MKIIA 850 carb needle: bean can vs peashooter

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AN has had MK3 balanced pipes made, these have been made using their MK3 as a template at the manufacturers premises.

Motorcycle Sport April '73 has a picture of the 850 and a long list of the changes from the 750, the bike shown has balanced pipes, standard filter and straight through pipes.

Around the time of the introduction of the 850 many US states had differing noise regs, what got shipped could be anyones guess and thus the variation in the pre-MK3 850's.

Without the venturi's, black caps and balanced pipe, but with FA head, I use the standard original early 850 jetting with the needle one notch higher. It runs better now than in anytime in the past 22 years.
 
Madnorton said:
Motorcycle Sport April '73 has a picture of the 850 and a long list of the changes from the 750, the bike shown has balanced pipes, standard filter and straight through pipes.

Around the time of the introduction of the 850 many US states had differing noise regs, what got shipped could be anyones guess and thus the variation in the pre-MK3 850's.
Click to expand...

Variation?

The only 850 in production at that time for all markets was the Mk1 (as Roadster, Interstate, Hi-Rider) with balanced pipes, peashooters/Interstate silencers, perforated metal airbox and carbs with 260 main jets, etc.
Mutes, self-tapping screws and smaller main jets were put in the crates so dealers could fit them if local noise regulations demanded it.

The Mk1A low noise emission model wasn't introduced until late in the year and was intended for sale on the European market which is why the US mainly had the Mk2 for '74.
 
Another minor update. Yesterday I did a manual check of the carb balancing – I have an electronic gizmo but didn't want to use it because of reasons – and it seems to be spot on. I also did a WOT plug chop – any plug chopping is a pain to do round my way because of the amount of traffic – and found the right plug showed a weak mixture. The left plug showed rich. But that'll be because (I know this'll sound a bit crazy) I'd raised only the left needle one notch. I did that because, of two new downpipes fitted only a day or two before, the left had gone blue already, while the right looked fine – I just thought I'd try it. Also, predictably, the engine bogged at sustained WOT. No great surprise there. Although it does all seem a bit back-to-front. What did surprise me was that both carbs have 2-ID-ring needles! With stepped spray tubes ... The carbs were fitted new by me in 1999, after I'd given up on what I presume were the originals, which did have 4-ID-ring needles. I will have been very specific about what bike the carbs were for when ordering them – I've found the invoice, it's from a highly reputable supplier, and for whole carbs, not parts – but anyway stepped spray tubes require 4-ID-ring needles, no? This very detail came up earlier in this thread. What were Amal thinking, I wonder, combining 2-ID-ring needles with stepped spray tubes ...? Quality control issues, or ...? Anyway I have a pair of 4-ID-ring needles on their way, we shall see.
 
NortonMKIIA850 said:
but anyway stepped spray tubes require 4-ID-ring needles, no?
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Well, the '73 parts book and 2/2A supplement clearly states for 850, "928/104 needle, (use with spray tube 928/107)".

'73 factory manual:

"850
..
Needle: 928/104
Choke (spray) tube: 928/107"
 
Thanks, that's what I'm seeing too – not that I would doubt you! :D Thanks for checking. The 'correct' needles should be arriving tomorrow ... When I fit them, I'll take the opportunity to check the needle jets too. Cheers.
 
Alternatively, you can use the 2-ring needles with the flat-topped spray tubes in an 850.
 
True, but the 2-ring needles I have look past their best, and the carbs have the stepped spray tubes, so it seems easiest to fit new needles. Cheers.
 
With methanol fuel changing a 0.117 inch needle jet to a 0.116 inch needle jet is the difference between sluggish and flying. With petrol you use about half the amount of fuel, so it is twice as critical to get the needles and needle jets correct. With petrol as fuel, that equates to half a thou wear in the needle jet being the difference between flying and sluggish.
 
acotrel said:
With methanol fuel changing a 0.117 inch needle jet to a 0.116 inch needle jet is the difference between sluggish and flying. With petrol you use about half the amount of fuel, so it is twice as critical to get the needles and needle jets correct. With petrol as fuel, that equates to half a thou wear in the needle jet being the difference between flying and sluggish.
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Just to be clear, you're suggesting using smaller needle jets with petrol?
 
L.A.B. said:
Alternatively, you can use the 2-ring needles with the flat-topped spray tubes in an 850.
Click to expand...
for NON balanced pipe systems

Finally the voice of reason..DUH

4 groove needles and cut away spray tubes for balanced pipes
In case you finally get it....They (balance pipes) act like open headers ONLY at low/mid rpms..... then the effect goes away toward high end/rpm requiring main jet tweeking based on the high end restriction or breathing.
Now I get it, I just didn't pay much attention before. :mrgreen:
 
NortonMKIIA850 said:
acotrel said:
With methanol fuel changing a 0.117 inch needle jet to a 0.116 inch needle jet is the difference between sluggish and flying. With petrol you use about half the amount of fuel, so it is twice as critical to get the needles and needle jets correct. With petrol as fuel, that equates to half a thou wear in the needle jet being the difference between flying and sluggish.
Click to expand...

