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MG MGB Technical - Gas mileage woes

Well, I was getting about 22-23 mgp city/highway. I drive the same every week with my daily driver 69 gt.

I've had the car since last fall and have been slowly going through it. The distributor shaft was worn, so I replaced it, along with points, condensor,etc. retimed it. didnt touch the carbs (previously adjusted them).

Now i get 20-21 mpg. I put in a tank of premium, and got the same. the car runs good. seems like i should be getting better mileage.

maybe i need to re-adjust the carbs? i'm confused, but that's nothing new.

thanks for any opinions.
Don
don g

When you replaced the distributor, did that include a new vacuum advance? Could be that the new advance doesn't have the same advance characteristics as the old. If it doesn't leak, try putting the old one on the new distributor.

Wayne
Wayne Pearson

Did you measure the timing on the old distributor? Or just set the new one to the book figure? If the latter maybe the distributor was set to a different advance. Did you reconnect vacuum advance? Have you sucked on the pipe coming from the distributor to check it is working? As per Wayne are the reference numbers on the old and new distributors and vacuum capsule the same? Even if they are there are charlatans out there that will sell rebuilt distributors with any old springs, weights etc. inside them as they are no longer available (BT, DT).
Paul Hunt

don,

there is not only the distributer and ignition setting you shold considre, but also your carbs.

For a quick chech, just pull the dampers (do not intercharge them) and check the needle for wear(bright spots that look posished to high gloss).

If any wear is found, new jets and needles should give you an average of 28 to 32 MpG, as long as the floats are O.K. and the butterfly spindles are not worn out (just rock the brass spindles, there must not be any movement). Overhauling the carbs yourself is easy, the bodys normally do not wear out and you will be able to do this job yourself within 2 or 3 hours for the first time. Repair kits are readyly availble.

Ralph
Ralph

thanks for all the ideas.

the "new" distributor is actually a used one in good shape. i did verify the vacuum advance works in it.

i did check the timing beforehand and compared it to what it is now

the carbs were balanced, but i am not confident of the condition of them- that might be the main area to look.

don
don g

If you were previously running on regular and the engine wasn't pinging, advance the timing a little at a time and then test drive it, advance as muchfar as you can without it starting to ping. Running premium you should be able to easily go 5 degrees more.
John H

Just a point of reference - My 73 with rebuilt and recurved distributor, and rebuilt carbs gets about 20 mpg. Be careful when comparing MPG figures on this board because the Imperial gallon is bigger than the US gallon. So higher figures in UK/Continent may be accuratefor their gallon, but you may never get those in the USA. Yet, you may actually be getting about the same mileage. Previous posts have discussed this.
David Burke

Of course it makes a HUGE difference whether your driving in town; on 55-mph "blue highways"; or on interstates. Routine stopping and accelerating absolutely kill your gas mileage - more than you think! Worse than that!! Given all that, I live more than 60 miles from the nearest interstate but I never get less than 25 mpg.

Start from the beginning. Check your compression. If it's not around 120 + or - 10% from highest to lowest, you may want to do some head work. Check your head-nut torque and then valve adjustments. Next focus on the ignition side: point gap, timing, wires, plugs (what do they look like?), any lateral wobble in the distributor shaft? Probably not, since you just replaced it. But are the vacuum and centrifugal advance mechanisms working properly?

Then, set your carbs to nominal position: jets flush with the bridge (and plus or minus six flats - I can't remember which). Balance the carbs and continue precisely according to Haynes. DON'T "tinker"! Make a note of each adjustment you make to each carb - making sure you do the same to the other. If you cannot find a sweet spot on the carbs, perhaps they need rebuilding. Your HS4s can leak enough through the throttle shaft bushings to make adjustment very difficult if not impossible. If so, it's time to contact Joe Curto. Otherwise, you live with whatcha got. My main concern is reliability. If it's reliable, I'm inclined to postpone the next step until I get a Round Tuit. Still, on the open interstates, I get WELL over 30 mpg.

FWIW,
Allen
Allen Bachelder

"jets flush with the bridge (and plus or minus six flats - I can't remember which)."

Flush with the bridge (*not* as high as they will go which some have said) then back down six flats or two full turns.
Paul Hunt

This thread was discussed between 28/08/2008 and 02/09/2008

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