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E10 petrol - Part 2: rubber seals and hoses

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RichCutler View Drop Down
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Post Options Post Options   Thanks (2) Thanks(2)   Quote RichCutler Quote  Post ReplyReply Direct Link To This Post Topic: E10 petrol - Part 2: rubber seals and hoses
    Posted: 04 Mar 2022 at 16:01
How to make your old Guzzi play well with E10 petrol...

The following mentions affected parts of big block Tontis, but I'm sure it will also be useful to owners of other Guzzis.

The significant ethanol content in E10 petrol will degrade original-equipment nitrile rubber components that come into contact with it – if the rubber is unsuited for alcohol. That's likely to be cheap fuel lines, and the rubber bits in older models of Dellorto carbs: O-rings, choke plunger seals, carb float needle tip. The petrol cap and interior tap seals are also nitrile rubber. If in contact with E10 petrol, these will swell, then harden and crack over several months to years.

However, these parts can be replaced with alternatives made of materials that are compatible with E10 petrol: nitrile rubber made to the German DIN 73379 standard for fuel hoses, and Viton for O-rings and other seals.

Viton is fluoropolymer, and as such it is very stable (e.g. to heat, ultraviolet light and chemical attack). Unlike nitrile rubber, it can last decades. It significantly outperforms nitrile rubber in ageing tests, retaining its pliability and shape. However, it's a lot more expensive than nitrile rubber (10–15 times), which is why it wasn't used much in vehicles until alcohol fuels appeared. So, using Viton seals not only provides suitability for E10 petrol but also means they'll likely never need replacing!

Fuel hose
The first thing to replace is the fuel hose: no one wants petrol dumped on a hot, running engine!

A lot of fuel hoses are cheap tat made of low-quality rubber from Asia that will swell and split with E10. High-quality nitrile hoses made to DIN 73379 type 2A (no overbraid) or type B (heat- and abrasion-resistant overbraid) are suitable for E10. 

Thinner-walled polyurethane fuel hose suitable for E10 is also available but seems harder to find.

The hose inner diameter is 6 mm.

VHB C carb
After a bit of experimenting, I found suitably sized Viton O-rings as alternatives to Dellorto's nitrile originals. The size of these such as the choke ring O-ring don't seem to be available (perhaps they're custom sizes), so the below may be one or two tenths of a millimetre different. However, all have been checked for fit, and work fine - any size difference is of no practical consequence.

O-ring sizes (internal ID × cross-section):
• Float bowl: 70 × 2 mm
• Choke lever plinger assembly: 8 × 1 mm
• Choke jet: 3.17 × 178 mm
• Mixture screw*: 2.5 × 1.50 mm

*Not standard on the T3 but I prefer to add this O-ring.

Viton O-rings are available from https://simplybearings.co.uk.

Viton seals for the choke plunger are available from eBay UK, or can punched out from Viton sheet using a leather/fabric punch (both also available from eBay). I bought mine from eBay: they are 2.5 mm thick and 6.5 mm in diameter. Leather/fabric punches are availble online. To fit new seals:

• lever out the old pad with a small screwdriver
• note the lip around the edge of the plunger that retains the seal
• insert one half of the new seal under this lip, so the seal is
  sticking out at an angle
• carefully push the seal under the lip.

The orange-tipped float valve needle available directly from Dellorto uses Viton. This replaced the wear-prone (nitrile?) black-tipped needle many years ago. 

Petrol cap and interior tap seals
The petrol cap seal for the early T3 with the flip-up non-locking chrome cap is held on by a central screw and is easily replaced. Buy a small sheet of 2 mm thick Viton sheet, cut a 50 mm circle (use a circle cutter) and make a 2 mm central hole surrounded by four equidistant 1 mm holes spaces 22 mm apart (use leather/fabric punches).



The remaining item to replace is the interior seal for the square petrol taps. Unfortunately, a Viton alternative isn't available, and as it's not flat but has raised rings around the outlets it can't be made from Viton sheet. My solution was to wipe this seal with silcone grease as used for O-rings to act as a barrier: this is alcohol and petrol resistant, and won't affect the rubber. It has had the added benefit of making the fuel tap nice and smooth to turn!


Edited by RichCutler - 05 Mar 2022 at 11:50
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Dave P. View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dave P. Quote  Post ReplyReply Direct Link To This Post Posted: 04 Mar 2022 at 17:12
Interesting and informative, thanks for posting.
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Mike H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Mike H Quote  Post ReplyReply Direct Link To This Post Posted: 04 Mar 2022 at 18:17
Yes thanks for posting these! Thumbs Up


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David Nimrod View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote David Nimrod Quote  Post ReplyReply Direct Link To This Post Posted: 04 Mar 2022 at 19:19
Excellent Posts... thank you!

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Ken-Guzzibear View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken-Guzzibear Quote  Post ReplyReply Direct Link To This Post Posted: 05 Mar 2022 at 09:56
Thank you really useful
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