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MG MGB Technical - Still Having Gas Mileage Problems

Okay, I've posted various stages of my trying to resolve gas mileage problems on my 72B since last year. I'm about ready to just accept that my gas mileage stinks on the MG.

My original first few gas tanks in Sept-Nov were (16, 14, 20, 20, 17).

This spring I did the following: valve job, replaced plugs, adjusted valves, replaced point&cond system with electronic, new cap, cleaned out fuel system with several carb cleaners in tank, cleaned carbs with spray cleaners while off engine, air filters look new, adjusted carb pistons (using engine oil).

I've done several other things to car (emergency brake cable, flush brake fluid with synthetic, oil change 20w50, filter change, lube front end, refilled empty front shock oil with hydraulic jack oil) that shouldn't have any affect on mileage.

Car starts and runs fine. Prior to all of above car used to spit out black stuff (oil? gas?) onto cardboard behind car. Of note: with 60+ degree weather car starts best with choke open., but does require a little choking to pull out of garage but a block later its full open. Since valve job car really has very nice acceleration at all speeds, even in 4th w/OD, but I'm not racing around or doing rapid acceleration. It does not have the rich fuel smell it used to have, but I still smell gas - even when driving on highway; but it doesn't stink up the garage like last year.

Last tank of gas: 19.4 mpg

I have not found a single sign of gas leaking out. I have cardboard under the car that shows no sign of leaking.

I have not taken the floats out yet to check setting, but I'm thinking I should set them a tad to the lean. I have not replaced fuel lines. Some previous suggestions were to advance timing (still set at stock) 10 or more degrees from stock.

What next? What else?
R.W Anderson

I would try a fuel pressure limiter, amd also drop the fuel tank to check for rust on the bit you can't see under the boot floor. We got 34 MPG over the aborted OSH run and then en route to the Arden run. This is per imperial gallon so about 20 to a US gallon. This should be easily achievable.
Stan Best

If the engine sounds right and pulls OK - My money would be on brake drag. As they get older the front calipers may stay engaged for a while - and the rear sometimes lock on too. Best test is for heat after a stop start drive.

On the other hand the needles and their valves could easily be the prob - get you exhaust analyzed - that will tell you straightaway if you can not adjust to near factory setting.

Let us know how you get on.

Roger
RMW

What color are your plugs burning? The electrode should be somewhere in the light tan to medium gray range as should the inside of your tailpipe. If they are dark brown or darker, you are too rich. Note to always check plug color after driving the car for twenty miles or so, Idling will give you false readings.
John H

Mine is running a bit rich, but I still get low mid 20's in mixed driving.

I'm running 20w 50 castrol in the engine & carb. I set my timing with 89 octane, advanced to pinking, then backed off a bit till no noise, + a bit for piece of mind and mislabeled fuel.

Mine starts the same as yours - just touch the starter and it fires right up, but needs a bit of choke to back out and get going.

I seldom throw money at mine, just gas, oil and hydraulic fluid for the leaky clutch slave. Maybe yours is expecting to have money thrown at it? Try parking it near a little econo box for a while and see if some of the econo part rubs off ??

I suspect richness and some dragging someplace.
C Holm

You should be able to get near 30mpg in mixed driving, and well above on highway cruise in OD. Stan's conversion is off - it would be more like 28mpgUS.

If it starts and runs that well cold, it is rich.

Dead vacuum advance unit eats gas.

As Stan said, Common for late B to have holes rusted in the tank top, letting gas leak out via slosh, and evaporate without drips. Common to see stains near the top of the tank, but not always. Gas also evaporates from the same holes.

Dragging brakes, front end alignment, low tire pressures are major mileage killers.

My standard check for drag is to come off the brakes when the car is just perceptibly moving on a smooth "flat" bit of road - if it stops quickly, it's dragging, if it continues to roll or goes faster - since the road is rarely actually flat - then it's good. If you can stop completely, release the brakes, and it starts to roll, it's very good.

FRM
FR Millmore

For my car, it has no overdrive, and probably 75%-25% slow town to highway driving. New fuel tank and affiliated works.

I've set mine for best power from 45 to 70 mph. Needles are probably not optimal as HIF6 was not factory for a B.
C Holm

RW. A couple of thoughts come to mind that are not covered above.

