Erratic Idling

[g1drp]

Having spent very little time playing with cars since my youth (1980s), I have a problem with my 1994 Mini Mayfair Automatic.
Its an odd setup, carburettor and a catalytic converter. The problem is that the tickover is not smooth and the revs go up and down once the choke has been pushed in. I plan to service it next weekend and I'm contemplating changing some of the key ignition components and checking the timing (assuming my Gunsons strobe still works). If you Rev the engine (in neutral) there is no hesitation, so I think the timing is possibly OK.

There seem to be quite a lot of vacuum tubes, and I'm wondering whether there is air getting in after the carburettor.

I only collected the car yesterday and I drove 85 motorway miles with no issues apart from an aching right ankle (I'm 6'5"). We had to buy an auto because my wife has an auto only licence.

Any help will be greatly appreciated.

Cheers,

Ian

[g1drp]

It seems to idle perfectly if I put my hand over the air intake. The revs rise and it's much smoother and the engine responds instantly upon opening up the throttle.
I tried easy start, as suggested by a forum member, but no obvious vacuum leaks were found. 
This car is odd, it has a catalytic converter but it has a carburettor instead of fuel injection.

Could it be that the mixture requires enriching?

[limby2000]

Have you tried screwing in the air screw, it still sound like its getting too much air. All uk cars from january 1991 have to have a catalic converter i beleive no matter what fueling source. Maybe carb or manifold gasket leak. Oh the fun that are mini,s .

[g1drp]

Hi Terry,
Thanks for your post. Where is the air screw please? I have no experience with SU carbs but I remember my Chevettes and Vivas had Strombergs and you could move the jet up and down to adjust the mixture.

[wimfournier]

Perhaps the damper piston is not moving freely up and down in the dome of the carburettor.
When it is not going down onto the bridge at idle than there is too weak a mixture to push down the piston in the cilinder.
When you put your hand over the mouth of tehe carburettor you write, the engine is running right. In this case your hand simulates the piston in the dome is in the right position.
When you put a finger -upside down- in the mouth of the carb you should meet the piston. 
With the engine at rest the piston should be down on the bridge. Only with a fingernail under the rim of the piston you can lift the piston up.
When you bring your finger inside and the piston is so far up that your finger can get easily under the piston, than the piston is stuck and did not go down far enough to maintain a appropriate vacuum at idle running.
With a finger under the piston you should easily push it up and it should come down without hesitation when you put your finger down again.
If this is no good, you'll have to take the dome off the carb and clean the stem of the piston and the hole where it moves in up and down.
And, you have to clean the rim around the piston that sits close tight against the inner wall of the dome.
If nessecairy you could use a little smear of silver polish to clean the wall where the rim might make contact with the wall. All has to be smooth.

Do you have oil in the neck of the damper bowl under the black nut?

[g1drp]

Hi,
Thank you for your suggestions, I'll have a look tomorrow. I remember changing the diaphragm, in a Stromberg carburettor, whitch was exhibiting similar symptoms. I know that the SU carburettor does not have a diaphragm but the principle is basically the same. I think that, if I took the air filter housing off and covered the air intake, the engine would stall. 
Anyway, I'll have another play tomorrow!

[g1drp]

I found out yesterday that the carb was spitting back so I had a play with the timing. My 1980s Gunsons strobe has died, so I set it up by ear. I found that, if I set the tick over at 1100 rpm it drops to about 850 rpm in D with my foot on the brake pedal.
The car is now more responsive and pulls more cleanly in top at low revs with no pinking, though it didn't knock before anyway.
I'll get a new strobe later this week.
If nothing else, I've learned a great deal this last week and I would like to thank anyone who answered my posts for their help and suggestions. I repair high end 3D printers for a living, so cars are somewhat different to work on.

[wimfournier]

Nice!