Thin Lizzy

6mpg. Is that normal?

26 posts in this topic

Hi All,

My 69 351w 4 barrel is averaging 6mpg at the moment.  Is that normal?  It's got an Edelbrock 1406 and has headers. I'm not thrashing it, just driving normally.

Is it running too rich?  Is there something I can do to improve the economy please?

Thanks for any help you can give.

Mark.

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If the plugs aren't all sooty, and your exhaust doesn't smoke or burn your eyes, then I'd say you have a fuel leak somewhere.  Any wet spots under the car when you park it?

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No soot on the spark plugs.  Exhaust very slightly smoky on start up but then ok.  No wet spots on the garage floor.

Wondering if something else has been added to the car as it does accelerate very quickly. Goes faster 0-40 than my Lotus Esprit Turbo.  Great for putting a smile on your face, but not so good for the wallet.

Ill have a look and see if it has the vacuum advance has been disconnected.

Thank you for the information and help.

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I have been chasing low mpg for years.  Has anything changed in your engine?  Did it formerly get much better?  Has it been this low for a long time?  I asked the same question a few years ago, and got some good replies.  I think you can search for my post ( or others) on the subject to see what they did. 

Start with the easy things, like plugs, wires, disti, coil, timing, vacuum leaks, carb mixture, plugged exhaust, fuel leak ( mentioned above), stuck float in carb?  Those are the simple things. It is easy enough to buy new parts ( and keep the old ones ) and replace them to see if any of those things fix it.

I went through the process with mine, and I think it could be the wrong cam was installed when the engine was rebuilt years ago.

Good luck it can be frustrating.  Let us know of progress.

Thin Lizzy likes this

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The timing was out and was sorted last week.  Wheel alignment is ok.  Haven't checked for dragging brakes, will do.  I'll check for a stuck float too.

Thanks.

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What has been done to the engine besides headers and carburetor change?  What rear end gear ratio does it have, and when doing 55 mph, what RPM does your tachometer  say?  What transmission does your car have?  What diameter pipe is your exhaust?

Thin Lizzy and reginalduj16 like this

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No tacho on the car. The trans is an fmx.  Exhaust is a Flowmaster.  I've owned the car for 2 months, car has no history so no knowledge of what's been done.

I'll check for valve over lap.  Thanks.

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Should get much better than that.

in my 66 conv. 289-4V ported, polished. Cam, pertronics and MSD I get an average of 20mpg if I drive easy and can cut that in half if I am driving energeticly.

 

the combo of parts you put together make a difference. Cam, intake, headers, all need to matched for the band they work in

 

bob

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1 hour ago, Thin Lizzy said:

No tacho on the car. The trans is an fmx.  Exhaust is a Flowmaster.  I've owned the car for 2 months, car has no history so no knowledge of what's been done.

I'll check for valve over lap.  Thanks.

When on the highway, does it sound like the engine is running at a really high RPM, or normal range?  When idling, does it have a rough idle or smooth idle?

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It's got a smooth idle.  Doesn't sound high rpm at 55.  Engine and car run nicely.

Used to get 18-20 easy from my 1972 302 vert.

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23 hours ago, moodster said:

Could it be that you have so much valve overlap that the fuel is getting burnt in the exhaust?

david

This is what I was wondering about.  How would I know?  I was guessing the CAM could have been doing it.  I replaced the timing chain 2 years ago, and double checked that is was correctly set.  How do you know about overlap?  What do you do about it? My car runs perfect other than the MPG issue and it runs a little hot. .

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If your engine has a smooth idle, then you probably don't have a lot of valve overlap.  How many times have you averaged 6 mpg on a tank?  

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???

Ah yes he is over the pond, whoa not only 6 mpg but super expensive gas.

Maybe something easy like a hung up brake pad?

 

Good Luck 

 

JIM

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The car roles easy so it's not the brakes.  And it's 6mpg (UK imperial), so more like 5 mpg US.  We pay £5.50 a gallon.  With the old exchange rate that would be like $10 a gallon, but the pound is weak at the moment so it's only $7 per gallon.

I've checked for leaks and there's none.

 

 

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Are you converting the odometer readings to match the fuel? 

Mph to meters?

kilo? To gallons

If not stored in a garage at night, may try to see if it improves with a locking gas cap

And my funny for the day..maybe be leaking from the carb overflow tube from being inverted, down under 

Back in the day in my HS years and money was tight, I put a 4x2,carb adapt and 2.85? gears  in the car, mileage was great. Now mpg are irrelevant to me, all the stares and thumbs up make that thought go away

grendi and Thin Lizzy like this

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The car is garaged.  After filling up and I put my foot down a few times I can see the fuel gauge needle move.  I drove 75 miles to Brands Hatch motor circuit and used half a tank of fuel.

Oz is down under, not us.  I thought we were the 51st state! 

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First off you have a beautiful car. It must a be a real head turner in your town. I would think it would be like driving a DB4 in my town. : )

Scanning quickly through this thread, I agree with the comments. Assuming all the above recommendations are addressed, you may want to look at the Edelbrock carburetor. A 600CFM is certainly fine for your setup, but personally I have always had trouble getting the Edelbrock carbs tuned properly, especially at WOT. At 6mpg you are using a boat load of fuel. So much so you should smell it in the exhaust. Your eyes should be watering near the back of the car. Something doesn't add up. Do you have access to a different carburetor for testing? In my opinion, a Holley 600CFM 1850 style would be a good start. 

You probably do this already, but my prefered method for checking mileage is to fill the tank, drive a distance, then fill the tank again. You then, divide the kilometers driven by the liters it took to re-fill . I would think you should be getting at least 5KPL not the 2.5KPL (6MPG) you are getting now. 

What is it like driving a left hand drive car in a right hand drive world? A bit different, I'm sure. I'm not sure my brain would know how to shift a 4spd if I had the drive a right hand drive car, Ha!

 

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Yes you're right, the exhaust does smell of fuel.

Thanks for your comments.  Driving on the left does take a bit of getting used to, you need to hug the kerb, otherwise you'll drift to the centre of the road.  It's not so good for overtaking though.

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