Pros and cons of different carbs? Keihin vs Mikuni vs Amals?

I'm using the latest Amal Mk1 Premier aluminum body carbs. You save 3/4 lbs per unit, compared to the zinc body originals. They are more difficult to get the throttle cables to fit the slides which were easy for the originals. (I had to disconnect the throttle lines from the two into one junction box), Other than that it went together well. I put 53 mile on the 850 today with them for starters. Still dialing them in. I will report back on how the mileage stacks up to the original sleeved Amal's that I was using.
Cheers,
 
CanukNortonNut said:
I'm using the latest Amal Mk1 Premier aluminum body carbs. You save 3/4 lbs per unit, compared to the zinc body originals. They are more difficult to get the throttle cables to fit the slides which were easy for the originals. (I had to disconnect the throttle lines from the two into one junction box), Other than that it went together well. I put 53 mile on the 850 today with them for starters. Still dialing them in. I will report back on how the mileage stacks up to the original sleeved Amal's that I was using.
Cheers,
Click to expand...
Did you wind the spring onto the cable after you hooked on the slide?
 
gortnipper said:
All this talk about FCRs needs to stop.

Please stop.

My wife's wallet is begging you!

No, carry on...
Click to expand...

suit you sir…

Pros and cons of different carbs? Keihin vs Mikuni vs Amals?
 
gortnipper said:
Did you ever do a dyno before after with the carbs?
Click to expand...
Yes and no…

Yes I‘ve had these carbs on the dyno a few times (not on their own… connected to a motorcycle)… but never in a back to back comparison with other carbs.

Some interesting learning though.

Like the fact that different length velocity stacks can vary the output by 2 bhp.

And the difference between the best velocity stacks and the stock stubs designed for filters (even without the filters) is over 4 bhp !

And that they need the next leaner needles when running the Maney replica pipe vs unbalanced peashooters.

And the fact that on my mk1 engine build, the Maney pipe added 5 BHP from the mid range upwards. And then on my mk2 build, it added nowt !

And the fact that the dyno man said it was the best fuelled carb bike he’s ever had on the dyno… and they normally see modern sports and race bikes !

I haven’t had it on the dyno yet in its latest 920 guise as covid stopped play. And now my old dyno has gone, so even when I do dyno it, comparisons between it and the previous runs and build specs are sadly irrelevant.

The trouble with playing around on a Dyno (as with any experimental endeavour) is that for every question answered, 3 or 4 are generated !!
 
gortnipper said:
All this talk about FCRs needs to stop.

Please stop.

My wife's wallet is begging you!

No, carry on...
Click to expand...
I’m sorry to say this… but…

IMHO, given the spec of your bike, you would see a nice improvement from fitting FCRs…
 
jseng1 said:
Mikuni RS34…flatsliders look good but the throttle shaft is toward the rear and because of that I don't think they will fit on a Commando (frame interference).
Click to expand...

jseng1 said:
Mikuni RS34…flatsliders look good but the throttle shaft is toward the rear and because of that I don't think they will fit on a Commando (frame interference).
Click to expand...
Jim, you are not quite right.

You are right that they do not fit when you use the original curved manifolds and rubbers without any modification.
In that case the lever assy/crank bell will interfere with the frame.
As I said before they do fit with some handicraft work….

I found 2 ways to fit them on my Commando:

Easy way:
Get some nitrile plate manifolds (got mine from Allen`s performance in the UK: nitrile plate manifold „PM03“) and stick the carburetors in.
So you can use the carburetors as they are with push-pull-setup without any modification. The venturi axes is perpendicular to the engine axes….so they got quite an angle, but they work without any troubles. This was my first way to put them on. But didn`t like the look and didn`t like the short way from carburetor to valve. And didn`t like the short air intake.

