390 gas mileage

[390owner]

I installed a hei distributor in my 390 with new plugs and wires a couple of months ago. The first thing I noticed was my mileage went from 8 to 11. I was surprised but happy. This was driving in town. Now I am back down to 8-9. What is different. I can not get any better than 8-9 now no matter what I do. How was it up to 11 for a few tanks right after all the new stuff?

[My427stang]

I installed a hei distributor in my 390 with new plugs and wires a couple of months ago. The first thing I noticed was my mileage went from 8 to 11. I was surprised but happy. This was driving in town. Now I am back down to 8-9. What is different. I can not get any better than 8-9 now no matter what I do. How was it up to 11 for a few tanks right after all the new stuff?

Any chance the distributor is loose and it turned?  Although I'll add an increase to 11 is surprising and it going back to the same number seems like maybe a math or filling error, but I can't see much else changing other than it retarding itself if it was loose

If you like the results of the ignition, make sure you have the right curve too, part throttle curve on a driver makes a heck of a difference, and depending which distributor you used, can be real easy to change

[390owner]

No math error. I fill it up the same each time and usually wait until I have 1/4 or below to fill up. I dont think the distributor is loose either. I thought maybe it was something to do with the plugs. I have the timing set with a light. I also advanced it a little but it was pinging up hills so I put it back where it was

[hwoods]

my 65 Galaxie got 10 mph back in 6, but that was when we had real gasoline

[My427stang]

No math error. I fill it up the same each time and usually wait until I have 1/4 or below to fill up. I dont think the distributor is loose either. I thought maybe it was something to do with the plugs. I have the timing set with a light. I also advanced it a little but it was pinging up hills so I put it back where it was

So no idea why it changed, but any of us can try to help with the right ignition curve for street driving if you want it.

Timing makes a big difference in mileage, especially part throttle. Pulling a couple degrees out because it pings could make a significant difference in what you see at the pump

Put the light back on it, unhook the vacuum advance, find out what you have for initial, then bring it up in 250 rpm increments until it stops advancing and tell us how much and what your total is.

Then post what the engine is, gearing, modifications, vehicle, and the type/brand of distributor you are running and we can try to help.

[Thumperbird]

Though they are supposed to be state certified, at least in my state, pump to pump can vary 10% easily.  Have to use your own can or the same pump and nozzle every time to really get good #'s.
Can't see this making up all of your difference but just another variable.  Have to admit I smirked when I saw the title, having had a 390 truck back in the 70's where the gas needle dropped an 1/8 every time I started it in the winter.  You are using whole numbers, what are the calculations out to a tenth at least? 

[Chrisss31]

There are huge differences in the percent of ethanol content in gas too.  Maybe the station got a load of gas that had a relatively low amount of ethanol that lasted a for a few fill ups.  When we first started getting ethanol gas at our station up in MA it was supposed to be something like 5%, but sales rep from the terminal told me that they had measured some loads up to 20%.

[Falcon67]

I'd go with the ethanol additive thing.  I used to see about a 10% drop in mileage in the truck between our "10%" here and the "10%" I'd by at a lower priced station in Fort Worth (180 miles east).  Mileage should be up there because the altitude and DA are much better there than here, so the engine should not have to work as hard.

Best mileage combo I have found for the street for an older style engine still using a carb is a Ed 600 model 1406 electric choke and a MSD (or Mallory) multi-strike ignition.  I've seen +1~3 change on the ignition and +3~5 on the carb back to back from points + Holley vacuum secondary.

[My427stang]

As I said, I think we need some real numbers for the ignition if we are going to help, but here is my first ever ethanol story

My weed wacker and blower, Stihl and Husqvarna, both seemed to be fussy this year.  Both seemed to run lean, didn't have any power, but both had a couple other issues too.  Broken fuel line on one, plug getting old on the other.  Got them back to 80% or so, then realized all the baloney started with this year's new gallon of premix.  I burned it up in the 4 stroke lawn mower, it didn't care, but got a fresh mix from the local "gas only" station and what a difference.  Both came right back. 

All I figure is in the land of corn here, they must have added a bit more squeezins to that last batch of Sam's Club gas. 

So I am with you guys, but 11 to 8 is a 27% loss, that would have to be a good dose of ethanol or other miserable additive

[Falcon67]

>So I am with you guys, but 11 to 8 is a 27% loss, that would have to be a good dose of ethanol or other miserable additive

Good point.  If the engine seemed to lose any power, I'd be inclined to pull the distributor and check the gear.  Could also be possible damage done in the carb from a load of "bad" fuel.  I have noted over the last several years that things like power valve gaskets don't hold up well when running pump fuel. 

