purpose of vent tube on carb manifold?

[hoghungry1]

Confuses me and may be part of my sputtering?

[Nige1100s]

It connects to passages in the head that join the inlet valve guides half way down their length. It should be connected to the small pipe connection on the original airfilter. When you shut the throttle the pressure in the inlet tract falls below atmospheric pressure and tries to suck air down the inlet guides bringin oil with it. This is meant to allow it to suck clean air from inside the filter. Its all explained in the manual.

When i first rebuilt my engine I connected this pipe to the breather on the engine valve cover , overpressurised the crankcase and popped the engine sprocket seal out.

If your not using the original filter I would connect to a pipe and cable tie a bit of foam over the end to stop the engine breathing muck in.

[Spokes]

Sputtering may be caused by an air leak at the insulator and gasket attaching the carb to the head. It may be just overtightened. Also, sometimes dirt in the fuel will cause the same trouble.

I concur about the port next to the carb on the insulator. I too was wrong about attaching the breather pipe to it. My next CA95 I will run a tube back to the air filter. If I use an aftermarket filter I will rig a way to attach it so it draws through the filter.

[Sam Green]

If the spluttering is caused by a weak mixture due to an air leak, check the mating surface of the carb. If the carb is over tightened, the mating surface can become bowed allowing air to get past the O ring in the joint.

Sam.