Single carb on 750 Commando 34 or 36?? (2015)

comnoz said:

I ran a 44mm Keihin carb from a Harley with a custom manifold on my Norton for several years. It was probably the best carb setup I ever ran. Jim

Click to expand...

Now that’s very interesting Jim. Was it a pumper or the earlier one? I happen to have one of each on my bench. I’ve recently fitted an S.U. To my bike and I like it a lot, but the Keihin is a much smaller unit so would love to try one as a comparison. I don’t suppose you still have that manifold do you?


Cheers,

cliffa.

I'll bet that accelerator pump was a nice addition!

As we all know, as long as sufficient air/fuel is available by whatever means - fuel injection, single or multi carbs - the engine can produce its maximum power.

A pair of stock twin 32mm 850 Amals can, in theory, anyway, flow a lot more air/fuel than any single carb that is likely to fit in the space available. But since the engine can only use what it needs, having more air/fuel capability available beyond that won't produce any more power. We tend to bad-mouth the OEM Amals - and they deserve it from a build-quality point of view - but from a power-producing standpoint, they were (are) excellent. Admittedly, carbs with accelerator pumps or CV carbs are friendlier in some aspects of the operational envelope, especially in a quick roll-on from low or mid-throttle to WOT!

cliffa said:

Now that’s very interesting Jim. Was it a pumper or the earlier one? I happen to have one of each on my bench. I’ve recently fitted an S.U. To my bike and I like it a lot, but the Keihin is a much smaller unit so would love to try one as a comparison. I don’t suppose you still have that manifold do you?


Cheers,

cliffa.

Click to expand...

I still have the whole setup. It is on a bike in storage and will be back on the road eventually.
Is is like this.

Jim, what’s the advantage to that carb over a similar sized FCR ?

I swapped from twin 36mm Amal Mk2s to a single 34mm DellOrto.

It is noticeably slower on acceleration and I'm thinking of going back to the twin carb set up.

No matter what size single carb you have, the route from carb to inlet valve is very tortuous. My single carb manifold has some serious tight bends in it which really must impede the flow.

The very best my commando has ever run is with a single tm 40 mikuni pumper
I was a bit taken back by the mid range and the top end performance it really wasn't what I was expecting as I have had many different single carburettor configurations on my norton
The only reason I removed it was because it could sometimes be difficult to start
So I went back to twin amals that are good for starting when you are limited to how many times you can kick a bike over this becomes important
But I will soon be fitting an electric start, and as soon as that is fitted the single mikuni tm will be going straight back on

Fast Eddie said:

Jim, what’s the advantage to that carb over a similar sized FCR ?

Click to expand...

Probably no advantage -other than the Harley carb is small and fit under the backbone with a manifold that was long enough to flow well.
The FCR is way too tall to fit a Commando with a long 2 into 1 manifold.

comnoz said:

Probably no advantage -other than the Harley carb is small and fit under the backbone with a manifold that was long enough to flow well.
The FCR is way too tall to fit a Commando with a long 2 into 1 manifold.

Click to expand...

Gotcha.

And the long manifold is needed to avoid restrictive tight bends I assume?

Fast Eddie said:

Gotcha.

And the long manifold is needed to avoid restrictive tight bends I assume?

Click to expand...

Yes, the short MAP and similar manifolds flow very poorly. That is what causes the loss of power with a single carb -it's not a restriction from having only one carb.

With carburation getting the midrange jetting right is critical. When you have dual carbs, you usually raise and lower the two needles together without considering the wear in the jets and on the needles and any manufacturing errors. When you jet a single carb, you are only adjusting one needle and one needle jet and one float level, so it is much easier to get it spot-on. Where the difference between good performance and best is concerned - when adjusting needle and needle jet, it is bugger-all. Especially with petrol as fuel. Methanol is much more forgiving. With dual carbs, one is always likely to be richer than the other, unless you are a fanatic.

