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MG MGB Technical - idle speed drops aprox 200 rpm after long drive

My 1976 mgb roadster has a minor problem.

After a long drive, not always, the idle speed drops to aprox 500 rpm and becomes a bit unstable.

When cold its about 800 to 900 rpm and very regular

I done a check on ignition and carb setup all seems to be good.

Question; What is the function of the bimetal in the HIF4 carbs and is it possible that there is some wear on these?

Overall there is a good performance with this engine.

Any ideas about this?

Jacob
J Halma

The bimetal strip adjusts the mixture according to fuel temperature. It is not adjustable and does not wear. It bends to do it's adjusting. (A bi-metal strip is two unlike metals pinned together, each of which has a different coefficient of expansion. As they heat up one metal expands more than the other and causes the whole assembly to bend. If thats teaching Granny my apologies)

If is conceivably possible that this is at fault but it is at the back of the possibilities.

I would check for air leaks. Then it's back to first principles and balancing the carbs and mixture check.

You have HIFs-these have a poppet valve in the butterfly that can cause problems. The butterfly can be changed or the valves soldered up. They no longer really perform a useful function.

If it is also difficult to re-start when hot this would point to an over rich mixture.

Does a quick blip on the throttle clear it?
Michael Beswick

I am having the same problem with my 1970 with dual HS4 carbs. Idle speed drops when the under-bonnet temperature rises. I have set the idle a bit higher for cold operation 1,200 RPM, and settle with 700 or so rough idle when hot. I also use a booster electric fan that comes on when water temperature in the cold side of the radiator reaches 90°C. This helps clear heat from the engine compartment.
Glenn Mallory

Jacob, I had this problem on my 1976 Roadster and fixed it by changing the throttle spindles, butterflies and spindle seals. The seals were faulty and that was causing the varying idle speed. Part Nos 7,8,28 and 32 from the webpage below.

http://sucarb.co.uk/carbspec/carburettor/spares/id/1606/

Andy
Andy Robinson

Well it seems either time or temp related!
1. Pin holes or loose hose clamps in the water system, spraying a mist on to ignition parts.
2. Sticking SU piston
3. Poor or missing heat shielding under carbs
4. Flooding carbs
5. Air leaks on induction
Some of the more obvious ones!
Allan Reeling

thanks all for your comments

i already performed a check on mixture setup and balance of the carbs. this where OK.

Also (one yaer ago) renewed throttle spindles and seals. But put back the butterfly with the poppet valves.

Actions to be taken; replace the butterfly valves and go hunting for air leaks.

Thanks again

Jacob
J Halma

I have used just a dab of two part epoxy on those poppet valves.Use just a light coat to seal the little plate to the large butterfly disc.
Sandy
edith sanders

This thread was discussed between 27/05/2017 and 29/05/2017

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