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MG MGB Technical - SU Carb Needles

I have a 1967 MGB with a 1868 engine, HS4 carbs, K&N filters and no.6 needles.
At my recent MOT, I asked the tester to run an emissions test for me, results where as follows:-
at idle CO = 6.15% HC = 261ppm (looks too rich) but at 2500rpm CO = 2.08% HC = 99ppm (possibly too lean?).
Can this be improved by changing the needles?
Regards
Graham
G Cherry

N0 6 are the rich option but only go rich higher up the rev range. You could try a No 7, that goes even richer higher up, but would allow you to lean at idle by screwing up the jet.
Allan Reeling

Thanks Allan that looks possible, but how do you find out which station the carb is seeing at 2500rpm. It doesn't appear to be very much throttle and may only be as far as station 5, so may need from station 3 to be richer (does that make sense?).
Graham
G Cherry

What is the condition of the jets? if they are worn you should renew them and readjust the mixture and see if the results are better.

The mixture on SU's is adjusted at idle.
If the jets are worn, more fuel will flow through them with a rich mixture as a result.
To compensate one would turn the jet up, making the mixture leaner. The mixture will now be leaner throughout the whole range, but maybe a worn jet has more impact on the idle mixture because of higher vacuum values etc.?

I've seen worn SU's with the mixture screw turned all the way up and still with a rich mixture at idle, it would result in a leaner mixture at full throttle.

Running too lean in normal driving conditions is harmfull for the engine.
Willem vd Veer

The carbs are fairly new and the needles look fine with no marks on them, its difficult to see in the jets, but I guess they are the same, as they came together. If I screw the jets up to reduce the idle CO, it will weaken the mixture at 2500rpm even more, which is why I think changing the needles will help.
Graham
G Cherry

Graham-
Willem has explained it well.
There are two types of needle, fixed and spring loaded bias.

Bias needles wear both jet and needle, all the time, but tend to wear all along the needle, which keeps things sort of even, up to a point. Replacement of both jet and needle is recommended at about 20,000 miles.

If you have the original HS4, they will be fixed needles. These require that the needles be accurately centered, or wear results. If centered correctly, neither needle nor jet wears over very long periods. However, corrosion from fuel/water frequently pits the jet and needles, at the level the fuel sits when the engine is OFF, and lower as fuel evaporates over time. This condition is worst on cars which sit for long periods; it is common to find concentric rings of pits on the needles. Greatest corrosion is at and just below idle position.
If the needles are worn from poor centering, the wear will also be at idle position, ie stations 1&2. Poor centering may also cause unpredictable deviations from established needle profiles, depending on which direction off center the needle is.

Either case means that idle will show as way rich, and attempts to adjust will give lean above idle.

Remove the pistons and examine the needles under magnification, looking for wear or pitting from stations 1-3. As the idle dimensions for all needles are .089-088 in a .090 jet, any wear or pitting is enough. Measure accurately. You can assume that pitted needles mean pitted jets, which is difficult or impossible to see. Bad jets tend to give overall rich, while bad needles tend to rich idle.

FRM
FR Millmore

A thought Graham, if you are keen on studying. Get the tuning manual for SU carburettors( off hand cannot remember the author!) but it is a wealth of informaion including how to modify you own needles and how to determine what station is doing what. I've misplaced my copy, Blast! so I cannot help further. Mike
J.M. Doust

Just remembered some of the writing in the SU book, and that was that you can start the engine and removed the damper rod from the pistons on the SU's, this then allows you to insert a light rod, or plastic straw that is graduated according to the stations. At gives revs the rods will rise to the 'station' operating. of course there is no damping, so easy as you open the throttle. Can you see what I am getting at? Tricky thing is that the engine is not under 'load'. I was trying to dream up a way of rigging a small camera to film the rods while out on the road or on a given patch of road...... oh yes,,, a rolling road does the same! Mike
J.M. Doust

How does it run? What's the plug condition like?

If you aren't getting flat spotting on part-throttle acceleration, or plug fouling, then there is nothing wrong, JFDI.
PaulH Solihull

This thread was discussed between 09/05/2012 and 11/05/2012

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