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MG Midget and Sprite Technical - Fuel smells

A bit like Anamns exhaust smell, but I get fuel smells with the frogeye. If it were a wiring 'leak' one could go through a logical sequence with a meter or test bulb to eliminate and narrow down the fault. But I dont know of a similar logical way to chase down a smell!

There is a faint wiff of fuel with the car parked in the garage, but more alarmingly one can smell it on the road at 60mph. The carbs (hs2s) don't have an overflow pipe and don't have any damp showing around the float chamber lids. So if it is overfuelling that will be going into the engine.

It starts and runs smoothly, though needs a bit of choke from cold,even in these summer conditions.

It has an electric fuel pump, fitted at the back as per a later car. Leaving ignition on, engine not running, the pump does its usual several rapid clicks and then settles to one every 40 to 60 seconds or so.

I never leave it parked with the choke out.

What should I look for next?
GuyW

And the plugs. This afternoon a 50 mile journey, much of it at dual carriageway speeds, though slower, 30's, for the last mile or s
#4 is the longer plug.




GuyW

And whilst I have the plugs out, a compression test. 15 spins, WOT. :

DRY. WET.
1. 172 195
2. 152. 185
3. 162. 190
4. 165. 190

1098 engine.
So wet figures ok, suggests valves are good?
Dry figures a bit low, but evenly so compared to wet so some bore wear but nothing catastrophic.

Anything else to be gleaned?
GuyW

Had a fuel smell for ages on the elan only after it sat in the shed for a couple of days and couldn't find it, the carbs (Strombergs) were dry ,the fuel lines were dry , fuel pump ok and had more or less given up trying to find it. Until I had the back end jacked up high one day doing some rear hub bearings. Fuel smell. Had a look around and there was a little fuel puddle on the floor up the front--further looking found it was weeping out from the gasket on the glass fuel bowl on the pump. Made the mistake of pulling the bowl off while the rear was up, so back together till it was on the ground again.--Removed the bowl and found corrosion had bubbled up under the rubber sealing ring, cleaned him up and new ring and no more fuel smell. I'm guessing the ring was sealing itself off under pressure when running but would weep when the pressure relaxed.

Might be worth a try jacking the rear up or parking on a steep down hill and have a looksee like that.

As far as your comps, yeah no.2 is down a bit but if it's going ok just drive on. Might pay to check them again after a month or so just to make sure something nasty isn't starting to happen in there.
Plugs--yep colour is good but it also depends on the colour and texture/dampness around the earth base--an end on or 45' pic is better for that.
William Revit

Annoying Guy. There have been a few threads recently on fuel smells haven't there?. No obvious cause.

Modern petrol for one thing seems to be one cause. No drips fom my Sprite, but my garage stinks. Must be evaporation.

But like you I also get strong whiffs driving, even with the roof down. In my case HIF. I stop and look for a leak, nothing.

My boot smells, but no sign of any leak. Rubber neck bone dry. Not weeping at the ends. I've dropped the tank twice now to check the sender gasket. Bone dry. I've checked the pump and breather. Bone dry. No signs at all of seeping or dripping, yet I smell petrol.

I sometimes think it's when I brake, maybe fuel surges and overcomes the float valve.

My hs2s used to flood even after I replaced the float valves. I gave up and swapped to the hif, and now I still get the smell of petrol.

Maybe petrol just smells more than it used to. Maybe I'm just more aware of it, as per the comment Willy made re exhaust gasses.

Good argument for electrification. Lol



anamnesis

Fuel stink has happened to me twice. Both times it was a rusty tank top and boot floor.
Allan Reeling

"Good argument for electrification. Lol" 😊
Wonder if we're beginning to see the effects of ethanol in the fuel?
Does anyone still have access to ethanol free petrol? Long gone in this neck of the woods (NW England)?
Jeremy MkIII

No, long gone here too Jeremy. Esso 99 is now 5% too.

If electrifying my Sprite was quick to do, and above all cheap to do, I would. Or, if I was likely to still be driving in 10 years I would. None of those are true. So I suppose I'll just plod on battling against being fumed by petrol and exhaust gasses for the next x number of years. ☹️.

anamnesis

Could rubber fuel hoses be permeable to fuel vapour, but not liquid?

