Terrible gas mileage on mopar 318. 5MPG!?!?!?!?

[TWS67'Fury]

I have a mopar 318 that i just rebuilt. Quick build sheet, new pistons (flat top), new bearings all around, new cam (Summit 1789 .454lift and 272 dur), new lifters, new timing chain and gear set DUH, new used heads (1974 open chamber heads with hardened valve seats, old heads were junk), stock headers, offy dual port, holley brawler 600cfm 66 main jets. Anyways im getting close to 5mpg!?!? like wtf i WAS getting 10mpg with the same offy intake. before i hear it i know the offy dual port isnt the best but its what i have. ive read some posts about carb fuel lines ect. my plugs look fine tan on the porceline and a little darker on the electrode im just here to maybe get another input to what can go wrong. i have already checked the oil and im not dealing with a (internal or external)leaky fuel pump because my oil never goes up. I am prepared to redo all of the fuel lines i do think my issue is at the sending unit but are there any other places i should look. Also instead of rambling i have a quesiton. Could not having a carb spacer cause such detremental gas mileage?


Sorry for the rambling

 

[413]

You are not rambling, just telling the story.

how many miles on this new engine? What kind of driving are you doing to get 5 mph without black plugs? Something is not right.

fuel lines get fuel from the tank to the carb without leaking. Can they make it get bad mileage? Not likely, unless it is leaking. The Carb controls how much fuel gets into the engine.

 

[65Port]

How does it run and idle? Is it hard to start? Timing- where is it at? Is your fuel gauge playing tricks on you? Have you thrown a vacuum gauge on it? I know it's rebuilt an' all but...

 

[TWS67'Fury]

You are not rambling, just telling the story.

how many miles on this new engine? What kind of driving are you doing to get 5 mph without black plugs? Something is not right.

im still in break in on rebuild about 250ish mi in. motor had 86k miles and i confermed it wasnt rolled over. driving is constant variety some cruising and some revving keeping it changing so the ring and everything get acclimated to a diverse driving style. And no kidding something isnt right.

 

[MoPar~Man]

What transmission?
Brakes are not dragging?

 

[TWS67'Fury]

How does it run and idle? Is it hard to start? Timing- where is it at? Is your fuel gauge playing tricks on you? Have you thrown a vacuum gauge on it? I know it's rebuilt an' all but...

starts fine when cold has the hot start issue my timing is sitting about 12-14 initial and i belive 36 total. I know my fuel gauge is not accurate at all but as with any car there is always a way to tell what level im at i know 1/4 on the gauge is 10-12gal (more than half) anything above 1/4 is not accurate i can also hear it in the tank and ive double checked by dipping a hose is and seeing what gets wet. im replaceing the sender this weekend (hopefully) that way I can have a semi accurate reading. and to answer about the running and idle its got a slight miss due to one of the cyl having the wrong sparkplug. driving is fine except if i try and hold an rpm it kinda sputters i assume due to either too rich or too lean main jets acceleration is perfect she pulls hard and throttle response is amazing.

 

[TWS67'Fury]

What transmission?
Brakes are not dragging?

brakes are not draggin and the transmission is a 727. the car also has the towing package idk if this helps.

 

[Knebel]

Powervalve stuck open/wrong value? Advance in the distributor working? Vacuum advance working? Sticky needle valve maybe?

 

[413]

What camshaft specs?
How much idle vacuum?

you need to fix the miss, it will affect ring sealing.

 

[Old Boomer]

Exactly how are calculating your mpg's?

 

[MoPar~Man]

Ignition? Original or aftermarket electronic?

 

[65Port]

starts fine when cold has the hot start issue my timing is sitting about 12-14 initial and i belive 36 total. I know my fuel gauge is not accurate at all but as with any car there is always a way to tell what level im at i know 1/4 on the gauge is 10-12gal (more than half) anything above 1/4 is not accurate i can also hear it in the tank and ive double checked by dipping a hose is and seeing what gets wet. im replaceing the sender this weekend (hopefully) that way I can have a semi accurate reading. and to answer about the running and idle its got a slight miss due to one of the cyl having the wrong sparkplug. driving is fine except if i try and hold an rpm it kinda sputters i assume due to either too rich or too lean main jets acceleration is perfect she pulls hard and throttle response is amazing.

Sounds like you have a couple of things that need attention first. If it were me, I would not replace a single thing until the idle, the wrong spark plug and sputter issues are resolved. Introducing new parts always mucks things up. Once those issues are addressed, then see where you are at. 5 mpg would leave evidence somewhere else so I have a hard time believing that is actually happening. Good luck and let us know how it goes.

 

[Justin Plant]

Leaking fuel from the feed line at the tank?

 

[TWS67'Fury]

Exactly how are calculating your mpg's?

Two fill method. 1. Fill up (all the way) record milage 2. Drive till close to empty. 3. Fill up (all the way) record mileage. Divide ect. Formula will look like this. Mi²-mi¹=mi³ mi³÷gallons filled=MPG

 

[Triple Pickle]

A 318 that only gets 5 mpg would run like crap. Or is pulling a heavy load uphill. Or is hooked to a very low geared differential. Is the odometer somehow inaccurate to the point of racking up 3x the miles it’s traveling? Just throwing that out there.,

 

[TWS67'Fury]

A 318 that only gets 5 mpg would run like crap. Or is pulling a heavy load uphill. Or is hooked to a very low geared differential. Is the odometer somehow inaccurate to the point of racking up 3x the miles it’s traveling? Just throwing that out there.,

Hooked up to a 2.79 or 2.94 diff and I've checked miles against a GPS tracker and there I'd no discrepancy.

 

[TWS67'Fury]

Leaking fuel from the feed line at the tank?

I don't know I'm going to check this weekend I did have anissue with a leaky gas tank when I first got the car. It stopped on its own but I wonder if the bumps and conditions of normal driving means it's only happening when driving.

 

[TWS67'Fury]

Sounds like you have a couple of things that need attention first. If it were me, I would not replace a single thing until the idle, the wrong spark plug and sputter issues are resolved. Introducing new parts always mucks things up. Once those issues are addressed, then see where you are at. 5 mpg would leave evidence somewhere else so I have a hard time believing that is actually happening. Good luck and let us know how it goes.

I absolutely will I have a feeling it's the feed line like another member suggested. I will get the idle and everything under control first before anything. I also am wondering if I need to regap my spark plugs for my cam.

 

[TWS67'Fury]

Ignition? Original or aftermarket electronic?

Original. Got a new dizzy and coil for the rebuild. Stock single point style.

 

[TWS67'Fury]

You are not rambling, just telling the story.

how many miles on this new engine? What kind of driving are you doing to get 5 mph without black plugs? Something is not right.

fuel lines get fuel from the tank to the carb without leaking. Can they make it get bad mileage? Not likely, unless it is leaking. The Carb controls how much fuel gets into the engine.

I've also decided to run the tank fully empty to see if my gas gauge is more off than its ever been. I assume if my mileage stays the same way I have around 15miles left but we will see