Jetting 932 Amals on 850 Comando

The notched spray tubes are fitted to Commandos to overcome low inlet depression
the inlet pulse is weak on both Commandos and tridents both models suffer low speed fuel pick up issues
Amal re worked the carbs for the later 850`s a combination of restrictive exhausts / air filters
do not help to purge and re fill the cylinders

Early models like the Atlas and roadster have free flow silencers and 30mm caburettors fitted
perhaps moving to the 32mm choke size also slowed up the air inlet speed further reducing vacuum
effect across the needle tube at low RPM

using the modified carbs on the road going 850 is beneficial or fit the Notched type spray tube with the 928/104 needle
to your stock 32mm 4 stroke bodies


I have also experimented with tridents and to get the bush type pilot system working better use the float bowls with the
pilot pick up drilling located as low as possible in the float chamber ( there are two variants of AMAL MK1 concentric bowls )
both share the same part number maybe Amal also came across this issue and modified the bowl without telling Dealers

Ludwig

The information came directly from Tom Ullman Grosvenor works ( AMAL ) back in the 1980`s when customers could meet suppliers face to face at trade shows

Triumph / Norton were struggling for cash in the mid 70`s and this was the quick economical quick fix Amal developed to overcome the fuel pick up issues

Ludwig

The notch tube increases the vacuum effect across the needle jet
drawing more fuel past the needle ( so richer mix at partial throttle openings )
the best test is to DYNO your bike using both the stock 32mm 932/300/301 with 622/124 needle and 622/074 tube
then try the 32mm Norton carb 932/35/36 with 928/104 needle and 928/107 tube
the AFR across the range is much improved using the Norton specific carbs with the modified tubes and needles
so it does work

Just for information the Wassell 9 series Norton carbs also fuel extremely well on a commando and are direct replacement
most of the guys on this forum are US based and seam to prefer the PWK / Mikuni / Amal Premiers
are the Wassell available in the US ?

ludwig said:

The 4 ring needle is 2.5 mm longer than the 2 ring needle , but this is IMO of no consequence ( I could be wrong , of course ) .
Otherwise they are identical , EXEPT that from 6- 7 mm on the taper , the 4 ring is thicker = leaner .
The difference is about one step in the clip setting .
With the needle in the lowest position , the taper on the needle only comes in effect at +/- 10 mm throttle lift , and the leaner section of the 4 ring at 15- 17 mm , or half throttle . so only from over half throttle the 4 ring will be leaner than the 2 ring needle .
At a constant 1/2 throttle opening , an average 850 Commando ,( depending on gearing , aerodynamics ..) will be doing over 100mph in top gear .
So much for small throttle openings .

Click to expand...

According to the account given by Norton development tester Bob Rowley in Mick Duckworth's book 'Norton Commando' the Amal modification cured: "the 850's flat spot under sudden acceleration" so this may not necessarily be all about whether one spray tube/needle combination supplied a richer or leaner mixture than the other, but that it's purpose was to correct a flat spot that occurred when the throttles were opened rapidly, so I think we could be barking up the wrong tree if we only think in terms of how the apparent differences between spray tubes and needles affects mixture at constant throttle positions?

I run a set of 32 mm Premiers on my race motor without the stepped spray tubes , and admittedly it is not used for cruising I certainly dont notice any flat spots when snapping open the throttle at lower RPM and the dyno runs proved I am getting the best out of those carbs, with current timing, exhaust sizes etc. I have the notched spray tubes to install if need be, but reluctant to do so as I would struggle to notice the difference with this motor.
Regards Mike

Mike

If you have already found a good set up on the dyno I would be reluctant to fit the notched tubes
as there will be no HP gain , I guess you are not doing 30mph in traffic on tick over ?
and if there is no hesitation when you open the throttle quickly my advice would be to leave well alone

Interesting stuff guy`s

has anyone else tested the effect of E10 fuel on the factory jet settings ?

I know I'm about a month late to this party... but had to chime in with my 2 cents:

It seems to me that in the course of this everlasting, raging debate one very basic element is completely overlooked. Even more than displacement (750 vs 850) a major factor in proper carb set up is intake and exhaust configuration. Early 850's had virtually the same air cleaner and muffler arrangement as 750's had. The major difference was the cross-over in the 850 header system and "mutes" (baffles to us) in the peashooters. Yet many early 850 Amals had standard spray bars too. (Go ahead ask me how I know this!)

The notched Spray bar was introduced to cope with the horrifying flat spot that came along with the (mandatory EPA) fitment of this equipment! Essentially... our government required a "pass a vehicle" roll-on test from 40mph. The Amals with standard spray bars could not pass this test. Cracking the throttle open at 40mph led to virtually nil acceleration. Working with Alan Lines at Amal led to the discovery and implementation of the notched spay bars as an effective "cure".

Exactly how the fitment of a head pipe cross-over and baffles would cause a two-stroke-like "short, sharp drop in vacuum" I can't say... although the cross-over probably had more effect than is generally recognized, since it effectively made the head pipes function more like a high-volume, big-bore header. (It might also circumvent the lean/rich argument, in the sense that the notched set-up was used to simply extend and flatten out, what was a short and sharp drop.)

My own early 850 currently has a single notched spray bar Amal (932/35) on an RGM manifold, cross-over head pipes and open (original) peashooters and while it runs "fine", I feel the mid-range response is a bit too "fluffy" and sluggish. Several years back, before I changed carbs and was aware of (or cared about) differences in spray bars, I had the original factory Amal (with standard spray bar) on this exact same configuration and when I rolled on - the bike would try to yank my arms off! This was long ago, so my memory might be a trifle "rose-tinted", but I know one way to find out! I think I'll dig that 932/29 out, re-install it and let you know how it turns out. :wink:

Be that as it may, it stands to reason that an 850 with individual head pipes, (non-muted) peashooters and free-flow air filters would run better with standard spray bars.

Sorry... "senior moment" on my earlier post!

My 850 has a STANDARD spray bar on it's single Amal (932/35) as of today. Hence the fluffy mid-range and lack of snap.

The "yank my arms out" punch I recalled from years back... was when it had a notched spray bar (932/29) fitted! I got it bass-ackwards in the earler post, but the point remains the same. With the cross over head pipes it should run better with the "correct" spray bar.

Of course I could always leave the carb alone and install new individual headers! :?

UPDATE: Yup! It runs better with notched spray bar carb! Back to back "tests" after the swap from regular spray bar carb = NOTABLE increase in throttle response! It lunges forward (as it should) now! Considering Amals do not have an accelerator pump, it even does okay at (what I would term) "lugging" engine speeds, like below 2500rpm. Anything above that and it wants to GO... instantly!

Shoulda done this a long time ago.