MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Gas Mileage

Okay I figured when I bought the 80B that is in nearly new condition; I would get much better miles per gallon than my 72B.

The 72B has the SUs, the 80B has the Zenith.

I started out with it running very rough from old gas when I got, and have put in fuel treatment with each of the 3 tanks so far in a month. It starts much easier now, and there is no sign of a problem.

I've replaced the air filter, which had field corn and mouse droppings inside of it. I've sprayed carb cleaner into throat of carb to clean soot. Replaced the plugs, flushed the cooling system, changed the oil, new fuel filter, new hoses.

The automatic choke works fine or at least it functions fine. Cold starts did have a 2000 rpm idle, but I've since adjusted than down to 1000 cold start idle and about 700-800 rpm warm idle. Automatic choke functions fine as far as how the idle comes down gradually as the temp goes up.

I have no indication the car is running rich-lean, or bad. It purrs along just fine.

I removed the air pump, and plugged the related openings in head and removed the valve at intake manifold. I've replaced several vacuum hoses after finding them so dried out they were not sealing.

So I keep cleaning and tightening things up, thinking that which each improvement gas mileage will improve.

The car keeps running better and better, but mileage went from 20mpg - 19mpg - 17mpg.

I essentially drive it the same routes, same traffic, same weather, same way - not hot footing around, just crusing along on the highway to work at 65mph, city speed of 30mph.

Was I supposed to have done something else to enhance air to carb when I removed the air pump? I know something obvious has to the issue. Car rolls very easily so brakes are not dragging, new tires are properly inflated.

I may replace cap and rotor just because they look original, and ignition system was replaced years ago with an Allison system.

Thoughts? I have time to think about this while I wait for my new heater control valve. After that I'm draining differential and putting in sythetic gear lube.
R.W Anderson

keep driving it, for setting up - check/adjust (as required) in this order - tappets, (CB points), plugs, timing then carb mixture, if you adjust any item in the chain then you need to check and possibly adjust the items that follow in the chain

I'd also change the oil and filter again after 500-1,000 miles to act as a flush, do a thorough change by getting the oil as hot as possible and leave to drain as long as possible to get as much muck and existing oil out as possible

when you put the new heater valve on I'd also suggest scraping out the cylinder drain hole (personally I'd flush/back/flush the rad, engine and heater matrix individually)

if you get trouble from the new heater valve get the next one from Ashley Hinton - http://www.ebay.co.uk/sch/ashleyhintonmgparts2002/m.html?_nkw=&_armrs=1&_from=&_ipg=25&_trksid=p3686

be very care about replacing the rotor arm as many modern made ones are very poor quality, buy good quality from somewhere like the Distributor Doctor - http://www.distributordoctor.com/

also check the HT leads, these are excellent leads - http://www.performanceleads.co.uk/

good idea about changing the diff oil, again hot as possible to drain long as possible also I'd suggest doing the same for the g/box oil

it would then probably be a good idea to open the car up a bit, give it more revs now and again, hold on the the gears longer, a few good 'blow out' runs

after 6 months give the car another full service and check/adjust (as required) the set up again

the only other reason for increased fuel consumption I can think of at the moment is a leak, tank or rubber pipes perhaps
Nigel Atkins

In order of your responses...

I'll keep driving, but with only so much time I'll delay tappets until spring, but I did forget to double check timing. Too much time just looking for something wrong or broken. Plugs already replaced.

I'll do another oil change late Fall.

Already flushed system, using the valve is what I think caused it to leak after sitting for years with no movement. I'd heard there were problems with some replacement valves. Didn't know there was an alternative. I'd prefer scrapping the standard version and plum something else together.

I think I'll get rotors from AdvancedDistributors, they have pretty good ones made for them.

I'll be removing cover on diff., and cleaning everything out, new gasket and new syn. oil.

No sign of a gas leak, my 72B has one and it is easy to detect. I think on that car there is a leak on the top side of the tank, somewhere above 3/4 full tank.

Thanks for thoughts. In the meantime the car is running quite well. Needs a better stero though, 1st new speaker is in drivers door, Wed it is passenger door - 5" shallow Sony speakers.

