Motorcycle carburettor insulators

[dmapguru]

I have an old motorcycle where some new parts can longer be procured from the OEM. I am considering casting/molding some carburettor insulators. These form a sealed connection between each carburettor and the cylinder head. The OEM items are made from some flexible polymer and look like the attached image:

They will have to be able to resist temperatures ~90C with excursions to 130C and be stiff enough to resist pumping loads but be flexible enough to deform to get them onto the head/carb stubs.

I am a mechanical engineer and polymers are not my usual choice of materials. I was wondering if you could advise me on an appropriate material, and if I am even attempting something doable with a cold casting approach, like an RTV silicon.

Thanks

 

[TugboatEng]

I would guess that Neoprene is the original material.

Silicone could work but it would require reinforcement. You'll want to use Aramid fiber at those temps. Silicone is very permeable and will sweat oil through so they may get dirty over time, especially if you are breathing your crankcase fumes.

Two other more modern materials that will work very well as solid rubber and not require reinforcement are HNBR and Aflas.

 

[dmapguru]

Thank you for this information. I am (vaguely) familiar with HNBR, but is Atlas a brand name or...?

 

[Pud]

I'd guess the original would have been natural rubber and also suggest a two part polyurethane to cast.



Politicians like to panic, they need activity. It is their substitute for achievement.

 

[TugboatEng]

Aflas, autocorrect got to that one.

For polyurethane, I know off-road motorcycles use a lot of it for vent tubing. It's listed as having good resistance to gasoline. But, in my experience it hardens quickly which is not acceptable for carburetor boots.

Obviously there are different types of polyurethane with wide ranging performance so I can't rule it out but only describe my experience.

 

[Pud]

Well, I used to make vanes for chemical pumps out it. Pumps were for the French army for jet fuel filling.
It was an injection molding variety though.


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[Pud]

Forgot to add: it was a polyester based PU rather than a polyether type.


Politicians like to panic, they need activity. It is their substitute for achievement.