Amal Carb Types: Concentric and ?

Now, that's how you answer a question. Short, simple, and to the point. And, hardly a number in sight.

Wait a minute L.A.B. I've been thinking about this (dangerous). You are still pushing the same volume of air through the engine. Let's say the engine is doing 5k, and pulling say 20 cubic feet of air a minute through the engine. You can change the amount of gas, but at any R.P.M. the engine will push basically the same amount of air. One carb with half the intake area should see twice the air velocity, which, I would think, would add quite a bit of resistance to the airflow.

O.K., what am I missing here?

Diablouph said:

O.K., what am I missing here?

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If the carb is only feeding one cylinder at a time and not both at once, then there isn't twice the velocity, only double the airflow in two engine revolutions, but remember with twin carbs on a 4 stroke 360 vertical twin, each twin carb is only working for approximately half of one engine revolution (the induction stroke), and then does practically nothing for the other 1.5 engine revolutions! So there's no lack of capacity in a single carb.

O.K. That makes sense. The carb does not have a constant flow, so my numbers would not add up. Makes sense now why Commando Owners don't all go out and buy 50mm carbs.

Real simple, each draw of a piston is always being fed by a single carb, no matter if carb hooked directly to a single jug or by a split manifold so each piston can share in its turn. So only thing that matters is size, either for bottom-mid range optimal size or WOT top end sizing or compromised size.
A 34 mm single can feed more flow than a pair of dual 32's, because the 32's are not connected up so both can flow at once on each piston draw.
Best deal would be like the old chevy Quadrajet, tiny primary throats for low and mid range response but big old secondaries to kick in with a lead foot.
A small and a big carb on a single manifold would be a treat if tunable.
Hang the small carb off to the side, no matter just more air tube inertia for the slower rpm needs, put the big boy direct a shot and close as possible-practical.
A carb must be small enough to restrict-narrow flow enough for venturi effect to lower pressure on fuel level to be drawn up.
A 32 single works very well for many BI riders.

Hobot, if I am reading your post right.

You believe that a single 34 Mikuni will give a 135mph top end, and two Amals will lessen top end to 125mph?

And also you believe that although the two amals give a crisper throttle response, the single mikuni is better for wide open throttle high rpm runs?

Did I read you right?

If so, this is the exact opposite of what everyone else believes.

Hobot, if I am reading your post right.
You believe that a single 34 Mikuni will give a 135mph top end, and two Amals will lessen top end to 125mph?
And also you believe that although the two amals give a crisper throttle response, the single mikuni is better for wide open throttle high rpm runs?
Did I read you right?

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Yes, that is the way it worked on my initial learning curves on Ms Peel-Combat.
Yes a bigger carb within reason favors the WOT hi rpm flow better than smaller.
For a couple of seasons after run it allowed WOT hi rpm use, I explored WOT HI rpm use. Got to meet fun folks asking for hot shots willing to compare speedo readings as I just couldn't believe what Smith clock read, in darn short order too.
After about 5000 rpm the 34 begins win the pull power equation.

On the other hand or side of the carb, the 28 mm standard head ports beat the snot out of Woverhaven 32 mm hi Compression Combat head. Expert who molded ports showed hogged big ports either by factory or afterwards.
I had been waiting eagerly for the Combat head performance but it just dogged Peel out from creeping and mid range pull. Big Combat head didn't do well on Peel until 6800, then felt like another piston kicked in. But Peel was not powerful enough to pull her 22T tall gearing but to 6000 in 4th, so Combat head was top end let down too. On Peel's 920 engine CH 0 head ports should be small enough to give good response with emphasis on torque rather than wait to near red line to kick in.




So in my older age I want it the way the factory had em set up, points fast rise timing and dual 32's or a single 32.

I haven't heard of any Commandos winning races with a single carb set up! :shock:
Please enlighten us Hobot???
Kenny, Doug or Jim could attest or input on to that.
CNN

I’m going to buy a Holly 4 barrel pumper. Where do you suggest I look for a manifold? We’ll talk jetting later.

Didn't Jim report 34HP for a single carb and 38HP for a dual on an 850 :?: I can understand having better throttle response at low speed with a single carb, but at the top end, the dual carbs would do better I would think.

Jean

I haven't heard of any Commandos winning races with a single carb set up! :shock:
Please enlighten us Hobot???
Kenny, Doug or Jim could attest or input on to that.

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Don't know anything of their carb set ups, just what I've experienced and in general the rest of the world in range of carb sizes that still start and run an engine.
I expected more over all power by dual 32's with straight short manifolds so was surprised on the 34 single working better to run with modern squids gangs in the opens. Peel had one other feature that may affect results, she had mismatch of 32 mm manifold meeting the lip rim of 28 mm head port. But likely not much as I got similar results in pure factory Combat going from improper Munki to Peels dual Amals that were new out of box 7000 miles she gave em up. On the factory Combat I liked the response of the duals to dice Gravel and zing back up to speed slowing for crests and blinds. On Peel who was built to race like level and liked to run in 5000 above I liked the single 34 better. I took more wrist turns
and slight delay from near idle but once engine spun up it out raced the duals.

The worse combo on Peel was 2s cam, Woverhaven head and dual carbs out open 2-1 header 1.5" pipe. Acted like a darn Turnip, made plenty of power sounds but just didn't deliver. Then over rev event so decided to start anew
when the 1098s started showing up in our area. But best set up was so good that I may will put her back that way and use the blown engine in an animal armored commuter. Tame for trail tricks yet few moderns would stay with her in the opens because of what was at end of opens. Wild as I seem I know better than fly too fast into or over blinds so slowed to 90's, that when they'd catch up.
If I was on a track who knows if they would regain anything before too late again.

I"m doing the experiment for ya, building a 920 with 750 size head parts and only 38 mm carb. Let them racers spin their guts out, I'm after the torque for turns.

CanukNortonNut said:

I haven't heard of any Commandos winning races with a single carb set up!

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Maybe not Commandos, but many "single carb" twin cylinder engines did win races.

If you take a look at the majority of "single carb" cylinder heads (BSA A7/A10 for instance) they are not always "twin carb" heads with a single carb manifold bolted on*-but have an inlet tract specifically designed for a single carb.
The Commando cylinder head was never designed for a single carb, so it's parallel inlet port design requires a rather squat and widely splayed inlet manifold necessary in order to fit a single carb-which is a rather poor compromise (in my opinion) and I suppose we should be thankful it works as well as it does?

If the Commando inlet ports were to be welded up and re-cut to the optimum shape for a single carb (with a single inlet tract entering the head and splitting into two much closer to the inlet valves?) then I'm sure it could be made to work better than the present setup at higher RPM.


*(if they are, then the manifolds are usually a better shape)

Doe any one know did the earlier commandos have a different manifold size 28/30mm compared with the 30/32mm on the later models, is this correct?

Does this part number sound right 06-0509?

yeh oldbritts has listed the 06-0509 for the 1970 but then they show an alternative 062819, does anyone know what the differences are between these if any2?

single carb 34 hp

dual carb 38 hp

This has to be at the rear wheel, and on a completely stock 850, as reported previously


Peak horsepower, from what I have read, is developed in the last 10% of usable rpm

so on a stock Commando that all agree runs out of poof around 5700rpm, peak horsepower is being developed from 5000-5700 rpm?

Dual carb setup flows more air at higher rpm, so makes the extra four horsepower above 5000rpm

I spend very little time at wide open throttle above 5000rpm, will stick with a single 34 Mikuni