Question about carb setup '73 MK2 850

Today I decided to take the carbs apart for 2 reasons: 1 I didn't yet take a look at their condition and 2 The right one had a slide that was stuck a bit and came down slow. Also that one would overflow a bit after tickling. I found a few things that I need opinions about:
1 The carbs had 220 main jets. The books all say that 850's should have 260's. I have some brand new 260's as the PO gave me the old carb's also as he bought a new set some time.
2 I have 2 different slides. Left is a chromed 3.5 slide, right is an Aluminum 3 slide. The cutaway in the chrome one is higer. The books say an 850 should have 3.5 slides. Boths carbs are 932's and I measured them to actually be 32mm's. The aluminum slide already had some marks where it was squeezing in the house. I found the old carbs had brand new chrome 3 slides in them, probably the ones that came with the new carbs.
3 the PO had reused the old needles. I didn't find any marks, but i found some brand new needles among the parts that came with this bike.

So I have 932 carbs, but with 220 jets, old needles and different slides. This bike shoud have ran rough just above idle, is my guess, but it ran. I gues it has always had this setup, with both the old carbs as the new ones. I have no idea what the extra number on the bodies mean. L and R are left and Right, but L36 and R300 make no sense to me.

Needles are in the middle position, should be top?

I see no reason to have different slides. As the alu 3 slide was already eating itself in the body and as I think they should both be 3.5's and because the chromed one had more play than the aluminum one, I think of buying 2 anodised 3.5's at Norton Holland works. And I think of putting in the 260 mains.

The choke slides are left out in this bike (but I have them in my stock). The 3.5's will make the mixture leaner, so I hope it will be no problem during warm up.

I wonder what the people over here think.

Some pics:

NIX said:

The carbs had 220 main jets. The books all say that 850's should have 260's.

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220 would be correct for the 850 Mk2A and Mk3 with the standard black plastic airbox and black cap silencers but larger jets are normally required for peashooters or other silencers/mufflers.

NIX said:

I have no idea what the extra number on the bodies mean. L and R are left and Right, but L36 and R300 make no sense to me.

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932/L36 is a carb from an 850 Mk2 and would normally have a 260 main jet (for peashooters).
932/R300 is a generic replacement carb.
Edit: 850 Mk2 carbs would be 932/R35 and 932/L36 (or R29 & L30).


Standard 850 slides are 3.5.
850 needles are 928/104 identified by 4 ID rings above the needle clip slots, however, the one in the photo appears to be a two ID ring 622/124 standard 4-stroke needle.

Are the spray tubes flat-topped or stepped (L36 would be stepped as standard)?

Got to make them the same jetting/slide type at a minimum. So would go for the recommended 850 spec jets/slides. When you say one slide is slow to drop back down, have you tried different slides in same body with same issue? You might have a distorted slide bore in that body, likely from over torquing of the flange, bowl or top cap fittings. There was a thread some time ago the forum on a method to straighten out the bore. Note you could also consider having both carbs & correct slides "sleeved" for a reasonable price.

You do need to really check the fuel & air passages, as these can be a real issue on original carbs. All but impossible to fully clear the fuel passages leading to the pilot jet. So even if jet itself flows well when spraying cleaner through, the fuel pickup at bowl and all the right angle turns within passage can be fouled, unreachable...which limits easy of fuel flow to the jet. Unless you can find a really good solvent or figure a way to work some wire through the passages, might be worth moving to a replacement carb purchase.

Look at the last section of this page: https://www.gregmarsh.com/MC/Norton/CommandoID.aspx There are three different setups that could be a 74 850. Depends on your exhaust and airbox. L36 is probably correct - those numbers define the jetting - 932 is the size. The one with 300 just means generic jetting - that's OK if a Norton-specific generic. Later Norton's have special spray bars and needles. If you buy a standard rebuild kit, you will not get a Norton-specific needle. Not sure which "books" say 260 main jets. Some certainly do but Amal does not for all bikes.

Do you have peashooter or black cap mufflers? If peashooters, are they very loud? Do you you have the "hamcan" air filter, or plastic air box, or something else. What is the engine number?

Add up the price of all the parts you're thinking about and then compare to a new set of proper Premier carbs. It costs a lot to rebuild them with new slides and then you may still have worn bodies.

I agree with idea of replacing the carbs with a new set of 932 Premiers!

Hi All, thanks for the answers. The airfilter is not the plastic box, but the older metal ham can type. Exhaust is loud pea shooters (no black caps). This is a US MK2 850 engine setup, so no MK3 with plastic box. I plan on mounting exhaust pipes with a balance pipe because I remember it was more quiet. I have a used set of pipes as it should, that I hope will fit with the current silencers. Until then I will ride it with the current imitation pipes. Reason why 1 slide got stuck is because it's both soft metal. That's why I want to buy good fitting anodised ones, hopeing they will last a bit longer. Besides that: I have four 3 slides and only one 3.5 slide.

I have the original Norton workshop manual and the Haynes.

I will have a look at the spray tubes and at both the old and the new needles and give the required info. (I'm sorry I didn't.)