First, what are you using for "gasoline"? The blended fuels we get in Arizona can contain ethanol which will reduce gas mileage as compared to pure petroleum based gasoline.

Second, how are you measuring your distance? The odometer may not be accurately reflecting the distance driven. On my Scout, the odometer is off by 25%. Thus, establishing the accuracy of your measuring device is in order. I drove on the interstate and plotted the mileage markers against the odometer to make my determination.

Beyond that, idle mixture, fuel tank condition and needle selection can make a significant difference in what the gas mileage is. Your model should have a charcoal cannister on the right side at the rear of the engine bay. If you are having a problem with the carbs over flowing you should have a strong smell of gas from the charcoal cannister. Detaching the over flow line from the cannister and running it into a clear bottle should allow you to see if the carbs are over flowing.

Les
Les Bengtson

Ooops, mea culpa, I meant to hit the 9 not the 0 so for 20 read 29! That makes more sense!
Stan Best

Richard, we get about 25 to 28 running regular fuel on the 70B. It takes a good while to come off choke, and the spark plugs are tan on inspection. I surmise that our tendency to run at 70 mph plus eats into our fuel milage a bit. I'm not a big believer in fuel savings at that speed from the OD, and I've never compared OD to non OD milage, but HP is HP, and the car has some frontal area to overcome. We run about 28 psi on the 14 inch 185R70 radials. Our speedometer is calibarated, and our odometer is dead on (they usually are) We've checked against a GPS as well.

I think you need to do Fletcher's front end check, and rear brake setting check. You also need to revisit the mixture setting on your carbs. If you need help, let me know. I like to adjust them by driving, tweaking, and driving some more.

One additional fuel milage / mixture killer is float height in the float bowl. Too high, and your fuel is too close to the bridge, and is not metered properly.

One final note, I think filling an MGB tank consistently is an exercise in futility. And the less gas you need to add, the worse the consistency is.

good luck,
dave
Dave Braun

Those mileages are way down, I can't see brakes, tyres etc. causing that unless they are way out, and for the brakes at least the drums would be on fire. The organic bit behind the wheel usually makes the biggest difference i.e. the driver on a correctly set up car, and of course the road conditions - I get as low as 11 or 12 if I use a tankful round town in my V8, 27 or 28 on longer runs, but have seen as high as 34 on a 300+ mile mostly motorway run at moderate speed. 33-35 (UK) is normal for the roadster, but again on long steady runs in France was getting close to 40.

I've found spitting black stuff out the back is normal when starting on choke. I'd say you shouldn't get a fuel smell, that implies leaks or very rich running. The latter leads on to correct setting up of valves, ignition and carbs. With carbs the adjustments are done at idle, so if the needles were way off i.e. a much more agressive taper than they should have, which could be wear grooves even if they are the correct needles, you wouldn't normally know about this other than from driving. Also choke sticking open (it is an enrichment system, not an airflow stangler) will cause rich running. After a dozen miles or so at a steady 50mph (or as near as you can get to that) what do the plugs look like? That is usually the first place to go in rich running.

I've found that in running at least there is very little difference between the float chambers overflowing and running out, when I had a float fail and was controlling the fuel pump from the OD switch. If it isn't leaking out of the overflow with the ignition left on but the engine stopped (disconnect the coil) after several minutes then the pressure is OK and a regulator shouldn't be needed, especially with an OE pump. The pump should click less than once every 30 secs in this test. However vibration can cause the float valves to leak under certain conditions, so I'd run for a while with a little catch bottle on the carb overflows and see if it, well, catches anything.

Paul Hunt 2

Following the above recommendations, I have started looking at possible problems with my fuel system that are creating this low gas mileage problem.

So first new problem: Can you identify this float?

I opened up my shop manuals and opened up my float bowl chambers ready to check the calibration on my floats. Both were less than 1/8" as specs require.

My surprise came when I tried to see how to adjust the float to specs. There was nothing to bend,twist, adjust! I looked at manual, and parts catalog and discovered that my float doesn't look like the one that is suppose to be in the car.

The tag number is AUD 265F, on my 72B w/SU HS4 carbs. Catalog suggests the 265 was on the 1968 car not the 1972? Is this my first problem?

I cannot see a single way to adjust this fixed float.

I removed the needle and seat and found that one carb had a fabric type washer under the seat, nothing under the other. I tried a tad thicker washer under the seat to see how this would affect 1/8" gap between float and float body, but this proved to really through the float adjustment off.