I also note that over the last 2 years, in order to keep the 351C 10.5:1 engine running on the tune up I have had to increase the mix of 91 pump to 110 race.  Used to get by with 1:4 (1 gallon 110).  Over the last 2 years that had increased to 50:50 and the performance remained erratic so I went 100% race fuel.  That cleared up any issues, at the cost of $7.50/gallon.  Replacing the intake gaskets during a manifold swap after two years on a pump fuel mix showed the intake, head ports, back of the valves coated with a dark brown goo.  So much for all those advertised "detergent additives".

[cjshaker]

Don't even get me started on ethanol. Less power means less efficient, causes issues with nearly all fuel systems (even though modern cars will 'tolerate' it), causes severe issues with small equipment (as Ross found out), starts to go bad as soon as it's produced, and most of all...from field to sales, it costs more to produce than it saves. Yeah, making the planet green my butt.

[390owner]

The engine is in a 79 bronco with 3.50 gears. I am running 32.11.50 tires. It has headers and a 600 edelbrock. When I first installed my new hei I set the timing at 12 degrees right where is was with the old distributor. The started driving it and noticed it went  longer than normal before I had to fill it up. I checked my miles from last fill up and I had went just over 200 miles on the tank.  25 gallon tank. So I thought I would advance it a little more so I bumped it up about 2 degrees then it would ping at light throttle on the highway when I hit a small hill. So I went back to 12. Runs great but the mileage dropped back to 8 or so after about 3 tanks of gas. I have tried several stations here in town but it does best from murphy oil. Anyway I was just hoping it would stay at 11 but it is a 390

[My427stang]

To simplify, there are 4 parts to your timing, initial, total (from centrifugal), rate, then on top of that vacuum.

Although you have found that more than 12 pings, that could be because any of those is not tuned for the engine.  Generally, factory distributors deliver with way too slow of a rate and way too much, that's to run on any fuel out there as well as an early form of emissions control. Performance distributors usually deliver with 20-22 of centrifugal, but they generally slow them way down so the average Joe doesn't ping

Are you running one of those Chevy type distibutors?  Of so, pretty easy to adjust.  I would likely put it at 12 initial, make sure you have 36 total (usually adjusted with a bushing) then have it come in no earlier than 2800.  Then you will likely either have to limit the vacuum advance or adjust it if possible, and feed it from ported vacuum

Depending where you are now, you may find some power and mileage back for very cheap money (adbance curve kit for a Chevy style HEI is probably under 10 bucks)

[Thumperbird]

Just have to chime in on the ethanol thing, biggest joke of a sustainable fuel idea ever.  They used to seek out huge aquifers and build the plants above as it takes some ridiculous amount of water, 7 gallons or more to produce a gallon of ethanol.

I run Premium in EVERYTHING all the time, never any issues with small equipment, even let the fuel sit over the winter, no problems.  Premiums can have ethanol but not as much and I watch where I buy.

[Falcon67]

Just to add to the confusion LOL - I run the low 86 octane 10% pump stuff in all the lawn equipment.  I never do anything other than add fuel or mix for the two stroke weed eater and chain saw.  No Stabil, nothing.  No problems.  Use the chain saw about once or twice a year, starts right up.  The mower is a 1993 Snapper with a 12 HP Briggs.  Been ours for 8 years, never had the carb off, parked usually 2 months over winter - such as it is sometimes here.  Same with the Sthil weed eater.  Prime, choke, pull.  Generator - same/same.  The 6500 is going up for sale, it sat about 5 months before I cleaned it up - started on 4th pull. 

[chris401]

I installed a hei distributor in my 390 with new plugs and wires a couple of months ago. The first thing I noticed was my mileage went from 8 to 11. I was surprised but happy. This was driving in town. Now I am back down to 8-9. What is different. I can not get any better than 8-9 now no matter what I do. How was it up to 11 for a few tanks right after all the new stuff?

If all else checks out, check the tension on your parking brake cable. You could have a slight drag from a rusty cable. If both back seem the same look at the lever and primary cable.

[Pentroof]

I installed a hei distributor in my 390....

Someone mentioned it could be retarded. I agree. Putting an HEI distributor on a Ford is retarded.

[Katz427]

After reading some comments, I wonder if any one will believe my old 63 Ford 390 P code would net me 16- 17 mpg running the pike between Syracuse and Hartford, and Boston. That is with 3.50 gears and a 3 speed stick.  Gas was a whole lot better back in the late 60's, early 70's, before the gas crunch.

[chris401]

After reading some comments, I wonder if any one will believe my old 63 Ford 390 P code would net me 16- 17 mpg running the pike between Syracuse and Hartford, and Boston. That is with 3.50 gears and a 3 speed stick.  Gas was a whole lot better back in the late 60's, early 70's, before the gas crunch.

I believe it. Although HP cost fuel these old engines do not have to be undependable gas hogs to run a car up the road.