Correct SU Needles For My Carbs?

[Tasng4]

I'm trying to find the correct needle size for my carbs. The problem I have is that the car lacks the 'get up and got' I would expect from this engine. I realise that with unleaded petrol I should expect some fall in performance. Currently when I floor the throttle the car doesn't really accelerate it merely gains momentum.

I have made the following changes to the engine/car:
- Performance exhaust.
- mild road cam.
- K&N air filters.
- electronic ignition (the kind that sits in the distributor cap.
- Skimmed head/block (nothing special just enough to ensure a good
gasket seal)

Everything else is standard including the carbs (1.5" HS4) which are new and still have the supplied AAU needles fitted.

Also, when I rebuilt the car I fitted a pair of 1.75" SUs which had BDR needles fitted, a trip to a rolling road recommended a pair of BDT needles which made no difference - the car was producing about 60bhp (any idea what my car ought to produce with the performance changes I've made). I was told my car was "over carbed" which I took to mean the std head couldn't suck hard enough to make effective use of the 1.75" carburettors. Is there a reccomended needle size for these carburettors which will make them work well with my engine?

I don't know anything about the relative differences between SU needles, there doesn't seem to be any corralation between the various profiles i.e. needle 1 may be 2 weak but needle 2 may not provide a richer mixture just a different profile.

Needle size AAB has been suggested for the std carbs.

Any help/suggestions greatfully received.

[Grease Monkey]

What car? what size engine?



John.

[Tasng4]

Err yes that information would be useful.

- MGB-GT Rubber bumper (this would not normally have SU carbs fitted but I swapped them)
- 1975, manufactured Sept 1974. It is one of the first R/B models with the C/B interior
- 1798cc
- UK car

[JPB]

...MGB-GT Rubber bumper (this would not normally have SU carbs fitted.....)

It should. Or did you mean that you'd swapped the SUs for something else?

[stagman]

this is the "technical" way to work it out - I think! may help??!

http://www.teglerizer.com/suneedledb/index.html

or I would email Burlen and ask them

http://www.sucarb.co.uk/

[Tasng4]

Originally it was fitted with HIF4 carbs but I changed these for HS4's Sorry if this confused.

[tritonofnor]

download a program called "winsu" very useful!
just run your car details through my copy and it comes out with ADX, which is a bit lean top end, or ABY which is a little rich all the way through... you should be around the 90 bhp mark...

[Tasng4]

Well, just to finish off this post I ordered 2 AAB needles (original ones were AAU) which I've now fitted. There is a deffinate improvement in throttle response though not as much as I'd hoped although I do realise there will be weeks of small tweeks to mixtures etc. before I get it right.

Thanks for all your replies.