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

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gillou

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Sep 25, 2014
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Hello,

I fit an 932 Amal single carb on my 750 Commando with standard head, combat camshaft and 1"3/8 exhaust tube and peashooters, with the 1"1/2 i haven't any succes to tune the carb.
For my using it's a good compromise but when the engine is more than 5000 he is short of breath....

Then i think fit and tune an VM 34 or TM 34 mikuni carb, why ?? Because it's cheaper than an MK2 and in France you find easyly VM or TM but no MK2.

First question : on a 750 i have to prefer 34 or 36 ?
Second : is the manifold 1/2 MK2 34 carb is the same for VM carb? If i take a VM/TM 36 where can i buy a manifold ?

I have this configuration for VM34 :

pilot jet 30-35
slide3.0
needle jet type 159
needle jet size p-4,p-6
needle 6dh3
main jet 260
air jet 1.0

But if i choose 36 is there any modifications?

Good day.

Gilles
 
If you look at thread about single carb conversion you will see some of the others single carb tune setups .... for my bike a VM34 works well ....I use a 2.5 slide and 220- 230 main .... I think a lot depends on your individual bike and it's condition ...
Craig
 
A 2 into 1 manifold designed for a 36-38 mm carb will flow better and make more power that the smaller manifold made for the 34mm carb. The size of the carb itself will not make a noticeable difference. Jim
 
comnoz said:
A 2 into 1 manifold designed for a 36-38 mm carb will flow better and make more power that the smaller manifold made for the 34mm carb. The size of the carb itself will not make a noticeable difference. Jim
Click to expand...
This is true but you will have to modify/enlarge the 34mm carb to manifold flange holes for they are narrower mounting hole on center then the larger 36 to 40 mm manifold and the 36 to 40mm flange is too big for the 34mm carb.
 
gillou said:
For my using it's a good compromise but when the engine is more than 5000 he is short of breath....
Click to expand...
Same here. With my stock '74 850 running a 22T countershaft sprocket, she pulls hard through all gears, but runs into a wall at around 5000rpm/95mph. That's plenty fast for my old bones, and I get a very consistent 57-58mpg in return. The smaller 34mm carb really promotes the 850's low end torque. Go by what Jim suggests as far as manifolds are concerned, since your Combat cam can use more air.

Running a quick bit of math to help put carb sizes into perspective:
30mm = 707mm2 throat area
32mm = 804mm2 - a 14% increase over the stock 30mm
34mm = 907mm2 - 28% increase
36mm = 1017mm2 - 44% increase
38mm = 1133mm2 - 60% increase

I grew up in a era where bigger is better, and that definitely rang true with carburetors. Everyone wanted an 850 Holley. If you look at the fact that the latest breed of NASCAR vehicles produce enough horsepower to push a tin box through the air at over 200 mph while having their inlets restricted to ~350cfm, you can appreciate that smaller carbs can be made to work fine as long as all other components are tuned to match.
 
I may get shot down here but some things to think about:

1). Assuming a stock engine there is only so much air that can be pumped through the engine. Carburetor sizing is based on CFM so it's pretty easy to calculate how much air can move through the engine.

2). There are charts to size carburetors based on engine size (CC's) and RPM. A Norton 850 comes up with approximately 35mm for carb throat size. (Numbers can be massaged based on RPM expected so there could be a range in size based on what RPM is used).

3). Any size carburetor within the range based on RPM can be made to work, but generally the smaller the carb the better the response due to higher velocity. This assumes the smallest carb can supply the needed amount of fuel (does not starve).

4). A larger than optimum carb can be more difficult to tune and can result in poor low end response and richer running at idle. In my experience it is much harder to get a larger carb to make the transition off idle smoothly.

More fuel usually means more power, but that assumes there is more air as well. Running a stock engine with a larger carb is not always better due to available air.
 
