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

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dennisgb said:
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.
I does appear to be fuel-level related on mine, as it pulls like a freight train up to the point in high gear where the throttle is held to the stop for an extended period of time (>5 seconds?). At that point, it pretty much levels out. Slowly closing the throttle at that point will return a slight, almost imperceptible, forward push, signifying a lean condition. Going larger on the main jet doesn't help, so I'm assuming the needle/seat assembly is the limiting factor at that point.

dennisgb said:
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.
Assuming it's the needle and seat causing the restriction, raising the float level would only buy you the fraction of a second that it would take for the main jet to pull the level back to where it was before. I still want to play with it this Spring as far as increasing flow into the bowl, similarly to what is done with the Amal float bowl eyebrow cut. IIRC, Mikuni makes ported variants of the seat that should allow a greater flow.

Nathan
 
Nater_Potater said:
Mikuni makes ported variants of the seat that should allow a greater flow.

Nathan

I didn't know this. Will need to look for them.
 
I try to anderstand all post about single carb but my english is not enough good!!!

But i andertand that it's not necessary to increas diameter abow 36 and it's better to use a 36-38 manifold two fit a 34mm carb.

I buy a 36-38 manifold and i am going tow by Mikuni VM carb 34 and 36 second-hand part not too expensive. With the configuration i read i tune the 34 (with the good flange for the 36-38 manifold) and try to find the good tune for my bike then i try with the 36 to see the difference.

I like the Amal 32 singl carb, if i can find another good compromise with a 34 or 36 carb with an engine fuller of at all RPM.
 
gillou said:
I try to anderstand all post about single carb but my english is not enough good!!!

But i andertand that it's not necessary to increas diameter abow 36 and it's better to use a 36-38 manifold two fit a 34mm carb.

I buy a 36-38 manifold and i am going tow by Mikuni VM carb 34 and 36 second-hand part not too expensive. With the configuration i read i tune the 34 (with the good flange for the 36-38 manifold) and try to find the good tune for my bike then i try with the 36 to see the difference.

I like the Amal 32 singl carb, if i can find another good compromise with a 34 or 36 carb with an engine fuller of at all RPM.


Yes, A Mikuni vm34 works well on a 36-38 manifold. You will need a bushing to adapt from the 34mm carb size to the 36mm rubber flange. I usually turn a bushing from PVC pipe. Jim
 
comnoz said:
Yes, A Mikuni vm34 works well on a 36-38 manifold. You will need a bushing to adapt from the 34mm carb size to the 36mm rubber flange. I usually turn a bushing from PVC pipe. Jim

I ask to a friend to machining it. Just near my home there is a people who sell a TM 35. What do you think about this? I read the parts are the same, jet needles etc..
 
gillou said:
comnoz said:
Yes, A Mikuni vm34 works well on a 36-38 manifold. You will need a bushing to adapt from the 34mm carb size to the 36mm rubber flange. I usually turn a bushing from PVC pipe. Jim

I ask to a friend to machining it. Just near my home there is a people who sell a TM 35. What do you think about this? I read the parts are the same, jet needles etc..

I had some slide sticking problems with the TM carbs on a Norton but I know some people report good luck. They do flow well. Jim
 
I have a TM34 on mine. Works good. Very responsive, clean idle. No slide sticking. Not many miles on it though.

Will need to look into the larger manifold.
 
A friend of mine told me that 2 different fuel bowls are available at Amal (maybe Burlen in the uk) to increase the fuel flow into the bowl:

Ref 622/056 with an aperture of 3,2 millimeters in the seat vs 2,54 in the standard bowl
Ref 622/054 (sold for use with Methanol. 4 mm vs 2,54 mm).

I'm going to try the 622/056 and will let you know.

Hope that's clear and helps.

Laurent
 
More at play here than just FUEL flow. The engine needs a CORRECT MIXTURE of fuel/air. The MORE of this MIXTURE that can be inducted into the cylinder, the better for power. If your inlet valve has reached it's flow capacity and creating a lean top end condition, someone has woefully mis-applied the carb size.
So, if a person insists they cannot live with multiple carbs, why would you dramatically DOWNSIZE? An original pair of 32mm carbs yields 1608 square mm's of aperture. A single 32 is obviously only HALF original, 34 is a piddly 908 mm, 36 carb is 1017 mm, a single 44 is ALMOST back to stock with 1520.
I just don't understand the attraction to dramatically DOWNSIZE the carburetion. Well tuned twins will start, respond and deliver economy similar to a single.
An asthmatic top end is a poor trade off for perceived placebo benefit of "response", "economy" "simplicity".

