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

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seattle##gs said:
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?

This is a bit misleading, particularly in light of those suggesting that a 44mm carb will work on a stock Norton. Not suggesting that is what you mean by "Motors will run on a wide variety of carburetion and actually run fairly well" but it is a very vague statement in light of some of the comments in this thread. While it may be possible to tune a 44mm carburetor, it will not run very well due to a huge drop in velocity. Higher velocity through the smaller carbs of 32-36mm will provide a much quicker throttle response and are sized properly based on the CFM of the engine. Fuel volume required doesn't change just because an oversized carb is used. Even though the throat is bigger the engine can only pull so much air based on displacement. That volume of air will only allow so much fuel to be burned at the proper mixture. It's much better to have the smallest carb that allows the optimum volume of air to flow, because of the improved response.
 
right, I didn't mention carb size. To clarify, using a 34 mm mikuni as an example, the standard pilot is a 25. you could run a 27.5, a 30, perhaps a 35. Which one is optimum? It still runs well on any of them.
The usual kit comes with a 6DH3 needle with a P-6 needle jet which seems too rich...I usually use a P-2 or P-0 with that needle. But it can be made to run well with either needle jet. Which one is optimum? Actually, I found the 6F9 and a P-6 needle jet works the best for me.
and the clip position? middle or one richer? hard to tell. which one is optimum? I could go on with more variables. And I could make it run pretty well with any combination of these. The main jet is probably the hardest to dial in because I am never at full throttle or up at those speeds, so I go with the general opinion on what size to use.

And, of course, the bigger the carb the poorer the carburetion at starting, idle and just off idle. Hence my earlier question about using a 36 VM and if there low end problems that can be solved with proper jetting.

Meanwhile....I am looking at a 48MM S&S shorty for a friends harley ...I could have run my old Ford 300 six cylinder with this.
 
seattle##gs said:
right, I didn't mention carb size. To clarify, using a 34 mm mikuni as an example, the standard pilot is a 25. you could run a 27.5, a 30, perhaps a 35. Which one is optimum? It still runs well on any of them.
The usual kit comes with a 6DH3 needle with a P-6 needle jet which seems too rich...I usually use a P-2 or P-0 with that needle. But it can be made to run well with either needle jet. Which one is optimum? Actually, I found the 6F9 and a P-6 needle jet works the best for me.
and the clip position? middle or one richer? hard to tell. which one is optimum? I could go on with more variables. And I could make it run pretty well with any combination of these. The main jet is probably the hardest to dial in because I am never at full throttle or up at those speeds, so I go with the general opinion on what size to use.

And, of course, the bigger the carb the poorer the carburetion at starting, idle and just off idle. Hence my earlier question about using a 36 VM and if there low end problems that can be solved with proper jetting.

Meanwhile....I am looking at a 48MM S&S shorty for a friends harley ...I could have run my old Ford 300 six cylinder with this.

An O2 sensor would help you with dialing your carb(s) in and insure that the carb size is correct.
 
seattle##gs said:
....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......
How does one deal with the balance tube between the twin carb intake manifolds when using a mercury tube or Twinmax type carb synchronization tool?
Bill
 
pantah_good said:
How does one deal with the balance tube between the twin carb intake manifolds when using a mercury tube or Twinmax type carb synchronization tool?

I remove the tube and attach the balancer (Morgan Carbtune) tubes to the stubs.
 
+1 on the carb tune. You can probably get shagged out twin concentrics to work ok-ish with a set of balance gauges. Tuning by ear, and throw in points ignition, I'm amazed we got any riding done during our youth.
 
gripper said:
+1 on the carb tune. You can probably get shagged out twin concentrics to work ok-ish with a set of balance gauges. Tuning by ear, and throw in points ignition, I'm amazed we got any riding done during our youth.

Or maybe we know too much and have too many choices now :D
 
gripper said:
+1 on the carb tune. You can probably get shagged out twin concentrics to work ok-ish with a set of balance gauges. Tuning by ear, and throw in points ignition, I'm amazed we got any riding done during our youth.
yes but back then who cared about a reliable tickover? worn amal carbs are fine when they are wide open !! points are also ok even with a worn out advance unit it may kick you back now and then but that was a right of passage! then fast forward and you know things have moved on a bit when you arrive at your destination with cleanish hands!! cheers baz
 
My hands get dirty just walking by my Norton .... used to happen with other Brands too ... the Duc just takes my money ( tires ) .... leaves my hands clean mostly though ...
Craig
 
No problem...set your frozen advance unit at full advance, and make your junk amals idle at 1500, bypassing the pilot circuit and it will run all day. In town it will be a bear and shifting might be a problem but who cares? You just saved about $500 - $600. Not to mention being completely stock British.

I used the local dyno to arrive at my carb settings.
 
In the end, doesn't cylinder filling depend on the standing waves which are set up in both the inlet tract and the exhaust ? The gas mixture 'flows' by mass transfer rather than by laminar flow, and it all happens at sonic speeds. So anything which causes a bump in the wave reduces mixture transfer. With the single carb, half the inlet tract is facing a wall. It might be OK on a road bike, however I would not do it on a racer unless I wanted to run a nitromethane/methanol mix. Mainly for ease of doing jet changes.
In the olden days lots of hot-rod cars ran log manifolds - always worked better with a blower.
 
