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Discussion Starter #1
i was thinking of replacing my stock cv carbs. what do you guys recommend for my xs400. cylinders are obviously about 200ccs a piece, stock size is 34mm cv. what i have read its best to go down a few sizes because of the better flow. but what type should i use?
 

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I am partial to mukuni vm carbs. Stock issue on a lot of suzuki's and kawasaki's, plenty of parts out there. Not sure what would be a good size but I know you can get new aftermarket VMs in 32mm.

you can get individual carbs new or off any of the kawasaki triple bikes (different sizes for different displacements). banked ones...well you don't need them.
 

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it's really not because of better flow per mm of bore

it's because many CV carb'd bikes come equipped with way too much CV carb



and a mechanical slide carb does not forgive simply because it is possible to over carb with a mechanical slide

and it simply is not possible to over carb with a constant velocity carb

which is why you can hang a 2 inch bore SU on a 74 inch shovel that will never see 6000 rpm and it will run ok

put a 41mm Mik on it and it turns into a soggy dog begging for 36-38mm

there isn't enough cam, port, compression, rpm potential, or much of anything feeding those 200cc cylinders that will even use all of a 32mm VM

I haven't done the math on it since I don't know all the specs but off the hip

I'd say a pair of 28's would make one very crisp and snappy and a pair of 26's may be about right

that's about half the flow potential of the big 34's

and it would get good fuel mileage too

just for kicks and grins, what are the intake port dimensions and shape?

my guess is they are round...... tell me the bore size
 

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the hole in the head is 31mm?
 

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then the next thing you need to know is the bore, the stroke, and the rod length

cam timing is helpful too

if it has a 31mm port

I'm guessing it is an over square engine (larger bore than stroke) and a small rod to stroke ratio

like around 1.5:1

that single ratio for a given displacement is one of the most important for determining optimum port and carb sizes
 

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Discussion Starter #7
69mm bore / 52.4 mm stroke i don't know the rod length, cam timing, and i don't know how to find them right now. i have a service manual but its at work. im sure it has all that.

what does a small rod to stroke ratio mean? how does it effect port diameter demands?
 

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say you have an oversquare engine with a 1.4:1 rod to stroke angle

it will not be a low rpm torque monster

it will be a high rpm engine and will require both a large port and carb

let's say you have an undersquare engine like a HD or many brit bikes

let's say it has a rod to stroke ratio of about 2:1

it will work best with a much smaller port and work just fine with a much smaller carb


why? because the amount of time the piston is effectively creating a depression for which the port/carb to flow into due to the rod angularity function in dictating where the piston is positioned in the cylinder bore relative to crankshaft rotation

but if you have an actual 31mm port in the head

you should be fine with a 30-32mm carb but 31's do indeed exist


just for reference and perspective

my short rod 750cc/45 inch Triumph puts 52 horses and 47 ft/lbs of torque, to the back tire, on a good day

it breathes thru 32mm carbs and 1 3/8 inch pipes, max power only drops off a couple ponies with 30mm carbs but the torque curve rises slightly and is much flatter (feels stronger in the seat of pants)

if it were a long rod 750 is would be almost impressive with the 30's

almost enough even to make you scratch your head and ask WTF

my very long rod 45 inch HD flatty does 34 very streetable horses (way up from a stock 15ish) and has broken 100 mph (much faster than a stocker which does well to bust 60mph) and it also is capable of returning 70 mpg at 70 mph (stock they did well to return 35 mpg at any speed)

it breathes thru a 28 mm carb and 1.5 inch pipes

a 1100 cc JAP flathead would scare 100 mph wearing a 26 mm carb

very long stroke and very long rods, tiny intake ports


however......

your 400 is breathing thru the same size carbs as an XS650

regardless of what it's internal geometry is all about

I'd feel just fine saying a 34mm is just too damned big for it and that is the real reason you wouldn't be happy with a 34mm mechanical slide carb on it

remember...... a CV carb will not let you over carb an engine

a Rochester Quadrajet (CV secondaries) is capable (slightly modified) of flowing 900 CFM

yet they were on very mildly tuned small engines as well as fire breathing monsters and worked fine on either

rule of thumb and you can slice this up to fit your engine

it takes 700 cfm to make 350 CID turn 7000 rpm

I'm thinking a 30 mm VM will flow about 110 CFM

somebody please correct me if that is wrong


oh and there are some very interesting engines out there whose future and usefulness was limited simply because they were over intake ported

the iron head sportster is one prime example

53 inches (900cc's) with 41mm ports

same size as a 74 inch (1200cc's) big twin

same rod to stroke ratio as the little 45 inch (750 cc) flathead

the guys that make them go fast weld up the huge intake ports

another prime example was the Mercedes Desmo from the 1950's

and in recent times...... BMW's 3.0 and 4.0 liter V-8's

they had real problems because of very long cam timing and huge ports
 

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Discussion Starter #9
i have the yamaha dealer service rep working on the rod length.

the cam timing though is
OPEN CLOSE DURATION OVERLAP
IN BTDC 30 ABDC 70 280 60
EX BBDC 70 ATDC 30 280 "

as copied out of the service manual.
 

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cool, now the only significant difference between conventional motorcycle slide carbs flow potential and butterfly valve CV's is that you do indeed have a throttle shaft and butterfly in the bore but the better designs only have small flow differences

but there is a huge difference of area and flow capability between 34mm bore (with butterfly in the way) and a 30mm mechanical slide

34mm has 908 square mm of area

30mm (88% of the 34's bore) has 706 square mm of area (77% of the 34's flow area)

28mm (82% of the 34's bore) has 530 sqaure mm of area (58% of the 34's flow area)

17mm (50% of the 34's bore) has 227 square mm of area (25% of the 34's flow area)

so it takes roughly 4 17's to do what one 34 will do

do you know how to determine your cam duration and overlap from those numbers?
 

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Discussion Starter #11
280 degree duration, and 60 degree overlap its given (if thats what it is). i tried to make it all neated up but my spaces were removed in the post. i don't know the significance of those numbers. (i know changing them moves where the power is best but thats about it)
 

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Discussion Starter #12
called yamaha and had them look into the archives. they had three people researching it but no one there could find the rod length. there isn't a way for me to know it without taking my motorcycle apart.
i think i will try to get some 28mm carbs see how that flows.
 
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