Tuning SU carbs on an 1850

[Bumpa]

Woah! Lots to think about here, thanks. I'll try to answer you each in turn.

DOLOMITE 135
No, there is no fuel coming out the overflows.
No, there are no air filters on the engine at the moment.
I only cleaned the carb components with petrol. The dashpot pistons slide up and down easily and without the dampers in place they snap down onto the bridge properly.

ALANH
Thank you for those links. I was looking for non-waxstat conversion pieces but couldn't find the handed pair, so I might consider those. They are a much more reasonable price.
I would love to give it a run but it has no MOT and won't get one until this fuelling issue is sorted. Oh, and it needs a new windscreen as the existing one is cracked.

GALILEO
Good thought about the choke linkage, but I have checked and double checked that, and the jets are free to go right up. As you say the SU HS is such a simple carb that the problem is most confusing. I have worked on MGs with twin SUs for years and never seen both carbs with the same problem before.

YORKSHIRE SPAM
The fuel pump is the original mechanical one that has been on the engine since 1977, so I doubt it is over-powering the carb needle valves. Or could it be? How could you tell? Anyway, I've ordered new needle valves.

ALANH
Pressure in the tank? How could that be? I have taken the gauge sender out of the top of the tank and inspected the interior with a flashlight and it is amazingly well preserved. Very little sign of corrosion. The tank to pump fuel line is new.

I'll post any new developments. Thanks again all.

[yorkshire_spam]

YORKSHIRE SPAM
The fuel pump is the original mechanical one that has been on the engine since 1977, so I doubt it is over-powering the carb needle valves. Or could it be? How could you tell? Anyway, I've ordered new needle valves.

Strike my suggestion of the list.... some of the new mechanical pumps (or electric pumps if somebody get's an EFI rather than carb pump by accident) deliver over pressure, which "overwhelms" the float valve and fuel just piddles out wherever it can. (either the overflow, the jet or both) - but if you are running the standard original pump that's not an option.

[DOLOMITE 135]

Are the top of jets smooth across the top, or do they have a recess / step? I once experienced a spitfire 1500 (twin HS4 carbs also) that had petrol flooding out of one of the carbs, it turned out that the top of the jet where the needle enters had a separate piece that was pressed in rather than the normal solid version, this separate piece had come adrift causing the issue.

Probably unlikely that both your carbs would have this fault, but if your jets were of the pressed type, and the inserts had come adrift (and been lost during their refurbishing) I do not think it would be possible to adjust the carbs such that they would not flood.

[AlanH]

Pressure in the tank? How could that be?

If you look at one of my recent threads, a non-vented cap caused pressure to build up in the tank forcing fuel out of the pipe.

Do you have or can you get hold of another pair of carbs?. Changing them would prove if they are the culprit.

[Bish]

Hi
Having just done the penny waxstat conversion myself, I can sympathise with your troubles! Simple enough but I found the linkage and choke to be a fiddly job.
I’m no expert but the fact that both of your carbs have the same symptoms must mean you have an issue with the linkage and or choke mechanism? You say you have fitted new butterfly’s as well as a rebuild so maybe something went amiss when reassembling the linkage? Also did you put the small silver bullet back in the housing with your two 2ps? Frustrating I know but you will find the answer!

Good luck.

[Slowmo]

Is the mixture rich during driving or just when sat at idle?

My sprint was OK when travelling, but very rich at idle, gradually increasing fron 2% to 7,8,9% depending how long you left it. Couldn't make the MOT levels at idle.

My conclusion was that the fuel level in the float chambers was rising slowly due to poor float valve sealing. Never any petrol from the overflows. Could be due to poor needles and/or crap in the fuel.

I fitten new float needles with 'rubber' tips, and an inline filter before the fuel pump. The problem seems to have been solved. Now stays at the same reading indefinitely.

Relatively cheap, and can't do any harm, even if it doesn't work for you.

Peter