So you smell it even though there is no evidence of a leak?

anamnesis

Well---had completely forgotten---the elan used to stink in the boot. Back in around 1985 Australia started with new cars having to run unleaded fuel-there was a long phase out of leaded fuel which was completely banned in about 2002.
Back in 1985 the popular Ford Falcon here had a one piece plastic fuel tank with incorporated filler neck, BUT the station wagon version had a flat metal tank inside the cabin as part of the floor behind the rear seats with a filler neck out the side connected by a rubber hose probably 4-5" long and prob. 2 1/2' dia. Even new you'd jump in one and they'd stink of petrol. There was an urgent recall and all the filler neck hoses got replaced with a silicone lined hose --no more smell----I improvised on the elan at the time and found a plastic drink bottle same size as the rubber filler hose, cut the ends off and slid it over the filler hose and did the clamps up--it was ok-ish but if you got in the boot and sniffed around you could still smell fuel at the ends of the hose---Got a Falcon silicone hose eventually and that completely sorted it.
Long story, short, but yes to answer your question the fumes can definitely get through old original non unleaded hoses
William Revit

Thanks Willy. I'll start looking for a new rubber neck then, one that states good for ethanol petrol, -- assuming somebody supplies them.


https://youtu.be/DART7FA9Krs?si=Hqf91nmqFiNBxzw3

anamnesis

I have a vague memory something like this came up before and the problem was down to fuel standoff or fuel leaking out of the carb inlet and wetting the air filter hence the fuel smell when running. I've had a quick look in the archives and couldn't find anything. IIRC there were telltale signs of fuel staining on the air cleaner plate.
David Billington

I think I may have found the problem. The clues were there all along. Warmish weather. 50 mile trip out in the Frogeye.

Analysis: 50 mile journey in the frog with a VRFG * Spare fuel can stowed in the enclosed frog boot. Which gets quite warm on a sunny day. It seems that the plastic fuel can cap doesn't actually seal that well so the warmed petrol expands, vaporises and finds it's way out of the can The fumes filling the frog boot are then drawn forwards by air currents at speed to invade the olfactory apparatus of driver and passenger.

* (variable reliability fuel guage)

As a test, the fuel can is now in the garden shed. Which is smelling of petrol.

🤣🤣🤣
GuyW

Vortex Random Flow Generator Guy?

Interesting though. If my boot smell is coming from my rubber neck, akin to your plastic petrol can, under the right conditions perhaps that's why I get a whiff too.

anamnesis

Amanm, the correct Frogeye fuel tank has an integral long filler pipe (no rubber connector). The tanks are a bit smaller and more expensive than the later versions, like yours.So I used the later type - already had a spare top section. Upshot is my fuel tank and filler pipe on the frogeye is the same as yours, though it shouldn't be! That said, it is hard to see how that rubber section, tightly fitted and clamped up, would leak. Certainly not like my fuel can which if inverted, leaks. Not even just drips, but a thin steady stream of petrol!
GuyW

Guy,

I've got a later tank in my frogeye but as it was new I welded the filler tube to it like the one that came out.

I think I would also be looking at getting a new spare fuel can. I've been in my shed a few times in this heat and both the mower and its fuel are in there and not a whiff of petrol, the cap is tight on the fuel can and it doesn't leak. I have though smelt petrol when standing near some petrol cars so maybe they're venting a bit in the heat.
David Billington

Our cars vent through the cap.

I checked my locking cap and the vent is clear.

But when I open my boot, there's a distinct smell.

That's what I meant Guy. However the smell is being created in the boot, it's passage to my nose must be folowing a similar route to yours, albeit a different source.

anamnesis

Here's what a bot has to say about older rubber hoses and permeability when using ethanol infused fuel.

Permeation of Ethanol Through Older Rubber Hoses
Understanding Permeation
Older rubber hoses are indeed more permeable to ethanol in petrol. This permeability allows fuel vapours to escape, which can lead to unpleasant odours and potential leaks.

Reasons for Increased Permeability

◦ Material Degradation: Over time, rubber materials degrade when exposed to ethanol, making them less effective at containing fuel.
◦ Ethanol's Properties: Ethanol is acidic and hygroscopic, meaning it attracts water. This can further accelerate the deterioration of rubber hoses.

Comparison of Hose Specifications

Hose Type Permeability Rate Notes

R6 Up to 600 g/m²/day High permeability, not suitable for modern fuels.

R9 15 g/m²/day Much lower permeability, better for ethanol blends.

PTFE Virtually zero Ideal for modern fuels, including ethanol blends.

Conclusion
Using older rubber hoses in fuel systems can lead to significant issues due to their higher permeability to ethanol. Upgrading to hoses with better specifications, such as R9 or PTFE, is advisable for improved safety and performance.

