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MG MGB Technical - HS2 carb needles

The engine in my 71 roadster was rebuilt about 5 years ago to 60 thoug oversized. Apart from fitting K&N air filters as far as I know it’s a standard engine. I also replaced the ABD needles in the carbs to AAA needles that were recommended..
The engine runs and picks up fairly well but tick over is quite lumpy. My question is have I-got the best needles in my engine.
I notice when buying an SU carb kit if they have the needles included they are AAUs . Thanks for any advice. Trev
Trevor Harvey

That should have read HS4s sorry.
Trevor Harvey

I have used the AAA needles for a number of years with good success. Do you have a build sheet on the engine? Best way to determine the exact requirements for your engine is on a rolling road. Then, pick the needle coming closest to what optimizes your engine performance. However, most commonly, what people describe as a "lumpy" idle is related more to the choice of cam rather than the needle since most needles have similar characteristics at idle.

Les
Les Bengtson

As Les says, the needle choice doesn't really effect the idle.
With the carburetors set up evenly it should idle fine unless of course someone has poked a modded cam in it.
A thorough carb. check/setup/balance should fix it.
But with any tuneup, valve clearances ,spark plugs, leads, ignition timing etc need to be done/checked before setting the carbs up.
willy
William Revit

Thanks for the reply’s. I have had the car now for 18 years and having rebuilt most things over that time including the carbs nothing has improved the tickover, it’s always been lumpy. When the engine was stripped down I never checked to see if it had a sportier cam in it as I thought the lumpy tickover was due to tuning, so maybe it is due to the cam. I have increased the tickover to 800 revs and it has improved it a little.
Trevor Harvey

What idle speed are you aiming for? Mine never idled well at the book speed even on 4* leaded, worse on unleaded. I have mine at about 1k, which also helps prevent bogging-down in a long idle in hot weather.

Otherwise as said if everything else is 'right' - many checks - then unless there is a carb fault any needle with the correct mixture and air-flow balance will idle as it should. It's off-idle you can get issues with incorrect needles.
paulh4

Thanks Paul, I do like a nice slow tickover, probably aiming for about 600 revs, since increasing it to about 800 it has improved, it’s not as lumpy as before. I have always wondered if it might have had a hotter cam fitted as it does go fairly well, I am now more certain that that’s what is causing the lumpy tickover.
Trevor Harvey

Personally I have never had a car with carburettors that could idle as low as 600 rpm. Even my modern computer controlled Honda idles at around 900 rpm. I'm with Paul on this and would set the idle at 1000 rpm. Much kinder to the engine to let it run smoothly.
Mike Howlett

I have the closest = to AAA available in fixed needle in my car. It was slightly weak at station 2 which accounted for why you could choose between plugs sooting at idle or a flat spot when accelerating. This was discovered on a rolling road and couple of passes with some fine emery paper solved it. As noted above the car is happy idling at 800 rpm, that is with a Piper 270 fast road cam.
Stan Best

This thread was discussed between 30/09/2024 and 22/11/2024

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