Morris Minor Owners Club

I cannot, as far as the UK goes, understand the obsession with fuel vaporisation (or 'fuel boiling' as mentioned only today) when dealing with a standard fit Morris Minor.

I know modern fuel is a far different blend to that of 50-70 years ago, but the simple fact is that these cars were exported by their 1000s to far hotter climates, in places like Australia, India, Caribbean, Ceylon (Sri Lanka) etc where they apparently served without problems and many of them are still operating on modern fuel blends.

Why do comparatively few vehicles seem to suffer with it?
How many people have actually fitted a Heat Shield?
Could fitting one simply be masking another problem?
If a Heat Shield were really required why was it not standard fit on exports to hot countries?
The one time I've thought my Minor might be experiencing a fuel starvation issue in mid 30s temps, it was actually a leaking pipe coupling to the pump allowing air to get sucked in. Probably caused by the diffential expansion rates of the aluminium pump housing and the brass pipe fitting.

Open for discussion.....

During Minors on Tour one owner of a sidevalve had terrible trouble with "fuel boiling". We thought the reason was obvious; lack of the plastic spacer between manifold and carburettor but fitting one reduced but did not eliminate the problem. His carburettor float bowl was too hot to touch and so was the rising metal fuel pipe. He went on to try various home made heat shields but I don't think the problem has been resolved as yet.

There is quite a lot of information on the subject on the FBHVC website.

Yes I agree it does seem odd when Minors were exported all over the world to places where our UK 35 C heatwave would seem relatively cool.

I have had it once and I immediately fitted a Grumpy heat shield kit and it has never happened again. I think the reason it happened was the tank being full of winter blend petrol and an unseasonably hot spring day stuck in traffic.

Stephen

Never had it happen, only thing done was shortly after purchase the fitted metal braided pipe from pump to carb was removed and quality rubber fuel line routed along inner wing behind strengthening plate then over to carburettor.
I assume the plastic spacer reference is visible in the attached picture?

Never been an issue for me. However, in very hot weather, the pump tends to work more rapidly at times, which I assume is related to vapourisation, even though it never effects performance.

Bill_qaz wrote: ↑Sun Aug 09, 2026 6:16 pm Never had it happen, only thing done was shortly after purchase the fitted metal braided pipe from pump to carb was removed and quality rubber fuel line routed along inner wing behind strengthening plate then over to carburettor.
I assume the plastic spacer reference is visible in the attached picture?
20250830_120229.jpg

Yes you have the plastic spacer there. It's between the manifold and the throttle cable guide bracket. PS: it looks like the top of your float bowl is wet. Petrol?

Stephen

90 mile round trip today in high 20s to 30 temps with several climbs up Aston hill area with no extra pump ticks noticed or lack of power. Top gear all the way up.
Got to be some common factor for those cars suffering this issue.

Stephen no not leaking had pipe off whilst doing some work but thanks for asking.

Bill_qaz wrote: ↑Sun Aug 09, 2026 6:46 pm 90 mile round trip today in high 20s to 30 temps with several climbs up Aston hill area with no extra pump ticks noticed or lack of power. Top gear all the way up.
Got to be some common factor for those cars suffering this issue.

It doesn't usually happen when you're on the move at a good speed even if the engine is working hard. It's when the air flow is poor and the weather hot such as when stuck in crawling holiday traffic.

Yes there must be a common issue. With the side valve example, the exhaust was rather odd and it went through the bulkhead rather high I seem to remember. Not all exhausts go through that bulkhead aperture at the same level. Maybe just too close to pipes and carb.

Stephen

Left over winter blend is by far the major contributor for vaporisation. All the conversations tend to be based on temperature but that is only half of the physics. Temperature and pressure are part of the equation that affect fuel vaporisation. Yes, as the temp rises the fuel starts to boil and when the pressure drops it will also boil. From the fuel pump to the fuel bowl the fuel is at higher pressure than ambient. The fuel bowl and carb are at ambient. The fuel pump should easily replace whatever small amount boils. It's very likely the pump ticks a bit more in warmer weather. All the fuel from the pump to the carb is at or above ambient pressure. However before the pump the fuel is below ambient. The tank is at ambient pressure if the cap vent is open. As the pump pulls the fuel through the long line, the fuel pressure will start to drop, especially if the fuel level is low. The worst area is the vertical rise to the pump. As the pump tries to pull the fuel up, the fuel pressure drops significantly and the pump starting ticking rapidly as if it there is an air leak. The fuel looks like a carbonated liquid where bubble just appear. I've seen it happen. If one is experiencing starvation, the vertical run to the pump if most likely the area it occurs. The cause could be any restriction in the fuel line.

This is a good discussion. I think we are narrowing this down. So far we have the culprits as:

1) Winter blend fuel in the tank used in hot weather. Winter fuel has a greater proportion of volatile petrol fractions and vaporises much more readily so that it can perform well in cold conditions.
2) Restrictions in the pipe from the tank. Dents, kinks, debris, partially blocked filler cap breather etc.
3) Different manifolds/exhausts that increase heat transfer to the metal fuel pipe from the tank.
4) Restrictions to the air flow in the engine bay
5) Low atmospheric pressure e.g up a mountain

Stephen

Google answered so question withdrawn

Winter blend fuels? Who is running a diesel minor?

XWL61 wrote: ↑Mon Aug 10, 2026 7:14 pm Winter blend fuels? Who is running a diesel minor?

See:

https://www.fbhvc.co.uk/fuels#vapourisation

There is a significant difference between petrol sold in summer and winter

Stephen

XWL61 wrote: ↑Mon Aug 10, 2026 7:14 pm Winter blend fuels? Who is running a diesel minor?

Thats what I thought but giggle says otherwise

Bill_qaz wrote: ↑Mon Aug 10, 2026 7:19 pm

XWL61 wrote: ↑Mon Aug 10, 2026 7:14 pm Winter blend fuels? Who is running a diesel minor?

Thats what I thought but giggle says otherwise

As does the Federation of British Historic Vehicle Clubs


Stephen

svenedin wrote: ↑Mon Aug 10, 2026 7:19 pm

XWL61 wrote: ↑Mon Aug 10, 2026 7:14 pm Winter blend fuels? Who is running a diesel minor?

See:

https://www.fbhvc.co.uk/fuels#vapourisation

There is a significant difference between petrol sold in summer and winter

Stephen

Well, you learn something new every day!

Thank you for the link Stephen!

Same here.
I mistakenly thought only Diesel fuel was summer and winter with additives to prevent "waxing" in colder temperatures.

It's good though because it explains a lot. It explains why I experienced fuel vaporisation on an unseasonably hot spring day whilst stuck in traffic with a tank of fuel that had been purchase in the winter. It also explains why we hear about lots of problems after owners "hibernate" their cars in winter. It is good practice to top up a fuel tank before storage so many will have been filled in the late autumn or early winter when the cars were put to bed.

Stephen

Perhaps the fuel vaporisation and the shoddy ignition coils are a global conspiracy to suck the joy from our lives and push us into electric cars or public transportation.