MKIIA 850 carb needle: bean can vs peashooter

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NortonMKIIA850

NortonMKIIA850

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Hi,

I've owned my MKIIA 850 (Europe) since 1998, and have recently got it out of storage after 12 years. (long story) It has the MKIII-style black plastic airbox, sectioned spray tubes and 4 ID ring needles that you'd expect on a MKIIA, but it was wearing straight-through peashooter silencers when I bought it, instead of the bean cans with which it would have left the factory. While rummaging through old Norton content on my computer yesterday I found a note from 2002 by Frank Forster, who might be on Access Norton...? Whether or not, I hope he doesn't mind me bringing this up. Anyway, he wrote: 'The 4 ID ring needle was made for the restrictive "bean can" mufflers used on the MK III Commando [and the MKIIA], and are correct for that use. For any Commando using mufflers of the "straight through" design, one must use the 2 ID ring, as otherwise there will be a lean spot when you open the throttle to increase from a 70 MPH speed.'

Is this still believed to be the case? If so, I'd better get some 2-ID-ring needles pronto – my Commando has always seemed lacking in power much above 70 mph, and doesn't like pulling much above 85 – 90. Even though I had the engine rebuilt by Norman White many years but only a few thousand miles ago. While I don't want to risk burning valves etc. by running lean at high speed, might this needle issue be behind the apparent drop in power? TIA
 
NortonMKIIA850 said:
'The 4 ID ring needle was made for the restrictive "bean can" mufflers used on the MK III Commando [and the MKIIA], and are correct for that use. For any Commando using mufflers of the "straight through" design, one must use the 2 ID ring, as otherwise there will be a lean spot when you open the throttle to increase from a 70 MPH speed.'

Is this still believed to be the case?
Click to expand...

Thanks to the testimony of Norton tester Bob Rowley (and the parts books) we now know for certain that's not correct, as stepped spray tubes and 4 ID ring needles were, in fact, fitted to (practically) all 850 Commandos regardless of whether they were Mk1, Mk2 (peashooters) or Mk1A, Mk2A, Mk3 (black airbox & black cap/bean cans) models, therefore, 4 ID needles should be used with the stepped spray tubes, alternatively, 2 ID needles and flat topped spray tubes can be used.
 
Ok great, thanks for that, it's a relief to be honest! So I'll be keeping those needles and sectioned spray tubes. Now I only have to ponder what I said about my bike appearing to run out of steam at high speed. Maybe the only way to be sure would be a direct comparison with another straight MKIIA. I don't think I've ever managed to get mine over 100 mph. I'm really not a speedfreak and generally observe speed limits (getting older, see...!), it's just nice to have power and know what the bike can do. Cheers!
 
My MK2A had peashooters but had the blackbox and ran out of steam over 3/4 throttle, the 230 main jets had been retained and when replaced with 260's (as per the MK2 fitted with peashooters as std) the bike was restored to full power. There is a difference of one slot on the needle position MK2 to MK2A but not checked to see what I did there.
 
NortonMKIIA850 said:
my Commando has always seemed lacking in power much above 70 mph, and doesn't like pulling much above 85 – 90.
Click to expand...

That does seem a bit down on power so maybe it needs a good going over?
 
Thanks for that L.A.B., so I'm not the only one thinking it might be a bit down on power. And kommando, for the suggestion that I might up the main jets! I'll see what I can see! Cheers.
 
If the stock configuration has been modified for free breathing, you'll need to step up the mains to get all it's got.
 
NortonMKIIA850 said:
It has the MKIII-style black plastic airbox
Click to expand...

Does it still have the rubber venturis fitted to the airbox trumpets by any chance as I believe they may restrict top end power.

https://andover-norton.co.uk/en/shop-de ... ke-venturi

The foam filter element degrades and eventually begins to crumble and the carb connector hoses can split so perhaps worth checking as well as the main jet sizes already mentioned.
 
Interesting point, GrandPaul. I remember something 17-odd years ago about the MKIIA inlets (and presumably the MKIII?) being tapered towards the valves, supposedly to improve torque, of which there was and is *plenty*, and the wisdom was to open these tapers up to straight through for more bhp. I had this done all those years ago in search of more bhp, just because the bike seemed to give less than it ought at higher speeds, but I felt it didn't make a difference. So, another reason to up the main jets, it seems. I see these jets are fairly cheap – they'll be the standard ones in there since the carbs were fitted new by me in a previous life – I wonder if it would be safe to go straight to 260 and see, or whether I should try it in steps. L.A.B., apart from the removed tapers in the inlets, intake is all standard MKIIA, so the carb connector hose from the airbox is two of these:

https://andover-norton.co.uk/en/shop-de ... ector-hose

The hoses are only a few weeks old, and the filter was new some years ago, I'll have a look at it.

Cheers.
 
Sorry L.A.B., I only just tumbled to what those venturi that you linked to are! I'd never seen them before ... They're not on my bike and never have been, since I bought it! Yet another reason to open up the main jets, I guess ... Cheers!
 
NortonMKIIA850 said:
Interesting point, GrandPaul. I remember something 17-odd years ago about the MKIIA inlets (and presumably the MKIII?) being tapered towards the valves, supposedly to improve torque, of which there was and is *plenty*, and the wisdom was to open these tapers up to straight through for more bhp. I had this done all those years ago in search of more bhp, just because the bike seemed to give less than it ought at higher speeds, but I felt it didn't make a difference.
Click to expand...

