My sons speedfight 100 has been laid up for two or three years. I can get it to run if I squirt fuel into the air intake but it dies of starvation after a few yards. On Saturday it actually decided to carry on and ran for 3km before I stopped it, behaving totally normal but next day it was back to the squirt and die routine. How does the system work? What is the thing just under the tank that looks as though it has a diaphragm in it? It has two pipes to the carb, one above the diaphragm and one below it. Is this the source of the problem?

David Burton, January 2007

if its been stood for a while then its advisable to take the carburettor apart and clean it out and all the jets as they only have a small hole in each of them especially the pilot jet.the fuel system works on a vaccum as the fuel tap has a diaphragm in it that only allows the fuel to flow from the fuel tank to the carb when a vaccum is created by the sucking of the engine, that then pulls open a valve attached to the end of the diaphragm and opens it and the fuel flows freely but when the engine is stopped the fuel flow stops,the amount of fuel left in the carb is enough to start the engine again and the valve opens again.sometimes the vaccum fuel taps do give problems and do not open correctly which may even be the problem.it is also worth checking the vaccum pipe that goes from the inlet manifold to the fuel tap as if it has a break or split in it it will not create the vaccum needed to open the fuel tap up.ok sounds a bit baffling maybe but its really very straight forward.ok david good luck with it p.s you can buy carb cleaner from a good car spares shop to help you clean it out if you like.

adam bike doctor, January 2007

P.S. from David Burton. Tonight I tried trickling fuel into the carb whenever it began to die, so I kept it running till warmed up. It then carried on unaided and ran for 8km including a stop and restart. We'll see what it does in the morning!

David Burton, January 2007