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MG MGB Technical - SU's And Altitudes

My recent Blue Ridge Parkway trip was cut short mostly due to weather. My 66-B ran like a pro with the exception of 4000+ ft altitude. I was fauling out plugs at a rate of 125mi per set. I stopped and replaced oil with a lighter oil in the dashposts but did not help much. I know this has been talked about but what is the best source for how to correct this problem on the road when I finish my trip this fall. The altitudes in the lower Parkway are 6000+. Steve, I am sure you can give me some insight on the Westcoast.
Steve C.
Steve C.

Some carbs have an aneroid bellows to lean out the mixture at altitude - http://www.tpub.com/content/construction/14264/css/14264_138.htm. You can do the same thing manually up to a point, but it will only be a compromise as it needs near normal mixture for idling and the weaker mixture for larger throttle openings as I understand it. Any tendency to stall at idle on the weaker mixture can be compensated for by screwing in the idle screws a bit, of course. For twin carbs whatever you do adjust both by the same amount in the same direction or you will completely muck up the settings.

If the majority of use is at altitude you would want to change the needles to 'weak' ones, i.e. ones that taper relatively less towards the tip than standard needles.
Paul Hunt

We have had both our TD and MGB over 14,000 ft passes in the Rocky mountains and other than running rich, have had no problems. While at Snowmass for nearly a week while attending a GOF West, I did lean the carbs out until we got down to lower altitudes. Living in Western Washington state, we regularly go over 4000 ft passes on our way to the east side of the state with no problem. Perhaps a hotter plug would be in order for those experiencing problems. Cheers - Dave
David DuBois

I would think that a properly tuned motor should do OK at 4000'. Flying small airplanes (carburated) we typically do not begin to lean the mixture until climbing through 5000'. Perhaps you are a bit rich at sea level.
Edd Weninger

Hi Steve- I regularly drive from Donner Summit, near Lake Tahoe California, to sea level in San Francisco. I'm running #6 in the SU's and only have set the idle down a bit at sea level. That's all! Vic
vem myers

If you are running with standard paper air filters, removing these will lean out the mixture sufficiently.

The SU compensates automatically to a large extent as the less dense air lifts the pistons less at a given speed and thus delivers less fuel.

An alternative is to replace the standard red piston spring (4.5 oz) with red springs (8 oz)
Chris at Octarine Services

Whoops - I meant replace with YELLOW 8 oz springs
Chris at Octarine Services

This thread was discussed between 29/05/2008 and 02/06/2008

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