Jetting 932 Amals on 850 Comando

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r5inmt

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I'm at 3500 ft and my 850 is running to rich, plugs are black and will even foul them after a short ride. It has 932s with 200 main jets. I'm new to Amal carbs and was wondering if any one has any jetting suggestions for me?

Thank you
 
r5inmt said:
I'm at 3500 ft and my 850 is running to rich, plugs are black and will even foul them after a short ride. It has 932s with 200 main jets. I'm new to Amal carbs and was wondering if any one has any jetting suggestions for me?

Thank you
Click to expand...
Lower the needle by raising the clip.
 
Lower your needles one notch ( move needle clip up one notch ), put in new plugs and give it a shot. Where are your pilot air screws? You can try turning those out a half-turn as well.
I got new Amal Premiers that came in with needle clips in bottom groove. I moved them to middle groove before installation. Ran lean and backfired in the pipes.
Went back to bottom groove and turned pilot air screws in a half-turn. It runs great now, perfect plug color, no backfires. I wasn't aware that a single groove move
could make such a difference. Tom
 
I live at 4000 feet field elevation and my bike is equipped with 260 main jets and the needle is position at the top groove. It runs really well. I have been toying with the idea of dropping the main jets down a size but living here in Tehachapi, CA if I were to drive any distance, I would be going down in altitude (600 feet to 4000 feet) not up.

You said in your post that you are running with 200 main jets, that seems really small for the bike. It sounds to me that someone has been trying to deal with an over-rich problem already. It makes me wonder if you even have the correct needles in the carburetors. The 850 carburetors have their own spray tube p/n (pressed into the carb body) and needle p/n's that are unique only to the Norton 850's. The spray nozzle has a step notch in it and the needle has four identifying grooves at the very top of the needle. I would think if you put in a standard needle that has 2 identifying grooves in it, it would run really rich. Anyways I think it is well worth checking to ensure you have the correct spray tube and needle configuration. I would also replace the 200 main jets with 260's. While you are at it I would make sure that the needle jets and slides are correct for your 850 engine.

Amal Carburetters has a good website of technical information and if you look up the carburetor for your bike it will tell you all of the correct jetting, etc. for your bike.
http://amalcarb.co.uk/technical/

If you happen to have a Mark IIA which is the 'special noise control' model, the main jet should be either a 220 or 230.

Keep us posted.
 
Thanks for all the good info guys. The bike had brand new 932s installed by a Norton shop in Denver just before I bought it. I also thought that the jets were to small, but it runs so rich I'm not sure. The needles have 3 notches and we're set in the middle, I'll move the clip to the top notch to lower them and see what happens. The bike itself is a 74 850 and is like brand new with just a hair over 5000 original miles on it.
 
Is there any way to know what spray tube I have? I know the needles and main jets are wrong, butt the pilot jets are 106 which is right. Does anyone know if I can just install the correct 4 notch needles and change the mains to 260 from the 200s that are in it now?
 
These pictures are from the amalcarb.co.uk website in the Mark I carburetor rebuilding section.

The spray tube that is pressed into the carb body is part number 928/107. It is the one marked Norton (3rd from the left).
Jetting 932 Amals on 850 Comando




This is an example of where you will find the spray tube. The step faces towards the engine or forward.
Jetting 932 Amals on 850 Comando




The fourth one from the left is the Norton 850 needle. It's part number is 928/104. You will note that all of the needles have 3 notches for the spring clip. What you are looking for is the identifying grooves above the top notch.
Jetting 932 Amals on 850 Comando




The slides themselves should have a 3 1/2 cutout. The needle jet is a 106 and should have a hole in the side of it just like the left hand needle jet pictured below. Make sure also that you have the same looking jet holder as the one shown on the left. Of course the main jets should be a 260 or for the Mark IIA a 220 or 230. Also check to see if your carburetor insulator is installed correctly. It should go between the head and intake manifolds and not between the intake manifold and carburetor (which I have seen from time to time). Make sure that the insulator is the correct size for your engine.
Jetting 932 Amals on 850 Comando



There is no replaceable pilot jet in the carburetor. However if the carburetors happen to be the "Premiers", they do have replaceable pilot jets which are screwed into the side of the carburetor directly opposite of the idle air adjustment screw.

