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MG MGB Technical - Filters and exhaust made performance go flat

Hi folks. May I pick your collective brains about su jet needles please? I have just fitted a pair of K&N pancake air filters and stub stacks, and a Bell big bore exhaust system with the long centre branch manifold, and I reckon that the performance has gone a bit flat, particularly at lower revs. I miss my low-down torque! (such as it was) I read somewhere that I may have to change the needles to richen the mixture - do you have any suggestions please? Many thanks, Rich
r j symons

If you improve the breathing then that does often cause flat-spots due to weak mixture when the throttle is opened. If pulling the choke slightly as you accelerate improves things, then that is what has happened. You will need 'richer' needles, you will have to compare what you have now with the others that are available here http://www.mintylamb.co.uk/suneedle/ and look for something a little thinner i.e. richer. For example if you have AAU now, AAA is richer, AAB richer still, with AAM between those two.

However that program is a little inconvenient in that you have to specifically select each alternative needle to see the effect, it would be much easier of you could just scroll through them.
Paul Hunt

Once you have upset the air/fuel balance by improving gas flow, getting the balance back through the entire rev range can be a long winded affair. But first set up the ignition, set the valve clearances and balance the air flows. You may be lucky and AAA will do it, but to accurately set the mixture through the rev range is better done on a rolling road. Go and see Peter Burgess. But other factors can affect the outcome, particularly the state of wear of the engine, the installed camshaft and how big is the "Big bore". Significantly bigger bore doesn't always improve performance, particularly at low revs.
Allan Reeling

Thanks for your advice, Paul and Allan. Much appreciated. Regards, Rich
r j symons

Allan is right. From Nottinghamshire it isn't too far to Peter Burgess in Alfreton. Ring him up and arrange to have it tuned by him. You will not regret it. Money very well spent. There isn't anyone else in the UK that could do a better job.
Mike Howlett

Thanks Mike, will do
r j symons

I had similar experience to you Rich.

I then fitted AAA needles (which I am still using after a rolling road session with Peter). Forum posts I had read suggested the even richer needles would be going too far.

The AAA made a very noticeable positive effect and seem to be the 'de facto' needle to first try with K&N's. Not to say that it will be ideal for every car...

A rolling road session is good advice, but as we all know, does add to the cost.

G
Graham Moore

Thanks, Graham - and assuming that I've got spring biased needles rather than fixed, I'm going to give the AAA's a go. Cheers, Rich
r j symons

What year is your car, and/or do you have an ID tag on your carbs - on the float chamber. Spring biased are very easy if you have them. Just remember to make sure the rubber 'grommet' that holds the needle is flush when installed, don't overtighten and make sure the needle bias points in the right direction. not sure how important the last bit is, but the Burlen standard install instructions mention it. No need to centre of course.
G
Graham Moore

"the rubber 'grommet' that holds the needle"?
It's certainly not rubber on mine!! I presume you are referring to the slotted and "Veed" piece of steel which holds the needle and spring in place.
Allan Reeling

Graham, my car is a '69 but the carbs are from a morris marina, I'm told! I believe I have AAU needles in at present. Rich
r j symons

"It's certainly not rubber on mine!! "

Ditto. I'm pretty sure you have no control over which direction the needle is biased in once the needle guide is correctly installed, i.e. with the its shoulder flush with the recess in the piston.
Paul Hunt

Paul, some of the needle holders are plain and not slotted.

Peter
Peter Burgess Tuning

This thread was discussed between 15/03/2016 and 19/03/2016

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