Carby jetting and plug advice

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croz2803

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Almost finished rebuilding my 850 commando and i just thought i'd get some opinions about how to set up my carby and what plugs would work best

I have a mk 2 850 commando with powermax pistons, a 4s cam, trispark ignition, an accel 140403s coil and a mikuni vm 34 carby. im not looking for "the ultimate tune" but im a little lost with working out what slide and jets i should have. i live in Melbourne, Australia so its rought 10m above sea level. the majority of my riding will be at speeds bellow 80 km/h.

Cheers

Andy
 
Concentrical car burator .

http://amalcarb.co.uk/downloadfiles/ama ... _guide.pdf

http://www.oldbritts.com/amal_tun.html

http://www.britcycle.com/Manuals/amaltune.

Note , ' spray tube ' s on this one http://amalcarb.co.uk/rebuilding-mark-1 ... arburetter

http://jerrydoe.com/norton_pdf/carb/carb_ebook_comp.pdf

plugs :

http://www.ngksparkplugs.com/tech_suppo ... aqread.asp

http://www.wallaceracing.com/plug-reading-lm.html

$s will come in at about 80 mph . :D :twisted:
on 23 T gearing . :D

Hope youve installed superior valve springs set / checked clearance .
==========================================================
Checking center electrodes not melting , as it will overadvanced or to hot a plug ( same thing )
220 to 270 mains . Will depend on exhaust too . try 240 ?

Not orrible Lucas 6 V coils , we hope . :lol: :p
 
One thing I've noticed, that never seems to be discussed, is the functional difference between the "notched" spray tube (and matching "four ring" needle) fitted to some 850's, and the standard spray tube and needle (as fitted to practically all other British bikes... including 850's). The notched spray tube was originally intended as a "work around" for machines shipped to the US market, equipped with "muted" mufflers for EPA noise compliance. The problem was that US spec 850's would not accelerate in the mid-range worth a damn because of the mutes in the peashooters! Using the notched spray tube apparently solved that. (Although according to Service Release N2/11 issued in February of 1973, dealers were supposed to install the mutes on new bikes and rejet to 220 mains.) Most Commandos (750 or 850) for other markets with OEM peashooters NOT equipped with mutes did NOT have the notched spray tubes and did not require re-jetting (retaining stock 260 mains).

The 2013 Amal Catalogue shows these notched spray tube carbs (932/29 right - 932/30 left) as correct replacements for 1973 850's. The same catalogue shows 932/35 right - 932/36 left as correct replacements for 1974 850's... (with standard spray tubes)... which came stock with "black cap" mufflers! Needless to say this can create some serious confusion!

Loosely translated... I interpret all this as meaning "Commandos are VERY sensitive to exhaust tuning and muffler retriction." Mismatching carbs to pipes, (and not knowing about the spray tube 'issue") might very well be a major reason so many owners switch to Mikunis and other aftermarket carbs, rather than frustrate themselves trying to properly tune the factory Amals.

FWIW: My experience has shown that using the notched spray bar carbs with "straight through" peashooters (and most likely any other straight though muffler)... results in really poor "fluffy" midrange response... no matter what you do with needle position. Switching to standard non-notched spray tube carbs (with matching needles) makes a huge difference. Midrange response becomes truly arm-straightening power.
 
Fit the exhaust first, set the ignition timing and plugs to spec. , then tune the carburettors to it. If you then change the exhaust, retune the carburetors. Many years ago we had a well tuned Thruxton Velocette which had the muffler with the fish tail silencer. The owner swapped it for a megaphone, and it immediately burned a piston.
 
As to tuning a Mikuni this might help:

www.mikuni.com/pdf/vmmanual.pdf‎

And this is a pretty good baseline:

On a 850 Norton for a SINGLE Mikuni VM 34mm dia, use pilot 30-35, throttle slide 3, needle jet type 159, needle jet size P-4, P-6, needle 6DH3 or 4, Main 260, air jet 1.0.
 
Kip said:
As to tuning a Mikuni this might help:

http://www.mikuni.com/pdf/vmmanual.pdf‎

And this is a pretty good baseline:

On a 850 Norton for a SINGLE Mikuni VM 34mm dia, use pilot 30-35, throttle slide 3, needle jet type 159, needle jet size P-4, P-6, needle 6DH3 or 4, Main 260, air jet 1.0.
Click to expand...

