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mk3 850 carbs

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I'm looking at replacing my MK3 Amal twin carb set up with either a pair of Amal premiers or a Mikuni single carb conversion, I must admit I do prefer the twin carb set up for originality and looks but I really want the reliability too, I have had so many conflicting opinions (the norm for Norton owners I suppose) in favour of both I am struggling to make the decision 

any tips, hints ,don'ts, do's, past experiences etc would be gladly welcomed and I will probably be just as confused at the end of this as I am now

roughly around £50 difference in favour of the Amal

cheers

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Lovely question Peter, should result in twelve different answers!

I have had a Mk3 from new, and used a Mikuni VM36 for a few years, but finally gave up on it. You are looking for comment so I will try and explain.

Twin Amals are difficult to balance exactly, if they are slightly off you get hesitation, poor low speed, etc. The fixed pilot jet Amals are also too weak on the pilot causing poor transition from closed throttle to slightly open.

A Mikuni solves all the above making it a lovely, docile, smooth runner. BUT, the Mikuni for me was a pig when trying to transition from choke on to off. I fitted a handlebar control but the choke system is a blanking seal that wants to be on or off with no halfway. You have to use the choke to start when cold, and if you switch it off too soon the engine just dies. Leave it on too long and it four-strokes (EDIT Ha! eight-strokes) badly. If you set off directly onto 40-50 mph roads this can be coped with. But if you set off into traffic or a local town it is a real pain until it warms up. Perhaps some people have solved the choke problem, perhaps the VM34 is less affected. I tried all manner of jets and pilots.

I have now drilled out the pilot jets on a pair of Amals and they work much better (up from 0.016" to 0.019"). The new Premiers can be fitted with 0.019" pilot jets.The secret to setting up, for me, was to use a pair of vacuum gauges - I wish I had discovered them years ago. With the gauges you can get the twin cable pulls to lift the slides EXACTLY together and this transforms smooth, low speed running.

Twin Amals let the Commando roar up to 6,000 rpm (on occasion). The single Mikuni dampens it down at 5,000 rpm.

Now for all the contradictory opinions....

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I am not an expert but bought a pair of Amal premiers which seem to work very well just be 100% sure you get the 19 size pilot jet.

Mine came with 17 pilot jet and caused all sorts of problems, until Amal and others informed me to use 19s.

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I,ve done over 400 miles since fitting new Premiers on my Mk3 and they,re excellent both in quality and function. Started and ran well from first fireup on choke (no messy tickling needed) and with minor tweaking of idle speed and mixtures, are perfect in throttle response, performance and economy

My old ones had #17 pilot jets while the new are #19 as Norman and John mention but my bike runs well on either. I also specified 3.0 slides from Burlens where I bought them which suits my bike (velocity stacks and K&N filter) but otherwise jetting is standard early 850 with 106 needle jets, 4 ring 850 needles with matching cutaway spray tubes, and 260 mains. I set them statically initially and then use vac gauges like Norman once they,re up and running but you can still do a good tuneup by ear as I did, just takes longer. Float heights were good as supplied

The only problem I had was the old 20,000+ mile choke cables so I made up a new set and I would recommend renewal if yours are long in the tooth. Make sure they,re adjusted to reach full throttle at the same time and choke slides are fully retracted when not in use

Checked everything over, no problems but initial 200 miles indicated it needed the needle dropping to the top or leanest position from the middle. Came back from a run today and averaged 60.13mpg for the last tankful including motorway mileage

It,ll pull way past the 6000rpm in top gear that Norman describes and I don,t believe for a second he didn,t go further....

thanks Norman for the reply and it was very informative

only 3 replies we were both wrong there maybe its been kicked around to often

still not sure but it does look favourable for the premiers

keep safe 

Pete

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Yes, I was surprised - not a single Mikuni enthusiast reporting perfect running :-(

There has been another Mikuni thread with discussion regarding jetting and I note one comment there about the poor choke transition.

When you see the severe bends in the manifold it is no surprise that airflow is impeded at high revs. Air leaving the carburettor meets a tee piece before it turns left or right to the cylinders. 

The cross sectional area of a single 36mm carburettor is 1017 mm2, while two 32mm chokes give a combined 1607 mm2. Another reason why a single carburettor inevitably limits top end revs and power.

Having said all that, the Mikuni was lovely when fully warmed up and pottering at low speed, and would be fine up to 60 mph, and gave me the same 54-59 mpg as I get with the Amals (typically two-up). 

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A great, inexpensive tool for setting twin (or multiple carbs) is the little plastic stirrers you get for your tea in motorway cafes. Place the end of each one under the carb slide and when you operate the throttle you get an exaggerated movement at the end of the stirrer that is sticking out of the carb mouth. I have used these for years and compared the results with my vacuum gauges and they are very accurate.

Personally I have always had twin carbs on my Nortons since the 70s and don't find they are a fiddle at all. However along with a friend some years ago we did have a problem with a single carb commando that vibrated badly and that was cured by fitting twin carbs.

