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MG MGB Technical - Ah for gods sake!

My overdrive just wont work at all. I changed the gearbox oil and it made no difference, and that was a pig of a job. I went to clean the filter and couldnt find it and when I went looking I ended up removing a solenoid from the bottom of it instead, which was a nightmare to get back in and I dont even know if I put it back in properly. Im just going to pay someone to sort it out.

Also, I tried setting up the carbs properly and failed miserably. It is idling very rough and doesnt seem to have as much power now. I followed all the instructions I was given but to no avail. The car is absolutely drinking petrol and for this reason alone I am getting a bit p!ssed off. I cant afford to run it with this level of fuel comsumption. Its using more fuel than a freight train! It used pretty much a whole tank of fuel for a 200 mile trip. granted, given that the overdrive isnt working a lot of that was at high revs. Im at my wits end! The smell of fumes in the car can be very strong sometimes as well.
Ross Kelly

As far as the fuel problem, since you can smell raw gas you are either leaking it or the carbs are set too rich. If you can smell it in the exhaust the carbs are rich due to adjustment of the mixture or choke or a flooding float chamber. There are also plenty of places it can leak from the lines and fuel pump.
John H

Leave the ignition on with the engine stopped to detect leaks John mentions. With the original SU pumps if it clicks it is pumping fuel, and with the engine stopped it shouldn't click more than once every 30 secs. If it clicks more often than that look for leaks, but it could also be the non-return valve in the pump leaking back which gives no external visible sign. This has no detremental effect on fuel consumption but in extreme cases can cause the oposite i.e. fuel starvation at speed, although if it were bad enough to cause this it would be clicking frequently.

I don't know what instructions you used for setting up the carbs, it isn't difficult but you must follow the procedure carefully and it does take patience. You might like to have a look at and click on 'SU Carbs' and 'Setting-up'.
Paul Hunt 2


OD doesn't do a lot for fuel economy, it just moves the horsepower to a lower RPM for more comfortable cruising.

When ever I hear of a set of carbs which won't lean out, I always think of fuel bowl float level first. Make sure your float level is correct, and the floats don't leak and sink, as too high a float level will allow the fuel to sit too high in the bridge, making leaning impossible.

I like to get 275 to 300 miles on a tank, depending on how fast I'm driving... 65-70 mph is usual for me.

Since you are smelling fuel in the car, you likely have other problems which Paul and John have covered, but the evaporative system should be in place, and may need to be dried out to eliminate the oders.

Finally, don't dispair. A properly set up fuel system on an MGB will stay set up for just about forever, and the next time you have running difficulties, you can almost be sure that it will be something as simple as new wires and plugs.

Good luck,
Dave Braun

You will save fuel with the o/d there is that portion going into internal losses in the engine due to changed rpm. Also every cycle must do more work (as there are fewer of them) so you must get better volumetric efficiency. It defintley does not move the power curve of the engine, thats fixed by how it breaths.
Stan Best

could be well off the mark here.....but anyway, its because you mention removing the solenoid. I helped a friend with his overdrive, and neither of us noticed the small ball bearing drop when we had a go at getting the solenoid out for a look. The change of oil, filter clean etc had sorted his overdrive problem out, then we had prevented it working by removing the solenoid and dropping a bit. Once the ball was replaced all was well - presume this was the cause, if not someone please correct me.
Hope you get the problems sorted out as for me having a functioning overdrive is all part of the fun of these cars.


You are correct in the pure sense, and I didn't mean to imply that the horsepower to rpm curve could be reconfigured with OD. However, the horsepower band is relatively flat and moving to a different portion of the band with a different RPM is not really a savings. The amount of horsepower required to move the car at a given speed doesn't vary much either (except for your very exacting point of volumetric efficiencies, etc.) given the relatively high Cd of our cars. Frankly, in practice the OD matters very little in fuel milage, and since we opperate our cars in the real world, practice counts more than theory.

BL didn't see fit to send many Bs with OD to the States. They did not wish to siphon sales from other makes in their stable (more fuzzy thinking on BL's part). The OD here is coveted for the ability to make the car quieter at our interstate speeds, and for the fact that in an earlier car it is really cool to have it available in third gear. I rarely (if ever) hear of anyone speak of any fuel savings. Incidently, the 70 B I care for has OD.

Dave Braun


If your trouble-shooting takes you to the floats in the near future, remove them and give them a good shake. I have had old HIF floats develop leaks and fill with fuel - causing them to sink, leaving the needle valve open and flooding things badly. Spare floats are now part of my on-board spares supply - always!


Your point regarding O/D is championed by John Twist. But overdrive is NOT for the car - it's for the driver! For those of us pathetic wimps (me) who cannot stand to listen to an 1800 whining away in excess of 3500 rpm, O/D is a godsend. I drove my 73 B/GT across Montana and North Dakota last summer at a sustained 80 mph for 1000 miles (75 mph speed limits). Without O/D it would have been about 63 and I don't think we'd be home yet...

Allen Bachelder

Allen, you've nailed the reason that OD doesn't save fuel: you'll drive much faster with it than without it, thereby squandering all the fuel you saved by having overdrive...

If the overdrive doesn't work 'at all' you shouldn't even glance at the hydraulics until you've absolutely proven that the electrics are working. Put it in fourth, engine off, ignition on, OD switch on (you should hear it click when you do this), and check for voltage at the solenoid. Trace backwards until you find voltage.
Sam Good

"BL didn't see fit to send many Bs with OD to the States"

They weren't *ordered*. Canada and California got more proportionally than the USA overall at over 25% to about 17%, and Japan got 100%. UK overall was 98%. Why they weren't wanted I don't know, maybe price, but I would have thought the UK would have been at least as price sensitive. It's considered highly desriable now, but unless you can ship one from the UK you are unlikely to find any lying around, and when you do they command a premium.

Whilst you *will* hear a click on the earlier D-type OD as you switch it in and out you won't with the later LH-type (4-synch cars) as it works on a different principle. To prove electrical continuity on those you must measure the current in the circuit (one of the very few occasions where a current measurement is required), and it should be about 800mA. Voltage at the solenoid isn't enough, it could be open circuit. Neither is resistance as it could be going open-circuit at the rated current. Easiest place to do this on a 76 or earlier is where the yellow from the main harness joins the yellow/red in the gearbox harness at a bullet connector in the mass by the fusebox. On a UK 77 it is where a white or white/brown goes from the main harness to the gearbox harness in the same location, amnd for a North American 77 you need to find the bullet connector where a white from the inertia switch joins the gearbox harness.
Paul Hunt 2

This thread was discussed between 18/03/2008 and 22/03/2008

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