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

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dennisgb said:
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
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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.
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having had a pair of worn out 32 mm concentrics then brand new 32mm concentrics then a single 32mm concentric then a 32mm cv mikuni then a 34mm vm mikuni then a 36mm vm mikuni i now have a tm 40 mikuni all i can say is the way the bike performs is way better in real terms for a road bike ie it will pull well into the red if i want it too,it has better mpg it has better bottom end power ,way better mid range and the top end feels the same as the twin concentrics ,also going from a 36mm vm to a 40 mm tm is like comparing apples and oranges ,i have always thought that maybe the flat slide mixes the fuel up a bit better but i'm no expert on this cheers baz
 
"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."

I kinda took a leap of faith that the VM34-6 being discussed was application matched by Suddco, Rocky Point Cycle and would be equipped for purpose. If it was pulled from Frosty Fred's snowmobile salvage yard via fleabay, kinda bears research, yes? You're right, using the wrong stuff, is, well, wrong. :idea:
 
concours said:
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.
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I have done it with a 44 mm VM Mikuni.... Still had power loss and the lower speed ranges were nearly impossible to deal with. Jim
 
comnoz said:
concours said:
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.
Click to expand...

I have done it with a 44 mm VM Mikuni.... Still had power loss and the lower speed ranges were nearly impossible to deal with. Jim
Click to expand...

Cool Jim, did you make a new manifold? Adapt a commercially available one? In the attic? No middle ground? VM38? VM40?
 
concours said:
comnoz said:
concours said:
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.
Click to expand...

I have done it with a 44 mm VM Mikuni.... Still had power loss and the lower speed ranges were nearly impossible to deal with. Jim
Click to expand...

Cool Jim, did you make a new manifold? Adapt a commercially available one? In the attic? No middle ground? VM38? VM40?
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I have tried lots of combos in the last 40 years -in search of the perfect setup. I have had 42 and 44 mm carbs [and several others] on a bored MAP manifold. I have also had the same carbs on at least 4 of my own designed manifolds.
Carb size didn't make a big difference on the MAP manifold.
I saw larger increases by boring the inlet side of the manifold than I did going to a bigger carb.
After switching to a longer, reshaped manifold I got pretty close to a dual carb setup.
 
comnoz said:
After switching to a longer, reshaped manifold I got pretty close to a dual carb setup.
Click to expand...

Can you elaborate on this Jim?

Also, have you tried a fuel pump on a single carb or a pumper single carb?
 
dennisgb said:
comnoz said:
After switching to a longer, reshaped manifold I got pretty close to a dual carb setup.
Click to expand...

Can you elaborate on this Jim?

Also, have you tried a fuel pump on a single carb or a pumper single carb?
Click to expand...

Pumper carbs are a bit funny, even with the Dual Keihin from CNW, or so I have heard. You have to be real carful when blipping off idle not to foul the plugs. Hitting the throttle when running down the road is OK but not a big deal. I needed to adjust the stroke and point of initiation to not be so active. These big breathing hemi's will hesitate if you pop hard on the gas and the pumper helps only slightly. However, the big MM flatsides totally over shadows any issues in this regard with overall performance. I am sure this is the case with the Dual Keihin as well.
 
pete.v said:
Pumper carbs are a bit funny, even with the Dual Keihin from CNW, or so I have heard. You have to be real carful when blipping off idle not to foul the plugs. Hitting the throttle when running down the road is OK but not a big deal. I needed to adjust the stroke and point of initiation to not be so active. These big breathing hemi's will hesitate if you pop hard on the gas and the pumper helps only slightly. However, the big MM flatsides totally over shadows any issues in this regard with overall performance. I am sure this is the case with the Dual Keihin as well.
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Pete,

I'm not sure I understand this. If the jetting and needle(s) are correct, how can too much fuel be delivered to the engine?

Dennis
 
dennisgb said:
pete.v said:
Pumper carbs are a bit funny, even with the Dual Keihin from CNW, or so I have heard. You have to be real carful when blipping off idle not to foul the plugs. Hitting the throttle when running down the road is OK but not a big deal. I needed to adjust the stroke and point of initiation to not be so active. These big breathing hemi's will hesitate if you pop hard on the gas and the pumper helps only slightly. However, the big MM flatsides totally over shadows any issues in this regard with overall performance. I am sure this is the case with the Dual Keihin as well.
Click to expand...

Pete,

I'm not sure I understand this. If the jetting and needle(s) are correct, how can too much fuel be delivered to the engine?

Dennis
Click to expand...
Accelerator Pump.
 
dennisgb said:
comnoz said:
After switching to a longer, reshaped manifold I got pretty close to a dual carb setup.
Click to expand...

Can you elaborate on this Jim?

Also, have you tried a fuel pump on a single carb or a pumper single carb?
Click to expand...

