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MG Midget and Sprite Technical - Runs rough and tick over speed varies

I have a problem with my 1974 1275 midget.

The tickover varies in speed depending on the engine temperature and is lumpy.

If I warm the engine up and drive it, it runs reasonably, with a tick over of about 900 to 1000rpm. However, it is not as smooth as it used to be. If I take it on a reasonably fast run for 5 or 10 miles or more, the tickover drops to about 500/600 rpm and is extremely rough. In this state it is pretty unpleasant to drive.

It have a full service about 1500 mile ago, had a compression test and rolling road tune. It ran well then. The engine was rebuild about 30000 miles ago with new carbs and distributor.

I have checked the inlet manifolds, engine breather and vacuum advance for leaks but have not found anything wrong.

Anyone any ideas what could cause this or where I should look next?

Mike Parris
M Parris

it will be the usual step by step logical diagnostic required

whilst waiting for more knowledgeable people to arrive lets start with what was included in the full service (forget about what I call a full service just put what was done)
Nigel At

Kind of sounds like the carbs are fighting each other....

one is running to lean compared to the other and one has a differant high speed idle comparered to the other

There is a pin at the front outside edges under the dashpots of each carb

Press them up and let them drop....one at.a.time

If it speeds up alot and takes along.time to fall ... To rich

If it falls and/or dies.... To lean,
If rise sliethtly and falls back to normal.... Perfect

Use a piece of rubber hose in the mouth of the carb ... Listen then compare to the other carb and adjust the high speed idle to match

Before making.adjustment...make sure to loosen linkage so the carbs are.independant of each oher.... Or you will have false readings

Prop
Prop

If it was good 1500 miles ago what has changed ?

If the carbs were set up and balanced why should they 'go out'.

Do you have points ? if so check gap and not pitted etc, points sometimes close up soon after renewal until settled in.
I would fit an electronic system i.e Ignitor or similar and avoid the points/condenser issues.

Asume you have checked the dashpots (carb dampers) have oil in them ?

R.
richard boobier

Richard has a.point.....

what changed for me was my fuel went bad over the past 3 weeks and now ive got the rear float needle sticking closed, and is running similar to yours

Pop open the floats and see if they both have fuel in them

Prop
Prop

Thanks for the comments. I have run out of time so have got my local old car specialist to look at it. Hopefully I will post the cause in a day or so.
M Parris

lets see if we can guess just to prove that logical step by step diagnostics is required rather than just guessing :)

I'll go CB points or coil
Nigel At

OK, so got the car back from the garage yesterday, and it runs really sweetly. Tick over now 750rpm and rock solid, smooth as silk however hard I drive it, and the sound is less rasping.

And the primary cause was a failed exhaust manifold gasket. The holes for the inlet ports were shot, with gaps in the sealing material. The other problem, that contributed to it, was the gaskets on the carb inlet were also leaking. Funny, but we replaced the exhaust gasket less than 2 years ago, so I ruled this out when looking for the problem.
M Parris

Mike, great to hear it's running well

just goes to show the need for checking, you did put you'd checked the inlet manifold

exhaust manifold has always given me the sound like a loose peice of metal

now it's running well time to catch up on driving time :)

Nigel At

This thread was discussed between 29/08/2011 and 04/09/2011

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