Couple of PS's

Grab some of these plasitc stirrers quickly - with all the environmental plastic horror stories cafes seem to be swapping over to wooden stirrers (probably ok but do seem a bit thicker to me, however are good as mixing sticks for two part adhesives)

Having used plastic stirrers if you should pop off to have a coffee after setting the carbs and then return to start the bike an aging brain may well have forgotten that the stirrers are still in the carbs. It appears there is no detrimental effect to starting the bike with these still in, the only downside is that you've lost your carb setting tools - well my 650 ss didn't suffer adversely anyhow.

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Does that mean your 650SS was both shaken and stirred? 

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I have 2 x Mk. 3 Commandos. 1 with Mikuni 34, and the other with original Amals. I agree that the Mikuni runs out of steam but at around 80mph on mine. I do like the convenience of the Mikuni and don't miss the tickling of the Amals as on the other bike. I really only keep the Amals on that bike as it is fairly original. The Mikuni equipped bike is my tourer, the other comes out for a good blast.

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As to what  carburettor is best it depends on what you want from the bike?

If you want top end speed you need twin carburetors, original looks stick with he Amals.

Disadvantage of Amals- poor quality material, primary circuit prone to crud build up, need to be kept balanced and a pain to get on and off for maintenance.

Single carburator- less top end speed, need a decent manifold ported to suit, needs to be jetted correctly. As for size most people seem to go too big and a 34mm single is often better that a bigger carb. As stated above carburettors with an enrichment device in this country can cause problems in the transition from cold start to hot running.

Mikuni offer a better range of jets but you have to get it tuned which can take a lot of effort and be expensive. I am running a VM34 on an 850 with a PW cam and for normal road use it is fine but I am having to run a slightly rich slide to cope with cold weather running from cold after starting.

Mikuni and Amal MK 2 caburetors are made of Aluminium and Mikuni carbs come with brass chrome plated slides. Although Surrey Cycles do I believe do brass chrome plated slides for Amals.

I do run twin Amal Premiers on my Triumph Bonniville. You pay your money and take your choice. But you have to know how you want to use the bike.

 

Christopher

"Disadvantage of Amals- poor quality material, primary circuit prone to crud build up, need to be kept balanced and a pain to get on and off for maintenance"

 A few redundant facts here, the latest Premiers are very well made and compare in quality to a new set of Mikuni RS38 Flatsides about to go on my Yamaha. Completely different type of carbs but both function very well. I still needed to reset two float heights on the Mikunis but the Amals were fine and they look great

Concentrics with heavy chrome plated slides are old hat and no longer needed with the excellent anodized aluminium slides that are standard on Premiers. Alot of pilot jet blockages were a quality problem with swarf but checking mine over showed all was good and they run fine as received

My last set did over 12000 miles and the bodies and slides were in great condition except where a cable broke and pieces of wire scored the slide and body but still ran perfectly once I,d fitted a new cable. A good air filter and clean fuel are a must. I use the standard nylon mesh filter in the banjos, no additional filters. These Premiers were only replaced because I "modified" them in various ways and decided on a fresh start, to good effect.

Out of interest, the carbs, ports, guides and valves were still clean and shiny after this mileage despite my oil tank breather going into the K&N, possibly due to the cleaning effect of modern fuels and a good air filter.

Maintenance is easy if you have the twin outlet K&N R-0990 filter and the carbs on my bike don,t need to be removed. All jets except the inboard pilots are easily accessed and a slide or needle position change can be done in 15-20 minutes, made easier with 2BA allen screws replacing the Phillips type. Float level changes are similar. I don,t need to synch them that often either, and a typical euro trip of 2000+ miles would not see any need for adjustment or drop in performance and economy. I tend to synch them with gauges more as a check rather than a necessity

The Amals run perfectly from idle to redline regardless of what you use the bike for, this will depend more on how much the throttle is twisted.

 

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A lot of original Amals were shot before they even saw a drip of fuel, they were made on knackered machinery. The Premiers are like comparing apples and oranges. 

With over 16K miles on mine, they are not even showing signs of wear, stay in tune for 4 - 5K miles. Final set when the engine is hot then use a little silicone to ensure the screws don't move.

If the carbs are off the bike you will not need vacumn gauges or stirrers. With the throttle stop screws in place and flush with its hole, drop the slides in and measure the height from the top of the body to the top of the slide. 

Then if the tank is off the bike, build the carbs up, and place side by side aft of the frame spine tube so you can see the slides, connect the cables, check the free play and slides lift together and clear full bore. If the tank is on the bike you may need stirrers if you can't feel the slides with fingers.

The final set up when at full working temperature is really worth doing, do not set the tickover too low.

 

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Brilliant, thanks to you all I have made my decision and to be honest I would have needed a lot of persuasion to come away from my original thoughts

its always good to hear from owners what their experiences have been you cant beat experience,and as always the choice is then u to yourself

many thanks- ride and keep safe

Pete

In reply to by jan_nelder

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Shouldn't that be a Commandocino?

 

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