Best carbs for Nortons

Doesn't seem like the heavy crank is much of a hindrance to acceleration, even with a stock bike.
The weight of the bike and rider is the main holdup, not spinning up the crank

Try free revving a stock motor to redline, you will quickly see that the weight of the crank isn't much of a problem for that motor. It goes from idle to redline in about 1 second.

I believe the Nourish racing cranks are heavier than stock Commando.

Glen

worntorn said:

Doesn't seem like the heavy crank is much of a hindrance to acceleration, even with a stock bike.
The weight of the bike and rider is the main holdup, not spinning up the crank

Try free revving a stock motor to redline, you will quickly see that the weight of the crank isn't much of a problem for that motor. It goes from idle to redline in about 1 second.

I believe the Nourish racing cranks are heavier than stock Commando.

Glen

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I have lightened a Norton 750 crank and the engine did go to the red line slightly quicker, but as discussed elsewhere on this forum a couple of years ago it appears to have not been the right thing to do. PW has mentioned that he never got to try out a racer with a lighten crank but wished he had. Since you have never tried it (and I'm not advocating that you should) as there appears to be no gain to be had, in the general consensus of opinion.

If you change from 3rd to top with a standard box and compare it with what happens when you change from 3rd to top with a close ratio box, there is a lot of difference in acceleration. With the wide box, you are spinning the motor up from a lower rev level, so it takes longer to get to peak revs. And you are demanding more from the motor. If you change the motor to deliver more torque, it is still better with close ratios. However often you do not know that you have more torque until you raise the overall gearing and the bike goes quicker. It is very deceptive. If the motor is pulling hard, it makes the same noise and has the same feel, as when it is having it easy. All you notice when you get it right, is the corners come up faster.
I don't know how accurately a dyno can measure changes in the torque the motor is delivering at various revs . Ultimate horsepower is all most guys talk about. My friend has an extremely fast 650 Triumph - it has never been revved above 6,300 RPM. I've ridden it and blew-off a very good race prepared 750cc Kawasaki two-stroke. With him, it is all about torque. With my 500cc Triumph, it was all about revs and top end. He was faster. You can have a motor which revs like crazy but goes nowhere fast.

Bernhard said:

I have lightened a Norton 750 crank and the engine did go to the red line slightly quicker, but as discussed elsewhere on this forum a couple of years ago it appears to have not been the right thing to do. PW has mentioned that he never got to try out a racer with a lighten crank but wished he had. Since you have never tried it (and I'm not advocating that you should) as there appears to be no gain to be had, in the general consensus of opinion.

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It helps you go faster if you use the energy which is stored-up in the crank. Once it is spinning high, keep it spinning high as you race-change up through the gears. A light crank stores less energy. There were two types of 650 Triumphs in the 1960s - the Thunderbird/Trophy and the Saint. The Saint had the light crank and was hopeless. If you are buying a one-piece crank for a pre-unit Triumph, the Saint crank is usually cheaper than the one with the heavy flywheel.

Two-stroke motors usually rev much higher and have lighter internals. If you turn one into a head-wind when you are racing, you can feel it almost stop. The inertia is improved by the much higher revs, but there is no substance to most two-strokes. Unless they have 6 gears, two strokes usually go backwards. And the 1960s 125cc Suzuki GP bikes had 12 gears. Gilera 4s in the 50s had 7 gears - light crank ?
When I bought the 6 speed box for my Seeley 850, it was to get two lower gears, so that off the clutch start I would be as full-on as I was with the 4 speed close box once the bike was rolling. What you lose before turn two of any race is very difficult to regain.
The Commando motor is quite quick enough, even in near-standard form. Without the right gearbox, it is hopeless.

I use methanol fuel, but all that does is keep the motor cooler and makes it easier to get the jetting right.

Going slightly off topic here as it's a norton website, a Yamaha engine tuner lobbed off 2lbs on a RD400 crank for a racing bike and I can tell you it really, no I mean really went on acceleration. Don't ask me how I know.

