Gas mileage LOW...help me out

Ok, so my son claims that he is getting only 6-7 mpg around town in his newly rebuilt slant 6 volare.

Running stock style carb - it was a new in box oem from years ago.....

The car seems to run well, to me.....he does let it warm up a bit before pulling out and he does a lot of short trips to and from school and work....2-4 miles each way, so really the car never has the chance to fully warm up.....

I know those short trips hurt mileage and arent great for the engine....same with the longer warm ups....

But to the tune of 6-7 mpg? That seems impossible.

He says he gets better on the highway.....like 17 ish.

When its idling it does smell rich.....haven't pulled plugs yet to look at em....

Hell, my Cuda with a tweaked out , cam'd up 4 bbl 318 gets 8-9 around town and thats with me hammering it all the time...LOL

First I thought it was his shorter winter tires causing the odometer to be off....but he just put his summer tires on and reported this lousy number.

Thoughts??

Choke opening too slowly?

Someone with knowledge will jump in, but Is it smelling rich because the choke it still on, cold? Seems from memory, his mileage isn't far off for short, 2-4 mile trip mpg. Carb should have been adjusted, and should improve some. It might need a little break in time to, depending. But that intake I think the carb pours the fuel on the intake floor and evaporation is the mixture till it heats. lol

How's the choke operation? What jet(s) in the carb? Knowing exactly which carb it is will help with tuning suggestions. Pull #1 and #3 spark plugs and snap a good, well lit picture.

This might sound overly simple, but are their any fuel leaks? either back near the tank or the fuel pump, lines, etc.? Could there be an issue only when it's moving and gas can slosh out? Just trying to eliminate all possibilities.

I should also ask what the timing is set to?

Vacuum leak on the carb?Those out of the box rebuilt carbs are not put together well,had a similiar issue years ago with one of those carbs

What heat range spark plug ? Are the spark plugs new? If so, pull a spark plug or 2 and see if it is black. If black not sooty, it would be a good indicator of it running rich. I usually stick with Champion plugs on those. I have seen old mopars run poorly on AC spark plugs back in the day. I would put in Champion or Mopar plugs and it would run great. You could spray around the base of the carb to see if the idle picks up to help ID a vacuum leak. If it has a egr valve I would check the operation of that. BTW, my 360 tunnel ram with two carbs and 4.56 gears would get around 6 -7 mpg but that was highway and around town. LOL

Verify timing and time in advance.

The car seems to run well, to me.....he does let it warm up a bit before pulling out and he does a lot of short trips to and from school and work....2-4 miles each way, so really the car never has the chance to fully warm up.....

Id say thats most of it there, and /6 have always been a mess on short runs with the external type manifold.

To compare a /6 and a V8 on how they heat up from a cold start is night and day.

If you have a heat gun, have him shoot the intake manifold in many locations at the end of a 4 mile trip. With the typical 2.90s type gears its never getting strained or any good rpm.

Id lean the choke out as much as possible, lean the carb out as much as possible and go with a hot T stat and the proper plugs. Adjust the carb for the lightest pump shot hole it will handle without hesitating from a start.

You also could easily fill it up and actually drive around town for 20-30 minutes a few times and them refill and see what pops then.

A good tune and a easy foot and not counting in only the 4 mile trips even with todays gas 10/20 should be obtainable.

4 bbl 318 gets 8-9 around town and thats with me hammering it all the time...LOL..... Is he driving like dad?

Not a really hard issue to sort out, good luck.

LOL he very well might be driving like Dad....LOL

And I am leaning towards the opinion its the short trips killing the mileage.....and will look into having him lean the choke out some and see what leaning out the carb does.

He says it pings under easy acceleration around town unless he runs 93 octane.....

If the choke thermostat is 10 to 15 years old, then replace it. There are many for sale on eBay, if your local parts store does not have one. The bimetallic spring on the thermostats seam to rust and just quit working well after 10 to 15 years.

1-bbl version

2-bbl version.

Make sure the choke thermostat is hooked up to a power source like this one. If it is missing or unplugged, then fix that (as well).

This is one of four different versions. The other version pair is about ½ this size, is round and is black (instead of white and rectangular). Both versions have the resistor on its side (or not).
All pictures were swiped from eBay.


If you have a new carburetor – then I doubt you have a carburetor problem.
If you have a rebuilt or reman carburetor – that that might be your problem. The carburetor rebuilders (retail box companies) tend to mix and match parts and in most cases, to dismal results.

With that said, if you take your original carburetor to a shop to get rebuilt (ie: you get your original one back) – then in most cases you are good to go.


Another possibility is stopped up catalytic converter, leaking brake booster or vacuum hoses broken/missing/mismatched. Also check the engine air filter - for a stooped up or restricted filter causes poor mileage.

Excessive engine idling (ie: warming up with no one in car, etc.) with car not in motion can also be a cause.


I would not drive any car for only 2-4 miles - except for occasional use only. Constant short trips are murder on engines for several reasons. I highly suggest finding a different route to work or school, so trips are at least 6 or 7 (7 or more is suggested) miles in length. It takes a while for engine to get hot enough to percolate out all water moisture as well as fuel vapors from it. Continued short trips WILL cause sludge. Also, short trips on carbureted vehicles allow for a lot of fuel to gather into the crankcase. Fuel is not a lubricate like engine oil is.