Last and least I would recommend reading the information on the amalcarb.co.uk website for rebuilding, tuning and balancing.
 
Peter thank you so much for all the help. Near as I can tell these are universal carbs. From everything I can find in the shop manual nothing is right about, the slides are 3 and should be 3.5, the needles have 3 notches instead of 4, the jets are 200 instead of 280, and the spray bar matches the first one on the left in your picture. The only thing correct is the 106 needle jets, at this point I think I have to purchase the correct carbs and hope it fixes the problem.
 
No need for new carbs, just buy the right parts and fit them, hardest bit will be the changing the spray tubes over.
 
The spray tubes are the problem, not sure if takes a special tool to press them out, and press the new ones in?
 
Okay, would you suggest I just install 3.5 slides and 4 notch needles and use the current spray tube that is not notched?
 
It's not hard to change the spray tube. Remove the jet holder and tap it out from above. The new one will locate in the surations and be retained when the jet holder is screwed back in.

The only tricky part would be if you were adding a notched spray tube and needing to set it facing correctly.
 
r5inmt said:
Okay, would you suggest I just install 3.5 slides and 4 notch needles and use the current spray tube that is not notched?
Click to expand...

No. The 4 notch needle is matched to the stepped spray tube.

Also you mentioned a 280 main jet, actually it's a 260 main jet that Norton calls out. When you replace the slides I would consider getting either Amal's anodized slides or chrome plated brass slides. I have actually operated my Norton with a non-notched spray tube with a matched needle that has the 2 identifying grooves (common setup for the Amal carburetors) and it seemed to work ok. As a matter of fact I was able to ride my bike up to altitudes over 6000 feet and the bike still ran fine with the factory recommended settings. However I went ahead and changed the spray tube and needles to the 850 configuration. Also I replaced the slides with the 3 1/2 chrome plated brass slides and the carburetors ran great.

If you decide to change the spray tube, they are not that hard to remove. When you install them, you need to have a drift that is the right size so that you don't damage the carb body. You have to make sure that the step is facing exactly in the right position before you start driving it into position. It is relatively an easy operation. I know that Clubman Racing Accessories sells the discharge tubes and needles for the 850 Commando.
 
ludwig said:
There is NOT ONE person on this forum who can tell you why you need a stepped spray tube , or what it exactly does .
Whatever answer they come up with , it is all hearsay ..
Click to expand...

Hearsay or not, John Healy gave what I consider to be a satisfactory explanation on the BritBike forum.

He is not a member of this forum as far as I am aware.


http://www.britbike.com/forums/ubbthrea ... Post204284

This post peaked my interest and I did a little asking... I understood that by cutting the spray tube you disrupted the normal Bernulli/venturi effect of the spray tube at low throttle openings. That is, it would present a softer signal (difference between atmospheric and manifold vacuum) to the needle jet and air intake around the needle jet. But why do we need to soften the signal one the needle jet/air at low throttle openings?

The answer came from Kevin Cameron. Kevin is considered by some to be a world class 2 stroke tuner. He is a hero among many Canadian snowmobile tuners. He spent years, along with Kawasaki factory engineers devloping their TZ750D race bike.

He explained that a 2 stroke piston-port engine (not rotary port) has a short, but sharp drop in vacuum as the piston opens the intake port. The piston is going up, which creates a vacuum in the motor's crankcase. This vacuum builds until it is released, in what he calls a "pop," when the piston finally opens the intake port. This short, but intensive decrease in vacuum would draw too much fuel from the needle jet if the spray tube was not modified (made less effective). This effect one the spray tube is most apparent at low throttle openings.

