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MG Midget and Sprite Technical - it hesitates above 70 mph

My standard frogeye (948 cc, 1 1/8 SU) hesitates above 70 mph while driving for more than 5 minutes lower than full throttle in warm weather. Full throttle cures the problem but I do not always dare to do so... I insulated the heat shield, inlet manifold. Fuel lines and installed an electrical fuel pump. It all helped a little bit but did not cure the problem. Under bonnet temperatures are 150 oF while standing in queue for the traffic lights and 140 oF while driving. Gasoline boiling point is in between 100 and 400 oF. What about a cool air box? Anybody experience with this?


Flip Brühl 948 frog 59

Hi Flip

It sounds like hesitation caused by lean-out - normally caused by over-heated float/jet/etc. giving rise to vapour locks. Giving it WOT fixes this by dragging more/cooler fuel through.

Can you let in more fresh air, e.g. I use 2 heater inlet hoses from front panel by rad to under bonnet to let cooler air into my RWA under-bonnet (one per side).

A
Anthony Cutler

Flip,

I wonder if insulating the inlet manifold is keeping heat in rather than keeping it out.

Charley
C R Huff

Anthony & Charley,

It is lean out, the spark plugs are white and so are the valves as I could see ( I parked the car on a gas station after it happened) It is not a lean needle as I controlled on a rolling road.

What is WOT fixes and RWA? I am not a native English reader. And yes insulating the inlet manifold helps to get it cooler. So I measured with a sensor. The hot spot outlet manifold is so close.

Flip



Flip Brühl 948 frog 59

Wide Open Throttle or Welly on T'carpet for those of us from God's Own County.

Round Wheel Arch aka '73.

A
Anthony Cutler

Flip, do you have spacers b'tween carb and manifold to insulate conductive heat?
Alex G Matla

I have heard of a chap who ran a new carb-to-carb fuel pipe around the front of the air cleaners, carefully keeping it level all the way. The standard position is pretty close to the heat.
Nick

Yes I have spacers and on the carb side of the heat shield. The fuel pipe is pretty close to the heat indeed but he front float chamber is 20 oF more hot than the rear one. (but all valves and spark plugs wher white when it happened
Flip
Flip Brühl 948 frog 59

I just mounted an other exhaust manifold, and believe it or not it hesitates less!

Flip
Flip Brühl 948 frog 59

Do you think the old one was somehow clothed/narrowed by dirt attaching on the inside of the pipe?

Could make sence that the modifications youve made demand a bigger bore pipe to improve the flow?
Arie de Best

This thread was discussed between 28/06/2009 and 06/07/2009

MG Midget and Sprite Technical index

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