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MG MGA - Misfire When Hot

I expect this is an age-old problem, but for me it's recent - after a twelve year affair with my MGA.

After a run, and when I get into traffic close to home, the engine splutters when asked to rev above idle - drawing away from lights, for example. Sometimes the misfire is momentary, sometimes a second or two, occasionally still there through second gear.

It feels like a fuel problem - perhaps a series of varying airlocks - and since it does not falter at idle, perhaps it might be one carburettor that's being starved.

That's as far as I've thought. What do you think?
Nick

Nick

Have you had anything apart recently - carbs, ignition?

steve
Steve Gyles

Is it when you have stood a little say in traffic and the engine gets hotter than normal, this is something that I have recently come accross at over 200 on the gauge, im wondering if it could be fuel evaporation in the carb barrels at the higher temp. just a thought. Vin
Vin Rafter

No, I haven't touched the carbs or ignition for years, Steve. Perhaps I should!

Yes, it is after standing in traffic, but the engine temperature this morning, for example, was a nice cool 175 at the lights. The fuel lines were pretty hot, especially the one that links the carbs. It runs in a curve, with a high mid-point.

I fitted a 'mike the pipe' tubular manifold about 8 years ago, and the heatshield had to be cut and shaped over it, so I should expect some local higher temperature, but I'm wondering why now?
Nick

If the carb dampers are very low on oil this can give misfiring if you accelerate hard from stationary.
J H Cole

Fuel vaporisation is common when the car comes to a stop after a run. The reason is that the heat generated by the recent high revs is not being disapersed by wind through the engine compartment. It collects under the bonnet and affects fuel lines and float chambers. It may have started suddenly because of a change in the fuel formula. Our cars were designed to use the petrochemical eqivalent of vin ordinaire, not the modern stuff which is prone to vaporise. In modern cars with fuel injection the unused fuel is returned to the tank to re-cool, so the heat does not accumulate. In our cars it just sits there absorbing heat. I would try insulating everything you can, even if you do it temporarily to start with just to establish if that's the problem. The manifold can be wrapped in heat bandage or in a bag designed for the purpose (see performance equipment websites). The engine temperature may be normal on the gauge but it's not the coolant temperature causing the problem: it's the air temperature. When my Magnette had a similar problem on the hottest days of our summer I found that just opening the bonnet (hood) for a few minutes dispersed the heat and off we went again. Some owners have used imaginitive solutions like PC cabinet extractor fans that get the heat out via a wheel arch but I regard that as a bit extreme unless the problem is severe and nothing else works!
Malcolm Eades

Thanks Malcolm. And JH. Checked the dampers, and the oil levels are exemplary.

But I did a little work on the fuel delivery today. The link pipe between float bowls had its high point right below the bonnet, and above the top lip of the heatshield, so maximum heat flowing up beneath it.

I've turned the banjos, shortened the pipe a little, and run it flat and right up against the dashpots; so better protected from high temperature, and no high point for airlock.

Anyway, it's a theory. Tomorrow morning I'm driving from London to Leominster, so we'll see.
Nick

This thread was discussed between 02/10/2009 and 04/10/2009

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