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MG MGB Technical - HS4 needles

Some of you may have gone this way. Having now set up the Air/fuel meter, I am now wondering what to do with the information! At tick over, I am a little rich, and the combustion colour ( with the colour tune) is a fairly bunsen blue. there is an even note to the exhaust. On the road under hard accelaration I still remain rich, which I think is good, but I am lean on the over-run ( normal? )Also if cruising with slight throttle in top, slightly rich but if I now push to full throttle, I am momentarily lean before getting rich again. It occurs to me I might get better accelaration if that momentary leaness was actually rich?( I guess this is where Webers and their accelarator pumps win out?)I run AAA needles with radiused throats under the foam air filters, Tubular exhaust extractor, 123 ignition, set up to optimum advance. I have temporarily mis-placed my SU tuning book,so cannot look things up, ( where it talks of getting an idea at wat station the needle is lying at certain engine load positions). I would need to go to a rolling road ( not that many here in Oz, and they often do not have current experience of SU's.) My dashpot springs are a little lighter, can't remember the colour without pulling them, engine oil in the reservoir is at correct level, an the little air hole at the dashpot cover screw is drilled out a little bigger. Maybe it's running ok but thought I would put it out there. thanks Mike
J.M. Doust

Mike
If your carbs have in fact been fitted with lighter dashpot springs, - this is probably what is causing your lean spot on acceleration

It is a common mistake to fit lighter springs hoping it will pop the carbs open quicker but the truth of the matter is that you get lean spots

The strength of the spring is selected so that the piston just reaches full travel (fully up) at the revs at which your ebgine is developing it's maximum power or just slightly before

If your engine develops best power at say 5000 rpm then the piston shouldn't hit the top till at least4500 and fully open no later than 5000 rpm

It's a real shot in the dark, but I would be refitting the std. dashpot springs, which should get rid of your lean spot but will give you an overall slightly richer mixture - but you can adjust a general rich by adjusting your idle mixture back a bit
Should work ok - Willy

What face is on your air/fuel meter
does it just have lean normal rich-----or
does it give actual air/fuel ratio
William Revit

Mike,
Bunson blue is about right! AAA are fairly rich, even for overbored, stage 2 engines, so you might expect rich except at idle. Light springs coupled with these rich needles should give good performance at the expense of economy. If you are going lean when you floor the throttle, this is mostly down to the damper, as this is what controls the initial lift of the piston.....what oil are you using in the dashpots? The springs affect the"ride height" of the piston.
Normal springs are red, but often it's difficult to tell with old springs.
It's easy to determine which springs you have with a set of sensitive scales and a piece of card. Google the info, it' all out there.
Allan Reeling

"momentarily lean before getting rich again" sounds more like insufficient damping than incorrect springs. The SU carb damper has a dead area where the main piston rises, lifting the damper piston through its free play, before the damping starts. This distance is more, and more noticeable, on my HS carbs than my HIFs as the free play of the damper piston on its spindle is greater.
PaulH Solihull

This will get interesting
William Revit

New report. Went out yesterday, having checked, dash pot level for oil etc. Using engine oil so, what 20/50? Can't find where I put the original springs but did not pull the dash pot to check what was in. Off I went. definite slight sluggish on initial pick-up.( all was warm, and this is in top, on a level) Can anyone tell me what the original damper spring colour was? Was it red? I think I have blue. Peters book will tell me the standard needle. Thanks Mike
J.M. Doust


Yes - red

also, another little point that an old fella told me once
He always used a straight 30 oil in the dampers as it gives a better control of the damper from cold to hot as the engine warms up where the multigrade oil tends to distort the required result a bit
Hope this helps Willy
William Revit

I doubt the carb warms that much, certainly nowhere near the temperature variation of engine oil cold to hot. The evaporation of the fuel when running keeps them noticeably cool, I have found.
PaulH Solihull

Thanks Willy and Paul. I will have to order a new set of springs. So I guess these blue springs are too light. I wonder for experimenting, could I 'pre-load them? A washer somewhere??? Have to open it up, see what rests against what? The original needle looks to have been AAU which stays fairly weak after station 2 ( where-as AAA richens up quite considerably after station 2.)I cannot find my old needles either, Blast!( my brain is going to mush,for sure!)Mike
J.M. Doust

This thread was discussed between 23/01/2012 and 30/01/2012

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