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MG MGB Technical - Gas Mileage Reality Check

I think my 72B gets awful gas mileage for 1800cc engine in a light weight car. Averages during the 4 months of ownership are 14-16mpg, mostly cruising around backroads at 40-50 mph. I've gotten the idle down from 1400 to 1000 rpm, it starts fast "like an old Chevy - barely need a turn of the starter", has a manual choke, runs fine, no feel of power loss at any rpm, tires are inflated properly.

I must qualify my inquiry, but offering that I plan on replacing head gasket and doing a valve job this fall, because I have a head gasket leaking oil between #2 & #3 spark plug area. Vehicle doesn't burn oil and feels tight, but I do notice typical oil burn at startup from what I suspect to be valve guide issues.

I must also state that to me it smells like it runs too rich. I don't have an exhaust analyzer so not sure what a reading would be. I'm not concerned about resolving this until after the valve job.

So, once I start improving mechanical components of the B, what is a realistic goal for gas mileage?

By the way I have a functioning OD, but it doesn't get used too much on back roads.

Thanks for your assistance.
BobA, Stillwater
BobA

You should be getting 25-30 mpg with the driving conditions you describe.
Clifton Gordon

FWIW ----- I get about 30mpg, mostly in town driving. But then I drive like an egg was between my foot and the gas pedal --- few quick starts and stay within speed limit etc etc...
glg

14-16 MPG does not sound realistic. i never checked my fuel consumption, but I guess I'm experiencing the same as Clifton & glg. You should be getting pretty close to double your figures. Check all tune up specs

cheers
Gary :>{D
79 mgb
gnhansen

Bob,

On my 73 MGB with OD, I was getting about the same gas mileage you described. I would drive about 80 to 90 miles and it would use half a tank of gas. Then, this summer, after doing a lot of research, I re-adjusted my timing and especially my air/fuel mixture, leaning it out. I also installed some new, slightly hotter plugs with a larger gap.

The improvement in my gas mileage was immediate. I am now getting double the mileage I use to get. I am still not getting 25 to 30 mpg as others have described, but I getting a over 20 mpg now, and most of my driving is short trips in town, so I can only assume that my mileage will be more out on the open road.

Check the archives on how to lean your carbs.

Good luck.

Robert
Robert Browning

Do check your odometer. You could be in for a pleasant surprise!

I don't think I've ever gotten less than 20 mpg with any MGB, and even that was an occasional exception. 24-25 in urban driving or on backroads where a lot of downshifting is involved, and 30-34 mpg at 70+ mph in overdrive on the interstates is pretty typical for me. Get your car to somebody who really knows how to tune it and do whatever is required.

Allen
Allen Bachelder

Bob,

I just noticed your address. No wonder you mileage is so low. You've been running up and down those notorious Stillwater hills. I spent most Sunday afternoons of my youth visiting family in Stillwater, and have to report that San Francisco has nothing on this town. I'm reminded of Bill Cosby's bit about abandoning his Volkswagen on Lombard Hill. In Stillwater, the main challenge, folks, is to get your right foot from the brake to the gas in less time than it takes to drift backwards into the St Croix River!

FWIW,
Allen
Allen Bachelder

Bob what I have dove 100s of times to the B motors to stop the oil leak is pull 1 cyl. head nut at a time & lube the threads up with anti-seize & tork the head bolt to 45 lbs, do all at 45 lbs & thin all to 50 lbs & thin to 55lbs 2 times. Set the valves at .015 cold, start the car & warm up to running temp & re-tork to 55 lbs. X 2 & now adj. the valves at .013 on the intake & .015 exhaust. (ex-valves wear more with no lead) & re adjust every 10k. I do this on ALL the cars I service & it has been a lot of years since & I have had a burnt valve or cracked head. I have had to re-tork heads 2 & 3 times in 1500 miles, to stop the oil leak. There is NO WAY I will do a valve job on a B motor with more than 30K on it! I have seen that you will do the valve job on a real tight motor & thin it will use a qt. of oil every 300 miles. Make sure that you are getting 38-40 deg. TOTAL advance on the timming! If you have no vacum adv., that works into 5 m.p.g. you will not get. I have seen the dizzy rust up & the motor will not run over 3 K & the m.p.g. droped off real bad. Get a vacum gauge & adj. 1 carb at a time for the highest vacum & thin rich up the mix 1/3 of a turn or 2 flats on the older carbs. Make sure you have at least a 180 deg. thermostat (I run 190-195 year round in my cars. 71 std. B 1800, with a webber & O/D in a 74 G.T. Del. to Wisc. & back, 60-80 mph @ 30 m.p.g. with the A/C om MAX! Robbert, I go to a colder plug, you will get spark knock & the plug may get hot enough to fire the mix befor you get the spark.
Glenn Towery

