I have a problem that recurs every 500 miles or so. The carb seems to "lose" its setting - completely. The mixture becomes way too rich, blows blue-black smoke like mad, fouling on the plugs causes some serious misfire...

I clean everything up. New plugs in. Take the car to a specialist with an exhaust analyser. Reset the carb and make sure all is running sweetly. 500 miles later...

This is the 998 motor with an SU HS2 carb. The carb was rebuilt when the engine was rebuilt - about 4000 miles ago. So this incident is now about the 4th time this has happened.

Any ideas?

Hi,500 miles between carb tune is definiteley not right,-Blue black smoke usually is a sign of oil burning-does the car consume oil?When you change the plugs do you notice an improvement and then more improvement when they tune the carb?seems strange they have to touch the needle/jet setting,hard to believe that the adjustment changes itself.check that your choke is not sticking on and check that thepiston is not sticking up ie when lifted slowly and then let fall it should give a distinctive clunk if it falls freely

Thanks. Pulled a plug - it is very sooty, but no oil. Ignore the blue - the smoke is black and you can smell the unburnt fuel. The oil consumption seems normal to me. I will check the choke and operation of the piston.

Yes, the car's performance improves markedly with new plugs and carbon fouling on the plugs.

Hi again-when you take it to tune the carb,do they just adjust the mixture and tickover,and do you actually see the readings on the analyser before and after they do the adjustments?What's the air cleaner like?

Check the floats, if original or old may be soaked with fuel. Once these carbs are set they last a long time. With todays fuels, corrosion internally makes them worse. If the brass floats are still there may have to change. Check needle seats too!

The air filter is new - just done a service (about 200mi ago).
They have a fancy gas analyser - so you get a before and after reading (about 3 or 4 different options). Taking the car back on Wednesday - so will let you know what the outcome is.

CO's are way! We are going to strip and rebuild the carb. Anything in particular I need to be aware of or look at that could be causing the problem?

Hi- as Larry says when set they should last a long time,check the things I mentioned before,as larry says check your floats are moving freely and not perforated,change the float needle and seat as a matter of precaution(cheap enough)and make sure it is closing properly and at the right height,and finally install an inline fuel filter or renew the old one and check your ignition timing and settings are correct before the final carb tune,and that the plugs are the correct ones for the engine.

You may also want to go with a better fuel filter- some of those common paper types let too much pass thru. You may try to find a very high quality (unfortunately also kind of expensive) but it sounds to me like you have crud getting thru and messing up the needle valve.

Bill
(former owner of several Mini's)

Check the needle's for ridges and the seat for taper. if there is either replace both.

Be sure you don't have an aftermarket electric fuel pump that is forcing fuel past the float.
Your car only needs about 2 or 3 psi to run properly. A higher pressure pump (say, 5 psi or more) will give exactly the reaction that you are getting.

Also, be sure ignition is good (points, cap, wires, etc). A weak ignition will foul plugs and may appear to mimic a flooding situation.

Additioanlly I have seen regulators loose there setting. Even the good Holley ones. I had to rebuild mine recently cause the pressure would flucuate too much one day to the next.