Am I correct that .106 needle jets belong to 2 ID ring needles?

Not planning on buying new carbs if these can be easily built to the right specs. And they are fairly new. For me it's important that I now know one is an original number and the other an original replacement, so I need to check more but they can be made a match.

This bike has some crazy features.

NIX said:

Am I correct that .106 needle jets belong to 2 ID ring needles?

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The .106 needle jet is the same for both.
The 2 ID ring needle is used with the flat-topped '4-stroke' spray tube and 4 ID needle with the stepped 'Norton' spray tube although you can use the 2 ring needle and flat top spray tube combination if (both?) carbs have the flat top spray tubes although the L36 would normally have a stepped spray tube and the R300 spray tube is normally flat-topped.

Re; 3 and 3.5 slide.. check that someone hasn't filed a 3 to a 3.5 cutaway.

NIX said:

Hi All, thanks for the answers. The airfilter is not the plastic box, but the older metal ham can type. Exhaust is loud pea shooters (no black caps). This is a US MK2 850 engine setup, so no MK3 with plastic box. I plan on mounting exhaust pipes with a balance pipe because I remember it was more quiet. I have a used set of pipes as it should, that I hope will fit with the current silencers. Until then I will ride it with the current imitation pipes. Reason why 1 slide got stuck is because it's both soft metal. That's why I want to buy good fitting anodised ones, hopeing they will last a bit longer. Besides that: I have four 3 slides and only one 3.5 slide.

I have the original Norton workshop manual and the Haynes.

I will have a look at the spray tubes and at both the old and the new needles and give the required info. (I'm sorry I didn't.)

Am I correct that .106 needle jets belong to 2 ID ring needles?

Not planning on buying new carbs if these can be easily built to the right specs. And they are fairly new. For me it's important that I now know one is an original number and the other an original replacement, so I need to check more but they can be made a match.

This bike has some crazy features.

Click to expand...

If they have the correct spray bars and if you buy the correct major repair kit, and if you buy the correct slides, they can be made the same. You need Major Repair Kit RKC/602 so you have the correct needle. RKC/460 looks the same but has the wrong needle.

It's very easy to spend a lot of money on old amal carbs and find they still won't work right
So if they are high mileage IE anything over 10,00 is high for amals (unless they have been used every single day)
You may be better off binning them and buying new
Just my two penneth worth

+1 on new carbs. I went the repair route when I got the bike and it was a waste of money.
New premiers solved a lot of problems and will last longer anyway.
Don't repair.

The carbs are less than 5000km's old (less than 3000 miles). I was hoping I had enough parts lying around to build something working out of them. It seems I have 2 different caerbs on the bike. All the needle jets I have are 106. The new needles are one with 2 lines and one with 4. And the 300 carb from the bike has the 4-stroke flat top spray tube, where the L36 has the stepped Norton spray tube. I also have a 301 used spare carb with tiny tickle knob.

It seems the old combination was L36 and R35, stepped spray tubes, 106 needles jets, 2-ring needles and 220 mains, which seems an odd combination as well. The new carbs however didn't bring what this bike is supposed to have, as it's 2 different kinds of carbs, mixed with old parts like the 2-ring needles and 220 mains. I guess this bike ran pretty hot as the pipes are rather blue (40 first centimeters from the head on).

I read the advises on new carbs, but if I try to match these carbs It seems best to rebuild the 300 to Norton standards (R35) wit a spray tube from one of the old carbs and get another 4-ring needle and 3.5 slide.

I'm even thinking of buying 2 anodised 3.5 slides, a 4-ring needle and reuse the old carbs. (Assuming they were set up wrong anyway and replaced for this reason.)

Another combination I can make is taking the 300 and 301, chrome 3 slides, 260 mains and 2-ring needles and see what position runs best.

Apart from buying new, of course.

Different spray tubes:


106 needle jet in the 300:


106 needle jet in the L36:


spare new 260 mains and 106 needle jets:


The spare new needles:


Olde carbes were L36 and R35:


Stepped spray tubes in the old carbs:


Spare 301 with flat spray tube:

NIX said:

Another combination I can make is taking the 300 and 301, chrome 3 slides, 260 mains and 2-ring needles and see what position runs best.

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That will probably work. The tiny tickler on the 301 can be replaced with the later type.

Check the 300 and 301 both have the flats on the unused inner throttle stop bosses and carb flanges like the original carbs as it's not a standard feature of the 300/301.
The 300 presumably already has the flats as it's been fitted but I can't see any evidence of it on the 301's flange in the photo.

Will do, thanks.

Looking at this site: https://andover-norton.co.uk/en/shop-drawing/66/carburettors-carburettor-pieces

Maybe I shouldn't look at these sites as mistakes are made everywhere. But I see they marked some parts specifically as MK2a:
5A Throttle valve 3.5
11A Throttle needle 4 ID grooves
22 spray tube cut away

The 06.7334 carbs pair is for MK2A mentioning the 220 jets.

