rebuilt my 66 DeVille engine and my gas mileage is worse

[huskertodd]

I recently rebuilt the engine in my 66 DeVille.  Before I did it I always got between 10-11 mpg.  Now I get between 7-8 mpg.  It is a drastic change and one that leads me to a gas station at every turn.

The guy that did the work blames it on everything but his work.  Am I wrong to think the new engine should be at LEAST as efficient as the old one?

I replaced the gas tank, the carburetor, engine and about every other item.  Any suggestions or do I just live with it?

[scotth3886]

I recently rebuilt the engine in my 66 DeVille.  Before I did it I always got between 10-11 mpg.  Now I get between 7-8 mpg.  It is a drastic change and one that leads me to a gas station at every turn.

The guy that did the work blames it on everything but his work.  Am I wrong to think the new engine should be at LEAST as efficient as the old one?

I replaced the gas tank, the carburetor, engine and about every other item.  Any suggestions or do I just live with it?

Are you getting any noticeable black smoke?  When driving and you get on it are you seeing a cloud in the headlights of the car behind you?   If yes and you also have a (rough) idle, you might have the same thing I had with the shim next to the intake instead of directly under the carb. 

My mechanic put mine back together after changing the intake to cure an exhaust leak in backwards order with shim next to the intake.  My gas mileage fell to 9 - 10 mpg from 12.

It clearly says how to do that in the shop manual and what will happen if you do it wrong.  I took it apart again last Friday and confirmed what the issue was.  I haven't checked gas mileage since I (hopefully) fixed it, but my gauge is moving slower. 

[chrisntam]

Give it some time to break in.  How many miles are on it?  How long ago did you start driving it?   Who did the initial start up?  Does it appear to run smooth and as it should?

Perhaps it needs to be tuned - jets, timing, etc.

If it's any consolation, my '70 gets 6 to 8 in town.  I did (imaginary) backflips when the car got 12 on the highway (500 ci engine).

[James Landi]

Make sure to check the vacuum advance on your distributor is functioning .  It's easy to do-- with a piece of 1/8 vacuum hose, simply suck in on the hose... with the distributor caps off, you should be able to perceive a tiny bit of movement from the vacuum you create.  If there is a leak in the vacuum advance diaphragm, you'll know immediately-- you can "pull air" through it.  This important little part is often overlooked and will negatively affect gas mileage and also cause your engine to run "rich" at idle.    Hope this helps.  James

[scotth3886]

If you don't already have one, a Mityvac comes in real handy.  It's packaged several ways depending what accessories you want with it.

I can't imagine owning an old Cadillac without one of these.

https://www.amazon.com/Mityvac-MITMV8500-Silverline-Automotive-Vacuum/dp/B0002SQYUA/ref=sr_1_8?dchild=1&gclid=Cj0KCQjwpNr4BRDYARIsAADIx9wYssVToSEeWv235OCoafeZ7YSOs1My-NxBcu9tFAtIeO7Fu9rJULYaAiggEALw_wcB&hvadid=409921603608&hvdev=c&hvlocphy=9014894&hvnetw=g&hvqmt=e&hvrand=2609973154558285964&hvtargid=kwd-63200551&hydadcr=26612_10407640&keywords=mityvac&qid=1595334336&sr=8-8&tag=googhydr-20

[fishnjim]

I don't recall anyone buying one of these for their gas mileage.
But the "rules" of troubleshooting say if something was changed and problem arises, it's most likely related to/caused by the change.
I'd take it somewhere with a tailpipe exhaust gas analyzer and see it's adjusted properly first, ie, not too rich.   The old by ear is not as good with this modern gas and untrained ears.   Also check for fuel leaks.   Especially, if the carb was accepted and not tested/adjusted.
It almost has to be fuel system related, but that's getting ahead of identifying the cause.
But the driver can exacerbate the issue, if you've been - putting the pedal to it, testing it since it's running better after rebuild.   Once you're into the four barrel secondaries, gas mileage is a laugh. 
It's possible rebuild tightness/wrong clearances would also be a factor but the magnitude is "great" and you'd have other issues, like overheating, hard for starter to turn over, etc..
I recommend anyone doing an engine rebuild these days of an old motor, to have it run on the dyno, at least engine test stand, before you accept it and pay.   The extra insurance cost is worth it.   Words are good, but mistakes happen, data is golden.

[walt chomosh #23510]

Todd,
  The "red flag" for me was the carburetor change.....walt...tulsa,ok

[35-709]

Yes, if that was an aftermarket, off the shelf carburetor, rather than a rebuild of your original, that is likely to have at least some effect on your poor mileage.