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

I tested quite a few exhaust systems on Dyno Hill and I can vouch for the inaccuracy of the butt dyno.
It is a very insensitive instrument, but when it finally does provide data, the data is generally incorrect.

Glen

Whilst a nice big hole has advantages for getting vast quantities of air in, emulsification is the name of the game too, no? Not overlooking the fact it needs to get out t'other side as well.
Horses and courses......

Nortorious said:

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?

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If fitting anything else than twin Amal Concentric carbs, say goodbye to the stock air filter (either Ham can or Bread box). That's the decisive factor to me.

- Knut

A butt dyno is all you get when riding. I happen to trust mine, foolish as that may be. I also use my ears to listen to the exhaust note. Not how loud it is, but the sound. It is a lower tone when too rich, and very nice and bright when optimal. Is that frilly knickers talk? I think a good tuner would understand it. Frilly knickers optional.

What does a dyno tune typically get you on a Norton if you've been tuning 2 and 4 stroke motorcycles for 50 years and think you know what you are doing inside a delusional bubble using your butt dyno and ears?

How about a reasonable example? Not an example of somebody with limited experience bringing a poor running motorcycle to a dyno tuner and getting it in the ballpark. An example from an experienced tuner that brings a motorcycle to a dyno tuner to see what improvement can be made, if any.

Very few people ever use a road bike in a way in which allows them to detect difference in performance. And most road going Commandos probably have rigidly mounted carburetors. Even the slightest bit rich will slow the motor - what happens when fuel froths in the float bowls ? All that needs to happen is for one carb to touch the frame or a bolt, and you can take the edge off the motor. It makes choice of carburetor pretty irrelevant.
When you ride a bike on a race track, you know when you are approaching corners faster. But on public roads, most circumstances do not repeat so often.

Schwany said:

A butt dyno is all you get when riding. I happen to trust mine, foolish as that may be. I also use my ears to listen to the exhaust note. Not how loud it is, but the sound. It is a lower tone when too rich, and very nice and bright when optimal. Is that frilly knickers talk? I think a good tuner would understand it. Frilly knickers optional.

What does a dyno tune typically get you on a Norton if you've been tuning 2 and 4 stroke motorcycles for 50 years and think you know what you are doing inside a delusional bubble using your butt dyno and ears?

How about a reasonable example? Not an example of somebody with limited experience bringing a poor running motorcycle to a dyno tuner and getting it in the ballpark. An example from an experienced tuner that brings a motorcycle to a dyno tuner to see what improvement can be made, if any.

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If exhaust note is an indication of good tuning, my bike must be a world-beater. It is just embarrassing. Some very fast race bikes are quiet and others make a lot of noise. The best measure of performance is how fast the revs rise between corners, when you repeat the exercise. But if you improve your motor's torque output, you usually do not know it until you raise the gearing. The needle jets in your carbs have a critical effect on performance. Most Commandos probably use needles which have a quick taper, so you don't need to control how fast you wind the throttle on. The tuning for best performance is probably what you do not need on a road bike.

I think a road-going Commando would be a very nice bike to own, but I would only try to make it go faster if I wanted to race it. You can buy plenty of modern road bikes which would eat it for breakfast. Tuning it to go faster is a pointless exercise.

To my eye that pic of the wide open Hi-Zoot carb looks about the same as wide open Amal. Again - like to see a dyno test.

Re trusting the butt dyno - I'm in Ronald Reagan's camp,"Trust but Verify!" (on the dyno/whatever).

acotrel said:

If exhaust note is an indication of good tuning, my bike must be a world-beater. It is just embarrassing. Some very fast race bikes are quiet and others make a lot of noise. The best measure of performance is how fast the revs rise between corners, when you repeat the exercise. But if you improve your motor's torque output, you usually do not know it until you raise the gearing. The needle jets in your carbs have a critical effect on performance. Most Commandos probably use needles which have a quick taper, so you don't need to control how fast you wind the throttle on. The tuning for best performance is probably what you do not need on a road bike.

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I thought your bike with you on it was a world beater period. Turn 2 at Winton awaits....

All that happens when I raise the gearing on my licensed Norton is make the bike difficult to ride in traffic. It's still quick with higher gearing, and a little too quick launching away from stop lights behind slow moving cars. Otherwise in the right environment away from traffic it is fun to ride. I can't ride at 100mph all day long, but I can ring the little 750's neck pretty good if I can see what the road conditions are like in the twisties. Really am sleeping dog slow in dappled shaded sunlit turns though. And almost crawling slow with the sun in my eyes dropping into a shaded corner in the early morning. A race track would be less stressful all things considered.