I have floats back together for now, and playing with tightening the needle seat gets me close to 1/8" for now.

I also decided to replace needles and seats as I can see a small ridge half way up the needle tip.

Your thoughts please.

Thanks.


R.W Anderson

RW,

it is not uncommon that carbs have been taken from another car, so it should not be the real problem if the numbers are not according to the publications.
The floats on very early B's were made of pressed brass halfs, soildered together and had to be adjusted, later they canged to the Nylon type shown in the image you sent.
This type has no setting facilities as given in the workshop manual. Beeing lighter than the early type and of all equal demiensions, they ar a little smaller and do not need an additional setting, as long, as they do not start sinking due to fuel finding it's way inside, than new ones must be used.
When I did a rebuilt on a set of 1968 carbs, I found out that the needles had got 'polished' trust patterns and the jets had become a little 'oval' too.
This seems to be very common on this 'fixes needle' type of SU HS 4 carbs.
The MpG results had been bad too, but with new jets and needles an average of 28 MpG was restored.
On my carbs i also had to change the butterfly spindles, as they were worn badly although there was no distortion of the bodys of the carbs themselfs.
If you need new needles, you should also buy a set of new jets too. Watch out as there are two versions, fixed ones and swinging ones, that are not interchargable, although the jets are the same.

It is not that lot of work to do an overhault and there are kits available for this job. If you do it for the first time, you might calculate 3 hours of work.
BTW, do not dismatle the fast idle cams and the springs in there, as it is an awfull job to rebuilt these small bits. Just use carb cleaner on these parts.

Hope this helps
Ralph
Ralph

All-plastic float heights are set with more or less washers under the float valve. But there is no way a small amount of float height change is going to change mileage. In fact with the typical problems with new parts you are quite likely to *start* getting a leak, and hence even worse mileage.

A ridge on the float valve 'needle' isn't uncommon in used valves, it doesn't harm the seal, although perhaps is more likely to trap debris. If there is no fuel coming out of the float chamber vents (have you put a catch bottle on them yet?) the float and valve action is fine, and changing things won't help mileage.

Like Ralph says, new needles and jets should be your first change, as well as confirming (with the catch bottle) that nothing is coming out of the vents.
Paul Hunt 2

Some additional info...

I have an aftermarket fuel pump mounted by heater motor. The fuel pump runs constantly, I have installed an inline fuel filter before fuel pump.

Upon opening up the float chamber, and removing the top w/float - the fuel level in the chamber is about 1/2 way up at most. Chambers are spotless & clean.

One can blow air by mouth through the fuel supply tube when float allows needle to be off seat.

One cannot blow air through when float pressure pushes needle into seat.

To me this implies the needle and seat are operating fine.

The gap between float body and float is just slightly below 1/8" spec. Which to me means a tad too rich. It takes a good eye to actually see the float move up when sliding 1/8" drill bit in between.

I've seen no sign of fuel coming out of the float, but not sure how to check. Should I have fuel supply line connected and car running, but have the hose going to canister disconnected to see if fuel is exiting where fumes should only be observed?

Car ran fine to work today. And last night a test drive up very steep river valley roads, resulted in no starvation, so slightly leaning float setting doesn't seem to have affected anything negatively.

So, should I be replacing the needle and seat or as hinted above will this just lead to more potential problems? Or am I to be replacing needles and jets?

And while I'm at it, please explain what is gained by tweaking the jet adjusting nut?

Thanks again..
R.W Anderson

I battled with my cars consumption, about 15mpg imperial. Did all the tuning, carburettor tricks, float chamber checks and rechecks. When all else failed I checked the speedometer and trip meter with a GPS. It was way out. I purchased a simple digital bicycle speedometer with trip meter, timer and clock. Installed it, measured the rolling circumference of the tyre at the correct pressure and set the bicycle speedometer. I'm now getting a consumption of 30 to 34mpg imperial and a spot on speed checked with a GPS and stopwatch against the milestones.

