :D
Trev Sellars
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Re: Anyone using an Amal Concentric MK 1 ?

Post by Trev Sellars »

Thanks for your help. A reconditioned 375 carb came with the bike, it's still in the box, so I might just slip that on. I was always led to believe that the
later Amal concentric carbs were superior to the earlier Mono block 375's and 376's, the biggest issue as far as I can see is the lack of needle choice available. I have never used Amal carbs before, so it is a new learning curve for me.

It may be, I revert back to the earlier mono block.
Last edited by Trev Sellars on Sat Aug 27, 2022 9:02 am, edited 1 time in total.
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Re: Anyone using an Amal Concentric MK 1 ?

Post by will_curry »

What size is the inlet port on your crankcase?
ian williams
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Re: Anyone using an Amal Concentric MK 1 ?

Post by ian williams »

i have a 626 concentric on my ariel racer , it starts easily and revs freely .
the engine is tuned along with a pair of expansion chambers and goes like stink .
i have the manual choke kit on the carb where there is a lever you press down on top of carb to lock it ON and then turn the lever 90degs to get it to release OFF , although on a leader youd need to use either no choke or get the strangler to fit it somehow..
sorry but cant tell you what the carb was originally fitted to but more than likely a bsa bantam carb as i know they use a 626 .

you could go for a mikuni carb conversion , join the leader and arrow page on facebook and the lads will tell you which mikuni , there is a files section on the facebook page with much info about many things leader and arrow and i think it has mikuni info .

keith harding can give you all the info you need ref mikuni carb .

i have a standard 375 carb on my 1958 leader and it runs fine except for the usual hot starting problems when the fuel evaporates in carb leaving an oil rich mix that wont fire .

when you pull up to stop for any reason on a hot engine , turn fuel tap off and let engine run the fuel out of carb before you turn it off , that way when you want to start it you can turn fuel tap on and wait a minute or so for it to fill carb up , then should have proper mixture and will fire up without oiling up .
cheers
ian
Trev Sellars
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Re: Anyone using an Amal Concentric MK 1 ?

Post by Trev Sellars »

Hi Will, the inlet port is a standard 7/8ths?
I do have a spare set of cases, barrels heads etc, and over the winter I will be building up a spare tuned engine. Lucky, the inlet port is larger in the cases, as it is from a later engine. The Barrels etc are going to be ported but for torque rather than revs, as it will only ever be used on the road.

I have just purchased a set of really nice barrels with original pistons and another set of original pistons +30 thou, with rings etc. 2 heads, 1 being from a 200cc machine. All on Fleabay for £150 delivered.

Thanks Alan for your tips, very useful. I will take a peek on the Facebook site and see what I can find out.
e bay barrels.jpg
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Re: Anyone using an Amal Concentric MK 1 ?

Post by will_curry »

I presume the 376 you have is from a Sports Arrow which will be 1"1/16.
How well will that work on a 7/8" inlet port?
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