This is the info you need. Then buy jets on either side of the recommendation so you can tune to your specific bike (might not be perfect but will get you close). So you should consider 32.5 as center for pilot jet. I would get 27.5, 30, 32.5, 35, 37.5 then put the 32.5 in there adjust the mixture and see what the throttle response and idle is. Do a WOT plug check. Make sure you have the right heat range plugs :D Adjust the pilot size based on plug color and performance. Main jet can be adjusted in the clip, so work with the recommendation size first and adjust as necessary.

Proper pilot adjustment is to open the screw until fastest speed is obtained and turn screw in 1/4 turn. I usually set the idle up to where the engine will keep running to make this adjustment. Number of turns out as stated in carb specs and manuals is the start setting with a new or rebuilt carb and is not the proper adjustment...you need to adjust to your bike and location once installed. If you have to turn the screw out more than 3 turns the pilot jet is too small.
 
As a follow up to the seemingly endless confusion and lack of hard FACT regarding notched spray tubes on 850's:

1 - Amal (Burlen) states that ALL their concentric 850 carbs with standard 260 main jets, are now fitted with notched spray tubes... (932/29-30 and 932/35-36) and that this is a result of information "on file" from original sources at Norton back in the day.

2 - I own original versions of 932/29-30 and 932/35-36 and I can tell you for a fact that the 932/29-30 carbs have notched spray tubes (as fitted to 1973 850's with original equipment US-spec peashooters with noise compliant "mutes") and 932/35-36 carbs (fitted to 850's in 1974 with black cap mufflers and the "quiet" airbox) do NOT have notched spray tubes.

3 - NORVIL shows fitment of notched spray tubes to post-MK2A 850's and "full circle" spray tubes for earlier 850's (pre MK2A).

The fundamental difference between these 850's are the intake and exhaust; black cap mufflers and "open" peashooters are not restrictive (and peashooters with mutes, and so-called "Interstate" mufflers are/were).

The full circle spray tube was the one used for proper mid-range performance with unrestricted intake/exhaust combos, by Norton, when the bikes were new.

Norton intended the notched spray tube to be used to prevent "flat' or non-existant acceleration when retrictive mutes were "mandated" for U.S. noise control.

Amal (Burlen) is currently in error. All 850's are not alike!

NOW... can anyone supply facts that dispute this? More importantly has anyone actually tried/used both types of Amals back-to-back to see what the ACTUAL results were, on an "unrestricted" 850? (Not really interested in "butt dyno impressions" as much as maybe real dyno charts... or some form of FACTUAL information on mid-range roll-on response, quarter mile times, etc.)

In other words... wouldn't 850 owners with free flow air cleaners and straight through mufflers be better off NOT using notched spray tube carbs? Can anyone prove it? Disprove it?

Sure would be nice to know... one way or the other... once and for all. C'mon... PLEASE? :?
 
Kip said:
As a follow up to the seemingly endless confusion and lack of hard FACT regarding notched spray tubes on 850's:

1 - Amal (Burlen) states that ALL their concentric 850 carbs with standard 260 main jets, are now fitted with notched spray tubes... (932/29-30 and 932/35-36) and that this is a result of information "on file" from original sources at Norton back in the day.

2 - I own original versions of 932/29-30 and 932/35-36 and I can tell you for a fact that the 932/29-30 carbs have notched spray tubes (as fitted to 1973 850's with original equipment US-spec peashooters with noise compliant "mutes") and 932/35-36 carbs (fitted to 850's in 1974 with black cap mufflers and the "quiet" airbox) do NOT have notched spray tubes.

3 - NORVIL shows fitment of notched spray tubes to post-MK2A 850's and "full circle" spray tubes for earlier 850's (pre MK2A).

The fundamental difference between these 850's are the intake and exhaust; black cap mufflers and "open" peashooters are not restrictive (and peashooters with mutes, and so-called "Interstate" mufflers are/were).

The full circle spray tube was the one used for proper mid-range performance with unrestricted intake/exhaust combos, by Norton, when the bikes were new.

Norton intended the notched spray tube to be used to prevent "flat' or non-existant acceleration when retrictive mutes were "mandated" for U.S. noise control.

Amal (Burlen) is currently in error. All 850's are not alike!

NOW... can anyone supply facts that dispute this? More importantly has anyone actually tried/used both types of Amals back-to-back to see what the ACTUAL results were, on an "unrestricted" 850? (Not really interested in "butt dyno impressions" as much as maybe real dyno charts... or some form of FACTUAL information on mid-range roll-on response, quarter mile times, etc.)

