Practically Classics Forum

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.

What car? what size engine?



John.

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

Tasng4 wrote:...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?

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/

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

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...

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.