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MG MGB Technical - Crusing lean

Have rebuilt my 66 GT engine and added the normal fast road modifications. The car has an O2 sensor and gauge fitted. The jetting (using a number 5 needle) appears to be fine at idle, acceleration and slowing. But cruising, the O2 gauge drops off the scale. The engine feels ok, but slightly lean and has a slight hesitation on acceleration until the O2 gauge comes up to normal again. Pulling the choke while cruising, the O2 gauge comes up again and can be controlled with the choke.
I can’t get the O2 gauge to run normal and the car to feel right at cruse. I have experimented with a number 21 and CS needles to no avail. What am I missing? Air leaks? Float levels? Ignition timing? Damper movement – how is it controlled?

Richard
Richard D

If you can correct the lean condition while cruising with the choke it sounds just like a weak needle. Fuel starvation i.e. fuel delivery problems wouldn't allow the choke to fully restore the mixture, and an air leak that is 'tuned out' at idle results in a *richer* mixture at wider throttle openings. Can't see how the float chamber would drop when cruising (short of fuel starvation which the choke thing seems to discount), even a quirk of vibration that affects the float valve usually causes it to leak and so the fuel level to rise. Can't see how ignition timing could cause it, although if you have carb vacuum (as you should have for that year) it's possible that the higher vacuum when crusing pulling harder on the diaphragm is opening up a small split which *would* weaken the mixture to the rear carb. You could check that my disconnecting the pipe and sealing the carb port and seeing what effect that has on the sensor. The carb dampers only come into play to richen the mixture when initially opening the throttle to accelerate, and only for a few seconds. With a steady throttle - cruising or accelerating - they have no effect.
Paul Hunt

The O2 sensor is an ideal tool for setting mixture under all conditions. I'm not surprised the no 5 is a bit lean, it's designed to supply the correct amount of fuel for the volume of air a standard engine will flow. It SUs are designed to be a bit rich when you are nailing it and a bit lean when you are cruising, this gives you good performance and good economy. Couple of questions
1) are you around Jo'berg on the High Veldt? I've driven several MGs there and they just do not like that altitude!
2) I take you are aware of the haystack and mintylamb programs that are avsilable for needle selection?
Stan Best

Thank you for your sound advice. Common sense prevails over a confused mind. I checked and resealed the vacuum takeoff on the rear carb (don’t have vacuum advance). I refitted the No.21 needles and was able to virtually tune out the lean problem I was having. I did notice that when it appeared right, one turn of the flat on the jet made a difference on the O2 gauge at example the 3000rpm mark. I think with a little more time and experimenting I should be able to get it right.
At the moment I am trying engine oil in place of light machine oil in the dampers. I think I know, but what affect should it have?
Yes Stan, I stay in Edenvale, on the eastern outskirts of Johannesburg. Normally aspirated engines don’t work as well here as at the coast. The altitude causes a 17% loss of power. But boy, do we enjoy the extra power when we do go to the coast.
I am aware and have some of the programs for SU needles but finding ones to experiment with here is a problem. Suppliers only seem to keep the standards.
Thanks for you help.

Richard.
Richard Davidson

Yes we have stayed at Edenvale, and liked it a lot. On the needle front, if you dont mind risking losing a pair you can always take some brass off to richen them up slightly this was a recognised technique in special tuning. They had a needle marking protocol to identify who had modified it and by how much. The material does not have to be removed evenly around the needle just abraded off one side with fine emery. The trick is knowing where to remove it, however the profiles available on the programs I mentioned do give thickness V station. The O2 sensor will help a lot here. There is a lot written about the dampers in SUs at altitude, from memory you can change the springs I think a lower rate on is used to allow a rich mixture for longer.
Stan Best

Here is a very good article on “polishing SU needles” that I am going to experiment with.
Thanks for your reply.

Richard.
http://www.terryhunt.co.uk/mini/pics/tech/picsb/pics.htm
Richard Davidson

This thread was discussed between 19/08/2008 and 20/08/2008

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