Tuning for performance is a hobby. Been at it for a while. Never sold a motorcycle anyone complained about. I usually get complemented on how well the bikes I've owned modified and sold perform. The old man always said if you are going to do something do it right, or don't do it at all. It is my way.

The butt dyno is one thing. But for actual objective proof it would be easy to compare smooth bore carbs to non smooth bore carbs on an actual dyno and look for HP increase. All it takes is time and money. You can compare Smooth bore carbs such as Keihins, Gardners, Flat slide Mikunis etc to regular Amals, regular Mikunis or any carb that has a non round slide shape at WOT. It would be much easier to simply measure the flow difference between smooth bore carbs and carbs with a non- round slide shape at WOT. Or you could ask the companies that make both types of carbs - the regular econo street carb and the high performance, high flow racing carbs (such as the Mikuni regular non smooth bore round slide compared to their smooth bore flat slides). I think the AHRMA classic racing rule book restricts the size of smooth bore carbs to prevent an unfair advantage.

The bike below used to have regular amals. I was too poor to afford expensive racing carbs, - so I made my own. When I installed the home made smooth bore guillotine slide carbs my lap times dropped. The bike was so fast that other Norton racers thought I was running fuel. Note the small clear plastic water bottle on the top fork yoke that I used for water injection (that's what created all the suspicion). But the water injection didn't seem to make any difference so I removed it (another failed experiment).

"It would be much easier to simply measure the flow difference"

True but a carb cannot flow more than the engine demands. If a specific engine needs X CFM to make maximum power, installing a carb that can flow 1.5X CFM won't increase power. IOW, flow testing can tell you that carb A can flow more than Carb B but it won't tell you if the engine can make any use of carb A's additional flow capability.

jseng1 said:

Look at the smooth bore Keihin in the photo. No obstuction - just a round smooth hole with optimum flow. Its the same with both the CRS and the FCR. The only way to improve flow is to get rid of the needle. Both also come with velocity stacks etc. The CRS is available in 31 33 35 37 39mm etc. The FCR starts at 35mm. The increase in flow of the both the CRS and FCR smooth bores gives an increase in WOT performance over the regular Amals. Its a noticable performance increase. The FCR accelerator pump gives a partial throttle roll on surge advantage but it also increases fuel consumption.

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Thats exactly what an Amal Smoothbore looks like inside.

The supposed benefit from modern carbs is a) better quality design, manufacture and materials and b) better and more tune-able jetting inputs.

At WOT there should be little or no difference between any true version of a smoothbore if all other aspects are equal, cos it’s basically a pipe !

I‘ve run CRs on a big triple and they’re great carbs. Obviously Keihin superseded them with the FCRs, I kinda doubt they did that for fun, so obviously Keihin believe them to be a better instrument.

robs ss said:

It's good for 124mph at 7000rpm and is a kitten around town.

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I confirm the same, it will comfortably pull 7,000 in top, that’s with a 21t sprocket and (very nearly) stock primary ratio (cNw).

At lower rpm though mine could comfortably pull a higher gear. Just gotta decide whether to try a 22 or 23 as I don’t want to have to go in there and do it twice !

Hi all,
it would seem to me that for us mere mortal road riding owners, the arse throttle can be the the ultimate arbiter of performance because irrespective of actual horsepower your bike is developing, if you are getting a ‘feel- good’ through the combination of acceleration, noise and crackling sweetness of your engine, then that’s all that matters.
I think for us it’s all about lower to midrange torque as even when we are ‘having a go’ we rarely approach 7 thousand revs irrespective of what we tell ourselves.
Again I say that correctly tuned Amals in good condition should be able to provide the majority of our riding needs with the possible exception of fuel consumption.
just a thought
alan

Alan L said:

Hi all,
it would seem to me that for us mere mortal road riding owners, the arse throttle can be the the ultimate arbiter of performance because irrespective of actual horsepower your bike is developing, if you are getting a ‘feel- good’ through the combination of acceleration, noise and crackling sweetness of your engine, then that’s all that matters.
I think for us it’s all about lower to midrange torque as even when we are ‘having a go’ we rarely approach 7 thousand revs irrespective of what we tell ourselves.
Again I say that correctly tuned Amals in good condition should be able to provide the majority of our riding needs with the possible exception of fuel consumption.
just a thought
alan

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Totally agree Alan!
The number I was quoting wasn't to say mine is a fire breather - because it isn't.
There's many modern 500s would eat that.
The point I was making is that the FCR 35s are sweet at low revs and will also take a Commando to "its" limit comfortably.

