I've had the bike carbs for bit, Keihin FCR flat slides 39mm. On a stock or stockish motor you're not gain to see a great gain, it may even go slower. Before I installed a more aggressive camshaft , the motor was borderline over carbed under camed. With the GX cam it did, as you'll see in the link Dave posted, make 73 RWHP. The addition of the carbs was worth at least 10HP, maybe more. At the track I run the speed at the end of the longest straight was almost 5 mph more. I've yet to dyno the car with the new cam, but it's likely up to about 80RWHP as I've picked up a couple hundred more RPM.

The reason I used bike carbs is due to left hand drive cars requiring moving the brake master cylinder in order to fit webers. I also got the whole set up for less money then the cost of a weber DCOE manifold.

Dave clarify the "don't work well" part of bike carbs.

For me they work great but I only use it on the race track. Also the bike carbs require a little more work as they are less common so not as much knowledge about them floating around. Basically it will require a custom manifold

Tom