Bob

Got to agree that something is not set properly. My 67 got 25 mpg going to and from Gattlinberg (around 2000 miles) Now that was mostly highway and freeway, but I set my odometer each time I fill up and routinly get 25MPG. I know my engine is not set as well as it probably can be.
Bruce Cunha

Bob
There are lots of things that can effect mileage. But you are WAY off and there should be some obvious things that can be identified easily. Are jets adjusted correctly; are carb floats set and working corectly; is the ignition timing, vacuum and mechanical advance all working and set properly. Do you have clean air cleaners? Are your brakes dragging? Do you have an obstructed exhaust? Is your choke/start enrichment on? Is your timing retarded? Is your exhaust system obstructed (a bad cat converter)?

A cam and cam timing can affect gas mileage. I have a modified cam that is failing and getting more conservative week after week. However my gas mileage is now up from 27mpg to 30 mpg and the idle is much better. A silver lining to every cloud.


Barry
Barry Parkinson

I would expect to get 35 miles per proper gallon.
Stan Best

We need to be careful what kind of gallon we are talking about. The US Gallon is 4 quarts, the imperial gallon 5 quarts.

For some reason cars in Canada get way better gas mileage than cars in the US

Barry
Barry Parkinson

In Canada, a gallon is 4.54 litres, whereas in the US a gallon is I believe something like 3.89 litres. My MGB used to get 25 MPG in the city (Canadian gallons), but since my rebuild with fast road cam and higher compression pistons, it is down around 17. I still am not sure the timing is set right, still playing around with it. Carbs might also be slightly rich. Also, I have been putting my right foot further down, as I am getting addicted to the increased acceleration it now provides.
Erick Vesterback

Erick, is that true in BC? How much is gas up there for large gallon or do they sell by the liters?
CW Strong

Bob-
Now that is truly awful fuel economy. My engine is 1924cc with a Piper 285 camshaft, Derrington crossflow head with dual Weber carburetors, Big Bore exhaust system, etc. Even flogging it through the nearby Blue Ridge and Alleghenny Mountains I get better fuel mileage than that! With things as bad as you describe them, I'd suggest that you go through the entire system with a fine tooth comb. Disassemble and clean both the distributor and the carburetors, adjust the valves, tighten up the exhaust system, the whole works.
Steve S.

Bob -- There's been many thing mentioned to check out but you're getting about half of what should be expected. Do you ever smell raw gas? You might check the float levels, that could cause a dribble of raw gas, but it'd seem to be more than just a dribble. Dirty air filters might be a contributing factor.

Another thought is what octane are yo putting in -- (just a wild guess, but if it's really low, you wouldn't be getting the performance.) What have you done with all the "emmission control" stuff? Is that all in place or still on. If it's still on the car, you might go through the whole system for air leaks or blockages, etc.

Maybe with a bit more information of engine/bcar number and how "stock" the engine is might help provide more detailed hints --- eg: emmission stuff, last major tune up and stuff like that. I don't know, it's early Sunday morning and I'm just thinking at the keyboard.
glg

If I drive it hard, maybe 20 mpg, if I drive it soft on the highway maybe 27mpg , Basically normal 22-24mpg. I do have a hot engine with a lot of head modifications. It is a fun car to drive.

I don't think 40 year old technology has a chance to match the new stuff. The newer cars are just much better for "mpg". But who wants to drive an econobox? The MG at least has style. My guess is that a car the size of the MGB with modern technology, if it could pass the crash and safety testing (if it could even be built) would be 40 mpg.
Frank Baker

My experience with modified cams (270 degrees approx)is that mileage is poorer in around town driving, but at a steady speed highway cruising it is superior to stock.

Lack of vacuum advance will hurt mileage, rich jets too. An overdrive or 5 speed will make a large difference at 70+ hwy speeds. The other obvious candidates have all been listed above.

Barry
Barry Parkinson

In 2004 we did a rally in SA which included an economy driving test. I was astonished by the results, our TD returned about 22 mpg, the MGBs around 30 and the Fs, TFs closer to 40. I was driving with due regard to the fact that economy was being measured so this is a measure of what decent aerodynamics and a properly mapped engine can achieve. As ever the MGB in terms of what it achieves with what it has is just unbeatable.
Stan Best

I have a 68 GT with its OE 18GF engine in pretty much original configuration (carbs, dizzy {with Crane XR-700 added} etc.), standard timing {20BTDC}, NGK BP6ES plugs, and standard valve lash. I get 20 MPG around town, about 30-33 MPG.

