Single Carbs for Viragos

When there are two intake events almost completely out of phase of each other in the same tract, there is always mixture on the move. One carb per cylinder experiences constant starts and stops in each tract. The speed of the mixture is like the top half of a sine wave: starting from nothing, picking up speed as the piston drops, slowing and abruptly stopping.

In the early '70s, I read up and found that the fastest production motorcycle then available was the Norton Combat Commando. So I bought one. Mine was the "Interstate" version, with a fiberglass seven gallon tank, glass fenders, side covers, and seat base. It weighed about 350lbs with a tank of gas. About 220lbs of that was engine/transmission. I won't describe it much except to say that I have never ridden a bike since that felt faster. And I've ridden many forced aspirated sports bikes.

Anyway, I managed to blow it up (Brit bikes had many weak points). During the rebuild, the dealer talked me into running a single Mikuni (36mm?) in lieu of the leaky dual Amals. The bike became a torque monster. Getting the hole shot put me hundreds of yards ahead of whatever I came against. The only thing missing was top-end urge. It still had the triple S cam, but it ran out of breath over redline (7000RPM -- these bikes had an 89mm stroke).

Now consider: two pistons moving together, on different "strokes." There was always vacuum and it was always in (essentially) the same direction. The heads were right next to each other and oriented in the same manner. There was no compromising on jetting. Because the vacuum was almost constant (the runners were long enough to preclude reversion) a smaller carb would do the job. I see dual Mikuni kits for similar bikes with venturis as large as 40mm!

Now what if those intake runners pointed in different directions? What if the intake charges over-lapped, and had a dead period mixed in? What if the exhaust header lengths were different? What if one head was inherently hotter than the other? What if the single carb was a constant velocity type? Using vacuum off which cylinder?

You would have an engine that had problems breathing at all RPM. Idle mixture would be a compromise, every setting would be a compromise. Every time the mixture changed direction, fuel would drop out of it and form puddles and streams on the bottom of the ports. Un-atomized fuel would pour onto the hot back face of the intake valve, some would evaporate, some would pour into the cylinder.

I've seen several different brands of one-into-two manifolds, and they are all very small in cross section. Good for torque. Good off the line. Not so good for high engine speeds. Say good bye to venturing past 5000RPM. Many people are fine with this. To these people, I would recommend swapping out to 700 Virago heads -- unless that's where they were starting. These have a small combustion chamber for a high compression ratio: excellent for torque. They have small valves; also a torque-boost. I would encourage them to run stock 1000/1100 cams as they are biased toward the low end, or possibly XVS1100 cams, but these would provide little or no gains and they are quite expensive.

More to come . . .

Topics