I don't like the thought of pumper carbs. What might be good in motor cars is not necessarily good in bikes. You might be able to whack the throttle open faster, but I don't usually do that when I ride a bike. I usually feed the throttle on in one smooth action. That way the mixture is always as lean as it has to be to get best performance and not too lean because of the loss of vacuum. With a pumper carb, the richer mixture probably extends to mid-throttle, where it needs to be leaner. You might be quicker off the start, but slower once you get mobile ?

I like the discussion, makes me feel fine about my plan for 2 Amal 34 MkII's. My other bike has 32's and works great, I was able to get 4 34's a while back and so may end up with the other 2 for sale.

dguzzi said:

I like the discussion, makes me feel fine about my plan for 2 Amal 34 MkII's. My other bike has 32's and works great, I was able to get 4 34's a while back and so may end up with the other 2 for sale.

Click to expand...

I would definitely feel fine about trying 2 Amal 34s at least you are actually trying it
Plenty of people told me that a 40mm single flatslide carburettor was too big for a 750
I do wonder if people have actually tried one!
I have said many times on here I have had twin 32 MK1 concentrics
A single MK1 32 concentric a mikuni 32 CV a mikuni 34 VM a mikuni 36 VM but the 40mm TM flatslide pumper was by far the best
The difference is night and day compared to the others
This is what I know because this is what I have tried
If I had listened to all the theories I never would have found this out

I use two 34mm Mk2 Amals, with the ports tapered 2mm per side for about the first 25mm - but with methanol fuel. With methanol there is much more leeway in the jetting than with petrol. The difference at mid throttle between slightly rich and correct jetting, is still immense as far as performance is concerned. With your Commandos on petrol, you have a choice between 0.106 and 0.107 inch needle jets, you probably need a size in between. But then the correct jetting for best performance, would become much more temperature dependent.

'but the 40mm TM flatslide pumper was by far the best
The difference is night and day compared to the others'

Any properly tuned carburettor is night and day when compared with one which is over-rich. The reason most bikes are faster when using methanol fuel, is that even if they are a bit over-rich, the methanol hides up the tuning errors. But even with methanol getting it really right is as difficult or as easy as it is with petrol. The most extreme situation is putting two-strokes on methanol. If they are slightly rich, they won't even start. And when they are going, if they are slightly lean they immediately seize. But in the end it is the same deal - when they are right, they are magnificent. When I ride my bike, I treat it like a two-stroke - I feed the throttle on. If I whacked the throttle open it would probably gasp. Some people ride around corners and wait until they are vertical before opening the throttle wide - point and squirt. You cannot usually ride a Commando-based bike like that and be fast enough to get anywhere in a race. With my bike, I brake a third of the way into the corner while cranked over and wind the throttle on from there.

Absolutely correct ! I use a 107 needle jet, 270 main with the 32's. That's probably a good start with 34's unless someone corrects me.
Sorry for intruding on the single carb thread...

I don't believe gas flow is the major issue when comparing single and twin carbs. With a twin carb system the gas in the inlet tract resonates at the same frequency as the exhaust system. In a single carb set-up it resonates at twice the frequency and the branch in the manifold interrupts the column of gas. The difference between the two situations is extremely small but on a race bike, the revs are usually much higher, so there is probably more benefit in a twin carb set-up. The main difference is that a perfectly tuned single carb will give more power than a poorly tuned twin carb set-up. A single carb is easier to get right, because with twin carbs you have to match the needle jets and float levels - so that when you raise or lower the needles, you don't have one side richer than the other. When tuning carbs, the main jets are largely irrelevant as long as they are slightly too rich. The other thing is - of you are doing plug chops to get the mains right - if you fit a bigger main jet and the plugs don't show an increase in richness, you are metering off the needles, not the mains. That is dangerous. If that ever happens, it becomes necessary to recess the needle jets and meter lower down, so that the tip of the needle remains trapped - but the mains do the actual metering at full throttle.
I have never had a four-stroke detonate through have the mid-throttle jetting too lean. When you race, you are usually only on full throttle for a short length of time, but the mixture needs to be a bit richer than what gives best power.
With my bike on methanol, I use 670 main jets. I could probably get away with 620 main jets. But it does not matter - the bike probably has slightly less power for 50 metres of track.