My economic 390 project
 

Just a quick overview, I got a 1974 F100 with a 390 from a friend of mine.

390 .060 over, fairly fresh build
compression ratio is unknown, been told it was built with extremely high compression, then told he was getting his builder to set it up for a blower... ??
Edel intake
Edel 650 (so I've been told) carb
Edel Performer cam
C8AE-H heads, not sure if they're ported or not, conflicting stories
Hooker Super Comps 1 3/4 longtubes
2.5 full duals
I think Flowmasters
C6
3:1 rear

When I got it, I put a set of points in it and drove it home. Rough mileage on it was 8mpg.

When I got some time, I set the points correctly, set the timing by ear, fuel filter, etc, just quick stuff trying to get it back running, got somewhere around 6mpg.

I cleaned the Edelbrock carb and put it back together, set the timing as high as it would go and still crank, any higher and the starter would bog. 4mpg.

I rebuilt my old Holley 600 that got 24mpg on my 302 on my Comet, decent little carb. I found the vacuum advance was busted, replaced it with a Crane adjustable kit. Swapped the carb, got it going, 6.9mpg.

Fine tuning the timing, finding and fixing vacuum leaks, 7.47mpg.

Further tuning the vacuum advance, dialing in the carb better, 7.8mpg.

I haven't gotten the engine to ping since I cleaned the Edelbrock carb. I'm pretty sure the only reason I got it to ping before was due to it leaning out because of the trash in the carb.

Right now initial is somewhere around 12-14*, mechanical advance is in the mid 20s around 1500-2000rpm (haven't got a tach on it yet), full advance with the vacuum shoots well past the 30* mark on the balancer.

I'm looking for advice on getting the mileage up a bit. My goal is 20mpg, I realize this is probably impossible, but I figure aim high and pull back as needed. I got 24mpg out of my 74 Comet, so I figure I can at least get this truck comparable to my sister in law's Jeep, 4.7 (or 4.8, whatever) that gets 14mpg.

Right now it seems I need to back the vacuum advance down a bit, kind of sputters when accelerating after fixing a vacuum leak. Before I fixed the leak it would burn the tires until it shifted into second @ 55-60mph.

The next thing I plan on doing is getting a vacuum gauge, tinkering with the idle and timing a bit, get a set of smaller jets and a looser power valve since this engine has more vacuum than anything I've ever worked on. I'm pretty sure the power valve never kicks in.

I am a little concerned/confused since I have not gotten the engine to ping at all since cleaning the Edelbrock carb. I've pushed the timing to ridiculous extremes and it might misfire, but it it will not ping.

I'm wondering if the jets that ran fine on the 302 might be a little rich considering how much more vacuum this engine has.

I'd appreciate any input.

Input? Sure. Get yourself some diagnostic tools and do it "right". the **best** you'll get, under ideal conditions, is probably around 15 mpg.

What diagnostic tools do you recommend?

To start, a timing light, vacuum gauge and dwell meter.

What type of ignition setup is the truck running? Stock style, or has it been upgraded to an MSD? New plugs and ignition components could pick up a small gain.

What system are you using to draw in air? Getting to cooler air could also pick up a small gain.

Check out another thread on here on tuning a 351. He has posted some really good, in-depth data on fine tuning.

Got a timing light already, I figured that would be clear since I said what the timing was above. Vacuum gauge on the way, I'm replacing the points shortly, so I'm not too worried about the dwell meter. Never used one to set points in the past anyway. Maybe I've just been lucky.

Stock ignition setup right now, figured on installing Pertronix or one of the other drop in systems. Pertronix ran fine on my Comet until I hotwired it wrong. Not sure on the coil, but I think it is a stock-ish coil. Previous owner had an MSD on it and swears that old engines require old ignitions (because he couldn't make it work right), so he threw it away and put a points dizzy in there.

Right now it has a K&N 4x14 filter, chrome breather sucking in hot air from the engine bay. Any idea of an aftermarket breather assembly with ducts like the factory stuff?

I think I may have read that or a similar thread, get a hotter ignition setup, gap the plugs to .45, higher gauge, low loss plug wires?

Got the Pertronix installed. Apparently the points were crap. I could set them and get it running fine but it all went to hell once I hooked up the vacuum advance. I worked on that for a few hours, thought I had it fixed. Regardless, the Pertronix has it purring like a kitten.

Got the vacuum guage, 18 at idle after I tinkered with the idle screws. Pretty sure it is still not quite there yet.

Initial is about 14, down from 20 on my test drive, vacuum hooked up to the manifold has it pulling more than 36 at idle. I'm fine tuning the vacuum advance again. I'll check the plug gaps later. My part throttle hesitation is just about gone. I'm hoping with the timing at 14 that has cleared it up.

Sounds like good progress. On the idle timing, not sure 36 degrees with vacuum helps it. I know its debated, but OE had the vacuum advance running on ported vacuum. May be worth trying both ways for your particular combo.

That is the plan.

It is an adjustable advance, so I'll be tinkering with it for the foreseeable future.

With it connected to the carb vacuum it ran extremely warm in city traffic, warmer than I'm comfortable with, especially considering it wasn't a hot Alabama day yet.

Be careful advancing it to much by timing with the vacuum gauge ..

Using vacuum gauge to set timing

Slacken the distributor clamp bolt, and with the engine still at idle, advance or retard the ignition until the highest steady vacuum reading is obtained. Then retard the timing until the vacuum gauge reading drops slightly, approximately one half to one inch. In some cases, you may need to retard the timing up to two inches to prevent pre-ignition (pinging).

Note: If you can't get the reading into the "normal" zone by adjusting the distributor then valve timing is the problem. Timing with a vacuum gauge will normally result in timing that is more advanced than what specifications call for, so you need to be alert for any pre-ignition (pinging) in the engine and adjust accordingly (retard the timing if needed).

I'm just talking about the vacuum advance. I set the timing with the light, the vacuum advance has to be set by trial and error as far as I know. If anyone has a better way I'm all ears.

The directions say to turn it in two turns at a time until it stops pinging at part throttle and the go back one turn. I haven't heard this engine ping since I fixed a lean condition due to trash in the carb.

Here is some good info on tunning the vacuum advance ..

I've been driving it a bit, timing is high, still can't make it ping.

I think the previous owner must have used bowls instead of pistons.

Still have a hesitation at part throttle, but I haven't narrowed it down to the carb or timing yet. I need to drive it with the vacuum advance disconnected and see.

Best MPG upgrade I did was a FULL CD ignition system. I use Jacob's products (which a lot of people don't like) a Duraspark II distributor and standard copper core plugs. I run my plugs at 0.090 and have tested with larger gaps but wanted to be safe in case of bad gas or towing over the mountains.

Full CD?
I probably know it, but it isn't coming to me right now.