Amal Tuning

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My Mk III is back on the road after swapping the 20 tooth countershaft sprocket for a 22 tooth sprocket - a truly great improvement! I still have a bit of carb tuning I need to accomplish and all will be well, at least for a while.

I had the stock 932s refurbished by AMR, which involves truing the slide bore and replacement of the original slide with a hard chrome plated bronze slide sourced from a Mikuni carb. The mod does away with the choke slides, which really isn't a problem. I had this mod done to a set of 930s and have run them over 20K miles on my 750 with no issues. But on the MK III, I have a problem with lean running at small throttle openings. The bike idles perfectly once warmed up a bit, but from just off idle (the most lean) to about 1/4 throttle it's seems a little lean. The plugs' porcelain insulators are clean/white but you can see a light tan circle deep in the plug where the insulator meets the shell. The pipes have blued a little bit, indicating a hot exhaust. And just off idle the engine will stumble a some, even spit back through one carb when cold.

The bike is equipped with a K&N filter, swept back 1 1/2" header pipes, and Dunstall replica mufflers. Ignition is Boyer Brandson. The camshaft may be stock, or it may be a 70's vintage Norris street grind, as that is what the local dealer often used for warranty work. The carbs are presently equipped with 106 needle jets, standard slide needles with circlip in the bottom/richest slot, and the slides are marked as being 3 1/2 cut-away. The spray tubes are the stepped units as was standard in a Mk III.

My first thought is that the cut-away is too big, but maybe there is some problem with fuel flow through the transition port in the floor of the venturi. My understanding is that port bleeds air into the idle circuit when the throttle is closed, but then will flow fuel during the transition from idle to needle jet circuits. I've read a number of years ago something about the size & height of the cross drilled hole in the needle jet affecting mixture at small throttle openings, but I've forgotten the details. I also found in my parts stash a set of 107 needle jets, but I've been warned that is a big step in enrichening. I still plan on trying the 107s first since it doesn't require removing metal, but I sure would appreciate some opinions on a good process to follow beyond that.

Thanks for your time,
 
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An off-idle lean spot almost always develops after changing to a more fee-flowing air cleaner and exhaust, assuming stock carburetor settings and jets. To get rid of that pesky lean spot, enrichen the pilot circuit by turning the adjustment screw on each carb clockwise a quarter of a turn or so. If that doesn’t work, raise the needles with thin washers.
 

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jumpjg said:
But on the MK III, I have a problem with lean running at small throttle openings. The bike idles perfectly once warmed up a bit, but from just off idle (the most lean) to about 1/4 throttle it's seems a little lean.
Adjusting the pilot air screws or shimming the needle (as mentioned by Jason Curtiss) could result in a partial cure?

As the problem occurs off idle up to 1/4 throttle then that is the area where the mixture is mostly governed by the throttle slide cutaway.
And I would expect from the engine's state of tune you have described that 3 1/2 cutaways could be too lean.
I have number 3 slides fitted (MkIII with peashooters and custom filters)?
 
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which spray tube(s) do you have fitted?, I believe Norton fitted tubes with a cut away to prevent the lean / dead spot -- not sure but I am sure other contributors will have knowledge.Good luck, James.
 

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james said:
which spray tube(s) do you have fitted?, I believe Norton fitted tubes with a cut away to prevent the lean / dead spot -- not sure but I am sure other contributors will have knowledge.

Jumpjg does mention that the carbs have cutaway spray tubes: "The spray tubes are the stepped units as was standard in a Mk III".

And they were standard on all 850 carbs I believe.
 
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My 73 Mk1 850 didn't have the cut away spray tubes. I'm sure it was fitted to later 850s to reduce the size of the main jet. (Its usually fitted to Amals used on two strokes to richen the needle and main). The more or larger drillings in the needle jet weaken it.
Spitting back when cold can happen cos the mixture is still too lean, or the ignition may be a little advanced.
If the stumble is just as you pick up the throttles when the motor is hot, that could be caused by plug fouling as well, try NY8s.
I do think the needles are too high for a near standard motor.
Cash
 
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The cut-away spray tubes were fitted to models with the black bladder air filter - i.e. Mk1A, Mk11A and Mk111.

The critical thing is that they are fitted together with the correct needles (They have four rather than two fine grooves machined at the top and are about 1 3/4" long instead of 1 11/16")

You should be able to get it to carburate cleanly with the cut away tubes but my preference is for the full type.

K&Ns can bring flat spot problems, I have tried the type where one filter fits on both carbs and never managed to avoid flatness grabbing a sudden handful at higher rpm although over-jetting could disguise it.

If you want performance then proper Amal velocity stacks and extended inlet tracts are the way to go but I find the best compromise to be the 750 type filter.
 

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79x100 said:
The cut-away spray tubes were fitted to models with the black bladder air filter - i.e. Mk1A, Mk11A and Mk111.

Yet "The New Big Norton" feature published in Motorcycle Sport April '73 edition (reprinted in the book 'Norton Commando Gold Portfolio') and well before any black box filter models were made it says at item 19:

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

We have discussed this before, some time ago, both angled and stepped spray tubes could have been used?

http://accessnorton.com/norton_commando ... ht=stepped
 
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L.A.B. said:
Yet "The New Big Norton" feature published in Motorcycle Sport April '73 edition (reprinted in the book 'Norton Commando Gold Portfolio') and well before any black box filter models were made it says at item 19:

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

We have discussed this before, some time ago, both angled and stepped spray tubes could have been used?

http://accessnorton.com/norton_commando ... ht=stepped

You had more patience than me in looking for the old thread.

I spoke to Amals many years ago when they were still in production and their technical adviser told me that the cut away tubes were associated with the black air filters.

The Motorcycle Sport article is obviously a straight lift from a press release so could also contain intentions as well as fact. Although the Mk 1 was released before the Mk1A, it would seem likely that the 1A was in the pipeline.

The 1973 850 brochure ("Superplus") shows mostly Mk1s but has a picture of a Mk1A Roadster in blue with Annular discharge silencers and the black air filter.

I remain convinced that the cut away tube was intended to be fitted with the black box but it would be a brave man who argued that he knew how everything left the factory during those troubled years.
 

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79x100,

Yes I'm sure what you say is correct, and I certainly wouldn't want to argue the point either way.
 

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One thing that's slightly odd is that the 928/107 stepped spray tube and matching 928/104 needle is listed in the 1973 750/850 (065034) parts book?

Yet this parts book lists no other 'A' model parts??
 
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