Basically using a scratch built manifold about 8 inches long with a 37mm Keihin FCR I got almost as much power as I did with a pair of 37mm FCR's.
No way could it be fit under the backbone of a Commando but I did have it mounted om my perimeter framed racebike for a while. Jim
 
comnoz said:
Basically using a scratch built manifold about 8 inches long with a 37mm Keihin FCR I got almost as much power as I did with a pair of 37mm FCR's. No way could it be fit under the backbone of a Commando but I did have it mounted om my perimeter framed racebike for a while. Jim
Click to expand...

I was thinking it would be too long. This is a race trick that I've seen before. I'm assuming this reduced or eliminated the turbulence you have been talking about along with other benefits. Thanks for the info Jim.
 
pete.v said:
dennisgb said:
pete.v said:
Pumper carbs are a bit funny, even with the Dual Keihin from CNW, or so I have heard. You have to be real carful when blipping off idle not to foul the plugs. Hitting the throttle when running down the road is OK but not a big deal. I needed to adjust the stroke and point of initiation to not be so active. These big breathing hemi's will hesitate if you pop hard on the gas and the pumper helps only slightly. However, the big MM flatsides totally over shadows any issues in this regard with overall performance. I am sure this is the case with the Dual Keihin as well.
Click to expand...

Pete,

I'm not sure I understand this. If the jetting and needle(s) are correct, how can too much fuel be delivered to the engine?

Dennis
Click to expand...
Accelerator Pump.
Click to expand...

My bad Pete. I didn't understand that an accelerator pump is an "enricher". For some reason I was thinking it just increased pressure. Makes sense now.
 
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
Click to expand...

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.
Click to expand...

Ok, this is all sound reasoning, I remember being first exposed to this, twin vs single Tillotson butterfly's on a Yamaha 433 sled engine in a Sno-Jet, circa 1971. (I know, piston port, reversion at low speeds, etc :roll: ) But it's a bit over simplified as explained here. This gaseous mixture at high speed isn't like a Play-Doh fun factory metering out 375cc volumes, stopping/starting, but rather pressure thus velocity drops/spikes. So, the theory of "it's only feeding one cylinder at a time, so a 32mm single should flow as good as twin 32's" isn't quite accurate. Relying on atmospheric pressure on downward (aka "vacuum") to fill the cylinder with properly ratio'd mixture is very much reliant on (at WOT) removing restriction and enhancing flow. Take away (for the sake of discussion) the branch tee/street ells/pipe flange type manifold that is designed for packaging foremost, and replace it with a well designed manifold, the 32mm carb will be the bottleneck (my original point :idea: ). I cannot argue whether it'll need a 44, 40, 38mm carb, but, as Jim found while race experimenting, a well designed manifold with a properly sized carb will work well.

JMWO :mrgreen:
 
Nater_Potater said:
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.
Click to expand...
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".
Click to expand...
"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
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You're right Nate, I do need to apologize for hijacking Gilles' thread, but it was a random act after reading through dozens of similar threads, and finally feeling the need to toss out some more food for thought seeing as it was a "size choice" thread, rather than the usual "single or twin" thread. I evoked some passion with your response. :mrgreen: I'm not the guy on the "keep it all original dammit!" soapbox.... I have indeed considered, compared dimensions and planned manifolding TWIN Mik VM32's.... but, haven't acted upon it, so, no input yet. The new Prem's are working well, so I probably won't. The VM is a great carb. As for repeated 57 mpg, that's great, :mrgreen: Not going to jail for speeding, for sure. :wink: One kick starts? Both carbs can do it reliably. Steady idle? Same. Enrichener Mik vs slide choke on Amal? Well, 2 outta 3 ain't bad... Mik wins. :oops: Maybe I'm set up/doing it wrong, but the cold idle isn't there for me with Amals.
 
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
Click to expand...

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.
Click to expand...

Ok, this is all sound reasoning, I remember being first exposed to this, twin vs single Tillotson butterfly's on a Yamaha 433 sled engine in a Sno-Jet, circa 1971. (I know, piston port, reversion at low speeds, etc :roll: ) But it's a bit over simplified as explained here. This gaseous mixture at high speed isn't like a Play-Doh fun factory metering out 375cc volumes, stopping/starting, but rather pressure thus velocity drops/spikes. So, the theory of "it's only feeding one cylinder at a time, so a 32mm single should flow as good as twin 32's" isn't quite accurate. Relying on atmospheric pressure on downward (aka "vacuum") to fill the cylinder with properly ratio'd mixture is very much reliant on (at WOT) removing restriction and enhancing flow. Take away (for the sake of discussion) the branch tee/street ells/pipe flange type manifold that is designed for packaging foremost, and replace it with a well designed manifold, the 32mm carb will be the bottleneck (my original point :idea: ). I cannot argue whether it'll need a 44, 40, 38mm carb, but, as Jim found while race experimenting, a well designed manifold with a properly sized carb will work well.

JMWO :mrgreen:
Click to expand...