In other words... wouldn't 850 owners with free flow air cleaners and straight through mufflers be better off NOT using notched spray tube carbs? Can anyone prove it? Disprove it?

Sure would be nice to know... one way or the other... once and for all. C'mon... PLEASE? :?
Click to expand...

All the data, charts or opinions based on an idividuals experience won't mean a damn thing. The only real and usefull definitive answer is how one or the other will work on YOUR bike.

There are too many variables depending on how your particular unit breathes,like airfilter, pipe, carbon build up or the lack there of, compression, wear of rings, valve train condition and on and on.

Realistically the only sure way to know... one way or the other... once and for all, is to spend the pittence on whatever spray tube you do not have and try it out for yourself. It takes about an hour to swap, is easy and straight forward to do and the information you get out of it will surely be definitive... one way or the other... once and for all!

I shit you not, the manufacturing processes of these bikes were drill press's and warn out jigs, at least that is what some reputable members tell me. I have no reason to doubt them.

Anyhow, please, let us know what you find out.
 
Amals are just bloody crude compared with a Mikuni. You would never use an Amal carb on a two stroke unless it had a horrible Villiers engine.. The only reasons I use Mk2 Amals on my Norton is they bear some resemblance to authenticity, and the slides don't hang up with the extra vacuum of the four stroke, also the methanol kit is readily available. I try not to use obvious Japanese parts on my Norton.
 
Pete,

Appreciate the response... but... been there done that...already know "what works best" on my bike. Misses the point entirely!

The idea is to get enough hard info from others to determine what is "generally" best and most accurate... for most 850's. Since Amal is not shipping (260 main jet) 850 carbs with full circle spray tubes, yet not all 850's actually used the notched spray tubes when new (see earlier post)... and the type spray tube (relative to exhaust and intake) makes a clear and significant difference in midrange performance... it begs the question.
 
My bad, kip, for not reading. But still, attaining solid info will still only be generally applicable to these 40+ year old motorcycles.

One other point I might have missed in the thread is the needle. I may be mistaken but i believe the needle to the notched spray tube is proprietary to it and same for the full circle spray tube.

As mentioned from Mikuni fans is that getting the best performance from an Amal system is certainly not the best performance attainable, of which i am a proponant. This is just simply the price of originality and just to be clear, there is nothing wrong with that.
 
Kip said:
2 - I own original versions of 932/29-30 and 932/35-36 and I can tell you for a fact that the 932/29-30 carbs have notched spray tubes (as fitted to 1973 850's with original equipment US-spec peashooters with noise compliant "mutes") and 932/35-36 carbs (fitted to 850's in 1974 with black cap mufflers and the "quiet" airbox) do NOT have notched spray tubes.
Click to expand...

According to the 1974 (Mk2/Mk2A) factory parts supplement, the "Mk2A" (black cap and black plastic airbox) models were fitted with Amal type 932/33 & /34 carbs.

The other carb option listed for '74 (obviously 850 Mk2 although it doesn't specifically say so) is 932/29 & /30.

Only the stepped "928/107 Spray Tube", and "928/104 Throttle Needle (use with Spray Tube 928/107)" are listed in the '74 parts supplement.

Mk2A (& Mk1A) noise reduction models NOT having the stepped spray tubes (and 4-ring ID needles) certainly seems to go against everything that has been said here in previous "spray tube" discussions.

https://www.google.com/search?sitesearc ... ray%20tube

http://www.inoanorton.com/docs/TheNortonCarburetor.pdf

Note the reference to:
"73-74 850 (with
black caps) 932/31-32"

"932/31-32" - were apparently fitted to the 850 Mk1A model.
 
L.A.B.

My comments (and associated part numbers) were regarding the only two Amal (sets) that come standard with 260 main jets... and BOTH (as supplied new from Amal/Buren) have notched spray tubes... period! All the others have smaller main jets... regardless of Amal number. In fact, one of the techs at Amal/Buren emailed me the "build sheets" (complete with hand written notes) and as near as I can tell by the component part numbers... all four 932 carbs with 260 main jets are identical, with the exception of being left or right "handed". In other words - internally as of now - there is no difference at all between 932/29-30 and 932/35-36!

Whats making me crazy... is that I cannot seem to get a consensus on whether all 850's SHOULD have notched spray tubes. NORVIL says no... Amal says yes... they cannot both be correct. AND... as I mentioned... my untouched originals have two different spray tube/needle configurations - 932/29-30 with notched spray tubes and matching four ring needles - the 932/35-36 with full circle spray tubes and standard needles! Weird and mysterious... but stock... which makes me think NORVIL has it right! They imply that free-flow air cleaners and mufflers do not require notched spray tubes... whereas "restrictions" on either of those do!
 