.....And is now considered not to be such a good idea! :wink:
www.accessnorton.com

RH4 vs RH10 cylinder head performance

I have read many posts regarding these two cylinder heads. I have not found any hard numbers regarding HP and torque. It would be especially interesting if HP and torque values v RPM were available. Any info or references will be appreciated. Thanks, Ed
www.accessnorton.com www.accessnorton.com

NortonMKIIA850 said:
I wonder if it would be safe to go straight to 260 and see, or whether I should try it in steps.

I only just tumbled to what those venturi that you linked to are! I'd never seen them before ... They're not on my bike and never have been!
Click to expand...


It's usualy better to start 'rich' and go leaner, so from what you've described I'd suggest starting with the 260s.
 
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Noted kommando! They do look restrictive, given how the air box is without them ... Thanks L.A.B. – well those inlets are opened up now, so...! I'll order a couple of 260s straight away. Although, my transmission is presently stripped out because it had got so noisy as to sound unsafe, so it'll be a while before I get to ride the thing again. The weather's not co-operating – I have to work outdoors – and I'm still waiting on one or two parts for the gearbox, so it'll be a week or two ... Thanks for all the tips!
 
Have you tried moving the needle clip to a richer notch? If the motor loses steam above 70 MPH I think the slide is only 1/4 open or less using a 20 or 21 tooth sprocket. Try the needle position first and see what happens.
 
I don't know how you guys ever get your jetting right when you are using petrol. With methanol the errors in jet size and needle are critical, however with petrol, they are twice as critical. I don't use Amal needles in my 34mm Mk2 Amal carbs, I use similar Mikuni needles because there is a better range of tapers available. If you have a flat spot in your carburation, that is a good thing because if you lose it by raising or changing the needles, you get best performance. If your needle jets are worn, you won't get best performance regardless of where your needles might be set. The motor will always be sluggish. It is important to keep your main jets slightly over-rich. If you are ever going to burn a piston, it will probably be when you are riding the bike on full throttle.
If you get your motor jetted right at medium throttle openings, it will probably be much more sensitive to changes in the weather. If you are running slightly over-rich, you will find the bike will go much faster when the weather is very cold. So you need to think about the prevailing weather conditions and jet to accommodate them.
 
More interesting stuff, thanks! Now you mention it, I do have memories from many, many years ago of bikes seeming to run more sweetly in cold weather – to be honest, what you say had never occurred to me! With this bike then, it seems to be a case of for richer or poorer, as well as in sickness and in health! :wink:
 
NortonMKIIA850 said:
Hi,

I've owned my MKIIA 850 (Europe) since 1998, and have recently got it out of storage after 12 years. (long story) It has the MKIII-style black plastic airbox, sectioned spray tubes and 4 ID ring needles that you'd expect on a MKIIA, but it was wearing straight-through peashooter silencers when I bought it, instead of the bean cans with which it would have left the factory. While rummaging through old Norton content on my computer yesterday I found a note from 2002 by Frank Forster, who might be on Access Norton...? Whether or not, I hope he doesn't mind me bringing this up. Anyway, he wrote: 'The 4 ID ring needle was made for the restrictive "bean can" mufflers used on the MK III Commando [and the MKIIA], and are correct for that use. For any Commando using mufflers of the "straight through" design, one must use the 2 ID ring, as otherwise there will be a lean spot when you open the throttle to increase from a 70 MPH speed.'

Is this still believed to be the case? If so, I'd better get some 2-ID-ring needles pronto – my Commando has always seemed lacking in power much above 70 mph, and doesn't like pulling much above 85 – 90. Even though I had the engine rebuilt by Norman White many years but only a few thousand miles ago. While I don't want to risk burning valves etc. by running lean at high speed, might this needle issue be behind the apparent drop in power? TIA
Click to expand...

the aftermarket bean cans are built the same as most mufflers not like the real ones
so there should be no big difference's in jetting.

(FYI I have real "bean Cans" with a very mild build and use a a 260 main)
 
I lived in Yerington, Nevada which grows a lot of hay, big fields being sprayed with water often. I was trying different jetting in my MK3 and the long straight roads bordering the fields offered great visibility and no traffic. I could actually feel the motor run stronger alongside the fields and then taper off when I left them.
 
When it is all said and done, the only part of the jetting you really need to put much work into is the needles and needle jets. If you use a number 3 slide in most carbs, the idle cycle and pilot jets usually have enough adjustment in them to get the very low throttle part OK. At the top end, it is normal to use main jets which are slightly too rich, so you don't burn anything when you do the big blast. So all you are left with is ajusting the middle bit of throttle opening from 1/4 to 3/4 throttle. What you are looking for is jetting just slightly on the rich side of too lean. If you go too rich, the motor will be slightly sluggish. Too lean and you will get the cough, flat spot or hesitation. If you get the jetting just right on the needle jet, you might get a real surprise just how good your bike will perform. It is the reason that the guys with the Manxes in the Lansdowne series and Goodwood Revival go so fast. The Brits have got the patience of Jove.

https://www.youtube.com/watch?v=o67ravmXl5U
 
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