Richard Davidson
RH Davidson

No fuel coming out of the float? Or the float chamber vent? Shake the float and listen for fuel sloshing around inside, or with a plastic light hold it against a bright light and see if it contains fuel. As far as the float chamber vent goes if you have a car with charcoal canister remove the hoses from the float chamber vents and attach temporary ones going down past the engine and exhaust and terminate in a little plastic bottle. If you tape the bottle to the pipes make sure you don't completely seal it up, the end of the pipes must be open to the atmosphere or you will pump neat fuel into the inlet manifold. You can leave the pipes removed from the float vents open until you replace them, although if you have an anti-runon valve as well this will be disabled.

'Tweaking the jet adjusting nut' adjusts the mixture! As least, it does on HSs, on HIFs the mixture is adjusted with a screw going sideways into the body near the float chamber vent. But it does raise the question as to whether the carbs are correctly adjusted, which can have a huge effect on mileage and driveability. There is no point looking for other running problems until you know the carbs are balanced for air and mixture flow, and delivering the correct mixture, and that can only be done after the valves clearances and ignition settings have all been checked and adjusted, and any electrical or mechanical (with ignition and carbs) problems fixed.
Paul Hunt 2

Robert, (sorry I typed Richard last time)

The procedure we performed when we went through the 70 B was to first warm up the engine and adjust the valves. As I've never done that before on a B, it took me three tries to get it really right. I ended up getting my best result with a clik-adjust tool, a slightly maligned but interesting piece which allows accurate adjusting when the push rods and rockers are slightly worn. We knew the valves were right when suddenly gobs of power appeared where we were lacking previously.

Our next step was checking out everything electrically... plugs, wires, distributor cap, rotar... and timing (which seemed to work best at 14 degrees advance) I know you've done all this, but check the timing again, and the color of your plugs.

The car ran better but still needed better milage. Only then did we attack the carbs.

The earlier HS4s that you are running will likely have the non spring loaded needles which do require more careful centering of the jets. After all your work, I think the float levels are fine for now. We needed new throttle shafts, as they were worn. The jets do adjust the mixtures, and although they adjust differently than the H2s I'm used to dealing with, I had to admire the better float to carb body connections. Randy at Quality Coaches can set you up with the correct kits, and new throttle shafts if you need them. In fact, if you bring your carb bodies to him, he will tell you if your shafts are marginal or not.

We put the carbs back on, set their initial setting (at idle) for balance and mixture. We found the color tune not very useful with the oxygenated fuels (we did this in February of '06) and then drove the car, stopping frequently to tweak the carbs, and back down the idle, as we eliminated a flat spot at 3500 RPM. We moved each jet the same amount as we tweaked.

The net result for us has been consistent, good milage. When the car's idle went to heck this month, it was the timing not the carbs. I was certain that the carbs were good because once done properly, you usually can ignore them!

I'm betting a total rebuild of your carbs is probably in order, given the mish-mash of parts you've discovered so far, and the fact that you don't really have a starting point to go by.

good luck,
dave
Dave Braun

R.W. Hook up a timming light, after you mark the crank pully & pointer. Start the motor & see how much TOTAL advance you have. With vac. adv. hooked up & at 2500 to 3000 rpm you should have 36 to 40 deg. total, if you un hook the vac. adv. it should drop the total timming around 7 to 10 deg. & if you get no drop the vac. adv. can or hose is bad & there is 4 to 6 m.p.g. Some m.g. motors like more timming than what the book calls for! I have a 71 G.T that I have played with & it will not run or give gas millage if it has less than 45 to 47 deg. total! & YES I HAVE CKED THE CRANK PULLY to make sure the rubber hasent moved. With 87 oct. I can hear her spark knock, just a bit on a hot day & hot motor (I run a 190 deg. thermostat for mileage) I have a down draft webber with 140 main jet & do real good with the state tail pipe test (60 on the co. & 80 on the hc.@ 2500 rpm & I think there should be a 0 in there, but not sure.) This motor has 145,000 miles on her total & I did the carb. & timming 90,000 ago. When I go I GO! on a turn pike or interstate I run + 10 mph over the speed limt & do 30 to 34 mpg!!! & that is U.S. gal!!! & yes I have a 280Z 5sp. trans. Your carbs are to rich, I think on a 60 deg. day you should need choke for 2 to 3 blocks to a 1/2 mile. Lots of luck & keep me posted. Sears use to sell a timming light with the deg. dial on the back of the gun for $70. & this is what I use to make things go smoother
Glenn Towery

This thread was discussed between 05/06/2007 and 17/06/2007

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