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MG MGB Technical - Serious fuel consumption problem with my MGB!!

Hi There -

I have something of a worrying problem with my 1981 GT, wondered if anyone could ofer an idea.....

It's a standard UK spec car in every way. The fuel gauge doesn't work properly (thanks to all who are offering advice on that!), so before going on a long motorway journey at the weekend I filled the tank. I filled it SO full that the fuel actually came up the filling pipe, just under the fuel cap.

I then took the MG away for a long journey, all motorway miles, no traffic. I kept to a pretty constant 70mph in overdrive top.

After 284 miles had been covered, I ran out of fuel, utterly and completely. This was something of a shock, as it means that I only acheived 23 MPG overall, ON THE MOTORWAY!!

This is badly wrong.....eighteen months ago I clocked 33-35mpg doing exactly the same speed, again on the motorway.

The car's the same in every way, but does feel slow and sluggish these days. Can an out of tune engine do SO badly on fuel??

The car's in the garage now, 'cos I just can't afford this much petrol for it!!

Any advice appreciated: thanks for reading.

Bruce
B. Mann

It could be that yours carb is need of a rebuild, you might want to looking into that and do a basic tune up of your car. oil change, plugs, wires, distributer tune up. Or it could all just be something as simple as a sticking damper or butterfly. The others will be along shortly to be of more assistance then I can since I'm not sure on the differences on a UK and US spec B.
CJD Dark

Check to see if you have a leak at the connection of the autochoke and the body of the carb. If you still have the autochoke consider changing over to a manual choke so that you can be sure the choke is off ( I am assuming you have a ZS carb set-up. Could the catalytic convertor be clogged?
Frank

Also check that the tank seam is not leaking - often this only becomes apparent when filling to the top of the tank. Most people drive with about half a tank and the problem is not noticed..
I discovered this after filling my B once and noticing enormous fuel consumption change....and drops of fuel along the road behind me!
Blue Max

No ZSs on UK cars, always SUs, and HIFs on later cars, and no cat.

If there are no physical leaks - check there is nothing running out of the carb overflow pipes and with the ignition on and the engine stopped the pump only ticks once every 30 secs or less - then it can only be hugely rich mixture (check the plugs - sooty black is rich, two plugs more so than the other will indicate the carb), and that would account for being down on power.

Is the choke mechanism working as far as you can see? Have someone pull the choke knob in and out and check the cams are moving the same amount. But its possible that the enrichment valve inside has stuck open which is a dismantling job. But do the set up first which will tell you if one carb really is rich.

Then check valve clearances, plug gaps, points gap/dwell and timing, and check that the vacuum and centrifugal advance mechanisms are at least doing something if not checking the actual curves. Then set up the carbs from scratch, checking the needle is correctly positioned in the piston first. You need to balance the airflows, then balance the mixtures, then balance the chokes. Rather than repeat it all here you might like to have a look at http://www.mgb-stuff.org.uk/wn_fuelframe.htm and click on 'SU Carbs'.
Paul Hunt

Good stuff Paul,

Either tank leaks or carbs drip + a full advance/mixture and choke check .
Renou

Is the timing badly retarded ?
Paul Hollingworth

Thanks guys -

This gives me some excellent ideas to try out. I'll check timing and fuel tank first, then go onto the carbs....

Cheers
Bruce
B. Mann

Whilst you're checking the timing, also make sure the ignition system (cap/rotor/plugs/wires/points) is up to snuff. An often repeated (and very true) adage is that most carb problems are in the ignition. Once the ignition system is sorted, tackle the carbs.

Paul K

Even when I drive my B like a gay lord I dont get more than 30 mpg normally I get about 25 in average driving, fast and its in the teens.
Jim

Check the hose/pipe connections at the fuel pump.
I had one leaking, but only while the pump was making pressure.
Turn on your ignition switch then watch for drips by the left rear wheel.
Pete W.

Ooops! Sorry, did I say left rear wheel?
I meant the Right rear wheel. The passenger side for North American cars.
Pete. W

This thread was discussed between 02/03/2004 and 07/03/2004

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