Inherited a 76 F250 2wd regular cab 390 automatic. Tag indicates a 3.07 open gear, so calling it a 2wd is a bit off. I have replaced the leaking factory carb with a new Holley 600cfm electric choke, new plugs, and all fluids flushed. Generally it runs pretty good, but has one glaring problem.

I can set the timing by ear so that it runs great around town, starts easy, crisp throttle response, sounds good. It even does well on the highway (55mph) and recently got just over 13mpg on a tank that included a 4 hour trip. It will not however climb a hill on the freeway (70mph) without popping, rattling, severe loss of power, etc. All I have to do is rotate the distributor counter-clockwise a bit, not much, and the problems are gone. Back in town, it sounds like a tank, idle is rough, power is down, and with it set like this the mileage drops noticeably. It still starts OK and is drivable but

Ignition seems the likely culprit, but it has been checked (timing light on the balancer) and the vacuum advance seems to be working. It has the Duraspark 1 system, and Grandpa says its stock except for the RV cam he had put in to help tow the travel-trailer. I'm sure this truck hasn't seen 70mph in over 20 years, but I can't imagine it pulling their long trailer around the western USA if the engine ran like it is now. Ideas?

 

Sounds like detonation to me...where is your initial (base) timing set? what is your total timing? at what rpm do you reach total timing? have you confirmed the vacuum advance is working?

Seems like things i.e. carb/dizzy just need to be "dialed in" but we need to know where you're at now so we have an established baseline to work from...

 

I will have to get my own timing light to answer those questions. I've always just set timing by ear and ran it. My 200 I6 Ford and 400 SBC never acted like this. With that said, I have some learning to do in this area. How do I confirm the vacuum advance is working?

 

What I found while verifying the firing order/plug wire routing was the #7 plug wire boot was shot, basically it fell apart in my hand and the internal connection was open and burned. #3 was just as bad, but the rest seemed ok. I put a new set of wires in and she runs better than I've ever known. Based on the condition of the wires, I doubt 3 and 7 were firing properly if at all, so there goes power and mileage and explains the noise. Now I wonder what damage has been done to the pistons and/or plugs, due to getting the air/fuel mixture but not burning it.

I still have no clue what the timing is set at, due to the fact that there are no numbers or other markings to use. With the timing light on I have a partial white line and a full white line across the balancer, and an angular tab toward the top at about the 10 o'clock position. If I retard the timing until the white marks are at the tab, the idle is bad and it runs rough. When I tune by ear the marks show down around 8 o'clock and it sounds good, idles smooth, etc. I flogged it up every hill I could find last night past the 70mph mark with no problems. Speed limits topped out at 50 so I had to be careful. Going from 40-45 steady to punching it up to 70 mid-hill produced strong acceleration without any knock. The drive to work this morning was rather fun.

I'd still like to determine the timing on this thing, but I don't see any useful markings.

 

Sounds like the dampener ring may have slipped. Pull plug #1, and rotate the engine by hand until the piston is at the top. If the dampener ring is clean enough to see the marks, the pointer (tab) should match the "0" position on the dampener.

If it's not, the ring slipped. Which happens quite a bit, apparently, especially when the rubber between the ring and the inner dampener gets old and dry.

 

I was able to clean the crud off the damper and found the timing marks had been painted over, but I do have the D5TE-DA damper. I made the markings visible and found that my initial timing is at 20* and goes up to about 26 or 27 with mechanical at a steady rpm. Tach is purchased but not installed yet, rpm was fairly high. Pushing it farther did not add any more advance. With the vacuum line attached, it starts out at 20* and goes up into the mid to upper 30s when revved. The timing marks stop at 30 so I'm estimating. It was probably near 40 when pushed to higher rpm, tach will be in place soon.

It is starting well, drives great, but I am noticing a slight surge when cruising at about 40 mph. 20* initial timing seems high but I have read that automatics sometimes take a bit more than the standard 12*. It's not bucking the starter at all, and no ping on hills outside town. The real test is a 1.5 hour loop on the freeway with some big long hills. No reason to make that trip other than to test. Without knowing the rpm, do those numbers for initial/mechanical/vacuum (total) seem OK?

 

20* initial is way to much crank it back to 8*or 10* , 6* is stock if I remember right .
then dial in carb from there .

 

OK, so I backed the timing down to 12* and adjusted the idle to 700 and tuned the idle screws to a best of 18.5 on the vacuum gauge. At 3000 the mechanical moves the timing to 25*. With the timed vacuum advance reconnected, it pulled about 34* at 3000. It is hard to be precise when the marks stop at 30. Sound good?

I will run it like this and post back if needed.