MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Fuel Vaporisation Query? Where and Why?

This might be a one-off that will never happen again, but, with global warming, perhaps it will recur.

1310cc Mk IV Sprite with Facet fuel pump at the rear, Filter King filter/pressure regulator and Weber 40DCOE carb.

It was 32 degrees in the shade at Loton Park Hill Climb on Saturday but a good deal more for the Sprite parked in direct sun on the tarmac paddock, albeit with a space blanket over the cockpit.

For the competition runs in the afternoon the car ran as well as can be expected, it's always a bit slower on hot days. The temp gauge almost reached N and the cooling fan cut in - I have it set at minimum. I had a short wait in the Top Paddock before returning down the hill, mostly coasting with the engine off. I switched on again as I reached the main paddock and on the on-board video you can hear the pump cutting in clattering away, clearly not pumping fuel. There was enough fuel in the line for maneouvering back into my paddock space, then nothing. The pump was clearly air-locked. Pressurising the tank to push fuel over the axle cured it.

The tank was >1/4 full but I put another 5l in. The same thing happened on the next run.

Yesterday was cooler and more humid and I had no problems at all.

I'm confused as to why I should get an airlock in the pump by the back axle - could it get hot enough back there to vaporise the fuel? I wouldn't have been at all surprised if it had occurred somewhere under the bonnet.

Colin
C Mee

Doesn't all the hot air from the engine bay exit via the trans tunnel ?


And where do you run the fuel line - down the trans tunnel or under it ?
Malc Gilliver

That's a pretty obvious one that hadn't crossed my small mind!

The pipe runs along the bottom lip of the tunnel after going over the top of the axle before the pump. I'll try insulating it.

Thanks Mark.
C Mee

Colin,

Worth a read ?

https://www.mambamotorsport.co.uk/l/stay-cool-more-power-and-longer-engine-life2/

Malc
Malc Gilliver

Malcolm (apologies for the Mark last time)

Thank you for that link, it is really helpful. I probably knew most of it already - I just hadn't put the pieces together and thought it through sufficiently.

I've started researching insulation for the pipework and thinking of ways to improve the air flow through the engine bay and transmission tunnel.
C Mee

Colin,

No worries, Malc is often autocorronged to Mark.

If racing my Sprite was my thing, I'd have either a saddle tank for fuel and a dam great big duct coming out of the trans tunnel - or vent it through the 'back seat' area or boot.

I have seen a lot of racing Spridgets with vents in the wings or if running a GRP bonnet, I'd be tempted to vent that.

Have you got a copy of Daniel Stapleton's book ?
Malc Gilliver

Malcolm I have a copy of Daniel Stapleton's book and dip into it from time to time but it is aimed more at the all-out racer. I should probably have looked to see what he says about the issue before posting.

My track activities are limited to a dozen or so club-level hill climbs each year as well as MASC and other social meetings and leisure runs. I am sticking to Interclub Class 1A Roadgoing Production regulations, so cutting vents and ducts are out of the question even though I know the car would probably perform much better with them.
C Mee

Nice, I plan to use my car to bimble around Warwickshire countryside with the gf, but have uprated it to not feel like a target on the motorway, trying so very hard to not turn it into a racer as I know after spending a fortune it will still be slow !

So you need to focus on getting heat away from your fuel lines, your fuel pump and shielding your fuel tank.
Malc Gilliver

Colin, will the rules allow a high pressure fuel pump and a fuel pressure regulator with a fuel return pipe. This is what modern cars do, keeping the fuel circulating via the tank which acts as a heat soak.
Rob
MG Moneypit

Thanks Rob.

That will be worth considering if I have a recurrence.

This was the first time it has happened for me and it's also the first time the Sprite's been in action with an air temp of 32 degrees so I'll try the insulation route first.
C Mee

I used to work in the car industry on the emissions / calibration side. We stopped using fuel recirculation as it tends to add heat back to the fuel tank - so if you go down this route you need to make sure the hot fuel re-enters the tank away from the fuel supply to the engine.

I'd go for a stand off heatshield on the tank to stop bulk heat hitting it and also shield / duct heat away from the fuel lines and pump
Malc Gilliver

This thread was discussed between 14/07/2025 and 15/07/2025

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.