Yet Another MPG Thread--19 mpg--What to do?

I've never gotten the best gas mileage in my '74 roadster, and on the last three tanks I've carefully kept track of the mileage. 19 mpg each time. Pretty lousy, considering MGB's should get at least mid-20s, if not more.

My measurements were standard: Filled the tank each time to click-off at the same gas station, wrote exact miles down, then did the math. Exactly 19 mpg each time.

- Condition of car is excellent
- Oil changed every 3,000 miles
- Air filters newer K&N's
- New Yokohamas properly inflated
- HIF4 carbs adjusted properly for smooth idle by a long-time Brit car mechanic who I've talked to about the gas mileage issue. He says they're spot on.
- No fuel leaks, no backfiring, no smoking (me or the car)
- No bricks or bodies in the trunk
- Driving usually without a passenger, a mixture of city and 65 mph freeway driving, overdrive transmission used on freeways, and always driven moderately.

The thing runs great and is totally reliable. What's the order of adjustments and repairs I should attempt from most to least important? Or is there no hope?

Less driving....more pushing....

tune

thats condition of points ,plugs, wires timing & whatnot in the ignition system ,
ii think it is in agreement that timing should be @20 [at least] degrees BTDC dynamic

i bet you to my taste the carbs are much to rich .. but when i went true the ultimate leaning it only gained 3 MPG or so , thus a normal power tune to the carbs should be adequate [ on the plug check thats one point of inspection , too rich ? the plugs will tell the tale]

tires properly aired .. try 31 rear and 28 front .. make them hard..

one gentelman tuned and added air and gained 3 MPG [BTW 3 mpg at 5$ the gallon is about 2 cents a mile ]

MODIFY DRIVER METHODOLOGY
i suggest hooking a manifold vacuum gage up to the ig nition spark and thence to your willie... wow will that work ?

this will gain a lot,,,

I don't know about an OD Unit but i was getting 27 less and went to 31. w/o OD

i think with an OD machine i should get pretty high in the 30's 35,, more.. sure want the chance to test that

Adjust the valves.
Adjust timing.
Adjust carbs.

You may have the worng needles in the carbs - some times AAA needles are too rich and just waste fuel.
You should work with someone who has an air/fuel meter to actually see what your mixture is under YOUR normal driving conditions. Tuning for that will optimize mileage. Setting CO at idle as most shops do doesn't tell the whole story.

If you're running rich in the high rpms, then you drive your car somewhere, idle it down, shut it off, and check the plugs, you're not really seeing the rich condition on the plugs. You're seeing the condition of the engine's state of tune just before you shut it off (idle.) To really check the plugs you need to kill your engine at speed, coast to the side of the road, and pull a plug on the spot. Not typically a safe thing to do!

jeff
i am not at all to sure about that .. if a guy is shooting so much fuel through at speed he is only getting 19 mpg i do not think a few seconds of idling is gonna get all carbon fluff offn the plugs ,,
i think i am going to try that shut down at speed method.. sinciei highly value your suggestions..

that shut down process is safe as church if one does it on an approach to an interstate rest area.. ,, just takes timing and conservation of momentum..

conservation of momentum is a principle of vehicle operation that is paramount to fuel economy..

Last check I got 21 mpg

No "interstate" rest areas out here in Los Angeles, but we do have a lot of freeway off-ramps. I don't think I want to shut off a car going 65-70 mph on an L.A. freeway to check the plugs as I'd surely die. But, I will drive it and shut down soon afterwards and see what they look like. When I have checked the plugs, they looked fine.

Valves, timing, carbs have been adjusted and readjusted -- not by me, but my local Brit car mechanic. Should I have him lean them out more and see what happens?

I never checked mpg with the old Cooper's can air filters, but I doubt the new K&N's would alter gas mileage. Have a Jet Hot coated exhaust manifold and added a slightly larger exhaust, but those wouldn't affect mileage.

Not sure what carb needles I'm running, but maybe changing them would have an effect? 19 mpg is no better than any of my other cars -- BMW 325 and Honda Accord (both much heavier and both getting low to mid 20's)

So far, leaning carbs and better needles (and "less driving"). Any other ideas?? Thanks for any thoughts!

My 73 is only getting 23 at this point but its still being dialed in. Carbs are about perfect and soon the distributor will pay a visit to jeff along with a new cam and freshly rebuilt head. I'd like to see it get at least 25 highway non od. My 75 on the other hand if I keep my foot out of it and keep it under 70 will get 28 in mixed driving. 25 if I keep up with traffic and 19 if I'm flogging it in the mountians.

Drew
What about trying a different Brand or Grade of fuel just to see if it improves. I know it might sound too simple, but it might make a difference. It would be interesting to try a different Brand at each fill up to see how they stack up just for the hell of it. It might be only slight change but who knows.

k&ns supposedly make them run leaner becaus they breath better.. i see lots saying they put a richer needle in subsequent to adding k&n..

i put a k&n in and dialed the mixture even leaner. but i am a contrarian

ramp speed merging to the fast track is not ideal fuel economy procedure but one must match traffic in a light car or suffer the consequences..

i was getting 27 at least when fuel was a cheap 2 dollar bill.and i enjoyed power up launches.7/8 th throttle through the gears to top cog.. .... . now i am afraid it will get to fourty or even fifty dollars the firkin and i use a firkin every four days ..

Even slowing down (decelerating) from the speed atwhich you want to check your plugs will screw up plug reading. Think about it. Your car runs EXTREMELY lean when decelerating. Upwards of 20:1 air/fuel ratio. During that time, the carbon deposited mere moments earlier will be wiped away. Accurate plug reading isn't done after a month of driving. That only tells you about something MAJOR that needs adjustments and here we're talking about fine tuning.

