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A single carb will be so big in order to replace two carburettors, your low speed response is going to be very poor.

The reason a twin-carb set up is is used (not just for the F650) is to improve fuelling efficiency, combustion and low speed running. Unless you are going racing and fit Amal Smoothbore's, I don't see the point of fitting what is now an ancient carb.

There are some guys who do race-spec engines that might offer an alternative carb, at a cost..... At that cost, you could fully refurbish the original carbs, bolt them on and ride away.
 

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Look up bombardier ds650 carb for inspiration.
Did think about it myself , would do it to a project F650 if I get one.
 

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One 34mm would flow more than enough for the power a Funduro puts out, and would work way better at low revs.
As advised previously, the DS650 guys did a single carb conversion, mainly because heavier RVs needed more pulling power, not peak horsepower.
For a road engine an intake velocity around 400 ft/min works well, and twin 34mm on a F650 give nowhere near that.
On a smooth bore, that is. Constant Velocity carbs like the Miki are designed with the aim of maintaining, err, a constant velocity in the intake and are much less sensitive to over sizing, two 34mm smooth bores probably wouldn't run under 6000 revs.
FWIW the old British 500 cc thumpers ran perfectly well on one 1" / 25.4mm carb and even when tuned for racing needed no more than 1 1/8" or 28.6mm to put out around 35 real brake horse power,
 

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One 34mm would flow more than enough for the power a Funduro puts out, and would work way better at low revs.
As advised previously, the DS650 guys did a single carb conversion, mainly because heavier RVs needed more pulling power, not peak horsepower.
For a road engine an intake velocity around 400 ft/min works well, and twin 34mm on a F650 give nowhere near that.
On a smooth bore, that is. Constant Velocity carbs like the Miki are designed with the aim of maintaining, err, a constant velocity in the intake and are much less sensitive to over sizing, two 34mm smooth bores probably wouldn't run under 6000 revs.
FWIW the old British 500 cc thumpers ran perfectly well on one 1" / 25.4mm carb and even when tuned for racing needed no more than 1 1/8" or 28.6mm to put out around 35 real brake horse power,
i know this is an old post,but i have recently fitted a single carb to my funduro,bought a ds 650 manifold from US, and a manifold to carb adaptor [ebay] from china ,38mm vm carb [chinese knock off] and so far so good.Had to modify throttle cable and used pod type air filter,the carb is a bit small on the inlet side for the rubber adapter so had to make an aluminium ring to get a good fit,ive got a lathe so this was simple to do, bike seems far more responsive than with cv carbs.once ive done some mileage and a few plug chops,if all is well will buy a genuine mikuni vm carb and put cv,s in bin[or ebay]
 

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i know this is an old post,but i have recently fitted a single carb to my funduro,bought a ds 650 manifold from US, and a manifold to carb adaptor [ebay] from china ,38mm vm carb [chinese knock off] and so far so good.Had to modify throttle cable and used pod type air filter,the carb is a bit small on the inlet side for the rubber adapter so had to make an aluminium ring to get a good fit,ive got a lathe so this was simple to do, bike seems far more responsive than with cv carbs.once ive done some mileage and a few plug chops,if all is well will buy a genuine mikuni vm carb and put cv,s in bin[or ebay]
Sounds like a good mod, be interested in the results once put a few miles on it. Few photos would be interesting to see set up
 
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