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MG MGB Technical - SU Needles

G'day.

I've seen the answer to this somewhere but can't find it in the archives.

I have an 18GBUH engine. The carbs have No. 5 needles which my manual says are for the 18GA engine. The manual says I should have FX needles for the 18GB.

I seem to be running very rich on the front carby and reasonably close to right on the rear, but on both carbs I have to screw the jet up until it stops. The throttles are pretty close to being balanced. Fuel level in the float chambers is within 1 to 2 mm of each other measured with the tops off. I am currently running without air filters whilst I get the carbs set up.

Would the needles cause this much trouble. The 5 is a bit richer all the way up the scale according to my SU needle chart.

Cheers

Tony
Tony Oliver

http://www.mintylamb.co.uk/suneedle/


Nicked from another thread...
K Harris

What an interesting chart. Shows what I gleaned from the SU manual that I am a bit rich all the way up. But is this enough to cause my problem?

Tony
Tony Oliver

Although your float heights may be within a millimeter of each other, what are they in relation to the throat bridge? Too high can cause richness.

dave
Dave Braun

Thanks Dave. When I took the suction chamber covers off and peered down the jets I couldn't actually see any fuel. If the fuel level is too low that would cause lean running?

I looked for the fuel in the jets before removing the float tops to check the level.

Tony
Tony Oliver

G'day. Hard to believe this.

I was looking at the jets again trying to see the fuel level. Funny I thought, the front one seems different to the rear. After a bit of measuring I found some PO had managed to fit a .100 jet to the front carb and a .090 jet to the rear.

Seems to explain why the front carb was so rich, and no doubt affected the rear carbs mixture to boot.

All I need now is a new jet BUT, do I also need the correct needles?

Cheers
Tony
Tony Oliver

Tony-
You will probably not be able to tell the difference between those two needles. I think the only difference in the engines, at least on this side, was the diaphragm pcv valve and the first smog regs for the GB engine. If the smog valve has gone, you'd want the G/GA needles anyway.
Check that the needles are not worn, are flush with the piston face per book, and set both jets .060 down to start. Be sure jets are centered correctly - doesn't sound like Ace was too overly careful!
FRM
FR Millmore

Thanks FR. I'll leave it as it is until I can road run it and check fuel consumption. Not sure what a smog valve is. I've only got the mushroom shaped PCV valve.

I found an old 090 jet in my magic box. Must have been from the last Mini I fixed up...20 years ago! The missus often wonders why I never throw things out.

I centered the jets and set up the throttles and mixture. It is purring like a kitten now.

Tony
Tony Oliver

This thread was discussed between 19/06/2007 and 20/06/2007

MG MGB Technical index

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