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MG Midget and Sprite Technical - Smells of fuel

Hi can anyone help with this please, as in the title the car stinks of fuel when running it’s quite unbearable especially for anything more than 20 mins drive . There are absolutely no fuel leaks and the smell dissipates when parked up . I have leaned off the carbs , see the photo do those plugs look ok1-4 right to left ? The exhaust is straight through with an RC 40 with no apparent leaks it has a 731 cam and that’s about it engine wise . It’s a shame because it runs well but it puts me off taking it for a drive .



Mike Fairclough

Mike, I know you say there are no fuel leaks, but if it smells that strongly there must be fuel vapour which really does suggest a fuel leak somewhere. From your vehicle profile I assume it has an electric pump. If you turn the ignition on but dont start the engine does the pump tick, maybe up to 10 time and then stop completely? Or does it continue to tick at all?

Another possibility is the fuel gauge sender unit cork gasket. If that leaks then fuel can creep out ( capillary action) into the well on top of the tank, and evaporate from there. You wouldn't then see any evidence of a leak. Likely to be worse with a full or nearly full tank with fuel sloshing around inside when travelling.
GuyW

Hi Guy , yes it has an SU pump located in the boot that ticks 4 or 5 times then stops . The fuel tank is in the boot and I can see all the rubber hoses and connections clearly. It has a copper line running through the cockpit to the engine bay . It lives in a garage next to the house and doesn’t smell at all when laid up. I feel it’s definitely a running based issue. The exhaust pipe is quite sooty but no visible smoke but the plugs don’t look rich . It’s got me stumped
Mike Fairclough

I hadn't realised it was that modified. I guess as a track car? I wonder if there is a way of isolating whether it is an engine running/ fueling problem, or caused when fuel is physically sloshing around because of the car movement.

Does it backfire at all on over-run?
GuyW

Have you observed the carbs when it's running, in case there is any overflow from the float chambers?
Dave O'Neill 2

This is a coincidence.

Maybe I have the answer.

I've been searching for the source of a petrol smell too. No visible leaks, but it stinks. I previously fixed the fuel sender gasket, and assumed it was leaking again. I dropped the tank yesterday, and it's still fine.

Then I took a sniff of the fuel pump vent.

On the pump body is a pressure vent. It shouldn't smell of petrol. Normally the pump is under the boot, and a pipe from that vent emerges in the boot. My boot stinks.

The pump works fine, but I suspect the diaphragm, item #1 in the drawing, has begun to break down, allowing a very small amount of petrol to leak into the pump body, and the vent pipe carries the smell to the boot.

On yours, with the pump in to boot, you'll smell it too if the diaphragm is breaking down.






anamnesis

I wonder if ethanol has caused my diaphargm to begin to leak.
anamnesis

Mike, your plugs look good but, are you saying you can smell petrol or are you saying exhaust fumes----------

William Revit

Thanks for all the replies , yes it used for sprints Guy hence the 2 gallon tank in the boot . There is no sign of leaking or dampness around the carbs Dave I’ve been all round them. I will have a look at the pump Anam , it’s relatively new but an electronic SU ( no points ) but I assume it still has a diaphragm but not sure if it has a breather. As for if it is the exhaust or petrol Will I was coming to the conclusion of un burnt fuel in the absence of leaks.
Mike Fairclough

"Smells like teen spirit". Anam should be able to come up with a youtube link.
Rob
MG Moneypit

Lol Rob.

Oh for a place of perfect peace and happiness. Where a Spridget doesn't sometimes smell of petrol, leak a bit of oil, and rust from time to time.

Sounds like Nirvana. 😉.

But smells sweeter. 🤣

https://youtu.be/Xqxtt-wJ18Y?feature=shared

Better than the smell of petrol I guess.

Hello, hello, hello, how low
Hello, hello, hello, how low
Hello, hello, hello, how low
Hello, hello, hello

https://youtu.be/hTWKbfoikeg?feature=shared

A band has to get inspiration somewhere. Lol.



anamnesis

There is a distinct difference between fuel smell and exhaust smell--- IF it's a raw petrol smell, with this 2 gallon tank in the boot, where is the breather for that--
If it's an exhaust smell, with a hardtop like yours, it creates a vacuum behind the rear window and if your exhaust is out the rear then maybe the bootlid seal is leaking-?
William Revit

Do other enthusiasts agree that it smells excessively? Now that modern engines run so cleanly its easy to forget what old ones were like - I was passed by an AC Cobra kit car while out on my pushbike yesterday, lovely to look at but left a real stink of burnt and unburnt hydrocarbons in its wake.

Drawing exhaust fumes into cabin would certainly make you feel unwell, maybe take a household CO detector out with you next trip?
AdrianR

Thanks Will the tank does have a breather pipe that comes out of the boot floor near the rear spring hangar , not sure if the smell would be coming from that . It doesn’t as I say smell in an enclosed garage perhaps if it’s sloshed around ? Adrian , the CO detector is a good idea I might give that a try but I certainly think your right we forget about how much old cars used to smell and you certainly know when your following a classic . One other thought is I use Castrol Valvemaster plus and e5 premium fuel is that particularly stinky ?
Mike Fairclough

This thread was discussed between 29/03/2025 and 31/03/2025

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