Anyone using an Amal Concentric MK 1 ?
Posted: Wed Aug 24, 2022 8:48 pm
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
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