How critical is the cut-out needle jet?

grandpaul · Dec 7, 2015

I ordered and received a new pair of Amal Premiers for my 880 project, but the needle jets do not have the square cut-out in the engine side of the needle jet, facing into the intake manifold.

How critical is this feature, and are the new Premiers no longer made with the Norton-specific cut-out?

The carbs were ordered specifically for the Norton 850.

L.A.B. · Dec 7, 2015

grandpaul said:
I ordered and received a new pair of Amal Premiers for my 880 project, but the needle jets do not have the square cut-out in the engine side of the needle jet, facing into the intake manifold.

Do you actually mean the stepped spray tubes (928/107) ?

grandpaul said:
How critical is this feature,

Not critical.

grandpaul said:
and are the new Premiers no longer made with the Norton-specific cut-out?

No, or should that be yes, :? what I mean is that Norton 850 carbs ordered from Burlen I would expect to be supplied with the 928/107 stepped spray tubes and 928/104 needles.

grandpaul said:
The carbs were ordered specifically for the Norton 850.

If they were 'built to order' by another supplier from generic (300/301) items then there's no guarantee they would come with stepped spray tubes.

grandpaul · Dec 7, 2015

L.A.B. said:
grandpaul said:

How critical is this feature?

Click to expand...

Not critical.

Yes, sorry, spray tubes.

What exactly is the practical running difference? You say non-critical, should I just bolt them up and go?

Thanx.

L.A.B. · Dec 7, 2015

grandpaul said:
What exactly is the practical running difference? You say non-critical, should I just bolt them up and go?

Unless you intend to fit peashooters with mutes and then replicate the conditions of the snap open full-throttle roll-on test conducted by Bob Rowley, then there's probably no difference at all except perhaps for main jet size.

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, but with the mutes
( small disk fitted to the end of the exhaust silencer ) , this caused a the muted condition, no matter what you say our suggest the final cure was to install a cut away spray tube, this allowed a much smaller main jet to be used this in turn stopped the eight stroking when trying to pass the USA acceleration test, think 40 to 60 mph in a certain time.

It is not difficult to see how this worked, FOR THE MUTED CONDITION

Most times with the mutes fitted we would snap at the 1/4 mile point at 30 and 40 mph and be doing the same speed, 30 or 40 mph at the timing lights, after 1/4 mile, of course you could feather the throttle back and judicially pick up speed and improve, but the idea was that a person when accelerating passed a long vehicle could not be expected to intervene and should expect a solid reliable acceleration.

You may not be aware that AMAL did not set up the Carburettors for the trade, BSA, TRIUMPH, NORTON but development worked closely with Amal, and the responsibility for final sign off and payment was down to the Test and Development department of each of the firms, as Deputy group test rider under Bob Mann's we carried such responsibility.

It's quite wrong to suggest that we would have changed or introduced things for no good reason.

One final thing that would vindicate the modification was the fact that we had over 150 bikes held up and not allowed in California test Authority and after the modification they and all subsequent Norton's passed straight in.

I am not sure if Alan Lines? of Amal is still alive but he would no doubt give you chapter and verse on how and why the cutaway spray tube was beneficial to the MUTED CONDITION.

One day I may find my notes on this mod but in the meanwhile could I ask you to at least give some benefit of the doubt, the cutaway spray tube was a category 'A' modification for all machines that required Mutes.

Regards

Bob Rowley "

Mr Rowley's Test Method

As previously noted, we've got another place for talking planes, aerobatics, inverted flight etc, so please keep those thoughts in that other repository. Thank you. Unfortunately, much of the preamble and context relating to this subject has been deleted and/or locked, thus it is difficult for...

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Brooking 850 · Dec 7, 2015

Hi grandpaul, I use a set of 32m Premiers on my 70BHP race motor, they also came with no stepped spray tubes, no issue for me with this bike.
Regards Mike

acotrel · Dec 7, 2015

It sounds as though the cut-away helps throttle response over 1/2 throttle when the exhaust system is restrictive. Probably changes the venturi effect and causes greater flow through the main jet. Perhaps it depends on how the bike is ridden ? Personally, I never snap the throttle open and expect instant response. I'm used to two strokes where the throttle must be fed on relatively slowly or the motor can stop.

acotrel · Dec 7, 2015

Thanks for this :

'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, but with the mutes
( small disk fitted to the end of the exhaust silencer ) , this caused a the muted condition, no matter what you say our suggest the final cure was to install a cut away spray tube, this allowed a much smaller main jet to be used this in turn stopped the eight stroking when trying to pass the USA acceleration test, think 40 to 60 mph in a certain time.'

I will make some new spray tubes, re-jet the mains and see if it makes a difference at wider throttle openings. One thing I've found is that even with methanol fuel - the slightest bit rich and the motor become slightly sluggish . I've always taken it for granted that the normal way of jetting Amals is sufficient to cope with every situation.
- every little bit of information helps.

htown16 · Dec 7, 2015

I've been wondering if the real difference between the 850 step tubed setup and the 750 flat tubes is due more to the needles ran with them and not the tubes themselve. The 928/104 4 ring needle ran with the 850 step tube is longer overall and also has a longer straight portion before the taper begins when compared to the 622/124 2 ring needle ran with the flat top spray tubes. To me, this would could keep the mixture leaner for a longer time as the needle is raised until the taper comes into play. Essentially this would mean the bike is leaner around the 1/4 to 1/2 throttle position where the taper of the needle in the needle jet is the main influence on mixture which also happens to be where a lot a street riding occurrs. Was the factory intentionally trying to create a lean condition in the midrange to make up for more restrictive mufflers and aircleaner?

