Best alternative carburetor setup?

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72Fastback

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I have a 72 Combat with stock internals and I'm looking to upgrade the carb. I am trying to get the most comparable performance to the 32mm Amals, which includes the higher RPM range. All the while trying to achieve 55-60 MPG if possible.

I can fine tune most any modern carb with no problem at all, hoping to be able to do that with my Norton. The following is what I currently know about the possible carb setups.....

Single 34mm Mikuni loses too much up top.... What about with the UFO?

Twin 32mm Mikuni's have fitment problems, if so how badly? Performance should match twin amals, along with the fuel economy, yes?

Twin 30mm Keihn PWK's, anyone know anything about this?

Suggestions welcome for what meets OEM performance, and still get 55-60 MPG.

Thank you all very much. And yes I know this topic has been beaten to death. I have read nearly all the other threads about the carbs, and I'm still a bit puzzled.
 
On my standard 850 Commando I went from twin 32 Amals to single 36mm Mikuni VM carb. The off-idle and bottom end is better. Mid-range about the same as well set up twins. Top end slightly less powerful. I get 50 to 65 mpg depending on the type of ride.
According to Allens Performance the top end can be restored comparable to twins by fitting a larger single 38mm Mikuni VM carb. I suspect there may be a slight trade-off with bottom end torque, but judging from my 36 I think there should still be loads of it, despite the bigger venturi. Allens do the kits but you will always have to play around with jetting, since no two engines are exactly the same.
 
From everything I've read, plus my own experience, best performance by far is from a pair of 35mm Keihin flatslide "FCRs."

Downside: price.
 
The UFO is a poor mans way to upgrade a round slide. A hunk of plastic fills in the recess at the bottom of the slide. This ups the air speed through the carb.
It takes a number of modifications to get it working properly#15 pilot jet. and a careful cross drilling of the spray tube. A lot of fitting of the plastic to get the slide to settle to an idle. Adding an Amal slide spring to the Mikuni stock one.
Don't attempt this if you are easily frustrated by methodical tuning that has you pulling down the carb over and over. Yes it's worth it but you will spend more than a day getting to work if you are good and lucky.
Very snappy throttle response and good MPG can be had. I did have to upgrade the fire at the plugs to take advantage of the new found flow. Better coil, copper plug wires,and a .035 gap on some AP64 Autolite plugs.
Start with a carb that is dialed in or you will go nuts.
In the end a well sorted pair of Mark two Amal's are much better at the top end but you need to do some modifications to get them working as well. Read old posts on the how to of this.
 
IMHO the best value for money is to mod 2 out of a gang of 4 Mikuni RS, almost an FCR but far cheaper. They're also available as a PWC/snowmobile pair but I don't know how the jetting, needle & seat etc. compare and may be expensive to change.
The JRC so far looks good, and I suspect it flows more air than a comparable sized Mikuni.
The bigger Mikuni VM will always something of a compromise in mileage vs. response due to no accelerator pump, and running on the slide cutaway at cruising speed.
 
I have found the 34 mm Mikuni to be the best all round carb although I have not used the exotic $$$ twin set ups. I fit a Sudco velocity stack to the mikuni and fit a K&N filter over that. I have bought the UFO but have not yet fit it. It does take a lot of set up but the concept is sound.
 
So the UFO will NOT restore the lost top end power? I read in another thread that the way to restore the top end on a single Mikuni setup was to fit the UFO??

If a 34mm Mikuni with the UFO will not have enough up top, what about a 36mm Mikuni with the UFO? Would the 36mm lose low-mid power on my 750 because the venturi is too big, or is it an acceptable size?

No one has commented on the twin 32mm Mikuni idea. I don't see how companies could sell a twin 32mm kit specifically for a Norton Commando, and have extreme fitment problems. Apart from fitment, this may be a great setup, and then add in twin UFO's if possible. I would imagine twin 32's would be near identical to twin amals, but I'm probably wrong.

Thanks for all the responses so far!
 