I still say find a portable air/fuel meter that you can drive with. I'll have one at MG2008 if anyone wants to see how it works. Anyone who buys a distributor there will get the opportunity to have me install it and run the air/fuel meter on their car as well.

i wish i would have brought my plug wrench i would check this PM when i get my Ferkin of Fuel.

that would look good ,, its on 2 lane blacktop us79 .. if i timed wrong and did not roll up to the pump after taking the bad patch transition from road to lot ,, i might have to get out and push it up to the pumps... ala.. grapes of wrath... there lots going down the road running on fumes these days /

rather than relying on the pump as a way of measuring, because they are off quite a bit, more than you'd imagine. I would just figure out how big your tank is and get a good estimate.

My 67 tank is 12 gallons, and I can stuff about 13 in there if its just about spilling out.

I usually go by that rather than the pump.

Here's why, during the summer heat the gas expands. I know they're supposed to be accurate, and they're checked by some governing body, but they're off.

I brought a 5 gallon can down to the gas station by my house, and I've had this can for 20 years, IT HOLDS 5 gallons, maybe a half a quart over.

Well, at this Shell on the corner of Adams & Brookhurst in HB, I ended up paying for 7.4 gallons of gas! Yep, 7.4!!!! I was so pissed! The A-hole actually accused me of putting the gas in another car. The only thing was, I rode my BIKE!

7.4 Gallons in a 5 gallon can, so they're not very accurate.

This could be some of what you're experiencing. This could contribute to the low numbers.

I've NEVER had a gas station error on MY SIDE! so why would anyone else get any benefits.

I'm pretty sure that your 74 holds 12 gallons too!

B

BManBrian67 Wrote:

Hap - what's your take on reading plugs for street cars???

B-racer Wrote:

And mash the clutch pedal.

I'm not sure that the mileage is so bad since it is a combination of city & highway. Interstate only should get you in the 25-27 mpg range.

"" FLYING CUT""

a phrase to remember .. seems it stirs my memory of a phrase not heard by me but by my ancestors.. da was a big radial wright mechanic .. granther was a steam engine mon ,, gg pa was a blacksmith in the years when they were men of power..
i .. i .. am only a dilitant... and a seeker of wisdom..

do not push the clutch ,, 'box the gears out " ie: select n nutreal[sp].. seems good advice to let the pistons go still instead of being driven by the shaft.. but no sence in excessive wear on thm throw out bearings

i read 31 on the last firkin .. 27 on this one , i had leaned another turn. watching the EGT my temps were a little high.. an oddity , some times on throttle reduction EGT climbs , i have to blip throttle a couple of times to make it start to fall.

i am starting to turn the mix towards rich now.. sure wish i had that flexi-shaft fixed to the mixture screw goingto panel.. that would be everso much easier...

I was told that this is the way they should look.

I'll drive it and give it a 'flying cut' to check the plugs, but not today or tomorrow as it's about 106 degrees here and I'm lying low in the a/c today or I'll pass out. Another reason to add a/c to the car???

I'm not sure different brands of gas are going to have any noticeable difference. I have always used Shell or Standard/Chevron or maybe Mobil. Whatever's available, and I've never seen any significant mpg difference. For this mpg test, I refilled using the same brand of gas, of course, so the figures (19 mpg each time) would be legitimate.

As for topping up, Brian, which is what you're suggesting, I generally do that. After it clicks off, I pull out the nozzle just a little and give it one last squirt until it clicks off. That way, I have the tank filled the same each time.

But, back to lean vs. rich: My MG mechanic who's been tuning MGB's for 25 years or more, says he doesn't want to lean it out anymore because he's got it running so well right now.

I switched to regular gas (from premium) and the 'popping' and/or backfiring problem I had went away for some reason. Popping from beneath the car or out the exhaust (not sure where it's coming from) would suggest it's too rich. But, again, that problem seems no longer to be there, and I've had the adjustment double and triple-checked for leanness, and Frank (my Brit car mechanic) says its exactly right. But, yes, I'll check plugs when the damn temperature drops a little and I can stand to drive the car again.

You think one of those magic magnets you clamp onto your fuel line would "double my gas mileage"? :) Or water injection? :)

19 mpg is not good .. a 110 CI in a light car car should do better..

... 25 at least.. unless every start is a full throttle to speed , then brake to light and back again.. , surely that not the case .. i do not know whats not right but something is not ...

ignition/timing / carb adjustment /engine condition ...something ... these are simple systems.. set the ignition with new parts , set the valves , lean the carb properl, no jack rabbit starts .. should do 25-27 looking at 30 ....

please return the solution so we all may learn

mrbarry Wrote:

there is all kinds of questions about flying cut ..

should one cut the switch while throttle is open in acceleration condition , while held steady in cruise condition, on the flat , on the grade ,,

i suppose each condition would test the look of the plugs under a different engine load condition ..
when i do this i am not going to be wheeling down the road holding the clutch pedal down , i am going to select neutral .. i have enough trouble chewing gum and shifting at the dsame time ..

i have a nice spot pulling the tennesee divide,,i think i will try a flying cut just before topping the grade try 4 th at 2500 , 3rd at 3000 ..

Seems like I always gravitate to this area of the car but...

Is the PCV system in good working order or hooked up at all? If it is set up wrong or has leaks it can act like a small vacuum leak. The leak is usually small enough to allow you to compensate by enriching the mix. I would check the following:

- oil filler cap snug or get a new one
- oil dipstick should have a tight fit in the tube, replace or use some silicone to get a good seal
- restriction in the valve cover breather
- valve cover gasket not leaking
- side covers tight and not leaking
- hoses to the carburetor take offs tight and in good condition