R.W Anderson

tappets don't take long and if they're out they can cause problems and they need to be correct to make checking/setting the timing fully worthwhile

new plugs great (I think you still need to check the gap is correct for your car before installation)

after the tappets (plugs?) and timing you can move on to carb mixture and hopefully increase your mpg

you could clean and lubricate everything from the pedal to the carb to make sure nothing is sticking or catching - other than that that's me out of ideas other than possibly with the changes you've made perhaps alterations/adjustment to the carbs, vacuum or breathing(???)

just in case I've caused confusion the heater valve that Ashley Hinton produces is the original (using original tooling I believe) but AFAIK doesn't leak on to your dissy

I think I put before the diff oils I'd suggest (for others they are -)
. Castrol Syntrax Longlife 75W-90 (GL-5 spec*)
. Mobilube 1 SHC 75W-90 (GL-5 spec*)
(*GL-4 spec oil is recommend for the gearbox but the more modern GL-5 is considered fine for the diff)

often the fuel tanks get perforated at a certain level because fuel is always filled or kept at this level and this is discover when filled above this usual level

it must be running well if you can afford to be listening to the stereo :)

good luck
Nigel Atkins

Nigel, The dif has bronze shims which are eaten by GL5 additives. GL4 only in the dif!!! RW, while you have the cover off the dif it's worth renewing the diff shims...easy job.
Haven't had a lot to do with Zeniths, but heard they are a lot more temperamental than SU's, and the diaphragm gets "pin-holed" leading to mixture issues. not sure about the US plumbing and effects of removal. My 72 GT never gets less than 28mpg............good old SU's!!
As Nigel said, start by setting all the regular stuff; tappets, dwell, timing, plugs. Then you can look for faults. Easy spots to start with; vacuum advance unit function (suck it and see!), centrifugal advance mechanism (you should be able to twist the rotor arm and it should spring back), air leaks on manifold or servo. Then do a compression test. "Purring" doesn't necessarily mean developing full power. You then have to spend more time with pedal down and in lower gears. As for the mixture you might have to borrow/hire an exhaust gas analyzer to see what's happening.
Allan Reeling

Give me my old 66 GT. Get 24-26 mpg in work traffic & 40-45 on a trip depending on terrain. Mixed driving usually returns 32-34. Car has done 300,000 on one engine rebuild & uses little oil between changes at 3000m. Out max speed limit is 100k. (+10?)
Garth
J G Bagnall

Have you replaced the vacuum diaphragm in the top of the carburetor that lifts the piston and needle. Its bound to be weak if the car has been siting around. If it has even a tiny leak it will run rich and as yours is gradually getting worse it sounds like your problem to me. Denis
Denis4

Hi Alan,
are you thinking of g/boxes, always a bit confusing about the GL-5 in diff, IIRC for a good few years now EP(X) 80w/90 has been offered which is GL-5 spec

I got Heritage (BMHL) diff oil a couple of years back and got 80w/90 GL-5

from a web site elsewhere -
'Meets GL5 specifications. Suitable for post 1960 hypoid and spiral bevel gear sets.'
Nigel Atkins

Nigel, No diffs!! Am wary of your website recommendation. Remember those BMC diffs were designed way back. The GL5 is probably great for the gears, but how many post 1960 diffs used bronze shims?
Allan Reeling

Hui allen,
well I know the GL-5 was dropped from use in g/boxes but it's said to be OK for diffs, it's what's been handed out for a good few years now IIRC

I can't remember the details of the differences between using it in the diff and g/box

if you suggest not to trust it that's fine

my diff was noisy before I put the fully 'synthetic'(ish) GL-5 in and it still is but I don't notice the noise now because I've got used to it, well with the roof down anyway
Nigel Atkins

Yes Nigel, I turn the stereo up!
Allan Reeling

Allan if you can still hear the stereo you've got no problems :)
Nigel Atkins

RWA - I suggest you include the brakes in your checks - the fronts may work but are given to sticking on - the rears generally warn you when they are not working properly but can sieze . The oil should be changed every two or three years anyway.
Roger
Roger W

I need to retire so I can start on the above list of things to or check.

I wish there was more certain proof on the yes or no to Mobil 1 GL-5 synthetic being ok or not for differential. I would never use synthetic in transmission - I stick with Castrol 20-50. But I've used Mobil 1 in other situations, like my 72B - not aware of any problems.

Amazing how improved and modern oils and lubricants cause problems for older cars. Like no or not enough zinc in motor oil.

Time to look at exploded view of Zenith to find the vacuum diaphram too.

Tomorrow is Brit Fest in Hudson Wisconsin, so now I have to spend time making my 80B look all purty - not time for repairs/maintenance.

Thanks, Thanks.
R.W Anderson

This thread was discussed between 06/08/2013 and 09/08/2013

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.