Ludwig,

Page 76 of Mick Duckworth's book "Norton Commando", He is relating a story he got from Bob Rowley who was on the scene at the time. Here is the rather lengthy quote:

"To help meet America's ever-tighter noise limits, the exhaust system now had a balance tube linking the downpipes. Machines shipped to the US also had additional mutes inserted in the silencers to further reduce noise. With these in place, the 850 proved very sluggish in sudden acceleration tests as was being demanded for the US market, to prove that a machine could safely overtake trucks on Interstate highways.

Rowley recalled that when testing an 850 with US silencing, snapping the throttle wide-open in the higher gears at 40mph (65kp/h), acceleration was virtually zero. 'At the end of a measured quarter-mile it would only be doing 41mph (66kp/h). No one seemed to want to know about this, but I mentioned it in front of Dennis Poore, who thanked me for telling him and said something should be done.'

Rowley spent weeks working with Amal's Alan Lines at that company's Birmingham factory. A breakthrough came when they tried the type of spray tube Amal fitted in two-stroke carburettors. with a cutaway at it's top outlet. By experimenting with different shapes and amounts of cutaway in an 850's Concentrics, they arrived at a situation where the main jet size could be greatly reduced. This had the effect of eliminating the flat-spot while maintaining low and high rpm performance."

(As an aside... I've seen a British owner's manual for 850's that shows a 280 main jet as standard! No wonder they wanted to reduce the size.)

In a way... since that info sure seems like a "band- aid" work around ... it got me thinking about all this spray tube stuff as directly related to exhaust restriction. If UK (home market) Commandos with peashooters and no mutes didn't need the notched tubes (just those bigger jets) and accelerated just fine, all this at least proves how touchy an 850 is about pipe choices!
 
2 tokes don't have the idle intake pull of a 4 stroke so the cut back spray tubes allow more easy fuel lift quantity for off idle low rpm response. Much above idle the spray tube don't matter much at all as air velocity is enough so should not notice type of spray tube but for very low down off idle low rpm response. Amal spray tubes vary in height carb to carb and Amal did research that showed it didn't change fuel mix to matter a whitworth. i've V-nothced 932's on my factory Combat and notice a bit better to go from no throttle 'slow' rolls to snap throttle w/o bog to kick rear out a bit for fun on pasture and THE Gravel. Also noticed can take higher throttle held on slow steep climbs w/o stalling or unintended over powering tire to spin. The couple-3 factory 850's I've ridden gave a nice initial torque thrust but then leveled off for disappointing pull to speed, not the continued building pull of standard 750 or Combat that tempts passing traffic so easy. I think all Commandos would slightly benefit by some spray tube notching for tight city and off road response but not hi way needs.
 
Hobot,

You've actually got that backwards. Two strokes have a hell of a hit - all at once - and the cutaway spray tube is designed to prevent over-rich mixture when you yank the wire at low engine speeds.

"The slanted spray tube is there on a piston port 2 stroke carb to prevent the engine from taking a big gulp of gasoline at low throttle openings just as the piston rises and clears the intake port.

Piston port 2 stroke engines develop a lot of crankcase vacuum as the piston starts up the bore. As the piston travels up, and before the bottom of the skirt clears the intake port, the rising piston increases vacuum in a non-linear fashion. When the port does open vaccum has increased to a point where it creates a "pop" in the intake as the air rushes in to fill the vacuum. If left alone, this rapid incoming charge at low throttle openings, would cause the mixture to be momentarily abnormally rich." (Quote from John Healy.)

Apparently - that same phenomenon, to some degree, was exactly what was happening to 850's with US pipes. Hence the notched spray tubes.

Lets be clear, this whole notched vs full circle spray tube debate boils down to the effects of certain exhaust systems on AIR FLOW... due to pulsations in the pipe... and it's effect on clean, sharp mid-range performance. Na-Da to do with fuel in and of itself. It's just that you cannot properly jet a choice of pipe if you don't know what the air is up to, and that depends a whole bunch on whether air cleaners and pipes are helping or hurting! Make sense?
 
Firstly.

To backtrack slightly.
Kip said:
3 - NORVIL shows fitment of notched spray tubes to post-MK2A 850's and "full circle" spray tubes for earlier 850's (pre MK2A).
Click to expand...

