Replacement carb for Amals on 850 Commando

Hello again. I have a stock 1974 850 Commando with only 3000 original miles on it. I would like to replace the twin stock Amals with a good single carb and manifold. The Mikuni kit is very good but pricey. Is there any good alternative replacement single carb out there. The Mikuni kit is close to $350.00 US. I am a casual rider who wants reliability and not necessarily speed. The problem I am having with the Amals are sticking slides, jets vibrating out, hard to keep adjusted. etc. etc.. I am sick to death of them. Any ideas out there????

IMHO go with the mikuni.. It transformed my ride and made the whole experiance much more enjoyable, especialy riding in traffic. Starts easy, idles good and you dont have to keep tuning it up every couple of weeks. You do loose a bit mid range, but if you're not out there for the out and out proformance it's not realy noticable. It may seem like a lot of $'s but well worth it......

Personally, I think the Amals are about as good as you can get overall. My original Amals on my 850 work just great. I think if you just get them tuned in you might be surprised. With only 3k on the bike they can't be worn yet.

I had a 750 with the Mik on it before because the Amals were worn out. Given how my current 850 runs with them I'll always stick with stock. They just work so well.

I agree re keeping the Amals - they work perfectly when sorted. Most problems are from POs that overtightened everything, warping all the surfaces and causing the sticking slides as well. I believe that if you go through them thoroughly, as is documented in the site in several places, you will be very happy with them.

However, though I've never used one, the single Mikuni seems to be the ticket for a single carb conversion. CNW uses that on some, if not all, of their bikes.

I have fought that battle for the past 25 years, one carb or two.

Even when my Commandos were new, I could never seem to get the Amals to behave.
Oh, with the right tickling, the bikes would generally start on the first kick, but it was always the erratic, hunting idle that would constantly irritate and frustrate me.
Back then, It seemed I would make bi monthly trips back to the Norton dealerships to let the shop experts re set them, again and again.
Dissimiliar mix of pot metals in the slide and bore, I was told, caused different rates of heat expansion. This supposedly let in differing amounts of air depending on the temperature, making the idle erratic, and maddening.
Unlike others here, I never seemed to have the gift to keep them balanced and performing properly.

My single Mikuni 34 starts every time on the first kick, idles like a rock, and runs the bike perfectly.
I may be wrong, but I believe the single carb delivers as good as, and maybe even stronger midrange performance.
It is only the rare occasions when I want to accelerate with wide open throttle above 5000 rpm that a twin carb set up will flow more gas.

I have just completed a major motor rebuild. New oversize valves, guides, pistons, and Megacycle 5600 cam.
No question that I would benefit from a twin carb over 5000 rpm, just have so many bad memories.
JS Motorsports has a great price on a pair of flat slides, I am very tempted to buy them and sell the Mikuni.
This is a new product he is offering. Has anyone bought them? How do they idle and how do you like the performance?
Here I go again.
Your question? I believe you would be happier with the instant bolt on no hassle of a Mikini. Want to buy mine?

Want to get rid of the stock carbs? I might be interested in buying them at a fair price. For some stupid reason, I like the twin Amals!

I got a new set of Amals for less than $300.I bought a set from www.britishbikes.us and my bike never ran better.The guy in charge Dave included everything necessary for my particular model.He offered to buy my old carbs but also added they may be worth more to sell elsewhere.

I would go with the Jim Schmidt dual flat slides, a few dollars more, but miles ahead in technology compared to a lowly Mikuni not to mention the "stone age" or is that "zinc age" Amal. I have a set of FCR 35s, but I think they are too much for day to day use, and not only in price.

Jean

Just a FWIW-sticking slides could be a throttle cable issue. I recently pulled mine, and lubed it by hanging it from the throttle end, poking a hole in a baggy, I then slipped the baggy over the cable and used electricians tape to form a crude funnel. Then sprayed some Corrosion Block in the funnel/baggy contraption and repeated until it was dripping out the other end. It made a huge difference in the return springs being able to do their job efficiently.

Russ

One of these will do it faster without having to take the cables off.

Jean

highdesert said:

My single Mikuni 34 starts every time on the first kick, idles like a rock, and runs the bike perfectly.
I may be wrong, but I believe the single carb delivers as good as, and maybe even stronger midrange performance.
It is only the rare occasions when I want to accelerate with wide open throttle above 5000 rpm that a twin carb set up will flow more gas.

Click to expand...

+1 on the single Mikuni 34. I installed the new TM34 Mikuni flat slide and the improvement was immediate. As Highdesert said, starts 1st kick...every time, smooth idle easily adjustable with thumb-knob, great acceleration. Never been happier with the bike in the 32 years I've had her.

My Amals now do exactly the same - start on first kick every time - even after two weeks without running, idle smoothly at 900RPM - they will idle smoothly at 600 RPM but I prefer a bit more to reduce the chance of stalling at a red light - and, as has often been said before, I don't think anyone will argue against the fact that they produce more power.

