Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Motorcycle carburettor insulators

Motorcycle carburettor insulators

Motorcycle carburettor insulators

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.


RE: Motorcycle carburettor insulators

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.

RE: Motorcycle carburettor insulators

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

RE: Motorcycle carburettor insulators

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.

RE: Motorcycle carburettor insulators

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.

RE: Motorcycle carburettor insulators

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.

RE: Motorcycle carburettor insulators

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close