Carburettor: please help!

[rsawatson]

I have bought a cheap 1989 Austin Metro to keep me going whilst my Morris is in for bodywork restoration during the next week.

However, having driven it around without any problems for a few days, the Metro has started playing up - and I've identified the problem back to the carburettor, but no further than that.

Fuel is getting into the carburettor, and the car will run smoothly on the choke. However, if the choke is pushed in, or if the the accelerator is applied at all, the engine simply dies.

What does this mean? Can anyone point me in the right direction before I pay a garage to mend it?

Cheers.

[aupickup]

well maybe the car is running lean

[bmcecosse]

Does the piston in the carb move freely (see my signature!)? It should lift easily with one finger and then fall back with a nice'clunk'. Or as Dennis suggests - maybe it's just set lean.

[minor65]

My minor done the same thing, when warm the only way to get it to run normally was to have the choke right out.

The cure was a new carb needle, but the piston in yours may not be moving freely enough.

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[rsawatson]

Many thanks for advice.

It was a combination of two things - (1) the piston needle not moving freely (the dash pot cover had evidently taken a blow in the past - sanded off the inside and it moved nicely again) and crucially (2) there was water in the petrol. This would explain why, even after mending the piston needle, the engine would only run on the choke.

Fortunately I am not working tomorrow - so I am ripping of the petrol tank to drain and dry - hopefully that's the last of the problem. In all fairness, the previous owner did say that there had been a problem with water being in the fuel - we suspect it originates from somebody perhaps jet washing the car at some stage.

Anyway, thanks again.

[autolycus]

Unless minor65's needle was grotesquely incorrect for the car, a new needle would, in itself, have made virtually no difference at idle - certainly nothing like the difference pulling the choke out fully makes. If, on the other hand, the old needle was bent, such that the piston wouldn't move freely, or wasn't seated correctly in the piston, or the jet wasn't centred... So I wouldn't recommend changing the needle unless you know there's something wrong with it. A bit of wear just makes the mixture richer, but by then the jet is probably worn, too.

On the subject of how to treat the suction chamber, the Book of Words says "Clean the inside of the suction chamber and piston rod guide with fuel or denatured alcohol (methylated spirit) and wipe dry. Abrasives must not be used."

So perhaps "sanding off" the inside was inadvisable.

Kevin

[rsawatson]

The suction chamber was physically distorted - I understand what you're saying, but can't see how it would have trued the bent metal?

Good advice about the needle and jet however - as it happened, when reassembled it started first time and ran well having changed the inline filter - it was only after 12 miles' driving that the water began to find its way back up into the carb.

[bmcecosse]

Strange - because SUs are very tolerant of a little water in fuel. One way to be rid of it is to add some methylated spirit to the fuel - this is miscible with water and so it is then carried through and burned!

[rsawatson]

That's a good idea to bear in mind actually - however, in this case I think the only cure is to take off the fuel tank. When we tipped out the float chamber, I'd say about 40% of the area covered by the contents was made up of globules of water - so it is building up quite severely, after a relatively short driving distance (12 miles from being cleaned).

The fuel filter was also laced with brown deposits, indicating rust, so this can't be a good idea to leave either.

[autolycus]

The suction chamber was physically distorted - I understand what you're saying, but can't see how it would have trued the bent metal?

Replacement really is the only answer - I suppose that if you had a Cricklewood Bentley you might consider boring out the chamber and making a new piston, but ordinary mortals would just fit a new (i.e. old) one.

Kevin

[minor65]

Ref autolycus's quote

Unless minor65's needle was grotesquely incorrect for the car, a new needle would, in itself, have made
virtually no difference at idle


The problem was not at idle, the engine would not rev. But did rev and idle with only full choke. Old needle was not grotesquely incorrect. New needle did cure the problem!!!

[Alec]

Hello Minor65

I'm a little puzzled by your post, as full choke is so very rich, I can't see how replacing the needle alone would cure what you describe. Surely there was some other defect or maladjustment that you somehow corrected in replacing the needle. Could it possibly have been a 0.100 needle fitted originally instead of a 0.090" needle?

Alec

[rsawatson]

The idling screw was virtualy fully unscrewed - after treating the inside of the suction chamber so that the needle could move easily, and tweaking the idling screw, the engine ran well - until the water found its way back up.

We removed the petrol tank on Monday - drained it out, and as expected a lot of rust stained liquid emerged. The source of the water ingress became clear however when we removed the filler neck itself - it looked like something from a shipwreck - so rusty that there are gaping holes in the pipe.

[kittyfell]

But luck on the filler pipe. But at least it's not a rusted out tank. On the 'side' subject, I agree with others, a NEW needle would not make any difference unless the old one was bent. More likely the fitting of the 'new' needle disturbed/reset whatever was actually wrong with the carb

[rsawatson]

All refitted and running okay - I've done a 100 mile round trip without any problems at all.