Is 800g/m2 a lot?

Stating what we already know mostly. But until PTFE hoses become available for our cars, R9 seems the most suitable. I changed to R9 a couple of years ago, wonder what its life expectancy is.
Jeremy MkIII

Just a thought if you have an original SU pump they were fitted with 2 vent pipes. One from points, one from the diaphragm chamber - both discharge into the boot.
As I have noted before the original diaphragm does not like ethanol and hardens - could be fumes from the porous/ hardened diaphragm letting vapour through ?

I have had issues with my SU pumps and hard diaphragms so currently running a Hardi which has no vent pipes.

I understand the new SU parts are ethanol ok but so expensive for just a replacement diaphragm I decided it was more cost effective to replace with an alternative.
richard b

And can you get R9 filler neck rubber Jeremy?

Richard, yep I checked both of those. Only my diaphragm vents to the boot. So I took it out to check and the smell remains.

It has to be the neck rubber, but nothing is visible.

And here's a thing.

If the rubber is permeable to the ethanol, I wouldn't be smelling petrol. Ethanol smells like wine or beer or hand sanitizer; according AI on google search.

So it must be petrol vapours. But I thought they'd at least leave a stain.

I'll have to buy a new filler rubber and see if the smell 'evaporates' lol.


anamnesis

I am pretty sure my smell, or rather, the smell of petrol in my car, came from the spare fuel van. When I inverted the can with the cap screwed hard down, I got a thin dribble of fuel coming out! Not exactly fuel or fuel vapour tight!

When assembling this car (Frogeye) I used R9 pipes throughout. But that still leaves suspect components in there. But I too use a Hardie unvented pump, as Richard.
GuyW

Cover it in PTFE tape Anam?
Correct me if I'm wrong, I'm sure you will, but isn't the ethanol in suspension with the petrol so with the ethanol escaping, won't it take petrol fumes with it? Or is that too simplistic?
Jeremy MkIII

I would have thought those porosity figures would have given a pressure at which the results were attained.

I would expect to see cracks etc in the filler tube if it’s leaking/ porous.
Also unless you fill to the neck or park on a very steep gradient fuel would not linger in the hose.

I haven’t noticed it on mine which is original 1972 rubber !

Leaking tank tops/ senders more likely, the seal to the boot floor is only a grommet so could try additional mastic seal.
richard b

I replaced the cork type sender to tank gasket with a viton one, which does seem to seal much better and resists fuel.
Bill B

an--it's the vapour from unleaded fuel, not from the ethanol content. The problem started back mid 80's with the changeover from leaded to unleaded fuel so it's something in the unleaded fuel that gets through the rubber. Probably the extra benzine or toluene they poked in there to replace/reduce the lead content.

It's always worried/amused me that years ago when we used to mix our own race fuels that benzine was a real danger ingredient. We used to treat it like poison (wonder why) with goggles gloves and a mask when handling it treating it like the devil it is. Then when unleaded fuel came out it was the saviour--great stuff and I can remember adds on telly saying --from one brand of fuel in their ad.---"It's got toluol too."

Now it's just taken for granted that you drive around with a tank full of carcinogens and people wash parts in it with no protection etc
William Revit

Just to repeat. No petrol smell under car, or any fuel drips. The smell is only in the boot. I've dropped the tank several times to check. No tank leaks. No sender gasket leaks. Logically, it can only be the filler neck inside the boot.

Thanks Willy. You've convinced me.

Although, as you say, unleaded came in a long time ago, but it's only in the last 6 months to a year that I've had the boot smell problem.

However, I found this too on an Elan site.

"Because the fuel escapes entirely in gaseous form rather than pooling or wetting the material as a liquid, it frequently leaves behind zero discoloration, wetness, or staining."

So I suppose the only way to know is buy a new filler hose. I haven't so far, only becasue I can't see any evidence mine's leaking. But reading the above and what you say Willy, I'll get a new one.

And this looks like a good possibility.

"Samco ProFuel silicone 45 degree hose elbow. Samco's ProFuel range of silicone hoses have been designed to be used in the permanent transfer of petroleum and diesel fuels. Standard silicone is not suitable for use with fuel. With this in mind, Samco came up with their ProFuel range of silicone hosing which features a unique ProFuel lining which means it is 100% safe to be used with fuels and oils.

https://www.merlinmotorsport.co.uk/p/samco-profuel-45-degree-silicone-hose-elbow-pe45


anamnesis

This thread was discussed between 01/08/2026 and 04/08/2026

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