Tony, you have hinted at what I mean by over fuelling in your reply, but I'll expand in case anyone needs more explanation.tony wrote:I'd agree with martin. The reed motors are nicer to drive too. I'm not sure what martin means by over fuelling tho..but he is right about the power delivery. What happens with a large inlet timed motor is bad carburation low down, poor port flow low down. You need some clever thought to get the thing to work well at low rpm with big inlet timing.
With a piston ported motor, the inlet can flow either way, unless the piston skirt has closed the inlet tract. Therefore, when port is open and the piston is ascending, it will suck in air and draw fuel from the carb. When the piston is descending, some of this mixture will be blown-out again until then port shuts. Any fuel-air mixture that is blown out as far as the carb can then pick-up more fuel on the way back in again as the port opens again and the engine starts to suck. The incoming air becomes saturated with fuel, making performance very sluggish.
To adddress this, carburettor manufacturer insert a small collar around the atomiser/needle. This is sometimes known as the primary choke. This weakens the effect of the incoming flow at low revs and reduces the over-fuelling. It isn't a perfect solution, but when the engine is running at higher revs, and especially when the expansion chamber is doing its job, the over-fuelling issue goes away.
Engines with valves, such as 4 strokes and reed valve motors, do not need the primary choke, which is why the carbs from motorcycles like the NSR125 or Aprilia 125's appear to be fitted with four stroke carbs.