JMWO

YMMV

:mrgreen:
 
Aside from the actual volume that needs to be flowed does the fact that the air flow stopping and starting at twice the speed have any affect with a single card.
With two carbs there is a longer pause between intake pulses. I don't know enough to say wether that is a good or bad thing.
Pete
 
Seems a lot of times in the past worn out Amals got replaced with a single Mikuni. At one time that made some sense as it did solve the Amal concentric problem of rapid slide wear, albeit at the cost of quite a few HP, from reading here.
Now that Amal Premieres with anodized slide and other improvements are available for relatively little $, I don't understand why anyone would spend money going the single Carb route and live with diminshed engine performance. If you happen to have the single Carb items on hand already, then it's a different scenario. However if you need to buy the manifold and Carb vs already have the manifolds for twin carbs, I think you could have new twin Premieres on there for the same cost or very little more. It is either free or very cheap horsepower.

There's really nothing much required keeping the Amal Premieres in tune. I've got 15,000 miles on a set and haven't touched them, perfect idle, easy starting. If I get careless the bike will pull past 7 k in the first three gears and pulls 6700 in fourth, 21 tooth sprocket.
I think Jerry (owner of the site) still sells the Premieres.

Glen
 
+1 Glen... And more so!

I also seem to have always enjoyed carbs staying in tune for ages.

I believe the 'myth' of constantly having to tune twin carbs is also down to the fact that people struggle on trying to keep worn out carbs in use, sloppy slides, worn out needles and jets, all add up to carburation that is all over the place. Chasing such issues is what, I believe, has become the folklore that twin carbs always need 'tuning'.
 
concours said:
If your inlet valve has reached it's flow capacity and creating a lean top end condition, someone has woefully mis-applied the carb size.
Not carb size; application. Mikunis are used in any number of applications, many of which incorporate a fuel pump. This was the case with mine. Inlet valves can be had with diameters ranging from 1.5mm, clear up to and including 3.5mm! When you look at the available "pressure" at the inlet, you can appreciate the fact that a larger passage is required when running strictly gravity feed from the tank. Even when full to the tank's neck, you're lucky to see 1/3 psi. Given most snowmobile/PWC fuel pumps run around 2 psi, you can appreciate that an incorrectly chosen inlet will now be the main jet restriction by proxy.

concours said:
An asthmatic top end is a poor trade off for perceived placebo benefit of "response", "economy" "simplicity".
"Asthmatic"? As said in an earlier post, mine runs hard right up to 95 mph. That's just five shy of the "Go Directly to Jail" speed in my parts. 'Plenty fast for my tastes, and, if I really need to go faster than that, it'll be on a race-prepped bike on a sanctioned track.
"...perceived placebo benefit of "response", "economy" "simplicity"". Perceived? Well, I won't make arguments about the "response" aspect since that can be somewhat subjective, but, as to the economy, we went from one full year's riding returning 46 mpg with the twins, to the next year of the single Mikuni at 57 mpg. Not perceived; documented, tank-by-tank, over a 7,000 mile season.
"Simplicity"? 'Don't really care. I don't shy away from dual carbs based on that aspect. In fact, I prefer the appearance of twins. However, since running the Mikuni, the bike fires first kick hot or cold, and, thanks to the enricher, gives a nice fast idle while warming. It also allows me to actually sit at a traffic light without wondering when it's going to cough and die.
Do you run any non-Norton-sanctioned gasket sealants because you're tired of oil leaks? How about oil? Tires? At some point, concessions are made depending on parts availability, cost, and function.
Gilles originally posted "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 ?
Let's see if we can help him out.

Nathan
 
concours said:
So, if a person insists they cannot live with multiple carbs, why would you dramatically DOWNSIZE? An original pair of 32mm carbs yields 1608 square mm's of aperture. A single 32 is obviously only HALF original, 34 is a piddly 908 mm, 36 carb is 1017 mm, a single 44 is ALMOST back to stock

Only one cylinder pulls fuel at a time so the volume required is half of what you calculated. Both cylinders are rotating at the same time. One is on intake the other is on exhaust stroke. Carb size is essentially the same single or duals because your only feeding fuel to one cylinder at a time. The single carb has to feed twice as many engine pulses so the fuel demand is double in the same time period. This is why it can run out of fuel at higher engine RPM's.
 
Is it possible that the Commando twin carb tuning or synchronizing headaches being mentioned can be traced to the one into two throttle cable splitter? I don't have experience with a running Norton yet, but enough Ducati and BMW twin carb bikes to know that if I decide to stay with two Amals on this 750 Commando I'm rebuilding, it will have a twin cable throttle set up all the way. The Twinmax electronic carb balancer makes synchronization quite easy.
Bill
 
dennisgb said:
concours said:
So, if a person insists they cannot live with multiple carbs, why would you dramatically DOWNSIZE? An original pair of 32mm carbs yields 1608 square mm's of aperture. A single 32 is obviously only HALF original, 34 is a piddly 908 mm, 36 carb is 1017 mm, a single 44 is ALMOST back to stock

Only one cylinder pulls fuel at a time so the volume required is half of what you calculated. Both cylinders are rotating at the same time. One is on intake the other is on exhaust stroke. Carb size is essentially the same single or duals because your only feeding fuel to one cylinder at a time. The single carb has to feed twice as many engine pulses so the fuel demand is double in the same time period. This is why it can run out of fuel at higher engine RPM's.

