Scooterotica

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.

Tony, you have hinted at what I mean by over fuelling in your reply, but I'll expand in case anyone needs more explanation.

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.

I thought that was what you meant. Basically the blow back screws up the low end carburation. I thought maybe you had gone off somewhere martin!

However... does this bring us back to low primary conversation again?!!

this is what i cant quite get, aprilias can get upto 33bhp out of a 125 piston on 2strock - ok they are perhaps lighter and compared to the lambretta aerodynamics are going to surely put the lam back a bit; but what is the "voodoo" that gets these bikes upto this speed with just a 125cc?

druist wrote:this is what i cant quite get, aprilias can get upto 33bhp out of a 125 piston on 2strock - ok they are perhaps lighter and compared to the lambretta aerodynamics are going to surely put the lam back a bit; but what is the "voodoo" that gets these bikes upto this speed with just a 125cc?

Powervalve, huge transfers in casings and therefore the cylinder ports too, liquid cooling allowing an overall higher state of tune, flywheel weight, clever ignition, being designed to work as a complete package and not just individual upgrades cobbled together onto a 50 yo restrictive crankcase

Correct design is about it. For a start they either use a square stud layout or no barrel length studs at all. This way the areas can be had with modern angles. The exhaust port can be made correctly with proper subs. Also look at the volume in the crankcase. The ignition really is only the tip of the iceberg. Its not the be all end all.