Foam in airbox?

[Donut 77]

Evening all. The two little yellow triangles of foam were perished so I have ordered new ones, in case the density was critical. I didn’t want to just make my own. Could someone please explain their function though? I’ve seen them described as secondary air filters?

Secondly, as air enters the box (the big rubber square trunk bit with a dividing wall at the front) there was two strips of grey foam inside that have perished. I intend to make my own replacement for these, as I can’t find them anywhere online. They were glued in but I don’t want to buy a new complete air box, so I’m making a pair.

Anyone know what this foam is for? If it is to stop the air becoming turbulent on entry, why did they not just make a smooth rubber shape?. Maybe it’s noise reduction? I need to understand it before making the replacements?

Cheers

[tony.mon]

The triangular ones are filters for the air that flows in and out on the other side (top) of the carb slides piston. Any open cell foam will do. If you can breath through it it's fine. I use Motocross air filter foam and cut it in half to reduce the thickness. Then lightly oil it.

The foam in the snorkel is for intake noise control, it makes no difference to running or power whether the foam is there or not but don't remove the snorkel.

[Donut 77]

Thanks Tony.

Why do you oil it?

[tony.mon]

Because it helps small particles which would be too small to be caught mechanically, to be trapped on the sticky surfaces in the sponge on their way through. Use filter oil, not engine oil.