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MG MGB Technical - Carb Needles for Overbore?

My '67 BGT has an engine with numbers that indicate it is a '73. The engine runs well without smoke or other drama and seems to have plenty of power. It has a metallic green paint job, and I assume it is a rebuilt engine. It looks as if the previous owner obtained a rebuilt block and head and moved all the ancillaries from the old engine to the new one, including the HS4 carbs.

Last winter, I noticed that the engine did not like cold weather. It had difficulty running when cold, even though I used the choke liberally. And even when warm, it gasped when pushed on the freeway during cold weather (in Fort Worth, that means below freezing). It seemed to be a mixture problem, so I replaced the stock #5 carb needles with the richer #6. This helped, especially on the freeway, but did not cure the cold-running-on-choke difficulty.

So I'm thinking that the engine has been bored out, maybe 0.30" or 0.40", and I may need carb needles designed to work with this overbore. Anyone know what works best? Thanks. -G.
Glenn G

Well Glen,

I do not know what is winter in Texas, but here in Canada with -30F sometimes I could give you wat I lived.

Long, long ago ( I like tales)I made the error of driving my B-GT for few winter. I remember having to enrich mixture at fall. And more horrible when I go to ski in the hills at 2,500FT, the poor B was without any guts. This is caused by SU, which were unable to compensate for air density variations caused by cold or elevation.

Your motor could be on the lean side. As SU are depression type, they should be able to deal with extra vacuum. But if not, I think for those years needle #5 were the rich ones suitable for your case. Easy to find at MOSS, VB or e-bay.

There is some needle program somewhere calculating needle difference, supposely to "help", but I think a NASA engineer could be unable to understand the logic behind that...

Cheers,

Jean G.
Jean Guy Catford

I've found HSs do need a tad of richening in the winter compared to sumemr as they don't have the temperature compensation on the jets that HIFs have. If it runs well in warmer weather then I certainly wouldn't start fiddling about with needles, or you are going to affect that running. Just try richening half or one flat.
Paul Hunt

I'll try richening a flat or two. If that doesn't work, I'll consult the needle charts in my Speedsport book, "Tuning S.U. Carburetters" to find a needle skinnier in the right places. I hate to spend a lot of time experimenting and was hoping there was some common replacement.

Experience with a big overbore on an SU-equipped Volvo tells me that needles sometimes must be changed when other modifications are made. Thanks. -G.
Glenn G

Glen,

Changing for Volvo carbs could be also a plus for a B, as they are 1 3/4" instead of 1 1/2". But an intake manifold must be find.
Remember that Volvo B20 motor were 2 liter. But my suggestion for needle #5 will definitely solve your problem.

Cheers,

Jean G.

PS: have a look on e-bay item 300183082559, but you need 2 needles
Jean Guy Catford

Or that item, they have 3 of them

300182874586

JG
Jean Guy Catford

This thread was discussed between 06/01/2009 and 07/01/2009

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