Amals vs Mikuni

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"Sons of Arthritus"
Jan 14, 2004
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If this is not the most it is at least one of the most controversial subjects on Nortons. I have had the good fortune to acquire several Nortons over the last 18 months or so. Every one of them were supposedly 'Good runners'. I guess some people don't know what a 'Good runner' is because they all had issues. Every one had gummed up clutches, some misaligned primary chains, and everyone had carb issues, including the two with
Mikuni's. The other irony is I still haven't seen a set of points. They all have Boyers.
I have heard all the jargon about smoother idle, loose a little off the top, but over all more reliable and better with a single Mikuni.
My first conversion was my Mark III. I had distorted the barrel of one of my bored and stainless slide Amals. So I pulled the 34 Mik from the 71, set it up according to the INOA digest, and it was fine. I further tuned it from there. It was a one kick bike and ran hard through all the gears. But it was always a one kick bike, is it my imagination but it doesn't seem to idle as smoothly and seems a little less peppy. But it ran great, I sold it like that. The new owner even called back later to tell me how great it ran and how much he liked it.
I rebuilt the orginal Amals for the 71. Ran great, till it set for awhile, the left pilot giving issues.

I bought a 73 Interstate from a bike shop in Florida. Sould run fine right?
Besides the gummy clutch and misaligned primary, it doesn't carburate well. Looky here, cutaway spray tubes, 3 1/2 sildes, 2 ring needle. Install 4 ring needle, #3 slides, much better. I also replaced the resistor wires
with sold copper. Better yet. Still fine tuning.

I bought a very nice 74 850 Roadster. This guy spent so much money. Stainless everywhere, everything polished. H/O alternator, new harness, H/O coil with Boyer, chrome ham can with stainless screen. He commented his only regret was he didn't put a Mikuni. It does run poorly.
Let's take a look. Cutaway spray tube, 3 1/2 slide and looky here, a 2 ring needle. Now I need to check the main and the needle jet, I'll deal with this later.

And now for the motivation for this thread. I also acquired a couple of Combats, one running, one kind of. The running one has a Mikuni, the non runner has brand new Amals with cutaway tube, #3 slide, 4 ring needle, 106 needle jet and 220 mains.
The Mikuni runner ran strong on the top but idled heavy and loaded up easy. No wonder it had a QO needle jet with a 250 main. I rejetted it, it runs fine. I wonder what a Combat is supposed to run like with twin carbs. This other Combat has fuel tank and electrical issues, I'll get too it someday. I stripped the carbs and cables off the non runner, replaced the Mikuni with em. For fun stuck on a set of bell mouths, threw on a set of old, blued 850 head pipes with cross over and pea shooters. N7CY plugs with solid core wires, fired right up, settles down to very smooth idle, no Tach.
Lets go for a ride. Feels stronger right off idle. If I open it too fast there is a little bogg, the Tommaselli quick turn throttle may have something to with it, maybe the cutaway tube. Roll it on a little slower, then let it rip. Mercy oh my does this thing fly. Single Mikuni on this bike, never again.
This thing pulls hard everywhere, and what a rush on top. I don't have a tach, so I better be careful.

Obviously the previous owner over carbed the single Mikuni to try to take advantage of the Combats top end, but in so doing, made it load up down low.

My conclusion here is exactly what it says in the tech digest. A properly set up set of Amals will give a good run for there money. I know there are better carbs, Mark 2's, Keihins etc. But this thread is comparing to a single Mikuni and my experience on two bikes is that properly set up twin Amals give a SMOOTHER idle, better response and more top end.
I believe people compare how much better their bike is running with a new Mikuni but forget or don't know how a bike runs with good or correct Amals. If you've got bad Amals, don't give up on em, fix em or replace em.
amals vs mikuni

Mike, I already like you, very percise and to the point. With more time on my hands than is safe one day, I swithched a set of amals off of my no miles, new Mk111 to my '72 209832 s.# Combat with 31/2 slide etc. as it came from the 850, except for the mains, which exchanged for 230's. It idled and accelerated to redline in top with a 21 sprocket, with a power hit between 4k and 7.5k, that had not been around for awhile. I will try a 3 slide and a diff. atomiser, as I felt that it is a bit sluggish until about 3k, once I get this thing rewired and plumbed correctly with my alloy central tank and carrier.
The head on my bike was massaged by a Floridian years ago, and I ran 36 Dell'Orto's on modified Duck manifolds from Syd's in St Petersburg. With that setup I changed to the 21 tooth sprocket for safety's sake! Sadly, at one point I put the 36's on a Ducati that is no longer with me.
The biggest problem with AMALs is premature wear of the throttle slide and carburetor body. About twenty thousand miles is the most you can expect to get out of an AMAL carburetor before it needs to be “sleeved” or repaired.

However, AMAL carbs are predictable, easy to work on and easy to tune. Moreover, I don’t believe there is carburetor available that will outperform twin AMALs that are properly setup.


I guess the next question is how long do the sleeved Amals last. I've got 14,000 miles on my '72 combat 750 and they're doing very well.