The not so easy way: this is how I got my carburetors now:
Cut down the Lever assy/bell crank on the rear and upper side…..so you got only a pull cable
Get rid of the exterior throttle spring
Make a new top bracket with a holder for the throttle cable
Make a new bracket for a exterior idle screw.
Make a idle screw and a stop on the crank bell
Fit some new throttle springs INSIDE!!! the carburetors.
I use Concentric MK1 springs cut to fit…works ! throttle closes and WOT works
Shorten the curved manifolds appr. 1cm

I did the modifications with some layman tools - handsaw, file, dremel-tools and drill.
Yes, there is some work to do, but it pays….
When you got a good machine shop or lasercut tools for sure the result will be more beautiful than mine.

My result is funtional, works great and most important: DIY! :)

chris
Pros and cons of different carbs? Keihin vs Mikuni vs Amals?

Pros and cons of different carbs? Keihin vs Mikuni vs Amals?
 
ludwig said:
I still stick with Amal carbs ( with a few mods) but rubber mount them.
They last longer and perform better that way.
I feel it is wrong to bolt an Amal, or any other carburettor directly to a vibrating Norton engine.
Click to expand...
I agree Ludwig.

I thought similar when Glen posted his long life results from his Vincent.

Two things came to my mind:

1) As the Commando engine is rubber mounted, it obviously shakes about a lot, therefore carbs mounted solidly to it will shake around too. That will casue the slides to rattle around, causing wear.

2) Glen has chokes fitted, someone pointed out before that the choke slides, under tension of that long spring, will press against the slide and prevent, or reduce, rattle, and thus wear.

I don’t know if either of these ideas are real contributors to wear, but it seems logical.
 
I have often wondered about rubber mounted carbs on a vibrating engine
Unless the carbs are rigidly fixed to the frame surely they shake about even more on the end of a rubber tube ?
I'm not saying it doesn't work I'm just curious
 
baz said:
I have often wondered about rubber mounted carbs on a vibrating engine
Unless the carbs are rigidly fixed to the frame surely they shake about even more on the end of a rubber tube ?
I'm not saying it doesn't work I'm just curious
Click to expand...
baz :
I think it is not the shake, but the high-frequency vibration that causes problems.
Here is an interesting video, all be it about cars.
Rubber mouted Amals may also need an extra support ( mine have..) :

 
Chris Zet said:
Jim, you are not quite right.

You are right that they do not fit when you use the original curved manifolds and rubbers without any modification.
In that case the lever assy/crank bell will interfere with the frame.
As I said before they do fit with some handicraft work….

I found 2 ways to fit them on my Commando:

Easy way:
Get some nitrile plate manifolds (got mine from Allen`s performance in the UK: nitrile plate manifold „PM03“) and stick the carburetors in.
So you can use the carburetors as they are with push-pull-setup without any modification. The venturi axes is perpendicular to the engine axes….so they got quite an angle, but they work without any troubles. This was my first way to put them on. But didn`t like the look and didn`t like the short way from carburetor to valve. And didn`t like the short air intake.

The not so easy way: this is how I got my carburetors now:
Cut down the Lever assy/bell crank on the rear and upper side…..so you got only a pull cable
Get rid of the exterior throttle spring
Make a new top bracket with a holder for the throttle cable
Make a new bracket for a exterior idle screw.
Make a idle screw and a stop on the crank bell
Fit some new throttle springs INSIDE!!! the carburetors.
I use Concentric MK1 springs cut to fit…works ! throttle closes and WOT works
Shorten the curved manifolds appr. 1cm

I did the modifications with some layman tools - handsaw, file, dremel-tools and drill.
Yes, there is some work to do, but it pays….
When you got a good machine shop or lasercut tools for sure the result will be more beautiful than mine.

My result is funtional, works great and most important: DIY! :)

chris
Pros and cons of different carbs? Keihin vs Mikuni vs Amals?

Pros and cons of different carbs? Keihin vs Mikuni vs Amals?
Click to expand...
Thats some excellent R&D with good results. I don't have to go through that much redesign for the Keihin CRs so I'm not sure its worth it. But I'm interested.
 
