403 Carburetter flange gaskets

[mikebro]

I've just rebuilt the carburetter using genuine parts. The Service manual tells me that the flamge gaskets are special harder material than standard and should not be replaced with ordinary ones. The rebuild kits have ordinary papare gaskets for the flanges. Originals are just over 1mm thick and of a hard black material. The new ones are paper and about 0.5mm thick.Anyone lbow why the Ser ice manual is so adamant about using these "special" gaskets?

On another subject, I was interested to find that the number 3 carburetter had a petrol inlet port that had not been drilled out like the others had. Since the port was still considerably larger than the orifice in the float valve, it probably didn't affect operation, but it was interesting to see what to me was a 67 year old manufacturing fault.

[Clive Foster]

Back in the day it was normal practice, especially on better quality machinery, to use heat insulating spacer - gaskets on carburettor flanges. I imagine that that is what the original thick ones are. Hafta say that, as I recall matters, they were normally rather thicker in most applications.

Allegedly there were also potential issues with petrol vapour dissolving into and weakening ordinary paper gaskets. Can't say I've ever seen any evidence for this but I do recall obtaining a New Old Stock aftermarket gasket set for "something" in the middle 1970's that proudly proclaimed petrol proof carburettor flange gaskets. As I recall it they looked just like the ordinary paper ones to me, which is probably why the memory has stuck. I imagine that set was actually mid 1950s - mid 1960s vintage.

On the motorcycle side Amal always supplied thicker flange gaskets for the Monoblock and Concentric MK 1 carburettors and, again, specifically warned against using ordinary paper ones. Even though official bike makers factory full service gasket kits frequently came with ordinary looking paper ones.

Clive