Cheers

I would be interested to know what type of needles come as standard in Amal carbs on a Commando. I use 6D Mikuni needles in my 34mm Mk2 Amals. With Mikuni needles, many more tapers are available than in Amals. If the nornal Amals on a Commando have quick tapers, when you wind the throttle on the mixture richens quicker than with a slow taper. I use methanol fuel which provides twice the leeway than you get with petrol. Even wihen using methanol, half a thou of an inch oversize in the needle jets will make my motor sluggish. Wth quick taper needles in Amal carbs using petrol, the difference would be very noticeable.
I don't know who sells needles and jets for Amal carbs - could someone please post a listing of the available Amal needles ?
The taper on the needles compensate for loss of vacuum, but Commando inlet ports are relatively small - so losing vacuum with slow taper needles should not be a problem, If you lose vacuum, all you might get would be a gasp.
My 850 motor had not had much done to it, but it is still surprisingly quick.
BTW I am not in the habit of bragging, I just say what it does. The bikes I race against are 1100cc CB750 Hondas which are on methanol. My 850 is quicker everywhere except at the very ends of the straights. And I run very high gearing. The close box keeps the revs high, so I don't wind the throttle so far on each gear change. It overcomes the loss of throttle response which is due to the heavy crank. My usable rev range is from 5,500 to 7000 RPM and a bit. The crank is balanced to run smooth at 7000 RPM.

Alan L said:

Hi all,
it would seem to me that for us mere mortal road riding owners, the arse throttle can be the the ultimate arbiter of performance because irrespective of actual horsepower your bike is developing, if you are getting a ‘feel- good’ through the combination of acceleration, noise and crackling sweetness of your engine, then that’s all that matters.
I think for us it’s all about lower to midrange torque as even when we are ‘having a go’ we rarely approach 7 thousand revs irrespective of what we tell ourselves.
Again I say that correctly tuned Amals in good condition should be able to provide the majority of our riding needs with the possible exception of fuel consumption.
just a thought
alan

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It’s not that straightforward guys, unfortunately most of us humans aren’t very precise instruments !

We get fooled by comparative feel.

Tweaking your tune in a way that accidentally knocks off a lot of top end power can make the bike FEEL like it has huge mid range grunt. The way it tapers off at the top end tricks the brain.

Similarly, tweaking things in a way that kills the mid range, can make the bike FEEL like it has a huge top end rush of power.

Another big problem we humans have is placebo. When we tweak our bikes we do it for a reason ‘Mr X tells me these 1.5” headers will give 5 BHP, so I’m gonna try them’ and once we’ve told ourselves that, it’s really, really difficult to remain objective (I did exactly this with 1.5” pipes, and I was CERTAIN they improved power, but the dyno said otherwise)

I‘ve set things up with the arse dyno before many times… and been happy… only to have my bubble burst when taking it to the dyno! The same applies to lap times, this is very objective and can be even better than a dyno (so long as the rider is consistent enough not to contribute to normal variation significantly), or any closed ‘controlled’ ride, like Glen does with ‘dyno hill’.

I guess at the end of the day that so long as YOU’RE happy with YOUR bike then it’s all arguably irrelevant how good it actually is.

But, if you’re looking for actual improvement, there’s no doubt, unless you’re gifted with ‘feel’ like Percy Tait or Peter Williams, the rest of us have arse dynos that are at best, almost useless, and at worst thoroughly deceiving !

acotrel said:

I don't know who sells needles and jets for Amal carbs

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= Amal.

acotrel said:

- could someone please post a listing of the available Amal needles ?

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Two needles were used on the Commando. 622/124 for the 750 and 928/104 (with the 928/107 stepped spray tube) for the 850.

acotrel said:

I use methanol fuel...

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Once again can you please try not to drift off-topic.

TOTALLY concur with FastEd! I did so many dyno/track tests where testing demonstrated that the butt dyno is literally worse than useless for exactly the reasons Ed mentioned. I say worse than useless'' because it will often tell you the opposite of reality. If it was simply "useless" it wouldn't tell you anything at all...which is better!

OTOH, any sort of test that can reliably determine results is useful - the hill dyno is a good example, time in seconds to accelerate from X RPM to Y RPM in (usually) a higher gear, etc, etc.

MexicoMike said:

TOTALLY concur with FastEd! I did so many dyno/track tests where testing demonstrated that the butt dyno is literally worse than useless for exactly the reasons Ed mentioned. I say worse than useless'' because it will often tell you the opposite of reality. If it was simply "useless" it wouldn't tell you anything at all...which is better!

OTOH, any sort of test that can reliably determine results is useful - the hill dyno is a good example, time in seconds to accelerate from X RPM to Y RPM in (usually) a higher gear, etc, etc.

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Yes - but if your butt dyno tells you things are good, then they actually are, despite what other precision instruments may measure.
Most of us aren't in this for absolute horsepower - we're here for the fun!