Without the aid of on-board computers an engine tuner is stuck witht the fact that his selections must be a compromise. He is trying to get, what some tuners call "flat" carburation. That is the carb delivers the proper mixture from idle to full throttle. There is no radical variations, as presented to 2 stoke tuners with the "pop"

He must do thisto meet rider styles, atmospheric changes, enviromental rules, and any anticipated modifications to the bike. One such change is Norton's use of the stepped spray tube to lean the bike out at low throttle opeings to meet EPA requirements. Other times, such as the sloped spray tubes in the T150, to effects on intake manifold pressure caused by exhaust systems.

As a general rule you can say that straight cut spray tubes, whether it be in a Monobloc, Concentric or MKII AMAL, are suited for 4 stroke application and some 2 stroke rotary valve. When you see a spray tube that is cut in a slope it will as above but for a 2 stroke piston-port appliocation.
Click to expand...
 
I've seen somewhere, long time back, a similar written explanation about "signal strengths" and cut/stepped spray tubes.
It came either from someone in Norton Villiers, or even Amal. ??

But now I'm confused.
The Commando uses the stepped spray tubes. (?)
Did it have a weak or a rich problem at low air velocities, that needed to be corrected ??
I sorta thought it was weak, and needed something to increase the fuel flow just around idle (??).

And thats why you can't just use 2 stroke Amals on 4 stroke bikes,
Commandos in particular ??
 
With reluctance I'm going to jump in here. First, r5, when you describe your current metering needles as "3 notch" are you speaking of the clip locating notches or the identifying rings above the clip notches? For all practical purposes all (there are exceptions) Amal needles have only three clip location grooves. The key is in the identifying rings. AND THIS PART IS IMPORTANT, if by three notches you do actually mean the identifying rings above the clip location grooves you are actually running needles meant for 2 stroke applications and they are almost .20" too short and much to skinny to work with the standard 4 stroke spray tube. You can use either a 2 identifier ring needle with the standard 4 stroke spraytube you apparently have, or the 850 spec 4 identifier ring needle with the 850 spec cutaway spraytube with good results, but you can't mix parts. By the way, did you get this bike at Harry's?
 
Rohan said:
The Commando uses the stepped spray tubes. (?)
Did it have a weak or a rich problem at low air velocities, that needed to be corrected ??
I sorta thought it was weak, and needed something to increase the fuel flow just around idle (??).

And thats why you can't just use 2 stroke Amals on 4 stroke bikes,
Commandos in particular ??
Click to expand...

Without the aid of on-board computers an engine tuner is stuck witht the fact that his selections must be a compromise. He is trying to get, what some tuners call "flat" carburation. That is the carb delivers the proper mixture from idle to full throttle. There is no radical variations, as presented to 2 stoke tuners with the "pop"

He must do thisto meet rider styles, atmospheric changes, enviromental rules, and any anticipated modifications to the bike. One such change is Norton's use of the stepped spray tube to lean the bike out at low throttle opeings to meet EPA requirements. Other times, such as the sloped spray tubes in the T150, to effects on intake manifold pressure caused by exhaust systems.

As a general rule you can say that straight cut spray tubes, whether it be in a Monobloc, Concentric or MKII AMAL, are suited for 4 stroke application and some 2 stroke rotary valve. When you see a spray tube that is cut in a slope it will as above but for a 2 stroke piston-port appliocation.
Click to expand...

This says it was too rich to me, and would not meet emissions regs. I used to have an '84 Subaru GL that I would flip the air filter lid upside down on, which would let more air flow in, in order to pass the emissions check.
 
Thats ringing some bells - 2 strokes need more fuel at low speeds than 4 strokes,
so if it was too rich in the Commando they stepped the spray tube.

But why wouldn't other big bangers need stepped spray tubes then ?
Atlas had Concentric carbs, after all....

Atlas didn't have paper air filter unit though.
 
A lot of great information here, thank you guys! If I decide to go with new carbs, does anyone have a suggestion as to which carbs to purchase. I might buy new carbs, and do the mods to my current carbs this coming winter.
 
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