Just to be clear, you're suggesting using smaller needle jets with petrol?
Click to expand...

Petrol needle jets in the same carbs would be 0.106 or 0.107 inch. But the step between the two is too big.
 
dynodave, it's hard to get *anything* when people say things like 'there's x and y', I'm no experienced engineer so I need things like 'x=y'. No-one's said anything about balanced pipes acting like open downpipes/headers before, or the effect going away at higher revs. I'd pondered what the effect of balanced pipes might be, but hadn't come up with that idea. If that's the case then I'm not terribly bothered, unless 4,000rpm+ falls under 'lower revs' – it's power output at higher speeds that I'm interested in, the bike runs smoothly at all rpms, and power output is fine for me at lower rpms. Thanks for the input, when a reputable supplier in the UK starts offering balanced pipes for pre-MK3 Commandos I'll certainly consider joining the queue!

acotrel, sorry, I wrote 'Just to be clear, you're suggesting using smaller needle jets with petrol?', but should have added 'Or just not worn ones?' I thought you'd previously espoused tinkering with the needle jetting, but the difference between .106 and .107 is too great for petrol? So fit new jets maybe, but don't change size?

BTW, whereas John Healy says that '[n]eedle jets are the only Amal jet where the number relates to an actual inch dimension. [So a] 622/122-105 is .105" i.d.' – at https://www.princeton.edu/ssp/65-cub-da ... itbike.pdf – Amal says that the needle jet number indicates the flow rate: 'The number e.g. 100 indicates that it will flow 100cc`s of fuel in one minute at a specified head', at http://amalcarb.co.uk/mk-i-concentric-s ... e-106.html – what to believe? Or is this some sort of cosmic coincidence?!
 
dynodave said:
L.A.B. said:
Alternatively, you can use the 2-ring needles with the flat-topped spray tubes in an 850.
Click to expand...
for NON balanced pipe systems

Finally the voice of reason..DUH

4 groove needles and cut away spray tubes for balanced pipes
Click to expand...

No.


dynodave said:
In case you finally get it....They (balance pipes) act like open headers ONLY at low/mid rpms..... then the effect goes away toward high end/rpm requiring main jet tweeking based on the high end restriction or breathing. Now I get it, I just didn't pay much attention before. :mrgreen:
Click to expand...

Obviously you (still) don't get it (DUH).

Bob Rowley said:
A new acceleration test was introduced for USA and from memory the test was 40 mph snap open throttle in top gear, this was fine without the mutes fitted.......
Click to expand...

But not with the mutes fitted......

With mutes....

Bob Rowley said:
..............................the final cure was to install a cut away spray tube,
Click to expand...
 
NortonMKIIA850 said:
Amal says that the needle jet number indicates the flow rate: 'The number e.g. 100 indicates that it will flow 100cc`s of fuel in one minute at a specified head', at http://amalcarb.co.uk/mk-i-concentric-s ... e-106.html – what to believe? Or is this some sort of cosmic coincidence?!
Click to expand...

It has the same caption (below) for all their jets.
All genuine AMAL jets are calibrated to a flow rate. The number e.g. 100 indicates that it will flow 100cc`s of fuel in one minute at a specified head. Beware of imitations as they are just drilled to a size. All our jets are branded and packaged with the official AMAL logo.
Click to expand...

It doesn't specifically state "needle jet number", so for needle jets it has to be wrong and is just an error in the wording.
 
Amal needle jets are numbered in accordance with their internal diameter in thousandths of an inch. A 107 needle jet has an ID of 0.107 inch. Main jets are numbered according to their flow rating. You can make your own needle jets on a lathe using brass hex, a 0BA die nut and number drills. Making main jets is more difficult. You really need flow meters and reamers.
As long as you have the two needle jets with close to the same internal diameter, moving the needles up and down gives adjustment, so the actual size is not critical. The important thing is that when you lower the needles in the carbs, you should be able to make the engine cough. If you cannot get it to do that, the needle jets are too large and the motor might be sluggish without you really being aware of it. It really only matters if you are racing your bike. The needle position for best performance is just slightly on the rich side of the cough. If you do that to a road bike, it will probably become more weather sensitive.
 
One thing I have not pursued with my own bike is changing the needles to see it I can gain more go as I wind the throttle on. I'm using 6D Mikuni needles in a 0.117 inch needle jet with methanol fuel and it seems to work extremely well. The other common Mikuni needle which will work in a 34mm Mk2 Amal is the 6F, which is richer. I can speculate what that needle might do to the performance, but you never know until you try it. Jetting for methanol is the same as jetting for petrol except the jets are bigger, so jetting for petrol is much more critical. The slightest bit too rich and you go backwards. I don't use Amal methanol needles - they are ridiculous.
 
Look ! - If I can help anyone with a Commando-based bike go faster, I am happy. There are not enough of us. When I built the Seeley 850, I never believed in it - now I think it is really great. I've never previously used a motor which is so responsive to tuning. It absolutely loves methanol. What I have done with it, I have done with other motors, but it has never worked so well.
 
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