MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 MGB Technical - Poor Running When Hot

Now that spring is in the air, the air temperature is in the mid-eighties and that seems to be too hot for my '67 GT. The engine is a later 18V with HS4 carburetors. It runs fine under all conditions except after starting when the engine is still hot from previous use. Under these conditions, it runs very rough at idle, as if only one or two cylinders are firing during each revolution. If I rev the engine, it smooths out somewhat. If I ignore the problem and simply drive the car, the problem goes away in a few minutes. But those few minutes can be anxious.

I thought the problem might be related to another one where fuel was spraying out of the rear carb overflow pipe, but I fixed that problem and still have the rough running when hot. I haven't done any troubleshooting, so open to all suggestions. Thanks. -G.
Glenn G

Poor starting when hot can often mean it's rich. With the temp being (for us!) high might it be fuel vapourisation. Plug colour will give you an idea for general running as a start. Are the heatshields doing their job? If it starts OK but runs badly, it might be the opposite-an air leak giving a lean mixture at tickover but a satisfactory mixture when you introduce more fuel to rev it! Though why this should only happen when hot would be strange.
Do you have access to a gas analyser?
Michael Beswick

At idle, the symptoms are identical to an overly lean mixture. I, too, am puzzled why this should only happen when hot. I'm beginning to think the fuel might be too hot and therefore vaporizing too quickly. The float bowl temperature measures about 135°F when the engine is hot. -G.
Glenn G

My B would start fine when cold but when hot could be a pig to start. I initially thought vaporizing because of the heat, however problem was cured when richened by a couple of flats.
S Longstaff

Have you got a decent heat shield between the exhaust and the carbs?
Mike Howlett

Just to add to this, I have exactly the same problem on our '76 GT. Would be interested in everyone's response as it is something I would like to rectify as well.

Regards

Martin.
Martin Clegg

Yes, my car has a good stock heat shield between the exhaust manifold and the carburetors. I've put some insulating wrap on the fuel hose for now -- we'll see if it helps. -G.
Glenn G

It sounds to me like you may have a sticking float that works fine, when cold, but may be binding when the carb reaches operating temperature. RAY
rjm RAY

This thread was discussed between 10/04/2011 and 15/04/2011

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now