Craig,
You wrote, "you will need to rejet the carbys or they will run rich as you are doubling the fuel intake which will be too rich at idle/low revs and foul plugs"
This is a common misconception, but in fact isn't true.
This got me thinking. Bear with me... I've been trying to dissolve some of this fluid mechanics stuff in ethanol for the last 30 years.
There is some complexity here when changing from singles to twins (or triples, or fourtles
). The inlet manifold flow path is very different, and gas density changes in venturis can screw with Bernouli's ideal world venturi model. Ignore that for now, as grey motors are not that precise.
Agree that the main jetting should be very, very close to needing no change when going from singles to twins or triples (or fourtles). The trick here is that the venturi pressure drop is dependant on the square of the gas flow through it. That same pressure differential is what the main metering jet sees. The liquid flow through the main metering jet is dependant on the square root of the flow through it. Simplifying the mefs, the square and the square root cancel each other out. This means that no matter how many carbs you add, as long as they see their equal flow of gas, they will meter the same amout of fuel.
The power bypass valve system is driven by the same pressure differential, so in theory no need to change the power valve either.
But...
The above applies to the main metering system, which is driven by the venturi. The idle system is not... it is driven by the inlet manifold pressure (this is because the main metering jet discharges into the venturi, whilst the idle system discharges into the throttle body. Simplifying a bit, the inlet manifold pressure does not change much with singles, twins, triples or fourtles. Adding more carbs, they all see the inlet manifold pressure, and they all deliver (roughly) the same idle fuel flow as a single. This means that when going to twins, you need to wind the idle system right down. There is an overlap between the idle system and main metering system, so getting the idle right not only effects the idle quality but also the main metering quality. If you are lucky, there is enough finesse in the idle mixture screws to rest the idle after going to twins, triples or fourtles. If you are unlucky the idle mixture screw is not so fine when it is close to it's seat. In that case a change to the idle tube is required.
The accelerator pumps are independent (roughly) of vacuum, but with twins, triples or fourtles you now have twice, double or four times the fuel shot each time the loud pedal is depressed. The pump circuit thus needs a tweak to deliver less fuel, though for a relatively rough grey motor the average owner won't notice a difference.
Cheers,
Harv