Al, if you ride your bike like that, ie keeping it in a narrow rev range (which is actually a more ‘correct’ racing style) I would venture to suggest that you would benefit from a lightweight crank.

And the Triumph example is not ‘black and white’ either, many racers preferred the light crank. I’ve got a light crank in my ‘68 T120 road bike and I LOVE it ! To be honest, I do think that a heavier crank would make a more sensible road bike engine, but the light crank makes a more FUN engine !

Bernhard said:

Going slightly off topic here as it's a norton website, a Yamaha engine tuner lobbed off 2lbs on a RD400 crank for a racing bike and I can tell you it really, no I mean really went on acceleration. Don't ask me how I know.

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2lbs... pah!

The Steve Maney Norton crank is 7lbs lighter than stock. Imagine that, half a stone less rotating weight in the engine... no wonder they give a different feel!

If we are really talking about the best carbs for Nortons, probably the major difference between the available type of carbs is in the mid-range jetting - needle and needle jet. Ken (fullauto) has a single VM Mikuni on his Commando. With that he can use the full range of Mikuni needles and needle jets for that size carb.
When I jet my 34mm MK2 Amals for methanol, if the needles are one notch too high, the motor become sluggish as opposed to brisk. With methanol, the jets flow twice as much. So for you guys using petrol, getting the mid-range jetting right, must be twice as difficult. If you change the carb and in so doing improve the mid-range jetting, you would get an improvement in torque. However for 34mm Mk2 Amals, the #6 needles for the VM 34 Mikuni work very well, and there is a much wider range of choices. All the needles are the same diameter at the parallel part - the tapers are different.

Fast Eddie said:

Al, if you ride your bike like that, ie keeping it in a narrow rev range (which is actually a more ‘correct’ racing style) I would venture to suggest that you would benefit from a lightweight crank.

And the Triumph example is not ‘black and white’ either, many racers preferred the light crank. I’ve got a light crank in my ‘68 T120 road bike and I LOVE it ! To be honest, I do think that a heavier crank would make a more sensible road bike engine, but the light crank makes a more FUN engine !

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The "fun engine' in my short stroke Triumph would grab you by the throat and kill you. It was so bad that I actually enjoyed racing with it. These days I don't have any anxiety until I remember what that was like. The Seeley 850 is worry-free, I just sit there like a blob and ride it. The interesting part is braking before the corners and powering right through them. You have got have your head around of the corners to where you expect to come out. - Know how much the bike is going to turn as you gas it. You still have control, but need to allow for what the bike does when it oversteers. There is always plenty of room and plenty of time to correct it, if it goes a bit wrong.

acotrel said:

The "fun engine' in my short stroke Triumph would grab you by the throat and kill you. It was so bad that I actually enjoyed racing with it. These days I don't have any anxiety until I remember what that was like. The Seeley 850 is worry-free, I just sit there like a blob and ride it. The interesting part is braking before the corners and powering right through them. You have got have your head around of the corners to where you expect to come out. - Know how much the bike is going to turn as you gas it. You still have control, but need to allow for what the bike does when it oversteers. There is always plenty of room and plenty of time to correct it, if it goes a bit wrong.

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I don’t really think that your horrid short stroke 500 should come into the equation at all!

I’d have thought you couldn’t get further apart than that and an 850 Commando, even with a light crank!

Bernhard said:

I have lightened a Norton 750 crank and the engine did go to the red line slightly quicker, but as discussed elsewhere on this forum a couple of years ago it appears to have not been the right thing to do. PW has mentioned that he never got to try out a racer with a lighten crank but wished he had. Since you have never tried it (and I'm not advocating that you should) as there appears to be no gain to be had, in the general consensus of opinion.