Again, it's JUST about ensuring they are set up properly. Back in the day, part of the problem with erratic idle had nothing to do with the carbs - it was the AAU centrifugal advance springs being too weak which causing the ignition advance to "flutter." This would make the idle jump around, often causing us to wonder why the bike would "without reason," suddenly go from idling at 1000 RPM to 2000 RPM. I spent a lot of time trying to "fix" the Amals on my '71 before an "old guy" (probably was not much more than 1/2 my CURRENT age) told me, "it aint the carbs, it's the automatic advance." And that was followed by my first hearing the expression, "95% of carburetor problems are electrical."

Do yourself a favour and go with twin 32mm PWK (Keihin) flatslides, I've got them set and dialed in and they simply cannot be beat for the cost/performance. When I initially bought them they were not pre-set for Commando's but with some help from knowledgeable folks around here, I was able to zero them in. I just wish I had known that were people pre setting these carbs for Commandos before I bought them locally, I could have just ordered them with the Norton needles and proper jet sizes. Anyway, lesson learned. The folks at JRC are SUPER helpful and sent me the jets I needed as well as the needles within a week... superb service that contributes to the overall value. There has been a lot of testing done on these and you can by them set up and ready to go for your 850 (I'm running 122 mains, 30 pilots with Norton needles).



http://users.gotsky.com/jimschmidt/jsmotorsport/

I have a 71 Triumph Bonneville with the original carbs. I have put 57,000 miles on those carbs so far. They weren't new when I got them. I did replace the slides once a long time ago. I have replaced the needle jet once as well. They wear and cause rich running. They are worn out again now. The bike still runs fine. The only problem is that if you open the throttle really slowly the engine slows down and stalls instead of speeding up. You get used to that. The carbs are worn out but it makes little difference to idleing or performance. It has a steady idle at 1000 RPMs and I never adjust it. I have never had to un plug the pilot jet, I think that is because I always drain the tank and carbs when I store the bike. I run a Boyer and that is why the advance mechanism is not a problem. I have experienced the problem described by one of the other posters to due with centrifugal advance mechanisms, not retarding the spark to slow the idle down, on another bike.

My Norton 850 though has an almost new 34mm single Mikuni! It seems like it is going to be great. I have only ridden it around the block once so far. It has always started first kick so far If you want I can sell you a fairly worn 32 mm Mikuni and manifold for $100.00. Or I will trade you for your Amals.

Nigel

Mark -

My 850 has a single 34 Mikuni and as a result I spend as much time thinking about it as I do the carbs/injectors on my Toyota and Honda cars. In other words, I have never touched it or given it another thought since installation 3 years ago.

Translation: install, ride, and forget. Ride more.

At least that's been my experience.

I've owned several Brit bikes over the years, and will never, ever, ever use Amal carbs again.

In my opinion, there are better things to do in life than fiddling with carbs instead of riding.

Good luck.

I'm an Amal guy , but you REALLY consider 350 bucks for a carb/manifold/cable/filter kit pricey :shock:

Am very happy (for touring) with rubber mounted single manifold with 32mm Amal Mk2 with chromed brass slide.
Cheaper alternative still I had before that: direct mounted single manifold (Stewart Engineering) with one of the existing carbs. Renovate this carb by having the slide fitted with a brass bush, and the housing milled to suit. No more synchronising aggro. The SE manifold has only 2 engine flange boltholes, both on the outside. Easy mounting.
It is the material of the original slide and body that causes all the harm (rapid wear).
Negative on the rubber: found a large crack in it the other day. Friend of mine even always carries one in his toolbag!
Aggressive todays petrol?
Weight of the carb (now further away from manifold) hanging onto it and trying to follow the vibes of the engine?
Am going to support it somehow in a flexible way.
Oh, Mk2's have the "choke" (it is not an air restrictor but opens an extra mixture bore) lever on the carb, no cable needed to the handle bars. No tickler either.
From an esthetic point of view, for some it looses points because of the required rubber+hose clips and the squarish shape.

I thought that single 32mm Amals wouldn't flow enough fuel to run a 750, let alone an 850.

Are there folks out there running single Amals on Commandos?


Tim Kraakevik
kraakevik@voyager.net
Three Commandos

Jeandr said:

One of these will do it faster without having to take the cables off.

Jean

Click to expand...

Ok, I'll bite. What is "one of these"?

Russ

kraakevik said:

I thought that single 32mm Amals wouldn't flow enough fuel to run a 750, let alone an 850.

Are there folks out there running single Amals on Commandos?


Tim Kraakevik
kraakevik@voyager.net
Three Commandos

Click to expand...

Yes, for over 10 years I run a single Amal concentric 32mm (250 main jet, 106 needle jet) with chromed brass slide on my 850 Commando. Very easy going on long trips (economical fuel consumption and no synchronising).