Gobbly Gook :shock:
 
Two things I have not seen mentioned...on the minus side of Mikuni VM carbs.......the idle adjust screw begins to loosen in the threads and can cause a variable idle.... try rocking the idle screw up and down while sitting at a stop sign. This comes from that nice heavy slide dropping fast and whacking the adjuster. Do it enough and it becomes sloppy. A design flaw but easily compensated for.

2) I have yet to come up with a routing for the fuel lines that I am happy with. There used to be a good fitting but it is no longer available.

On the + side for Mikunis....a single Mikuni 34 VM makes it run a lot smoother IMMEDIATELY....if tuned correctly. I have experience with different kits and settled on the 6F9 needle / P6 N-jet / 2.5 slide combo. Works very well.

2) long life from the carb. much better than Amals. Do not know if the premiers last longer. Amals are good when brand new, but a worn out pair is a lesson in aggravation.

3) Very easy to install and remove. Much quicker than amals. This is a giant + when you have to work in that area. And R&R an air cleaner is much easier.

4) There is a lot more room to work in , back of the cylinders.


A single Mikuni is also good for the Norton owner that does not know a lot about Nortons, does not have a synchronizing tool or the knowledge to use it. I have seen quite a few poorly running british bikes and the owners have no idea what a properly running bike should be like. It sounds like a lot of Norton owners on this forum have spent a decade or two or three really studying these bikes and are therefore much more able and willing to keep their bikes in the best state of tune. But that is not the average owner
 
seattle##gs said:
Two things I have not seen mentioned...on the minus side of Mikuni VM carbs.......the idle adjust screw begins to loosen in the threads and can cause a variable idle.... try rocking the idle screw up and down while sitting at a stop sign. This comes from that nice heavy slide dropping fast and whacking the adjuster. Do it enough and it becomes sloppy. A design flaw but easily compensated for.

2) I have yet to come up with a routing for the fuel lines that I am happy with. There used to be a good fitting but it is no longer available.

On the + side for Mikunis....a single Mikuni 34 VM makes it run a lot smoother IMMEDIATELY....if tuned correctly. I have experience with different kits and settled on the 6F9 needle / P6 N-jet / 2.5 slide combo. Works very well.

2) long life from the carb. much better than Amals. Do not know if the premiers last longer. Amals are good when brand new, but a worn out pair is a lesson in aggravation.

3) Very easy to install and remove. Much quicker than amals. This is a giant + when you have to work in that area. And R&R an air cleaner is much easier.

4) There is a lot more room to work in , back of the cylinders.


A single Mikuni is also good for the Norton owner that does not know a lot about Nortons, does not have a synchronizing tool or the knowledge to use it. I have seen quite a few poorly running british bikes and the owners have no idea what a properly running bike should be like. It sounds like a lot of Norton owners on this forum have spent a decade or two or three really studying these bikes and are therefore much more able and willing to keep their bikes in the best state of tune. But that is not the average owner
Click to expand...

I'm a huge fan of modern carbs and run FCRs on my Commando, so what I am about to say is not based on Amal religion...

I struggle with the practicalities of your final paragraph, a point of view often repeated. I fully understand that some owner riders are far less mechanically savvy than others, but Amals are just about the simplest carbs ever. They were OEM fitment to our bikes, so correct settings, that work perfectly, can be obtained directly from the manual. They will work just fine even with only 'approximately correct' settings and with (almost) any old fuel. I therefore fail to see how someone who cannot do this would be MORE able to dial in and set up a none standard modern carb correctly !!
 
Yup I agree ... my bike came with the single VM 34 and I like it .... once I tried a pair of Mikuni VM 32's worked even better but I lost my great fuel mileage ... really this thread is about preference as most of the carbs mentioned can be set up to run a Commando pretty well .... some here like to get into the hair splitting and I do like to read it as some of it is educational .... for a thick head like me ...
Craig
 
Motors will run on a wide variety of carburetion and actually run fairly well. Most people are satisfied with that. And don't know any better. How many people really know what a stock Norton in top tune is like?

Anyway, dual amals work well if NEW and certainly look better on the bike than a single Mikuni. Tuning is not a real problem if the owner is somewhat mechanically inclined and has bought a carb synchronizer. A word on synchronizing tools...I have worked on dual carbs since the late 70s. I can get perhaps , at best, 90% of the way by ear if I am lucky. To get that last 10% and to eliminate doubts, I highly recommend a synchronizer tool. Even then you need a fair amount savvy to dial them in. But once you're there, they tend to stay in tune longer. Please forget the "remove one spark plug lead" method recommended in the shop manual. It requires a lot of guess work. I know several people claim the tool is not necessary and a waste of money but not so!

Forgot where I was going with this but let's get back to the original topic : 34 or 36? I have always used a single 34 Mikuni. Once I tried a 36 and had the usual problems of space...It will just squeek in but if you want to attach a throttle cable....

To the people who have used a 36 VM and got it dialed in, I have some questions. Did the bottom end suffer? Did you get good reliable starting and idle? Good pick up just off idle, no bogs? any flat spots through the range? And what was the power increase from say, 1/4 throttle on up?
 
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