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To the contrary, if you want greater acceleration, a lighter (less rotational mass) mass crankshaft yields more useable torque and acceleration hands down every time. Not only will the motor rev up quicker (more useful torque) and you will have less translational mass to accelerate down the track.

NIgel, there are two types of motor and gearbox. You can have the short stroke, light crank, but then more speeds in the box becomes more important. My Triumph had the light crank and was always revved very high. If it had six speeds, it would have been a winner. With four speeds, you could gear it either high or low. Gear it low and you could outride the rest of the field around the slow parts of the circuit, but get beaten at the ends of the straights. Gear it high and be slower coming out of corners, but be with the rest at the ends of the straights. I usually geared it high. With the Seeley 850 I still need six gears because of the clutch start, but everywhere else four gears close ratio is perfect. With the clutch start, you get the bike to jump from a standstill. So it is the worst case scenario with a wide ratio. However if I rev the motor to 6000 RPM and dump the clutch, the bike is fast enough off the start. I don\'t like doing that with a Norton box. If it gives up, I can get a shunt from behind me.
I think it takes a lot of racing before a rider really becomes sensitive to what their bike is doing. With my Triumph 500 , I learned to work from a position of disadvantage - the Seeley 850 is easy. It just amazes me that it is so good, because I never believed in it. One look at that crankshaft should be enough to put anyone off.

With the standard gearbox, the Commando engine is hopeless. With close ratios, it is good because you don't rely on throttle response so much. If I wanted to build a world-beater commando, I would probably build it with a lighter 75mm stroke crank. That way, the crankcases might last longer. But I would also use a high balance factor on the crank. What you pick up on the hurdy-gurdy, you lose on the merry-go-round. A lot depends on the inlet port size and the exhaust system . With a Commando engine, you are more playing with torque than top end power.
I think a light crank makes the bike more nimble.
However, upon reflection - I think the heavy crank helps my riding to be smoother and that is what going faster depends on. With the Seeley 850, high speed is easy, I don't have to be ten tenths on top of the bike. It comes down to tyres. If you are ragged, the contact patch of the rear tyre is affected so you get signals which make you apprehensive. When I ride my bike, there is no anxiety - I just do it. I've ridden a few bikes with light cranks and they are usually much easier to lose. Modern tyres make things easier, but in the extreme the result is the same. If you go fast enough, you are back in the era when the tyres had no grip.

The last time I raced, there were about 5 of Rex Wolfenden's 1100cc methanol-fuelled CB750 Hondas on the grid. In two races the Seeley 850 kept up with the leaders easily. In the last race, I was passing under the leader at turn two, when a fuel line came adrift. To me that says something about the Commando 850 motor with the heavy crank. It is not all bad. I am fairly experienced, but it is not me, it is the bike.
It is a bit of a joke - those 1100cc Hondas cost the guys a bomb. They are full of HRC titanium. Rex's brother is Honda Australia.

acotrel said:

If I wanted to build a world-beater commando, I would probably build it with a lighter 75mm stroke crank. That way, the crankcases might last longer.

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Too the contrary, with a lighter crankshaft the cases are taking a greater beating for a given balance factor and reciprocating mass. All governed by dynamics and kinematics.

Dances with Shrapnel said:

Too the contrary, with a lighter crankshaft the cases are taking a greater beating for a given balance factor and reciprocating mass. All governed by dynamics and kinematics.

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True - as long as its well balanced. But if the balance is imperfect then maybe not. Something else to consider - taking a few pounds off an airplane prop reduces stress and allows the engineer to take a lot more off the frame - even though both props were balanced within 2 grams - the prop stress is worse than the engine vibe stress.

Even if it is not well balanced, any given condition (other than a perfectly balanced rotating mass e.g. plain shaft) in conjunction with a lighter crankshaft will render more forces due to acceleration upon the crankcase. All governed by dynamics and kinematics. A heavier crankshaft offers dampening to the out of balanced conditions of a crankshaft.

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/

I should say some smaller carbs have 3 bolts