Reference to the stepped spray tubes appears in a promotional article in the April 1973 edition of the UK publication, "Motorcycle Sport" (reproduced in the book Norton Commando - Gold Portfolio), the feature outlines the various detail improvements of: "THE NEW BIG NORTON" (850 Mk1) one of those features being:

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

Therefore it's reasonably safe to assume that the stepped spray tubes were in use before Mk2A.

Kip said:
My comments (and associated part numbers) were regarding the only two Amal (sets) that come standard with 260 main jets... and BOTH (as supplied new from Amal/Buren) have notched spray tubes... period! All the others have smaller main jets... regardless of Amal number.
Click to expand...

If you had raised this matter a few years ago, then I'm fairly sure the consensus of opinion would have been that: 850 models with peashooters (850 Mk1 & Mk2) had carbs with flat top spray tubes, standard "4 stroke" 2-ID ring needles and 260 main jets, and the low noise emission (Mk1A, Mk2A & Mk3) models had carbs with stepped spray tubes, 4 ID ring needles and 220/230 main jets. However subsequent "spray tube" discussions did throw some doubt on that point of view, as there is no mention of the flat top spray tubes or 2-ID needles in any of the available "850" factory parts lists.

Kip said:
In fact, one of the techs at Amal/Buren emailed me the "build sheets" (complete with hand written notes)
Click to expand...

Phil Beresford sent me a copy of the Norton Commando factory carb build list some while ago (no hand written notes though), which shows stepped spray tubes & 4-ID needles listed for all 850 models, and that again seemed to reinforce the idea that all 850 models had carbs fitted with the stepped spray tubes & 4 ID needles.

Kip said:
and as near as I can tell by the component part numbers... all four 932 carbs with 260 main jets are identical, with the exception of being left or right "handed". In other words - internally as of now - there is no difference at all between 932/29-30 and 932/35-36!
Click to expand...

It is likely that the 932 35/36 carbs superseded the 932 29/30 on later Mk2 models (as 932 35/36 are not listed in the 2/2A parts supplement for Mk2) however I get the feeling that the 29/30 and 35/36 carb specifications may not be exactly identical, as I've an idea that 932 35/36 carbs may have the 622/056 float bowls with 0.125" diameter float needle seats (instead of 0.10") as standard 932 33/34 carbs apparently do. To quote Phil Beresford from a previous "Spray tube" thread:

"All 850 Commando carburetters are built using the original Amal build sheets and are fitted with the 928/107 spray tube and the 928/104 needle. This applies to standard and Premier carburetters. All the bodies have the correct machined flanges and throttle stop bosses and the 932/33 and 932/34 specification carburetters are fitted with the 622/056 float chamber which uses 0.125” needle valve seating rather than the standard 0.100” seating. Again this applies to standard and Premier carburetters and is as originally specified."

Phil Beresford, Technical Sales Co-ordinator .
Click to expand...
 
L.A.B.,

All your points are well taken. All the same, there are some points that remain unaddressed (and were the purpose of my original "hi-jack" of this thread):

1 - The "carburetor specification" (build sheets) supplied to me less than a week ago by Amal/Burlen shows float chamber/bowl part number 622/055 and chamber body insert 622/072... for BOTH 932/029-30 and 932/35-36! Phil's specs and these cannot both be correct.

2 - I own unmolested original 932/29-30 carbs and 932/35-36 versions and the spray tubes are different. However, right, wrong or indifferent, if you order 932/35-36 carbs from Amal today, they will come with notched spray tubes... same as 932/29-30. WHY - if they were different when 850 Commandos were in production (with different models and specs for different markets)... are they NOT different today?

3 - One can speculate that maybe Norton didn't have any valid reason to keep too many varieties of carbs coming from Amal and narrowed the selection they ordered for spares. The US market was 80-plus percent of their sales back then and that market required the notched tubes... so?

4 - Regardless of how it wound up the way it is... the way it is doesn't make it the way it should be! NORVIL seems to agree with this, since they basically recommend full circle spray tubes for unrestricted "applications". I tend to favor the opinion and experience of a company that has specialized in Norton all along, rather than one that bought out an original supplier just a few years ago and is essentially going by dated/second hand info that came along with the deal.

5 - Whether or not "all" 850's came with notched spray tubes is moot. The real question is whether they SHOULD have them and THAT depends on air cleaner and exhaust configuration more than "displacement".
 
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