Good explanation.

If going to a single carb meant that you were cutting the effective carburetor area in half the HP loss would be far greater than the 5 or 6 horse loss generally observed when switching to a single carb. That loss is seen because of the turbulence caused by the shape of the runners in the manifold. Jim
 
worntorn said:
Now that Amal Premieres with anodized slide and other improvements are available for relatively little $, I don't understand why anyone would spend money going the single Carb route and live with diminshed engine performance. If you happen to have the single Carb items on hand already, then it's a different scenario. However if you need to buy the manifold and Carb vs already have the manifolds for twin carbs, I think you could have new twin Premieres on there for the same cost or very little more. It is either free or very cheap horsepower.

Glen

My bike came with a single Mikuni VM. The carb was junk so I switched to a Mikuni TM flat slide because there is improved performance over a round slide carburetor.

From the Mikuni web site:

"Mikuni's TM Series flat slide carburetors provide a significant performance improvement over older round slide carburetors. Air flows faster and smoother through the TM Series venturi and jet blocks due to the flat slide design which helps create a smooth bore effect. The high velocity air flow means stronger vacuum at the needle jet for more precise metering, better throttle response."

This is true of all Flat Slide carburetors and from my experience the response improvement is obvious.

While these changes are not keeping with the perfect original restoration, the technical improvements in carburetion since the Amal design are worth it to me personally.

It's not totally about keeping it simple or cost.
 
dennisgb said:
concours said:
So, if a person insists they cannot live with multiple carbs, why would you dramatically DOWNSIZE? An original pair of 32mm carbs yields 1608 square mm's of aperture. A single 32 is obviously only HALF original, 34 is a piddly 908 mm, 36 carb is 1017 mm, a single 44 is ALMOST back to stock

Only one cylinder pulls fuel at a time so the volume required is half of what you calculated. Both cylinders are rotating at the same time. One is on intake the other is on exhaust stroke. Carb size is essentially the same single or duals because your only feeding fuel to one cylinder at a time. The single carb has to feed twice as many engine pulses so the fuel demand is double in the same time period. This is why it can run out of fuel at higher engine RPM's.

I never "calculated flow required", ( that's fir the flow bench and dyno owning guys), but rather only what aperture size was AVAILABLE. :mrgreen:
 
comnoz said:
dennisgb said:
concours said:
So, if a person insists they cannot live with multiple carbs, why would you dramatically DOWNSIZE? An original pair of 32mm carbs yields 1608 square mm's of aperture. A single 32 is obviously only HALF original, 34 is a piddly 908 mm, 36 carb is 1017 mm, a single 44 is ALMOST back to stock

Only one cylinder pulls fuel at a time so the volume required is half of what you calculated. Both cylinders are rotating at the same time. One is on intake the other is on exhaust stroke. Carb size is essentially the same single or duals because your only feeding fuel to one cylinder at a time. The single carb has to feed twice as many engine pulses so the fuel demand is double in the same time period. This is why it can run out of fuel at higher engine RPM's.

Good explanation.

If going to a single carb meant that you were cutting the effective carburetor area in half the HP loss would be far greater than the 5 or 6 horse loss generally observed when switching to a single carb. That loss is seen because of the turbulence caused by the shape of the runners in the manifold. Jim


I didn't suggest it was a 1:1 exchange, or anything like that.

As for the shape of the runners being the power loss theory, well, that's kinda hard to prove unless someone fabricates a manifold for a VM44 and the 5-6 horsepower loss is still present.
 
concours said:
dennisgb said:
concours said:
So, if a person insists they cannot live with multiple carbs, why would you dramatically DOWNSIZE? An original pair of 32mm carbs yields 1608 square mm's of aperture. A single 32 is obviously only HALF original, 34 is a piddly 908 mm, 36 carb is 1017 mm, a single 44 is ALMOST back to stock

Only one cylinder pulls fuel at a time so the volume required is half of what you calculated. Both cylinders are rotating at the same time. One is on intake the other is on exhaust stroke. Carb size is essentially the same single or duals because your only feeding fuel to one cylinder at a time. The single carb has to feed twice as many engine pulses so the fuel demand is double in the same time period. This is why it can run out of fuel at higher engine RPM's.

I never "calculated flow required", ( that's fir the flow bench and dyno owning guys), but rather only what aperture size was AVAILABLE. :mrgreen:

I read it that you were saying the single carb needs to be as large as both of the dual carbs together, which is wrong. :shock:
 
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