Carburetor needles.

I am interested in finding out what taper needles are fitted as standard in the Amal carbs on an 850 Commando which uses petrol. And what would be the corresponding Mikuni needles.
Throughout the 1970s, much work was put into getting the mid-throttle jetting right on two-strokes. Tuning a Commando to get performance is not much different.

acotrel said:

I am interested in finding out what taper needles are fitted as standard in the Amal carbs on an 850 Commando which uses petrol.

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928/104.

Page 10.

I am probably opening a can of worms with this but has anybody tried the 622/124 2 ring needle in an 850? my reasoning being was the change to the 622/104 4 ring needle solely for emissions and sales to America. A few months ago [just coming out of winter here] I was out on my 750 Fastback in the morning and my 850 in the afternoon and thought the 850 more sluggish in the mid range, It could be the weight and gearing difference but having been down this road with Harley carburation some years ago I have my suspicions, and yes I have gone from 3 1/2 to 3 slide and there is a slight improvement.

possm said:

I am probably opening a can of worms with this but has anybody tried the 622/124 2 ring needle in an 850? my reasoning being was the change to the 622/104 4 ring needle solely for emissions and sales to America.

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Nothing to do with emissions as the US EPA regulations didn't come into force until 1978.

The reason for the 928/104 4-ID ring needle and 928/107 stepped spray tube on the 850 was to pass a US acceleration test requirement when mutes were fitted to the exhausts.

possm said:

I am probably opening a can of worms with this but has anybody tried the 622/124 2 ring needle in an 850?

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I'm positive many have. When I was selling a lot of Amal, the repair kits and major repair kits (RKC 559, RKC 560, RKC 561) came with standard needles and the Norton-specific major repair kit (RKC 602) cost a good bit more. Many Norton owners ordered the standard kits.

possm said:

I am probably opening a can of worms with this but has anybody tried the 622/124 2 ring needle in an 850?

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Yes, but in carbs with the flat-topped 622/074 spray tubes (Generic 932/300 and /301) with no problems.

Is the taper on the standard needles quick or slow ? Commando ports are relatively small and if the taper is too quick, it might cause the motor to be sluggish in response to the throttle., With the heavy crank, that might be a major disadvantage. I would never lean-off the jetting by fitting slides with bigger cutaways. I don't think that has much effect above one-quarter throttle.

acotrel said:

Is the taper on the standard needles quick or slow ?

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The needle dimensions are on page 10 of the link I included in my reply to your first post.

I have down-loaded the PDF. But I thought somebody might have done the equivalents. With my 34mm Mk2 Amals, I use 6D Mikuni needles which are the leanest. I cannot whack the throttle open, but those needles give the best acceleration. One thing I have found with the Commando 850 motor is it tends to spin up at the same rate regardless of the overall gearing. However if you get an improvement in pulling power, you do not usually know it until you raise the overall gearing. When your overall gearing is high, you usually use more throttle, so the taper on the needles has more effect.
I use a close ratio gearbox, so as the overall gearing increases, the gears tend to spread, but smaller steps between gears means smaller loss of revs and I come up through the gears.
With most motors, as you raise the overall gearing, they tend to accelerate slower. The Commando motor is different. You don't find out how fast it is until you raise the gearing. The steps between gears in the standard gearbox are too large. As soon as you lose revs with the heavy crank,, you need to wait for them to come back.
I think with Commandos, a lot of things were done to cope with the worst case scenario. The taper on the carb needles might have been specified to cope with heavy-handedness.

Have you ever tried these needles ? ;
concentric - 622/278

MK2 - 2c3

They seem to have the slowest tapers.
Mikuni give more options.

acotrel said:

Have you ever tried these needles ? ;
concentric - 622/278

MK2 - 2c3

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No, and no reason to do so.

I have never run my 850 on petrol - only ever methanol. Methanol jets flow about twice as much fuel as petrol, so errors in jetting do not have as much effect. When I was jetting my motor, I made my own needle jets. I started with a 0,117 inch needle jet, and found I could not make the motor cough by lowering the needles. So i then made two more with IDs half a thou of an inch smaller. I was able to induce the cough, then raised the needles one notch. The difference in performance was very significant. I use 6D Mikuni needles in my Mk2 Amals - the leanest taper. If you are jetting for petrol using fast taper needles, you might not be achieving the possible performance your motor can give. The problem is, the motor can still run well, but just not as well as it can.
If you lower the needles in your carbs, you should be able to induce the cough. Raising them one notch will put them in the correct position. But that does not take the taper on the needles into consideration. The taper is there to compensate for loss of vacuum as you open the throttle, so port size has an effect. It is easy to set up and get good performance, but you might not be getting as much performance as is potentially available.
I think the needles and needle jets are the only reason my 850 is quick. I ride the bike as if it is a two-stroke - feed the throttle on - it is much quicker that way. Old habits die hard.
Most guys who race using methanol, run their jetting miles too rich. Petrol is ten times worse when it is rich.
For a road bike, it probably does not matter much.

L.A.B. said:

No, and no reason to do so.

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Thanks for that Amal carb tuning document. In the 1970s, I would never have been able to get that. I have never trusted Amal. I have often wondered how the racers in the 1950s in England, got their bikes going so fast on pool petrol. It must have been like playing cricket - needs lots of patience ?
Australians who went there, found Brit Manx Nortions were as fast as ours were when we used methanol in Australia. But our guys were used to the speeds in the UK. Also Australian Manx Nortons had Symco con-rods, so could rev to 8000.