Tuning SU carbs on an 1850

[Bumpa]

My 1850HL has the twin SU HS4 carbs. I am not unfamiliar with SU carbs having owned several MGs. I have had the carbs to pieces, cleaned them, fitted solid butterfly discs, and put two one penny coins in place of the waxstats. The pistons both move freely and I have fitted new ABK needles. The linkages are all working properly and the jets are not being held in the lowered position.

BUT the engine is incredibly rich. Even with the jets wound right up to the highest position, so they are actually sticking out slightly above the bridge, the mixture is much too rich. Pressing up the lifting pin a tiny amount just increases the revs, and my Gunsons gastester shows a CO level of over 6%. It will never pass an MOT.

I'm baffled. Any suggestions?

[yorkshire_spam]

Is the level in the float chambers about right? It's not over-filling and peeing fuel out of the jets?

[AlanH]

You need to find out which carb is running rich, a Colortune would do it.

I had something similar which was a duff jet.

[Bumpa]

Is the level in the float chambers about right? It's not over-filling and peeing fuel out of the jets?

Well I'm not sure what the right level is, but the plastic floats aren't adjustable anyway, or are they? In the SU instructions which I have it says that "The fuel level is not critical and need not be treated with meticulous accuracy - the normal level is 3/8 inch under the rectangular inner facing known as the jet bridge, but this is rather difficult to observe even with the suction chamber and piston removed and the jet fully dropped."

It then goes on to say how to use a 5/16 inch rod to check the float position when the needle valve is shut off and to bend the metal support if it needs correcting, but you can't do that with plastic floats.

[Bumpa]

You need to find out which carb is running rich, a Colortune would do it.

I had something similar which was a duff jet.

It is both carbs. That's what's so strange. All plugs are sooty black even with the jets on both carbs turned right up as high as they go.

[VanIsleSprint]

The float level can be adjusted by placing shim washers under the
needle and seat valve housing.

[RodR]

You’ve changed the needles, are you sure the shoulder of the needle is flush with the piston? If they’re set high in the piston it’ll be the effect as having the choke pulled out.

Worth double checking.

[Bumpa]

You’ve changed the needles, are you sure the shoulder of the needle is flush with the piston? If they’re set high in the piston it’ll be the effect as having the choke pulled out.

Worth double checking.

All good thoughts, but the needles are mounted exactly where they should be. I don't know quite what to do. Should I splash out on new jets and/or float valves, or am I throwing money away? The jets aren't exactly cheap at around £80 the pair.

[DOLOMITE 135]

The jets aren't exactly cheap at around £80 the pair.

That is a bit rich for jets (pun intended), http://sucarb.co.uk/abk-090in-jet-needle-spring.html .

What was the original reason for servicing the Carburettors were they running rich before?
If you have changed the needles did you swap the spring that mounts to the top of the needles over from the old ones?

[Bumpa]

The jets aren't exactly cheap at around £80 the pair.

That is a bit rich for jets (pun intended), http://sucarb.co.uk/abk-090in-jet-needle-spring.html .

What was the original reason for servicing the Carburettors were they running rich before?
If you have changed the needles did you swap the spring that mounts to the top of the needles over from the old ones?

I'm not sure what you're getting at with your hyperlink? Your link takes me to the needles, not the jets. The jets from Rimmers are just under £40 each.

Anyway, this car, and it's carbs, have been off the road for 26 years, so I have never heard it running. I have rebuilt the engine, carefully cleaned and reset the carbs and got it running. However, it is fantastically rich. The carb needles didn't look too good, so I bought new ones but they have made no difference. To answer your other question, yes I did transfer over the small springs on top of the needles.

I thought it might be the old fuel in the tank, which must have been 4-star leaded, but having swapped that for some new 97 octane unleaded it is still the same. Thinking about it, it's amazing that it started at all on the 26 year old fuel, but it did, quite easily. They don't make petrol like that any more!

The ignition timing is spot on, as is the camshaft timing. I am going to try new float needle valves as the old ones have a groove round the conical seat.

Thanks for the input. It all helps to organise my thoughts.

[DOLOMITE 135]

Sorry for the confusion, this is the link I intended http://sucarb.co.uk/hs-jet-assembly-090in-11927.html, however having double checked I see that the waxstat version is indeed double the price http://sucarb.co.uk/waxstat-jet-assembl ... 12863.html (my car is pre waxstat so I was quite surprised at the price difference)

The wear on the float needle valves could cause an issue, was fuel coming out of the overflow? I think they normally come with the seat as I normally replace both at the same time.

If the car has been standing for 26 years were you running it with the air filters fitted? if they are the originals are they clogged? as this would cause the car to run rich.

Additionally how did you clean the inside of the carbs as they should not be cleaned with anything abrasive as this can put them out of tolerance, further are the piston assemblies free to move fully along their entire travel.

If you are looking to possibly replace the waxstats, and the float needle valves it may be cheaper to buy a service kit:

http://sucarb.co.uk/service-kit-for-hs4 ... 12087.html
http://sucarb.co.uk/service-kit-for-hs4 ... 12086.html

[AlanH]

You can get non-waxstat replacements which use the existing waxstat linkages from Burlen
http://sucarb.co.uk/waxstat-jet-conversion.html
http://sucarb.co.uk/waxstat-jet-conversion-10871.html

But whether you need them is open to debate.

You don't say if you have given it a run, perhaps worth trying an Italian tune up (bit of a long shot).

[Galileo]

A worn jet will look oval, so if the jet is fully wound up, the needle carrier flush with the piston and it still runs rich then maybe you have a float needle valve not seating properly, only thing I can think of that causes really rich running. An argument somewhat ruined by the fact that both of your carbs have the exact same issue.

Is it safe to assume that there is no issue with the cold start mixture enrichment device setting? Seriously though SU Carbs are such a simple device that in reality there is not a lot that can go wrong with them, so it's always component wear when there is a problem.

[yorkshire_spam]

It's a billion to 1 shot.... You don't have a fuel pump that's pushing too much psi?

[AlanH]

It's a billion to 1 shot.... You don't have a fuel pump that's pushing too much psi?

Or pressure in your tank.