Best carbs for Nortons

Fast Eddie

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FCR carbs have 4 bolt and 3 bolt float bowls. The 35's have a 4 bolt some smaller carbs have the 3 bolt .
the problem with the 3 bolt carbs is they dont have a Leak Jet in the float bowl. This is very important
for tuning. I'm using a 3 bolt (28mm) carb and may try to drill and tap for a Leak Jet so I have some adjustment
with different size available jets. Some tuning tips here for the 4 bolt carbs https://www.instructables.com/id/Fixing-the-dreaded-lean-bog-on-Keihin-FCR-MX-carbu/
That’s a good article, thanks for posting.
 

madass140

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yes it is . he quoted the adjustable "jets" by 2 manufacturers on Ebay but around $90 for my 3 bolt carb
but unfortunately I dont have the clearance underneath for it. so I'm canning the FCR and will run a VM
style round slide which I am aslo using on a similar project, very easy to tune.
 
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I seem to remember when I was riding dirt bikes a lot (KTM 530) one of the hacks was to extend the metal nipple on the diaphragm and wrap an O ring around the timing screw/spring and actuator arm.
 
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Re;

https://www.instructables.com/id/Fixing-the-dreaded-lean-bog-on-Keihin-FCR-MX-carbu/

The no 1 picture for Keihin FCR Carburetors shows just what a massive size it is, did anybody see the expanding aperture camera shutter type carbs that were fitted to some of the earlier (1990s) works WSB Ducatis that had no slides but the camera shutter just opened and closed as a throttle? I thought it was a brilliant idea! . . . .it would solve a lot of slow running problems.


 
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I would never use an adjustable main jet. Mikuni and Amal main jets are usually calibrated with a flow meter, their size does not change. I drill needle jets using a combination of metric and number drills to get the size I want. The trouble is, you cannot drill a smaller hole in a needle jet. With Amal carbs on Commandos, I believe there are two sizes of needle jet specified - 0.106 inch and 0.107 inch. The correct size for best power using petrol, is probably somewhere in between. That thou of an inch difference is big even when using methanol fuel - with petrol, it is huge. Back n the olden days, whenever we bought a second-hand motorcycle - the first thing we did was replace the needle jet in the carb. The slightest bit of wear will slow the bike.
If you replace the carbs on a Commando, you probably get most improvement in performance if you get the needle and needle jet right. You can get the same result with old carbs if you jet them right. Main jets do not usually matter as long as they are slightly rich.
 
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Re;

https://www.instructables.com/id/Fixing-the-dreaded-lean-bog-on-Keihin-FCR-MX-carbu/

The no 1 picture for Keihin FCR Carburetors shows just what a massive size it is, did anybody see the expanding aperture camera shutter type carbs that were fitted to some of the earlier (1990s) works WSB Ducatis that had no slides but the camera shutter just opened and closed as a throttle? I thought it was a brilliant idea! . . . .it would solve a lot of slow running problems.


Since this was an apparently (earlier) works WSB Ducati effort, any chance this camera like aperture was used in conjunction with fuel injection? The reason I ask is I cannot figure out how you would get the flow signal to act on a jet, mainly in the lower, partial and mid throttle range. Neat approach as at WOT it should leave a relatively unobstructed passage.
 
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At a slight tangent, but carb related. Has anyone any experience of TT carbs. I have mounted 2 on my project, it’s a hell of an effort on the twist grip through the last 1/4 turn, is that normal? I have the upside down cone shaped springs and brass slides.
 
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Since this was an apparently (earlier) works WSB Ducati effort, any chance this camera like aperture was used in conjunction with fuel injection? The reason I ask is I cannot figure out how you would get the flow signal to act on a jet, mainly in the lower, partial and mid throttle range. Neat approach as at WOT it should leave a relatively unobstructed passage.
Yes it might have been fuel injection, just don't have any information about it other than it was at Brands Hatch and the Ducati won.
 
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Since this was an apparently (earlier) works WSB Ducati effort, any chance this camera like aperture was used in conjunction with fuel injection? The reason I ask is I cannot figure out how you would get the flow signal to act on a jet, mainly in the lower, partial and mid throttle range. Neat approach as at WOT it should leave a relatively unobstructed passage.
Putting the jet on a post to the center of the carb wouldn't be much of a sacrifice if it worked as well as it seams like it would work. Needs some out of the box thinking.
 
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Certainly anything is possible but why would one want to put a relatively big honking spray tube half way up into the middle of the airway. Even AMAL figured this out with the GP carbs way back in the day. My best guess is the camera apparatus throttle was used with fuel injection which “makes sense”.
 
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Doesn't have to be a "big honking spray tube half way up into the middle of the airway" It could be a skinny thin tube with no more cross section than you already have with a regular needle that you see in all carbs. But plenty of design obstacles to figure out. Maybe go with a diaphragm pumper type carb.
 
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Just going by what I have seen over the years. If so good I suppose we would be seeing plenty of these....but we don’t. With a tall(er) “spray tube” you are now faced with needing a signal that is likely double what you would need with a conventional (because of the extra vertical lift). This could be overcome by using a remote float chamber and raising the fuel level but now you are going back in time and reverting as far as float bowl location strategy which is discussed elsewhere on this forum. A pumper might get you through transitions but won’t help you steady state.
 
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A pumper type carb might be good in situations where you have to whack the throttle open. As you open the throttle, you lose vacuum, so you don't suck as much fuel and the motor can gasp. With the correct taper needle, this is minimised - but if you are jetted for best performance, you need to feed the throttle on in a controlled matter. Also the performance becomes weather dependent, so you need to consider what you want to achieve. For road use, it probably does not matter if you run slightly rich on the needle. All round, fuel injection with programmed ignition is probably a much better way to go, but you lose simplicity. If you have trouble with an Amal or Mikuni carb, what are you going to do with a computerised engine management system ?
With our motorcycles, we might be in a similar situation as with cars, where you cannot work on one which has not got chromed bumper bars.
 
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When I race, I always use methanol fuel because if you are running slightly rich, you still get good power. There is a big difference in power if you get the needles and needle jets right. However because methanol jets are much larger than petrol jets, jetting for petrol is much more critical if you race with it. But for road use, petrol is OK, even if you are running slightly too rich. The extra power you get from using methanol when racing, often comes from the way it hides-up tuning errors. Petrol does not do that.
 
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I think pumper carbs were originally developed for MX and enduro bikes where the riding is more stop-start rather than smooth.
 
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What I mean by a pumper carb is a carb with a diaphram that gets a pulse from the engine sump. The pulse hits the diaphram and the diaphram forces a squirt of fuel to the intake charge, after the pulse a needle shuts off the plumbing until the next pulse - sort of like fuel injection but without electronics. No float bowl. These type carbs were popular on McCulloch go cart engines and I saw a magazine article where one was adapted to a BSA 441. That BSA used a pulse from the exhaust pipe with a line tapped in near the riders foot. They measured a HP improvement but that's as far as it went.



Come to think of it - this is one of the few carbs that would fit in the Norton twins restricted space and it might be a great alternative as a twin carb setup.

I used to cut wood for a living and I have one of these racing motors in my chain saw and it absolutely kicks ass as shown below.

 
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The pumper carbs you are referring to are so that they can carburate an engine in various orientations (up, down, left, right, maybe even upside down) like a chain saw needs to. Great if you need to operate your Norton like that but who does? Come to think of it, where is hobot.
 
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