Derek Wilson said:
I'm ready to change my carb setup and I'm looking to get the current wisdom on all the best carb setups out there along with the the good, bad, and ugly of each. I read the old threads and I didn't see any recent updates on this that compared the merits of each setup.
Click to expand...
Nortorious said:
I'm getting my engine rebuilt and with that I want to mate it with the very best nee carb setup out there. It seems the common options are dual Amals (current setup that is failing me), single Mikuni, dual Mikunis, or the Dual Keihin Flat Slide FCR 35.

Right now I'm leaning towards the Keihins but am really not sure yet. Matt at cNw will be installing and tuning whatever I end up getting so I'm sure I'll be getting the best of whatever I choose. I just need some help figuring out the best choice. (@BrianK I wonder if you are still happy with yours)

On this particular issue I'm willing to spend whatever it takes to get the best setup with regards to great performance,.usability, and easy maintenance. I would love a "set it and forget it" carb but I know that doesn't exist. I'd like something that is as close to that as I can get though so I'm not having to clean and fiddle with my carbs a lot.

I understand the Keihins are more expensive, but cost aside do they beat out the Mikunis and Amals on all categories? Are they harder to start up and get the bike warm?

Does one of the carb setups start easier over others? And does one of the carb setups shine for delivering even power from 1000 to 7000 RPMs?

Matt is recommending the single Mikuni or dual Keihin but I see some people go with the dual Mikuni instead of single..

Am I asking the right questions?

In other words, if money is no object what's the best carb setup for a 73 Commando 850 with an owner that tries his best but isn't a carb expert?

Thanks for the advice!
Click to expand...


Still very much loving and recommeding the FCRs. They did take a little dialing in - I've posted my final settings somewhere here and can dig them out if helpful.

EDITED to add from a prior post:

The current settings are the result of a lot of experimentation over an extended period of time. Took quite a while, but these work great for me. Power, mileage, and plug appearance are all very, very good.

The FCRs are awesome carbs!

140 main jets (came with 152s)

Needle clip in #2 slot (from top; there are seven slots; the needle clips came in #5 slot I believe)

Stock pilot jets

Slow air screw out two turns (were originally out one turn)

Slow fuel screw out 5/6 turn (Right side was originally about 2/3 turn out. Left was originally 1.5 turns out)
 
Last edited:
BrianK said:
Still very much loving and recommeding the FCRs. They did take a little dialing in - I've posted my final settings somewhere here and can dig them out if helpful.

EDITED to add from a prior post:

The current settings are the result of a lot of experimentation over an extended period of time. Took quite a while, but these work great for me. Power, mileage, and plug appearance are all very, very good.

The FCRs are awesome carbs!

140 main jets (came with 152s)

Needle clip in #2 slot (from top; there are seven slots; the needle clips came in #5 slot I believe)

Stock pilot jets

Slow air screw out two turns (were originally out one turn)

Slow fuel screw out 5/6 turn (Right side was originally about 2/3 turn out. Left was originally 1.5 turns out)
Click to expand...
Thanks for that
Would be even better if you told us about your bike (750/850, anything done to engine (mods)?)
Cheers
 
ludwig said:
baz :
I think it is not the shake, but the high-frequency vibration that causes problems.
Here is an interesting video, all be it about cars.
Rubber mouted Amals may also need an extra support ( mine have..) :

Click to expand...

Reading this post I'm reminded of an article in an early /mid seventies Motorcycle Mechanics magazine on Mick Hemmings tuning advice for Commandos , based on his proddy racer . IIRC there was a commando engine on the cover ? Anyway , he recommended and sold modified inlet manifolds that included a length of car radiator hose . Presumably this benefitted heat and vibration insulation as well as allowing inlet tract length of your choice (within reason )?
Given the number of modified components around these days compared to back then , surprised someone doesn't still make them given relative simplicity ? "Just about every carb that followed the amal Mk1 was rubber mounted .
 
People interested in Keihin 35mm FCRs should be careful running a high mileage street tune on a race spec motor. Just saying
 
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