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Valves and MPG Question

Posted by Burrell77

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Valves and MPG Question #1
Burrell77 Cburrell77 Burrell New Hampshire, USA 1977 MG MGB MkIII	Posted: March 31, 2012 09:19AM Registered: 2 years ago Posts: 135

OK, I am confused, last year I consistently got 28-32 mpg with my car (9000 miles). Now I am getting 20-24 mpg this year.
the only changes I made were, new wires, cap, plugs, set the valves, re-tuned the carb, no engine mods or other changes. Adjusted the carb to weber specs. Why did I have such a drop of mileage?

Last year I put 9000 miles on the car, always had to adjust the carb with change in temp, the exhaust popped as I down shifted.
Burned oil as it went past the valves, Now that I adjusted the valves the car runs great.

Here are the details of the car

1977 B with 18V motor, 4spd with OD, weber 32/36 carb, 25D lucas distributor with pent. ignition, champion plugs gapped to factory specs, Valves were set to .013 warm, timing is 32 BTDC with idle at 3200rpm. The engine idle is set to 650 rpm. Compression is beautiful, 150 per cylinder, I run castrol 20-50W oil. (Only change this year from last is the valve clearance)

The PO set the valves last spring so I thought they were correct and the engine had very loud tapping sound, so this winter I set the valves and there were a few that were way out. The engine purrs now, it runs smooth idles beautiful,
So here is my question would adjusting the valves tighter affect my MPG that much? If not then why would I have such a huge fluctuation?

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Re: Valves and MPG Question #2
rrmgb robert schau Reston, VA., USA 1973 MG MGB 1973 MG MGB	Posted: March 31, 2012 09:24AM Registered: 6 years ago Posts: 8,984

Weber specs are a tad rich?
RS

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Re: Valves and MPG Question #3
pmaland2000 Paul Maland Leduc, Alberta, Canada 1973 MG MGB "Bea"	Posted: March 31, 2012 09:49AM Registered: 6 years ago Posts: 568

Maybe the valves a bit tight? Many say .015" hot... I dunno. I just set mine yesterday and they were all a little tight. I noticed that after, the engine ran stronger but I did hear more valve train noise with the added gap. Normal.
Back in the day, we set valve lash on some cars (with adjustable rocker studs) while the engine was running. You could watch the vacuum (gauge) drop and the engine lose rpm with even the slightest turn too tight. It can be that sensitive.

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Re: Valves and MPG Question #4
pinkyponk Adrian Page Nova Scotia Canada, Canada 1971 MG MGB GT 1973 MG MGB MkIII	Posted: March 31, 2012 11:35AM Registered: 2 years ago Posts: 2,736

32 sounds mighty high for a B. The 24 sounds more reasonable.

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Re: Valves and MPG Question #5
Rod H. Amity, Oregon, USA 1964 MG MGB 1968 MG MGB GT	Posted: March 31, 2012 12:01PM Registered: 9 years ago Posts: 14,987

The 28 to 32 sounds like highway mileage, and the 20 to 24 sounds like suburban driving.

I'd look at the fuel mixture and driving habits before a change in valve clearance, especially considering they sound like they are set correctly now.

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Re: Valves and MPG Question #6
rundjk David Kinsey Virginia Beach, USA 1980 MG MGB "Runaway"	Posted: March 31, 2012 02:24PM Registered: 3 years ago Posts: 1,476

What kind of time period is "this year?" One tank? Two? Six? Have you changed gasoline brands or octane levels? Did you use Sta-Bil last year and not this year? I have noticed a few gas stations in my area have cut away the "up to" on the label that used to read, "This gasoline contains up to 10% ethanol." If it went from 7% to 10% that may affect maybe 1 or 2 mpg, but not 4 mpg (I don't think) but a combination of poorer gasoline overall and more ethanol might do it. Also, low tire pressure can take away 1 or 2 mpg. More agressive stop light driving (revving to 4K in 1st and 2nd vs. 3K) could also be worth 1 or 2 mpg if you do a lot of stop and go. If I am consciously trying to see how good I can get my mileage to, it will go up; if I'm out having lots of fun driving, it will go down.

David

David J. Kinsey
Virginia Beach
Tidewater MG Classics
1980 MGB "Runaway"

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Re: Valves and MPG Question #7
Burrell77 Cburrell77 Burrell New Hampshire, USA 1977 MG MGB MkIII	Posted: March 31, 2012 09:38PM Registered: 2 years ago Posts: 135

I have gone though 5 tanks of gas this spring so far, I am running the same octane 87 here in NH we have been at 10% ethol for a quite a few years and back then I did notice a drop in MPG. I will readjust the carb again to make sure I am not running too rich. I am also going to check my timing chain to make sure I did not jump any gears. I will also just check the clearance to make sure that my nuts did not tighten or move after I have driven it a few hundred miles.

I do know that 32 sounds high for a B but it was nice and consistent for 9000 miles the PO also told me he would get 35 MPG. I do quite a bit of highway driving .
Thank You
Craig

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Re: Valves and MPG Question #8
oily-hands Owen Frankland Stockton on Tees, Cleveland, United Kingdom 1937 MG TA "Numbum" 1971 MG MGB GT "The Bruise" 2002 MG ZR	Posted: April 01, 2012 02:58AM Registered: 3 years ago Posts: 5,183

I wouldn't bother checking the timing chain. It's almost impossible for them to jump a tooth and if it did, the almighty clatter would alert you to the fact. It wouldn't run great either.

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