I'm interested in the use of 850 Amals with cutaway spray tubes and 4 ring jet needles. What was the reason for switching to these on the combat - any improvement? The original Amals for the '72 combat were R932/19 and L932/20. Main jets - 230, throttle slide cutaway #3 and jet needle position - middle. Mine is setup that way, idles good and accelerates without any hesitation with RITA ignition.

I have been trying to decide for awhile whether to switch my MKIII over to Mikunis or MKII's or leave them stock. After everything I have read on the subject I have come come to the conclusion that nobody really has the anwser. Therefore,I think I will go with what the factory felt adeqate.Hell ,the bike always started fine and ran fine.
Re: carbs

norton75 said:
Therefore,I think I will go with what the factory felt adeqate.Hell ,the bike always started fine and ran fine.

That was one of the reasons for my post. There are many other fine carbs but a well sorted set of Amals are very good. They have their faults true, but all the parts fit and when one starts tuning carbs, you'll find out how much work is involved in getting them right. The factory has already done most of it.

Regarding the cutaway tube on a Combat. It is what came with the bike and I don't have straight tubes to compare with. I was looking for feedback from others if they new what the difference is in performance. What I have learned is just because someone says 'New Carbs' doesn't mean they are correct. Also I have learned you have to match the needle to the spray tube. A 4 ring, longer needle goes to the cutaway tube, the 2 ring needle goes to the straight tube. I don't have info to say one is better.

I was thinking maybe cutaway tube was newer, better design as the Mikunis I have are cutaways. I am not so sure thats the case. What I do have now is more respect for the factory and there original settings. How do you know where you are going if you don't know where you are? Modifications and alterations are fine, but how do know if you gained anything if you don't know how it's supposed to run?

Update 74, 850. I put the 4 ring needle in. Runs perfect. Stock ham can with stock paper filter, 3 1/2 slide, 260 main, pilot screws @ 1 1/2 turns, idles smoothly, pulls hard. The P/O couldn't get it to run right after 'rebuilding' the carbs with kit supplied by some vender. He was sold the wrong needles.
Another AMAL carburetor setup example:

· 3-1/2 slide, chrome-plated brass with no choke provision
· 4-ring needle with clip in lower most groove
· Needle raised another 0.005” with a thin washer
· Stepped spray tube
· 280 main jet
· Air screw backed out 1-1/2 turns
· K & N air filter with free-flowing peashooter silencers
· Note: More restrictive “bean can” silencers were stock on my '75
· No crossover pipe

This setup works very well on my bike.

Interesting, sounds a little rich. You have obviously worked with this. You must be at sea level, maybe by the ocean with dense air?
Is your K & N the small unit one or the insert for the Ham can?

Anyone else? I'd love to hear from others regarding their set ups and combos.

The K & N filter mounts directly to the carburetors. And yes, I'm at sea level, some 70 miles from the Guf of Mexico. The main jet could be a bit rich, but not by much. If you suddenly wack the slides wide open, the bike accelerates smoothly and strongly.

Got just a quick question.
Situation: new Amals 932s, 300 and 301, right and left.
Having a heck of a lot of problems getting them tuned adjustment screws up all others. Just not cooperating. Find that the carbs don't quite fit on the bike. They are too close, which makes them kind of overlap at the middle of the carbs, where the carb body has a place for a slide adjust screw, if there were a hole drilled there, there is no hole because this is the inside of the carb. The old carbs, 932s too, have this spot on the carb body flattened/filed off so that they don't overlap/touch each other. Without this flat space on the new carbs, one is actually sitting 1/8 inch higher than the other, when viewed from the back(air filter position). Easy to see ,as the float bowls are off at the moment. Inside the bore you can see the carb doesn't line up with the intake manifold correctly, and the heat insulator is off center too, causing a restriction in the intake path. Maybe all this misalighnment is causing an air leak too. Is a Commando supposed to have these flat places on the carb body to avoid all this out of alighment thing, or have I got the wrong carbs on there...anyone else notice this problem with the new Amals? Wonderful quality control apparently....thanks for any help, I will be fileing the flats on tommorrow unless I hear otherwise...
Great question. I forgot to point out on this Combat that I got that had new carbs set by a 'Brit Bike shop', I had the same problem.
Original Commando carbs are indeed machined flat on those inner bosses.
File em flat, line em up and you'll be fine. This is also a great way to check if they are orignal carbs. I don't know if Amal makes new carbs with the bosses machined flat. Does anyone else know?
Thanks for the info...will get the file out today.
These new carbs have been a pain in the backend since I got them and really rather poor quality compared to the old ones, which by the way were not 300s but rather a 28 and 27 or 29. I have those put aside for a resleeve in the future, but does anyone know what these second numbers mean? I sent in a good reference to Jerry whichhe kindly put up for others to look at, but it doesn't for me, download...don't know if it works for you. But anyway...the 28/29 carbs are listed as being from 1973...but being still 932 size...what does the other set of numbers mean. My old ones ran fine, except for the idle obviously, but this new set is not so nice. Even the chrome slides I got, from which I had to return already one, are going to pot. One of the "OK" ones I had in now for two having the chrome bubble up on it too...great they say. So, which guru knows what these second numbers mean? Thanks!
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