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 - Testing Waxstats

Hi all,

I have an entertaining problem, which may be down to the waxstats.

1978 midget 1500, standard spec. Engine runs quite sweetly when cool and up to normal temperatures, problem hits when it gets hot - warm day, stuck in traffic(!). Idle speed rises on it's own to 1200-1500 rpm, and I get terrible spluttering and loss of power when trying to move off. All fine again once cool.

This suggests an air leak opening up at high temperatures, or a similar effect. So far I've replaced the manifold gasket (and had the manifold faces skimmed), replaced most of the studs and fixings (PO had 'improvised', not all studs were clamped!), reset both carbs, checked timing, points etc, and replaced coil and sparks. Vacuum and breather pipes all seem OK.

Knowing that waxstats can cause problems, before I go all-out and replace with standard jets, is there a practical test which would verify the jets as the cause?

I'm thinking of a mug of boiling water - just immersing the bottom of the jet - this should cause the wax to expand and replicate the problem at lower temperatures.

Any thoughts?

Thanks in advance, Sean
S W

""You re-set both carbs""....does this imply you have the SU HS4 duel carbs and not the stromberg 150?

I cant help you, Just trying to bring a little clarity to your situation


Prop
Prop

Do you have a heat-sheild between carb/float/fuel line and exhaust manifold?
Anthony Cutler

Prop: Yes, twin HS4's - should've said 'standard UK spec'...

Anthony: Yes, the heatshield is new(ish) - replaced last summer, as the old one had disintegrated.

Cheers,

Sean
S W

Sean,
This describes almost perfectly the problems we had when my wife bought her Midget back in 1981, she bought it in February and all was well, as the weather got warmer the problem got worse.
I tried all sorts to cure it, including putting plain throttle discs in, new float jets, damper springs plus manifold gaskets, and even a new fuel pump, all this did not cure the problem.
eventually, i discovered what caused the problem was fuel vapourisation, and it was caused in the main by the inlet pipe to the fuel pump crossing from the drivers side of the car to the passenger side of the car, across the top of the gearbox, as a metal pipe which was also clamped to the top of the gearbox with a metal P clip.I replaced the metal pipe with rubber and rerouted the pipe so it crossed from the driver's side at the rear of the gearbox opening and then followed the line of the brake pipe and then up to the fuel pump, the difference was amazing, it did not completely cure the problem but it was far less.
Flushed with this success, and the leftover rubber pipe, i replaced the metal pipe which went from the pump to the rear carb, and also rerouted it behind the battery, which was spaced from the bulkhead by a strip of wood.
This arrangement was as it left the factory as the car was only 3 years old when we bought it, I would be surprised if after this length of time yours is still like this, but it might give you a few ideas to try. let us know how you get on.

Alan
A Pritchett

Alan -

I took a similar approach, but I routed an all-rubber fuel hose from the pump to the carb (ZS) by running it between the heater box and battery. It's a bit early to tell if this will make a difference, but I had been having some issues with stumbling and cutting out during long runs at highway speed, and hopefully this will do the trick. By the way, another one of our local 1500 owners also did the rubber-hose-behind-the-battery bit.

-:G:-
Gryf Ketcherside

Interesting thoughts - I still have the metal pipe in the tunnel and crossing the gearbox, but have used rubber from the pump to the rear carb. And rubber again between the carbs. It all runs in front of the heater box though, so there could be something there.

I've certainly got a linkage problem though - it doesn't completly return to idle after driving unless I rev slightly - regardless of temperature.

Idle and mixture still isn't quite right, need to look at that next.

Cheers,

Sean
S W

This thread was discussed between 01/06/2009 and 04/06/2009

MG Midget and Sprite Technical index

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