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MKIIA 850 carb needle: bean can vs peashooter

Classic Norton Commando Motorcycles.

Re: MKIIA 850 carb needle: bean can vs peashooter

Postby Madnorton » Mon Aug 07, 2017 1:43 pm

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.

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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby Fullauto » Mon Aug 07, 2017 1:53 pm

Your cheque's in the mail! Well done.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby L.A.B. » Mon Aug 07, 2017 2:08 pm

Madnorton wrote: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.


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.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby NortonMKIIA850 » Tue Aug 08, 2017 3:53 am

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.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby L.A.B. » Tue Aug 08, 2017 7:55 am

NortonMKIIA850 wrote:but anyway stepped spray tubes require 4-ID-ring needles, no?


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"
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby NortonMKIIA850 » Tue Aug 08, 2017 8:08 am

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.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby L.A.B. » Tue Aug 08, 2017 8:13 am

Alternatively, you can use the 2-ring needles with the flat-topped spray tubes in an 850.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby NortonMKIIA850 » Tue Aug 08, 2017 8:23 am

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.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby acotrel » Tue Aug 08, 2017 3:00 pm

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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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby NortonMKIIA850 » Tue Aug 08, 2017 4:18 pm

acotrel wrote: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.


Just to be clear, you're suggesting using smaller needle jets with petrol?
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby dynodave » Tue Aug 08, 2017 5:00 pm

L.A.B. wrote:Alternatively, you can use the 2-ring needles with the flat-topped spray tubes in an 850.
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:
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby ludwig » Tue Aug 08, 2017 11:02 pm

dynodave wrote: ...
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. ..



And of course you have extensive test data ( like your avatar pic seems to suggest ..) to back that up ?
If not , it falls into your catagory of " worthless free forum information " .

Dave , you know a lot about Nortons , but you always remind me of this little fellow :

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Last edited by ludwig on Wed Aug 09, 2017 1:56 am, edited 3 times in total.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby acotrel » Tue Aug 08, 2017 11:46 pm

NortonMKIIA850 wrote:
acotrel wrote: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.


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


Petrol needle jets in the same carbs would be 0.106 or 0.107 inch. But the step between the two is too big.
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby NortonMKIIA850 » Wed Aug 09, 2017 2:18 am

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?!
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Re: MKIIA 850 carb needle: bean can vs peashooter

Postby L.A.B. » Wed Aug 09, 2017 2:26 am

dynodave wrote:
L.A.B. wrote:Alternatively, you can use the 2-ring needles with the flat-topped spray tubes in an 850.
for NON balanced pipe systems

Finally the voice of reason..DUH

4 groove needles and cut away spray tubes for balanced pipes


No.


dynodave wrote: 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:


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

Bob Rowley wrote: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 not with the mutes fitted......

With mutes....

Bob Rowley wrote:..............................the final cure was to install a cut away spray tube,
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