Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - carb needles

hi all. i have had my 1972 bgt for 4 yrs and has never run quite spot on.i seem to remember that before i fitted a rebuilt engine it always held back a bit and was always changing points and condensers thinking it was them. the rebuilt engine 18v has had head and block skimmed,bored +60 and has standard cam. the car has always had the k n air filters on it since iv had it. fitted the accu spark ignition and new coil. the car holds back and farts at about 3000 revs in 1st and 2nd and in 3rd is farting all the way more or less. feels just like points gap too small, but dont have fitted.i know nothing about carb needles but does it sound like it could be them as i dont know what else it could be now.
many thanks bob.

the needles in carbs at the minute are AAU which i believe are standard. is that right.

A professional inspection and tuning is often of great benefit when setting up an engine. And, you in the UK are fortunate to have a few specialists like Peter Burgess who not only have years of experience with these engines but also have available a chassis dynamometer (rolling road), allowing better tuning to take place.

As to your question about needles, I was advised by Roger Parker to try a set of AAA needles many years ago. They seemed to work better than the needles which came with the new set of carbs. (I think the needles may have been AAU, but do not have time to dig them out for my storage parts lockers.) Over the years, many other people here have mentioned that they either were using the AAA needles or had them recommended to them for general driving use. The AAA needles seem to be the starting point for most MGB use and it might be worth your trouble to pick up and install a set to see if they would make an improvement in how your engine runs.

Best of all worlds would be to go to a competent tuner, having a rolling road, and be able to test out the performance of the engine while making fuel system and ignition system changes.

Les Bengtson

You can check out whether or not it is a mixture problem by slightly pulling out the choke during the time that the engine is running unevenly. If this is the case then it is almost certainly the needles that are at fault. I too use AAA needles and have no problem in that respect.
Iain MacKintosh

thanks lads put aaa needles in and now runs fine.
thanks bob.

The AAA is marginally richer than the AAU over the useful range of the needle. I say marginally, because once you compensate for its richer state at idle, it is much less rich than it would initially appear.

It looks like a good choice if you feel you are slightly 'starved' at the higher, constant load areas when Highway cruising. I have my doubts that it would do much under richer acceleration loads as I believe the dampener system would overwhelm the slightly larger annulus.

Dave Braun

This is all interesting; When does one start to alter damper spring rates? indeed damper oil viscosity, and even the little hole at that top of the dash pot the black screw top, that keeps the central rod and damper in the piston center tube?
My understanding is that the biggest difference between a Weber DCOE and the su is that the accelarator pump on the Weber can give good 'take off' for the B engine. The SU not having accelarator pumps has to rely then; initially engine vauum to start to lift the piston, then, air velocity to draw enough fuel through the jet. too strong a spring, slows piston so not enough fuel. Too light a spring, might draw piston up too quickly so perhaps an over rich situation ( still a bog down?) the oil simply damps quick movement of the piston etc. In a well set up situation, one can get a su to almost match the Weber for quick take off. Then of course you have all these emulsion tube and main jets etc in the Weber but the way the twin Su's are set up with the intake manifold, the cylinders are balanced in terms of 'shock waves'. The Weber cannot match this arrangement, for straight forward situations. Out and Out racing is a different story. Mike
J.M. Doust


Your understanding of the SU is flawed. The purpose of the dampener and fluid is to retard piston movement upwards only, as a one way check valve it does not do anything about downward movement. So I think we can do away with the 'dampen quick movements of the piston' theory.

Once the upward movement of the piston is dampened, the resulting squeeze of air flow across the bridge as the mass flow increases to satisfy the butterfly opening causes a decrease in local pressure at the jet. Despite the needle being down in the jet, blocking some of the opening, the air pressure sucks up additional fuel for accleration then would otherwise be possible under steady state conditions at the same needle position. So, the air flow does not have to 'initially engine vauum to start to lift the piston' and then 'air velocity to draw enough fuel through the jet'. The very fact of the piston being lower is the enrichment tool.

The air pressure in the piston chamber has to be equalized with the air pressure under the piston, so the piston does force its way up until it reaches a steady state of pressure, despite the dampener. In fact, the carburetors are often termed 'Constant Depression' carburetors because of the fact that at steady state they seek a constant pressure. In ofder for all this to work of course the machined tolerances between the piston and the suction chamber has to be very, very fine.

The original SU carburetors had very heavy brass pistons. Eventually light alloy was selected, and a iron ring was added to the pistons to replace the weight loss. Neither of these set ups require springs. Eventually it was realized that a spring would do just as well to mimic the weight and the final condition of allowing an alloy piston with a spring selected for a specific 'weight' was devised. It became an additional tuning method. This mass (or spring) is the method used to smooth out the action of the piston, not the dampener.

I hope this helps,
Dave Braun

Thanks Dave, Damn difficult to get one's head around a problem sometimes. Your explanation makes sense, now lets see if I can apply it! Cheers Mike
J.M. Doust

This thread was discussed between 04/09/2011 and 11/09/2011

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.