L.A.B. · Dec 7, 2015

htown16 said:
Was the factory intentionally trying to create a lean condition in the midrange to make up for more restrictive mufflers and aircleaner?

But this was a full-throttle problem (brought about by the throttle being snapped wide open and held there at low RPM in a high gear) and only when mutes were fitted to peashooters (no change of air filter) that the stepped spray tubes and 4 ring needles supposedly cured.

htown16 · Dec 7, 2015

Can understand the step tube helping the throttle snap on by changin venturi size. But still having a problem understanding the effect of the needle change. At full throttle the needle is out of the jet, so should have no effect on the snap on case. Still seems to me that the 4 ring needle would lean up the mid range when compared to the 2 ring due to the taper coming in at a higher throttle position due to the longer straight portion and the needle staying in the jet until a higher throttle position due to its greater length.
Reason I ask is I have a late 72 750 that has Premier 32MM on that came with stepped spray tubes and 4 ring needles. The bike is running slightly lean with the needles in the lowest position. I'm switching it to 2 ring needles and flat spray tubes to see if it has any affect. Has anyone done any comparison of the 2 ring versus 4-ring performance over the throttle opening interval the needle/needle jet has primary influence. Not talking about WOT.

Rohan · Dec 7, 2015

htown16 said:
The bike is running slightly lean with the needles in the lowest position. I'm switching it to 2 ring needles

Why don't you just lift the needles a notch. ?
They are not generally intended to be as low as that.
Lovely performance if they are set correctly....

htown16 · Dec 7, 2015

Sorry wasn't clear, clips are in the lowest groove so they are as rich as you can go.

L.A.B. · Dec 7, 2015

htown16 said:
Can understand the step tube helping the throttle snap on by changin venturi size. But still having a problem understanding the effect of the needle change. At full throttle the needle is out of the jet, so should have no effect on the snap on case. Still seems to me that the 4 ring needle would lean up the mid range when compared to the 2 ring due to the taper coming in at a higher throttle position due to the longer straight portion and the needle staying in the jet until a higher throttle position due to its greater length.

Is it possible the 4 ring needle simply compensated for changes in mixture caused by the stepped spray tube at anything less than full throttle?

htown16 · Dec 7, 2015

Here's the comparison between the two needle types. The 4 ring is longer overall, more straight portion before the taper and thicker at the tip. It seems to me this would result in a leaner condition over the effective range of the needle but I could be wrong.

Rohan · Dec 7, 2015

htown16 said:
It seems to me this would result in a leaner condition over the effective range of the needle but I could be wrong.

Nortons always stipulated that the 4 ring needle for the 850 was used with the stepped spray tube, and vice versa.
They were designed to work together.
Lovely performance, with a fairly stock bike.....

This was discussed here on Ludwigs and needings threads, not that any real understanding emerged...

This was covered in the press someplace back when they were introduced, anyone know where that might be ?
I have read this in some detail, but where ?.
Did Nortons put out a bulletin about these 'new' carb setup.

grandpaul · Dec 7, 2015

Brooking 850 said:
Hi grandpaul, I use a set of 32m Premiers on my 70BHP race motor, they also came with no stepped spray tubes, no issue for me with this bike.

I have very standard cam and porting, so don't expect to be producing that much horsepower. However, I've heard read several knowledgeable replies regarding the non-criticality of the stepped cutouts (esp. w/ non-muted muffs), so will mount the carbs and fire it up tomorrow.

acotrel · Dec 7, 2015

Do the spray tubes in Premiers have a means of locating them so the cut-away stays towards the valve ? It is very interesting as I believe the 2 into 1 exhaust systems also run with a bit of back pressure.

L.A.B. · Dec 8, 2015

Rohan said:
This was covered in the press someplace back when they were introduced, anyone know where that might be ?
I have read this in some detail, but where ?.
Did Nortons put out a bulletin about these 'new' carb setup.

This appeared in the April 1973 edition of Motorcycle Sport magazine which listed detail improvements of :"The New Big Norton" (850)
(Article reprinted in Norton Commando-Gold/Ultimate Portfolio)

19. Carburettor ticklers are now of "water-proof" Spanish type. The float bowls are fitted with drain plugs. Cutaway spray tubes are fitted.

http://www.cartechbooks.com/norton-commando-up.html

Frankie17 · Dec 8, 2015

I think with open free flowing mufflers you should be able to get good results with the stock 622/074 spray tubes you have fitted

As some of the guys have already commented the cut away tubes 928/107 were designed for use with restrictive silencer fitted to the stock 850
these restrictive mufflers cause a poor inlet vacuum pulse due to slow cylinder purge ( low vacuum depression equals poor pick up of fuel through the needle / jet when the throttle is opened quickly

if you are fitting non stock free flowing silencers i think you should be able to work around this problem

grandpaul · Dec 8, 2015

Yes, I will be fitting the new Premiers later today.

How critical is the cut-out needle jet?

grandpaul

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