72, I wrote that piece years ago on the use of UFO's with a 36MM Mikuni yes it helps the top end no question. Does it work like a sorted pair of Amal Mark2's no.
To use the Mark two's you have to file the unused bosses off the inside of the bodies to get them close enough, and that's only one of the mods.... Mikuni's are far too wide to run without splaying so some more issues to confront. No free lunch sorry.
Don't forget the air cleaners they play a part in your plan as well.
 
So, twin 32 Mikuni's are out of the question, I don't want to hassle with fitting them all crooked and what not.

Single 36mm with/without UFO sounds good so far, anyone with a 750 Combat have this setup, if so whats your opinion?

Any comments for the twin 30mm keihn flatslide PWK carbs? On ebay they are brand new for $240 and it says direct replacement for amals, even uses amal throttle cable and old airbox or air filter. Anyone have experience with these?

I have NEVER ridden a Commando with sorted Amals, so if I don't know what the top end rush feels like, maybe I won't even notice that its not there with the carb setup I get, unless it is really THAT bad. Theres always brand new 30mm Amals for $330.
 
I have a set of 34 mm Mikuni's on my 850 cafe racer (lot's of head work and cam). They are on ACK cast manifolds matched to the ports and use oval, tapered K&N filters that fit nicely between the frame tubes and clear the battery box. That was the biggest issue. I have run these for about 12 years and they seem to work fine as the bike pulls strong on top end.

I'd love to try the FCR 35's, but the cost has kept me away. I have set of 41's on my 2-valve rubber Ducky and I was amazed at how it was transformed. I expected finicky starting without chokes, but it actually starts much easier than with the CV Mikuni's.

For a stock motor, I still like a set of sleeved Mk1 concentrics. They are all the carb the motor can use until you start tweaking it. If it's mileage you are after, a single Mikuni kit may help, but it would take a long time to offset the cost of the kit by the savings in gas.

Just my $0.02 :roll:
 
Ok a wacky idea maybe but if this UFO device works on Miks and Amals have a similar recess at the front of the slide which must disturb airflow what are the chances of improved flow if the recess was filled in and profiled nicely with epoxy? Clearance would be needed for the spray tube and a hole for the needle. Stupid idea or what are yr thoughts?
 
i have fitted the UFO to my 34 mik and i like it (took some fiddling, but was actually kind of fun) - there is a noticeable but not startling increase w/ low end performance and MPG, but i can detect no change at top end, which is what the folks at thunder products told me what to expect- so unsure about claims about increasing this area of performance ? anyone care to speculate :?:
 
Thirteen years later and surprised no one has made a less restrictive manifold to suit a single 39 mm or 41 mm FCR Keihin.
 
Didn't see any mention of jsmotorsports Keihin CR's. I have one of his PWK's on my mark three and it helped start it up after 25years, in the PO's shed, back when I first got the bike. He sets the CR round slides up with custom manifolds and a modified cable pull linkage that fits right on to commandos. Only seen good reviews /results, from his customers with these.
 
I've been running a pair of 33mm CRs on my 920 for about eight years or so. They work really well, but are really no better than a unworn pair of MK1 Concentrics. I had to re make the throttle cable set up to clear the Interstate tank, but other than that they fit okay. If I were you I would buy a pair of Premiers or a pair of MK 2s. I'm just in the process of fine tuning a pair of the later on my Atlas based Domiracer & have to say that I rather like them.

Martyn.
 
BrianK said:
From everything I've read, plus my own experience, best performance by far is from a pair of 35mm Keihin flatslide "FCRs."

Downside: price.
Click to expand...
I have this set-up and can confirm it's great but best performance can be achieved by disabling the "pumper" - i.e. install a shorter push-rod.
 
mikie3117 said:
I have found the 34 mm Mikuni to be the best all round carb although I have not used the exotic $$$ twin set ups. I fit a Sudco velocity stack to the mikuni and fit a K&N filter over that. I have bought the UFO but have not yet fit it. It does take a lot of set up but the concept is sound.
Click to expand...
Please point me to source of velocity stack for Mikuni 34VM , thanks very much !
 
Please point me to source of velocity stack for Mikuni 34VM , thanks very much !

me want to know too
 
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