Racing is a whole other matter. I think you could spend a lifetime tweaking and twiddling and still have more to do. Pipe length...intake length....this could go on forever. I wish I had the method and the funds to do it but XXX. picture the intake being pulsed as the valve opens and closes. Imagine the single manifold acting as a plenum where the original didn't. Throw in constant variations in air speed of the fuel mix...there are so many variables and no way to get data other than the local dyno. All I know is a great running stock commando is a joy to ride and hear. I can take off from my driveway and I am happy within a mile. Down over the bridge, point it east or north and go. Summers are best around 1 AM when it's warm and the traffic on the freeway has really thinned out.
 
gripper said:
+1 on the carb tune. You can probably get shagged out twin concentrics to work ok-ish with a set of balance gauges. Tuning by ear, and throw in points ignition, I'm amazed we got any riding done during our youth.

About riding in our youth: I once blew up a 750 engine, and the 850 that I replaced it with had obviously been down on the right side. Didn´t show on the timing cover, but it was impossible to get the points to work properly. When trying to get a grip on what was going on, I discovered that the advance unit was wobbling around, and then I saw that the end of the camshaft with the cone was not round. So naturally I didn´t change the cam, which would be the way to go today, instead I cut out a small piece from a Coca-Cola can and moved it around in the cone hole until the advance unit looked to be turning around without any wobble. That worked several years until I bought the Luca-Rita, and then I did the same thing with that, cheap fix :D . And I was running my CHROMED Amals, today I can not understand what I was thinking :? . They never did work properly, today I think I know why :lol:
Tommy
 
baz said:
gripper said:
+1 on the carb tune. You can probably get shagged out twin concentrics to work ok-ish with a set of balance gauges. Tuning by ear, and throw in points ignition, I'm amazed we got any riding done during our youth.
yes but back then who cared about a reliable tickover? worn amal carbs are fine when they are wide open !! points are also ok even with a worn out advance unit it may kick you back now and then but that was a right of passage! then fast forward and you know things have moved on a bit when you arrive at your destination with cleanish hands!! cheers baz

Seattle - " No problem...set your frozen advance unit at full advance, and make your junk amals idle at 1500, bypassing the pilot circuit and it will run all day. In town it will be a bear and shifting might be a problem but who cares? You just saved about $500 - $600. "


+1 Totally agree.

My 850 had what I could afford - The most worn out Amals in the world :shock:

$45 kept me alive for a week in 1978 !

Today the same bike can have anything I want - but I'd still sooner be 25 again :D
 
Can you elaborate on this Jim?

Also, have you tried a fuel pump on a single carb or a pumper single carb?
I'm trying out a Mikuni pumper with a Mikuni 34 VM. If it works I will move to the TM. Has anyone used a pumper with a return line to the tank for fuel overage and control of fuel pressure to the carb?
 
I have used a Mikuni vacuum operated fuel pump with a 2 bbl Weber carb but not with a single Mikuni.
I am still using a Mikuni fuel pump on my injected bike to circulate fuel between the tank and the backbone where the high pressure pump is located.
 

The fuel pump is now mounted on the top tube. It draws negative pressure from the carb manifold which activates the pump. Fuel is pulled from the remote float (it collects fuel from both drop tanks) and pumped to the carb and back to the tank to keep pressure down at the carb.
 
+1 on the carb tune. You can probably get shagged out twin concentrics to work ok-ish with a set of balance gauges. Tuning by ear, and throw in points ignition, I'm amazed we got any riding done during our youth.

YES...it does sound that way nowadays but in reality, I'll bet we got a LOT more riding done then than we do now. Now we seem to agonize over stuff that was of no consequence at all back then when, I suspect, most of us (certainly me) rode these bikes FAR harder than we do now.

Back then I never knew about all these "problems" with Commandos that we seem to discover almost daily that MUST be corrected. Bizarre ! ;)
 
This is a bit misleading, particularly in light of those suggesting that a 44mm carb will work on a stock Norton. Not suggesting that is what you mean by "Motors will run on a wide variety of carburetion and actually run fairly well" but it is a very vague statement in light of some of the comments in this thread. While it may be possible to tune a 44mm carburetor, it will not run very well due to a huge drop in velocity. Higher velocity through the smaller carbs of 32-36mm will provide a much quicker throttle response and are sized properly based on the CFM of the engine. Fuel volume required doesn't change just because an oversized carb is used. Even though the throat is bigger the engine can only pull so much air based on displacement. That volume of air will only allow so much fuel to be burned at the proper mixture. It's much better to have the smallest carb that allows the optimum volume of air to flow, because of the improved response.

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.

There was one problem that took some work to get around. The carb had an accelerator pump that shot across the venturi and all the fuel ended up in the LH cylinder. Then the left plug would get black and the right cylinder would make a lean backfire on low speed acceleration.

I ended up making a twin squirter nozzle to replace the single nozzle.
This setup actually made as much power as any twin carb setup and was still very docile at low speed. Jim
 
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