About the throat area calculations, did them myself a while ago, if you have lets say 32:s for a start, you have 2 carbs, right? Which means 1608mm2 area. If you then go for a single 34, it will be only 907mm2. I don´t quite get these numbers to add up. My only conclusion is that this is not the correct way of calculating this, I mean it gets very obvious if you try to achieve the 1608mm2 with one carb, then you need a 45 mm throat diameter, which according to everyone is way off the charts. So: how do one calculate this, I´m lost :?
 
fiatfan said:
About the throat area calculations, did them myself a while ago, if you have lets say 32:s for a start, you have 2 carbs, right? Which means 1608mm2 area. If you then go for a single 34, it will be only 907mm2. I don´t quite get these numbers to add up. My only conclusion is that this is not the correct way of calculating this, I mean it gets very obvious if you try to achieve the 1608mm2 with one carb, then you need a 45 mm throat diameter, which according to everyone is way off the charts. So: how do one calculate this, I´m lost :?
Click to expand...

Only one cylinder draws air at a time. Unlike an engine with 4 or more cylinders there is no continuous airflow through the carburetor at large throttle openings.

In essence one 32 mm carb will deliver the same amount of air as two 32mm carbs -except for the restrictions of the manifold and the lack of ram effect that you can get with two carbs. Jim
 
Yeah, not sure I was completely awake this morning.... :oops: I naturally know that there´s one bang at the time, just thought that...well I don´t know right now :? . BUT; does the shape of the intake matter much? I mean could it be an advantage or not to make a "box" that you attach the carb to? To equalize the pulses? Or doesn´t that matter on this engine configuration, you get better control by splitting the intake close to the carb?
Tommy
 
Salut Gillou & Gents,

I have experienced exactly the same phenomenon as Gillou and Nater (no way to get higher than 5000ish rpm) in approx. the same context :

1972 750 Commando with Combat head but CR back to standard via thicker cylinder base and cylinder head gaskets,
JS Motorsports "street" camshaft and pistons (and rods of course)
Single 32 mm Amal Premier carb (slide N°3, mainjet 250, needle 106 at lowest position, RGM 1 in 2 manifold)
Pazon Sure Fire and 28° advanced timing / NGK BP7ES sparkplugs looking OK.

Except for that problem, it works really well from idle to 5000 rpm.

I'm with you Jim in saying that in our 360° twin engines, only one cylinder is fed by the carb at a time. But does that mean that there is no benefit in increasing the carb size?
Should we stay with our 32 mm's and look for other causes for that 5000 rpm wall?

Cheers,

Laurent
 
fiatfan said:
Yeah, not sure I was completely awake this morning.... :oops: I naturally know that there´s one bang at the time, just thought that...well I don´t know right now :? . BUT; does the shape of the intake matter much? I mean could it be an advantage or not to make a "box" that you attach the carb to? To equalize the pulses? Or doesn´t that matter on this engine configuration, you get better control by splitting the intake close to the carb?
Tommy
Click to expand...

The shape of the intake manifold matters a lot. It could flow a lot better if the manifold was much longer -making it difficult to fit to a Commando. I have built and tested several different single carb manifolds for my bike. Jim
 
laurentdom said:
Salut Gillou & Gents,

I have experienced exactly the same phenomenon as Gillou and Nater (no way to get higher than 5000ish rpm) in approx. the same context :

1972 750 Commando with Combat head but CR back to standard via thicker cylinder base and cylinder head gaskets,
JS Motorsports "street" camshaft and pistons (and rods of course)
Single 32 mm Amal Premier carb (slide N°3, mainjet 250, needle 106 at lowest position, RGM 1 in 2 manifold)
Pazon Sure Fire and 28° advanced timing / NGK BP7ES sparkplugs looking OK.

Except for that problem, it works really well from idle to 5000 rpm.

I'm with you Jim in saying that in our 360° twin engines, only one cylinder is fed by the carb at a time. But does that mean that there is no benefit in increasing the carb size?
Should we stay with our 32 mm's and look for other causes for that 5000 rpm wall?

Cheers,

Laurent
Click to expand...


There is a small benefit to increasing carb size. Just like increasing the carb size when running dual carbs.
Bigger carbs mean less air drag through the carb - The power increase is small. Once you go over about 36mm the low speed tuning becomes trickier.

