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MG MGB Technical - Suddenly running lean? ideas?

Hi everyone,

Been running my '71 daily driver back and forth to work (70 miles round trip), but suddenly this morning it began to sputter and hesitate somewhat, correcting with a bit of added choke (but not much). Continuing the drive in this morning, I tried removing the choke a couple of times, but it went back to sputtering.

It was a bit cooler in the NorthEast this morning (around 46), but don't know if this could have any affect. This started well after the engine was warmed up, about 20 miles in. Hoping it's better on the way home...

Any ideas of what this could be? what to look for?

Thanks,
Dave

Dave Cardoza

Weak fuel pump, pluged fuel filter, dashpot oil low, possibly a vacuum leak at some point in the intake manifold or carbs. Before you dig into the carbs, check your ignition system out. A weak spark will sometimes respond to a richer mixture. Ignition coils, condensors and electronic ignitions will sometimes only act up while hot.
John H

Dave - For a fuel delivery troubleshooting guide, see my article by the same at: http://www.custompistols.com/cars/dave/ddFuelDeliveryTroubleshooting.htm
Cheers,
dave
David DuBois

Carbs rarely go out of tune, unless the throttle shafts wear, causing leaness. More likely spark or timing.

dave
Dave Braun

What if there is a failed float? Would a cracked float that was causing a rich mixture show the same results?

BH Davis
BH Davis

Remove the air filters and see if the carburetor vacuum pistons are dirty. If so, you might try cleaning the carburetor vacuum chambers and pistons with WD40 and paper towels. Remove them one at a time so as not to get them mixed up. Be careful not to damage the needles.

In my twenty or so years of daily driving SU-equipped cars, this has been the most common cause of lean mixture. The piston can get dirty enough to impede movement inside the vacuum chamber and the throttle body. -G.
Glenn G


Thanks everyone,

As usual, I appreciate the ideas and comments that everyone takes the time to bring forward.

Dave BuBois, thanks especially for the fuel delivery guide... very helpful info.

I'll start checking these all out and report back with what I find.


Dave
Dave Cardoza

This thread was discussed between 08/06/2007 and 11/06/2007

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