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MG MGB Technical - Interchangeable SU parts

A while back I swapped my troublesome HIFs for HS4s I bought 2nd hand as a complete pair, bolted them on and it cured the overflow problems, but they came with standard needles AAU spring mounted.
The engine is stage 2 with a kent 715. In my box of SU bits and pieces I had a pair of dashpots pistons and springs that took fixed needles. I bought a pair of No6 needles. and replaced the AAUs piston dashpot springs with these, figuring that the HS4 should have fixed needles.
But even with the jets wound fully up it is still running rich.
Should I put the original pistons back on and fit richer spring needles.
c cummins

You probably have either jets that dont match the No6 needles or are worn out. Spring loaded needles do tend to wear the jets a bit more than fixed needles.
The AAU and No6 needle are from a different range and I think they go with a different size jet. Denis

According to Mintylamb (a good web site) the AAU and 6 needles both use the 090 jet and are almost identical.


Tony Oliver

I always had it in my mind not to swap parts in carb circles? I would check with Minty lamb (and roast potatoes) for the correct starting point, then move up or down. Presumably you are discribing richness at tick over? Are your filters standard or freeflowing, one usually jets up in this is so. Another good book is 'Tuning the SU carburettors" can't remember the author. Mike
J.M. Doust

1) Don't swap carb bits!
If you put fixed needles in, did you centre the jets?
2) You don't know what was done to the carbs before you acquired them, get back to standard bits, later HS4's did have bias needles, by the way, biased towards the engine. Replace the jets,(they could be worn), piston springs and mate up the pistons with their original dash pots and dampers and check they don't bind.
3) Drop the jets down about 1mm
4) Make sure the pistons hit the venturi bridge after lifting with your finger. They should also drop at the same rate as well!
5) Check the floats.
6) Balance. THEN tune.

There is no absolute standard for needles for tuned/flowed engines. People always quote AAA's for even just K & Nfilters. Not so!! The correct needle depends on so much about your particular engine, i.e., state of tune, cam, standard of gas flowing, size of valves, type of exhaust system and air filters, and so on, and ideally need setting on a rolling road.
For example, my +.060", large valved, stage II, LCB'd, HR270 cam'd engine runs on my carefully profiled, approx, ABD needles, AAA ran far too rich!!
Allan Reeling

Thanks for the advice, I will invest in new jets and needles. Do I have to center the jets if I use bias needles?
Also I noticed on the left carb the bush moves when I adjust the jet, and on the right one just the jet moves. I've undone the lock nuts on both, but I'm hoping it's just one of them sticking.
c cummins

With sprung needles you have nothing to centre the jets against, unless perhaps you fit a solid needle first. The book says no centering is required.
PaulH Solihull

"Also I noticed on the left carb the bush moves when I adjust the jet, "
That indicates that some parts are missing or incorrect. When the jet retaining nuts are tight, the bush cannot move. Missing washers result in the jet not being in the correct position.

FR Millmore

This thread was discussed between 09/04/2012 and 12/04/2012

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