You may find that your 5000 rpm wall is due to the inability to get gasoline into the bowl fast enough. A single Amal carb will only flow enough fuel through the inlet needle and seat to make around 35 - 38 horse. Jim
 
comnoz said:
It could flow a lot better if the manifold was much longer -making it difficult to fit to a Commando. Jim
Click to expand...
about how much longer?
 
Do these single Carb bikes only pull to 5 k in the intermediate gears as well as in fourth, or do they pull right to red line in first through third, then only pull to 5 k in fourth?

Glen
 
Mark said:
comnoz said:
It could flow a lot better if the manifold was much longer -making it difficult to fit to a Commando. Jim
Click to expand...
about how much longer?
Click to expand...

I had pretty good luck with the manifold length at about 8 inches with 2 to 3 inches of common "plenum" between the carb and the split. Jim
 
worntorn said:
Do these single Carb bikes only pull to 5 k in the intermediate gears as well as in fourth, or do they pull right to red line in first through third, then only pull to 5 k in fourth?

Glen
Click to expand...

They usually start out pretty lively and then as you shift up through the gears you get more and more ahead of the fuel flow. By the time you get to high gear they can be pretty flat. Jim
 
comnoz said:
There is a small benefit to increasing carb size. Just like increasing the carb size when running dual carbs.
Bigger carbs mean less air drag through the carb - The power increase is small. Once you go over about 36mm the low speed tuning becomes trickier.

You may find that your 5000 rpm wall is due to the inability to get gasoline into the bowl fast enough. A single Amal carb will only flow enough fuel through the inlet needle and seat to make around 35 - 38 horse. Jim
Click to expand...

I'm quite surprised that the drop off in HP is this much. Agree with what you are saying tho, particularly the tuning issues over 36mm.

Is it really inlet needle restriction (wouldn't that be the same for a given bike with single or duals)?

I would assume it is more related to volume of fuel in the float bowl being used up, but I could be wrong.

Is it possible to adjust the float to improve fuel delivery amount on a single carb set-up? I suspect there is a point of diminishing return given that with dual carbs your only sucking every other stroke so there is essentially double the fuel available.
 
comnoz said:
worntorn said:
Do these single Carb bikes only pull to 5 k in the intermediate gears as well as in fourth, or do they pull right to red line in first through third, then only pull to 5 k in fourth?

Glen
Click to expand...

They usually start out pretty lively and then as you shift up through the gears you get more and more ahead of the fuel flow. By the time you get to high gear they can be pretty flat. Jim
Click to expand...


That is exactly how I find my 850 fitted with a 34 mik, it's strange that when I had a 750 with the same carb it did seem to go better at high revs.

sam
 
dennisgb said:
comnoz said:
There is a small benefit to increasing carb size. Just like increasing the carb size when running dual carbs.
Bigger carbs mean less air drag through the carb - The power increase is small. Once you go over about 36mm the low speed tuning becomes trickier.

You may find that your 5000 rpm wall is due to the inability to get gasoline into the bowl fast enough. A single Amal carb will only flow enough fuel through the inlet needle and seat to make around 35 - 38 horse. Jim
Click to expand...

I'm quite surprised that the drop off in HP is this much. Agree with what you are saying tho, particularly the tuning issues over 36mm.

Is it really inlet needle restriction (wouldn't that be the same for a given bike with single or duals)?

I would assume it is more related to volume of fuel in the float bowl being used up, but I could be wrong.

Is it possible to adjust the float to improve fuel delivery amount on a single carb set-up? I suspect there is a point of diminishing return given that with dual carbs your only sucking every other stroke so there is essentially double the fuel available.
Click to expand...

Sure it's the same no matter how many carbs you are running. With the Amal about 35 to 38 horsepower per carburetor.

With Amals you can modify the inlet needle for increased flow. You can drill the orifice in the seat larger and cut a window in the side of the seat to allow more flow. Of course fuel control suffers. [dribbley carbs]

With Mikuni's this restriction is not the same. They have different size inlet valves for different horsepower requirements. Jim
 
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