Short trips after engine is hot, is not of any issue or concern. It is any trip where the engine temperature gauge starts off anywhere but in the mid area of gauge.
BudW

I have a Slant Six -- and have been putting some brain cycles on your problem. In your other post, you have an under-hood shot. It's clear to me that there has been lots of tinkering under there, and several systems are missing. In particular, the carb heat systems is missing. I can tell you that having the correct temperature air coming into your carb (in this case, 90 degrees), is critical to having the car drive correctly. During warm up, the choke system provides the rich mixture to allow smooth driveability, and upon warm up, the carb heat system provides 90 degree air to the carb so the rest of their systems can do their thing. The carb heat system is: The heat stove on the exhaust manifold, a piece of flex pipe that goes up to the air-cleaner horn, a vacuum operated flap on the horn, and finally, a thermactor on the air cleaner that measures temperature. The thermactor varies the amount of vacuum going to the flap on the horn to keep the temperature in the air cleaner at 90 degree. It's seems like such a little thing, but indiscriminately ripping off "emissions junk" will often result in driveability problems if one doesn't know how to compensate for it. If you could find a donor vehicle, putting Humpty Dumpty back together again would, I'll wager, solve many of the problems you're having. Barring putting it back to factory specs -- You could also consider "starting over." Get yourself a new 4 BBL intake and small carb... Or you can just do this: Slant Sickness! How To Add 77 HP To Your Mopar Slant Six! - Hot Rod Network

BudW also noted the missing OSAC valve. That very-possibly could be the cause of your ping... Because, what it does is delay vacuum advance for roughly 15 seconds (depending on temperature, that is why it is on the air-cleaner.) Your distributor is getting an advance signal earlier than what the timing curves were designed for. If you could find a Slant 6 in a bone-yard, go part shopping. BudW ALSO pointed out the the choke thermostat and resister -- also very important! The "little black box" is an electronic thermostat. It provides full voltage to the spring to assist with choke pull-off. The resistor provides a lower voltage once the thermostat shuts off to make sure the spring doesn't cool off too much and re-choke the engine. All these things work together to make your engine happy. My Slanty didn't run right until I got the heat stove and electronic choke pull off working correctly... When I presented the parts to the check out counter guy at the bone-yard, they sent me on my way without charging me. By way of PS -- The electronic choke thermostat, the OSAC valve, and the thermactor were used on V8's, too, so the parts don't have to be from a Slant.

I told this story to to a friend just yesterday.
My mom bought a 1966 Plymouth Belvedere II in the spring of 74, /6 3 on the tree, now my mom had a way of driving that Plymouth that would make most car guys cringe. She be thru first second and into third by 18 mph or so. The dash would shake at every gear change, but she always got 20 mpg around town. And at 55 the Plymouth would get 25 mpg.

Here is a picture of the black round choke control.

This one does not have the resistor on it.

There are versions of the white rectangular as well as the black round choke controls that have resistors on them and versions with no resistors.
I just couldn’t find pictures of all four versions in a short time frame.

I think any one of the four versions could be interchangeable, possibly. I do know that any version will work better than “nothing”.
BudW

We got the carb in a bubble of air formed by the plastic it was in, with a cardboard base. I'm going to assume standard jets and equipment for a super six 2bbl.

The engine was rebuilt. We milled the head and bored a little. Plus a very mild cam, nothing crazy. Compression was obviously increased a bit.

We decided to convert to the super six package, which we did. Had a rough experience with a gasket, but we fixed it.

We changed the choke, to an updated electric choke. New temp controller.

We cleaned the carb with carb cleaner, spraying the outside and inside a few weeks ago. It seems to be leaking from the rod leading into the top of the carb. I don't know the exact term, but there is a picture.

Just standing next to the car, I can get a wiff of gasoline, hood shut and everything. I'll assume it's a combination of running rich and the leak.

Everything that is on the car was there when we got the car, aside from a make shift heat shield for the carb.

Summer's rolling around, so gas mix is going to change? I've got a bit of knocking under light throttle, on flat and up hill.

I'm going to re-time it based on vacuum, and retard untill no knocking. Then look into vacuum advance. Tune mixture and set final idle speeds.

I notice the idle speeds did get lower since the last tweek, but I'll assume that's just things settling in? We didn't put the gasket back on for the distributor, but I highly doubt that's causing any harm, asside from seepage.

A Carter BBD.

The_Red_Barron said:

It seems to be leaking from the rod leading into the top of the carb.

Click to expand...

Do you mean the location indicated by the white arrow?
If so, the fuel level is way too high.

The fuel level should be around the blue line (above).
Matter of fact, you can remove the carburetor top with base and body still on car (without any fuel leakage/spillage), if you wanted to.

Get the fuel leak fixed first, then go back in and reset idle and timing (for it may affect the other settings).
BudW

No gas should be leaking from up there. As stated remove the cover and check the function of the metering rods. If needed, those rods can be adjusted but I would search online for the correct procedure. Maybe some dirt is in the needle and seat causing issues also.

I wonder if some trash is keeping the needle/seat open, or something like that.

You said this carburetor is how old or has how many miles on it sense last rebuild (or new)?
BudW

Condition of the gas tank, rust, can directly effec carburetor performance. Have had rust in the carb after canister fuel filter.