You might want to use a vacuum gauge (at most auto parts stores) to dial in your carb adjustments. If unable to pull at least 16-18 inches of mercury at idle, start over in your tuneup procedure, always doing the carb adjustments last. And do be sure to check the action of your dashpot/air piston assemblies on your carbs (do each, one at a time) by performing the piston drop test as outlined in the emmissions section of the Bentley manual. If the two assemblies differ significantly, you will be chasing your tail on carb issues until your get them matched as close as possible. You can variances of a second or two in their drop times, but more than that seems to spell continuing aggravation.

If they are not close at all, you might find it a better investment of time and money to simply find a good pair of carbs to replace yours or buy a new set from the likes of Burlen (who make them). Just depends on what your time and patience are worth.
Bob Muenchausen

Sorry, that is 30-33 MPG on the highway in O/D fourth.
Bob Muenchausen

Wow, the above is a lot of good information... Thanks.

To clarify a couple of questions that my inquiry raised. The majority of the emissions items have been removed (no convertor, no air pump). Some items remain (vapor separator, carbon canister, gulp valve). It has been suggested I may as well remove the gulp valve too.

Yes, there is raw gas smell. Yes, there is a gas smell in trunk, but filler neck is intact - so I'll be replacing the 2 vapor lines in trunk that run to Separator. Yes, the car appears to run rich.

No, I haven't been able to find a gas leak anywheres or the sign of a gas leak.

The vehicle starts very well, with manual choke pulled, then idles within a minute without choke.

Several recommendations at past weekend's Wings & Wheels in Osceolla, WI - were to FIRST get the valves set right, as there is no sense adjusting carb lean/rich mixture until valves are just right.

I've printed the above responses, so I can develop a check list.

I am concerned about the response to not do a valve job as this may just make things worse. I will do a compression test first to assure the rings are fine. If rings are bad - we're talking a whole new process. Oil consumption is 1 pt to 1 qt every 3-4 tanks of gas (since I do have odometer error, I'm not sure just how many miles that is, but around 600-800 miles. Which for an old daily runner doesn't appear too bad.

So once again, thanks alot for all the responses. This is a fantastic web site. Reminds me of all the information I've received and given on TTOL-BVI's (TravelTalkOnLine, British Virgin Islands) bulletin board. The members make it a fun experience.

BobA
R.W Anderson

Hey, R.W Anderson, Stillwater, Minnesota, USA

[shameless plug]

my son Brian has a shop in Linwood MN near Stacy.
Steve Rixen is over in Hudson, and so on-------------

And me?
Well I'm a retired mechanic and can help fix that problem for you.



SF
Dwight
DCM McCullough

MGBs around 30mpg? If I get as low as 30 I wonder what is wrong.

After adjusting the valves always check/adjust the leads, cap and rotor, plugs, points gap/dwell and timing before moving on to the carbs. You will not get correct adjustment of the carbs if you have ignition deficiencies.
Paul Hunt 2

and to the above you might also check your brakes are not binding.
Roger
RMW

Hey Paul Hunt, your galleions [sic] are bigger than ours!
[LOL]

And we have roads over here, where you can let 'er go.
A heavy right foot is an American trait.
[More LOL.]

Best mileage I ever did, was a 180 mile trip, waaay back in the sixties in my '65 'B'
6 US gallons, in 180 minutes, on both freeway, and two lane balcktop, with lots of stop signs.
Ok, I drove a steady 60 'per.

SF
Dwight

DCM McCullough

Yeah I know, but are Stan's SA gallons smaller too?
Paul Hunt 2

>We need to be careful what kind of gallon we are talking about. The US Gallon is 4 quarts, the imperial gallon 5 quarts

That's not quite right -- 1 Imperial gallon = 1.201 US gallon. Both are 4 quarts (from the same root as "quarter", i.e. 1/4), but the Imperial quarts are larger. 1 Imperial gallon is about 4.8 US quarts....

HTH!
Rob Edwards

Our last trip in the 79B full emmisions, single Zenith, 2900miles. We averaged 37.6 MPG, now that is a Imp gallon (4.5 litre). And yes in Canada they do sell gas by the litre.
Cheers Ric
R.A Willis

I converted the SA gallons at 4.5 l per. My tuned car with the ST spoiler overdrive and economy driving techniques is capable of returning 60 MPH average speeds and 40 MPG economy. This would be roof up on a free flowing motorway tyres 26/28 PSI
Stan Best

"And yes in Canada they do sell gas by the litre.
Cheers Ric " --- but are your miles the same as our miles or should we resort to nautical miles or something we all understand such as leagues !
RMW

CW - Here in BC gas is sold by the litre, right now the price fluctuates quite a bit - between $1.01 to $1.19. Depends on how the Iraq war is going? That works out to about $3.90+ per US gallon. I am fortunate that I can walk to work in 10 mins, or take my bicycle which is 3 minutes. We don't consume alot of gas, the MG is purely for fun.
Erick Vesterback

This thread was discussed between 08/09/2006 and 14/09/2006

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