It seems the MK2a is a much leaner set up than the normal MK2. Just wondering what spray tube the normal MK2 requires. If it's the flat, rebuilding the 300 and 301 seems really the logical choice with 3 slides, 260 mains and 2 groove needles.

But on this site however: https://andover-norton.co.uk/en/shop-drawing/206/carburettors-carburettor-pieces
The cut away spray tubes, 260 mains, 4 grooved needles and 3 slides seem to be combination for an 850 32mm.

I see a lot of combinations, so I wonder if it changed per year/airfilter/silencers. Both the 3 and 3.5 slides are used in 32mm's, both 2 and 4 groove needles and both 220 and 260 mains are used. Only thing that seems the same everywhere are the 106 needle jets.

The 3 slides will run richer from idle to 1/4 throttle, the 4 groove needles will run leaner (I think mainly above 1/2 throttle and the influence of the spray tubes I have no idea about. 260 will run richer than 220's, that's obvious.

I probably need to check the idle jets also if I can find them.

NIX said:

Will do, thanks.

Looking at this site: https://andover-norton.co.uk/en/shop-drawing/66/carburettors-carburettor-pieces

Maybe I shouldn't look at these sites as mistakes are made everywhere. But I see they marked some parts specifically as MK2a:
5A Throttle valve 3.5
11A Throttle needle 4 ID grooves
22 spray tube cut away

The 06.7334 carbs pair is for MK2A mentioning the 220 jets.

It seems the MK2a is a much leaner set up than the normal MK2. Just wondering what spray tube the normal MK2 requires. If it's the flat, rebuilding the 300 and 301 seems really the logical choice with 3 slides, 260 mains and 2 groove needles.

But on this site however: https://andover-norton.co.uk/en/shop-drawing/206/carburettors-carburettor-pieces
The cut away spray tubes, 260 mains, 4 grooved needles and 3 slides seem to be combination for an 850 32mm.

I see a lot of combinations, so I wonder if it changed per year/airfilter/silencers. Both the 3 and 3.5 slides are used in 32mm's, both 2 and 4 groove needles and both 220 and 260 mains are used. Only thing that seems the same everywhere are the 106 needle jets.

The 3 slides will run richer from idle to 1/4 throttle, the 4 groove needles will run leaner (I think mainly above 1/2 throttle and the influence of the spray tubes I have no idea about. 260 will run richer than 220's, that's obvious.

I probably need to check the idle jets also if I can find them.

Click to expand...

All 850s require the Norton-specific spray bar and needle. 300/301 were never used by Norton. They are generic numbers. The filing required to fit two carbs together on a Norton should not be on a 300/301 but simply filing them could make them work. Are you sure both carbs are 932 (not 930)? The main jets, unless you'll be riding very fast, are the least of your worries!

NIX said:

I probably need to check the idle jets also if I can find them.

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There aren't any to "check" on the standard carbs - they are built into the carb. That's one of the important improvements in premier carbs- removable pilot jets.

What head do you have on the bike? A 73 wouldn't be a MKII, that was a 74 model with RH10 head and 30 mm intake ports.
I have used the flat top spray tube and 2-ring needles on my MKII without issues. Always with 260 mains and 3-1/2 slides.

NIX said:

Maybe I shouldn't look at these sites as mistakes are made everywhere. But I see they marked some parts specifically as MK2a:
5A Throttle valve 3.5
11A Throttle needle 4 ID grooves
22 spray tube cut away

Click to expand...

Yes, mistakes are everywhere and AN is no exception.

NIX said:

The 06.7334 carbs pair is for MK2A mentioning the 220 jets.

It seems the MK2a is a much leaner set up than the normal MK2.

Click to expand...

Yes, because the Mk2A was the low noise model with a different airbox and black cap silencers.

NIX said:

just wondering what spray tube the normal MK2 requires.

Click to expand...

All 850 carbs had the stepped spray tubes and 4 ring needles as standard...

NIX said:

If it's the flat, rebuilding the 300 and 301 seems really the logical choice with 3 slides, 260 mains and 2 groove needles.

Click to expand...

...you can use carbs with the flat top spray tubes and 2 ID needles on an 850 (long story) but it isn't the original 850 specification.

NIX said:

Both the 3 and 3.5 slides are used in 32mm's,

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3.5 were standard on all 850s, 3s on 750s, however, you might find 3s are better with the flat tops and 2 ring needles, and that's something you may need to try.

NIX said:

both 2 and 4 groove needles and both 220 and 260 mains are used.

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The standard 850 needle was the 4 groove fitted with stepped spray tube.

2 groove is standard '750' but that doesn't mean the 2 groove needle and flat top spray tube doesn't work on an 850 (again long story) and my 850 Mk3 works fine with that combination (932/300 & 301 carbs and '3' slides).

Of course, if you enjoy the experimental nature of what you are describing and are prepared (and looking forward to) the fiddling around that will be required, that's great! Often we do things to "make something work" rather than buy new parts just because it's challenging/fun to do so. It, of course, also takes a lot more time.

OTOH, if you are not looking at the project as fun/enjoyable, a pair of new carbs will deliver the goods with little or no frustration!