Best carbs for Nortons

jseng1 · Dec 31, 2019

In my opinion the Keihin 31 or 33mm CRS smoothbores have been the best carbs available for Nortons so far. The 35mm FCRs are nice but over carburated and just too bulky and cramped in a Norton. Now I find out that the 33 FRC is a smaller bodied carb than the 35mm. That would make it a better fit and still more than enough carb for most Commandos.

Unfortunately neither the 33mm CRS or The 33mm FCR are set up for Nortons and even though there is a big demand for the CRS carbs - they take up too much of my time to make them available any longer.

But the little 33mm FCR is very tempting. The necessary parts should be available to gang them together similar to the Sudco 35mm FCR dual carb kit. For anyone with enough ambition - this should be a sweet deal if you've got the time.

madass140 · Dec 31, 2019

yes Jim I have CR33's on one bike, kinda expensive and takes a bit of fitting but great. For another project (not Norton) I bought that
exact carb you pictured a 28mm FCR . not bad for $120

jseng1 · Jan 1, 2020

madass140 said:
yes Jim I have CR33's on one bike, kinda expensive and takes a bit of fitting but great. For another project (not Norton) I bought that
exact carb you pictured a 28mm FCR . not bad for $120

$120 is a steal - was that 2nd hand?

acotrel · Jan 1, 2020

Do long bell-mouths make any difference ? I have them, but I suspect they just look good. Although with the 2 into 1 exhaust and methanol fuel, the 850 motor seems to pull hard from zero revs, all the way to 7000. I only use the top 1500 RPM, except if I get balked.

Craig · Jan 1, 2020

In an old Norton manual I have (Haynes maybe) the last chapter is on performance tuning , without looking I think they mentioned best intake track length was 8.5” , they claimed 80hp at crank with all the mods they discribed in detail with drawings , never really gave that chapter a good read ....

Craig · Jan 1, 2020

Just took a look in shed, found the manual mentioned above ... it is the ”Clymer Norton -Service-Repair Handbook” .... it states 80hp plus at rearwheel possible without the use of exotic parts ..... 750cc Nortons “built and campaigned by Rob Wood and Alex Jorgenson in AMA National Circuit racing , they used no special factory pieces that cannot be purchased” ..... going to have to read this last chapter .... from open end of velocity stacks to gasket face on cylinder head should be 9.5” or 240 mm , however they may need to be shortened to clear frame .....

jseng1 · Jan 1, 2020

Craig said:
In an old Norton manual I have (Haynes maybe) the last chapter is on performance tuning , without looking I think they mentioned best intake track length was 8.5” , they claimed 80hp at crank with all the mods they discribed in detail with drawings , never really gave that chapter a good read ....

Further development by Ron Wood and Axtell found the best ram effect to be approx 10" from the head.

ashman · Jan 1, 2020

My old Norton manual has that performance chapter in it and I used most of the ideas in the stage one motor section for my 850 motor that I built for the Featherbed frame back in the 1980 when converting to the Featherbed but I also got the crank balanced for the frame, my motor performs great and has been very reliable for over 38 years now, has never let me down and once the cam kicks in it will just keep reving if you let it get away from you, so you got to watch what you do with your throttle hand.

Ashley

lcrken · Jan 1, 2020

jseng1 said:
Further development by Ron Wood and Axtell found the best ram effect to be approx 10" from the head.

If I recall correctly, he said 11.5" from valve head to inlet bell. That might actually be around 10" from the manifold surface of the head, or at least pretty close.

Ken

madass140 · Jan 1, 2020

madass140 said:
yes Jim I have CR33's on one bike, kinda expensive and takes a bit of fitting but great. For another project (not Norton) I bought that
exact carb you pictured a 28mm FCR . not bad for $120

From China on Aliexpress or Alibaba, just forget off hand. But their ad is the same pic you posted and the same one I got, that included some main and pilot jets as well but the pilot jets were wrong for this carb. The more you buy the cheaper they are

jseng1 · Jan 3, 2020

madass140 said:
From China on Aliexpress or Alibaba, just forget off hand. But their ad is the same pic you posted and the same one I got, that included some main and pilot jets as well but the pilot jets were wrong for this carb. The more you buy the cheaper they are

So is that a Chinese knock off?

Fast Eddie · Jan 3, 2020

I’m sure I’ll be bombarded by folk telling me how awesome they are, but, for me, I like my Keihin’s to be made in Japan...

Craig · Jan 3, 2020

I have no clue what the best may be .... I can say other than briefly experimenting with a set of 32VM Mikuni , my bike came with with a single 34 VM and it has been flawless over many years , I know other types should allow more speed , but as is, it will go 100mph, it's easy on gas and that good enough for me .... set and forget is nice too

jseng1 · Jan 4, 2020

The images below show one FCR carb with a Sudco sticker on the side and one without. Its true that Aliexpress shows these carbs at a cheap china price so I will contact Sudco next week and find out if they're being copied or what. Not sure about the info I've been given so far - most concerning is that the 33s may actually be the same overall size as the 35s and that would be a deal breaker.

This photo looks shows the blue sudco sticker on the bowl and a different accelerator pump linkage compared to the photo at bottom.

This photo is off the aliexpress site with some subtle differences to the photo above.

Here's a carb I'm not familiar with - a flatslide with a round screw off top. Its less than $50. Its a flatslide and I'll bet it blows away the Premiers (If anyone has the gonads to jet it and give it a try - and doesn't care about the knockoff stigma). Has to be a knockoff because it matches the fake mentioned on this Sudco website:

http://www.sudco.com/genuine_vs_fake_carburetors.html

Some even come with a fake Sudco sticker

Bernhard · Jan 6, 2020

The pic of the flat side carb looks like a power jet designed for a racing two- stroke- these do not normally come with any kind of pilot jet system as their use didn't need it.

jseng1 · Jan 6, 2020

Bernhard said:
The pic of the flat side carb looks like a power jet designed for a racing two- stroke- these do not normally come with any kind of pilot jet system as their use didn't need it.

I'm not familiar with the round screw top PWK in the photo above but I am familiar with the other PWK copy with the flat bolt on top and it does have an idle jet but its also designed for a 2 stroke and you have to adjust it by closing the idle mix screw all the way, then open it slowly and stop as soon as the idle picks up (at about 1/2 turn) and then stop - if you keep opening it then it idles like shit. If you can tune them they work MUCH better than Amals with an increase in performance. I had to design needles to work with the 4 stroke Norton.

Bernhard · Jan 6, 2020

I think I'll retract my earlier statement , having looked again at the pic, there is no power jet in sight on the bell mouth. Soz.

jseng1 · Jan 6, 2020

Check out the knockoff FCR carb with the rough casting in the photo below and compare it to the FCR carb photo above. Looks like they are showing a photo of a genuine Keihin carb but are actually selling a lower quality knockoff. I just talked to Sudco and yes there are knockoffs of these FCR carbs and others.

And yes the genuine FCR33s have smaller over bodies than the 35s so there is a market for some ambitious entrepreneur who will offer a ganged together twin FCR 33mm carb set.

MexicoMike · Jan 6, 2020

Best carb for stock or for a modified Commando?

If you are talking about a modified engine, that is, of course, a different application than stock and a different carb might very well provide more power on the modified engine than the Amals can. But on a stock engine? Does anybody have any Dyno proof that other carb(s) produce more power on a stock Norton than a properly set up pair of Amals?

Craig · Jan 6, 2020

My dyno is the seat of pants , my bike goes just great and starts easy ... slower than the Duc , prolly give the Griso a serious run , say on a city block distance , light to light ...

Best carbs for Nortons

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