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MG MGB Technical - Lean needles

I am running AAE needles in the 1970. After a Dyno run, it was clear that mixture needed to be leaned considerably for maximum HP over 3,000 RPM. With the jets leaned up level with the bridge, the HP increases at high RPMs but generates a very poor running condition at low RPMs under load.

The car is running at 5,280 FT above sea level so altitude is certainly a factor.

If there is a lean needle available for this model year, please advise. Moss offers only the AAE. Richer taper (narrow) at the damper end with a leaner taper (wider) towards the point seems to be appropriate. This would allow the jets to be lowered to the proper level while avoiding the lean "bump" off the line and excessive fuel to air ratio under a load above 3,000 RPM.
Glenn Mallory

Glenn. Contact Joe Curto. He is one of the better SU rebuild shops in the US and he knows pretty much everything about SU's. He will tell you what needle is better for you and sell it to you if you want.

http://joecurto.com/

Bruce Cunha

Depends how much leaner you need the needles!

Selection of possibles in picture below - use the Minty comparison tool at http://www.mintylamb.co.uk/suneedle/




Chris at Octarine Services

Do you have the fuel curve Glenn, or readings at specific rpm WOTs?
Peter Burgess Tuning

The number of available SU needles is very extensive. Unless you conduct some expensive trial and error, the most accurate set up method for mixture at various revs is still a rolling road session and graph. A "dummy" damper rod, graduated at 1/8" intervals, will show which parts of the needle need to be altered to correct the mixture at various revs. Probably start with a totally lean needle then "polish", as appropriate to richen where indicated.
Allan Reeling

FWIW some HIFs won't run correctly, and maybe not at all, if the damper cap is not in place.
paulh4

Disagree Paul, my carbs are always balanced by measuring the piston lift, using various methods, but all done without dampers. After all the damper is there to prevent sudden rise of piston causing an over-lean mixture on acceleration as the fuel catches up with the, quicker to respond, air. The graduated rod will show exactly which part of the needle needs to be altered.
Allan Reeling

I am on travel and will post the dyno results when I return. Looking at the chart,

http://www.teglerizer.com/suneedledb/090needlesortjava2.htm

it appears that the ABC needle would be a good starting point. It goes rich and then lean which would allow me to lower the jets to normal levels. I have a set that I tried once in the midget.
Glenn Mallory

Not the case with the HIF on my pal's supercharger, Allan. My V8 HIFs do, and my roadster HSs, which is why I said 'some'.
paulh4

This thread was discussed between 15/02/2017 and 20/02/2017

MG MGB Technical index

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