I need to know the specification for the timing for a 1978 F-250 4x4 351M, auto, with dealer installed A/C. Don't know if the A/C makes a difference just trying to give all the variables. I bought a Haynes manual to get the information and in the tune-up section it says see decal in engine compartment. If I had the decal I wouldn't have bought the book. Any help would be greatly aprreciated.

 

Welcome to FTE!
Please read:

https://www.ford-trucks.com/forums/s...d.php?t=248773


14*BTC
choke setting at 3 rich
Vacuum advance hose removed & plugged
650 rpm.

 

with the vacuum advance unhooked and plugged, set the timing 12-14 degrees BTDC at 650 rpms....test drive and adjust timing in 1-2 degree increments to find which setting better suits your driving needs.....12 degrees is what both of my ford(351m&460) call for on the stickers, on a general rule for both of them 12 would get me better economy but less power and 14 degrees was better power less economy.....but if the timing chain is worn its hard to say where it will run best,i run my 460 at 14 degrees for years, recently i had to bump it to 18 to keep the same preformance, i think the timing chain is to blame....

 

I've got a '78 351m set with base timing at 8deg (no vac). It advances via distributor weights to 20deg at higher rpm (no vac). When I hook up the vacuum line, my advance at idle goes up to 28deg, and somewhere around 40 or so at rpm. I have vacuum at all RPMs. Is this right? I'm running around w/o the vacuum hooked up, but she gets doggy if there's load on. Any ideas?

thanks!

 

Sounds like you are connecting the vacuum line to manifold vacuum. It should be connected to ported vacuum. What type of carburetor ?.

 

I've wondered that too, but the vacuum set up is a bit of a mystery to me. I have a 2150 carb, and I'm hooked near (what I think is) the choke assy, passenger side fwd, behind a small shield. It reads ~20in vacuum at all rpm - regardless of idle. I've searched around and every thread I could find suggests attaching here, and I haven't found a vac line that does anything else than ~20in at all time.

Thanks for any help - I've been chasing this problem for weeks!

 

I should add, there's no label on my valve covers, so I'm totally guessing at the 8deg base timing. This all started early in the summer when my oil pump jammed up, and I sheared a roll pin on distributor. I spent too long just learning about the differences between 351c, m, w (thanks again FTE!). I thought it was a windsor, until "my oilpan gasket isn't that shape" and "that oil pump doesn't look right", etc.

 

As you know the mechanical and vacuum systems are independent, they work together but are checked and tuned or adjusted separately. When the vacuum advance is disconnected and the port plugged the engine mechanical timing should move smoothly to around 36° BTDC and back down to the base timing when engine is revved. If it doesn't it will never run right. 20° is not nearly enough. Make sure it's done advancing all the way. The vacuum advance helps with all around driveability but it only really gets wound up in terms of advance degrees when cruising on flat ground. It pulls in a lot of extra timing under low load conditions. It will be around 50° at highway cruise. Sounds like the distributor is lunched.

 

So, my 20 deg isn't enough...good to know. I put in a new distributor - that's what necessitated the timing in the first place - so I THINK it works well (truck seems to run ok on it - I can draw vac manually and see it advance, etc). I haven't fiddled with the vac adjustment screw at all inside the diaphragm jobby. My base set to 8 is just a guess from what I've read. I'll play with different RPMs to see that just the mechanical advance is topped out (I don't have a tach). I also need to find a ported vac source. I'ved tested all I can find and just haven't found it yet. My "as found" vac set up may be smoked - still learning!
Thanks!

 

A tach is very helpful, but where the distributor tops out at is important. The initial timing is not critical, but the mechanical advance is not easily adjustable on a Ford distributor. As a practical matter rev up the motor and adjust the distributor wherever it needs to so it stops advancing at 36°. This would be without vacuum advance. Take it for a test drive and bring a 1/2" box wrench with you.

See if it pings or knocks on hard acceleration. Add another deg. or two if it doesn't. Back off slightly if it does. Then when satisfied re-connect vacuum advance and tune that, another test drive or two, though don't mess with the distributor timing at this point, adjust only the vacuum can.

Since you've a new distributor, it could be curved for a different application. Hope it wasn't for a dump truck. So long as the initial timing falls somewhere between 10° or 20° and there's no starter kickback when hot, should be good to go wherever the initial timing lands. Manifold connection for vacuum advance can be a little more tricky, but there's no voodoo to it, just don't be alarmed if you see 30° at idle, it won't hurt anything.

 

Good morning -
Wanted to give an update - I was checking where my timing problem. A couple weeks ago, I found that my timing peaked out around ~22deg. Was going to advance timing, liker you said, Tedster; and it poking around doublechecking vacuum lines, I (finally) found an empty carb port on the front drivers side of the carb, down underneath. Ported vac, uncapped, unused, open. Measured 0in at idle, ~20in when revved. So I hooked it up to my distributer, and revved up my advance went up to the mid-30s. Much better. So, I've been running like that now for a bit.

My question now - is 36deg a good target for total advance (mech+vacuum)? or should I try creeping up until it the I get some starter kickback?

 

34° to 36° BTDC is what you want to see. "Total" = initial + mechanical. This would be without vacuum advance.

Set the mechanical first with the vac advance disconnected. Test drive it. Get it set how you want, then don't mess with the distributor timing any more.

The exact number it will tolerate depends on engine compression ratio, gasoline quality, altitude etc. Don't really want starter kickback, the idea is to be just short of ping or engine knock on hard acceleration. This tuning isn't a performance or racing thing, it is how all engines are tuned. They always run best with optimum advance at all times under all conditions.

Re-connect the vacuum advance when satisfied with how the distributor is curved and tune the vacuum canister only. They work together but are adjusted separately. In other words, if the vacuum advance is pulling in too much timing, don't try to compensate for this by retarding the initial distributor timing. Make sense?

"Total" timing is an old drag racing term, which don't even have vacuum advance. Remember mechanical advance is RPM based. Vacuum advance is load based. There's only going to be 34 or 36 deg. when you really get on it. Cruising in high gear down the highway on flat ground there might be close to 50° timing or more. That's fine, this is how they are designed from the factory. Just about every OHV V8 ever made is timed like this.

 

Great explanation - thanks a ton. I'll get in set up per those instructions, and report back.

 

ok - played with it a bit this morning. Before I started, I had;

No vac on distributor:
8deg base
26deg tached out

With vac:
8deg base
40deg tached out


Then I adjusted advance per my understanding of your last note
No vac:
20deg base
35deg tached out

With Vac
20deg base
55deg tached out.


Took it for a drive - runs down the road fine. The only really obvious difference is that it's way smoothing at idle. Not sure if that says more about the motor or about me. Anyways.... So - I THINK I'm set up better than I was, but certainly still more to learn. This is how it's set as we go into winter. I hope it's good for a spell - I had snow on the hood this morning.

Thanks!

 

If that works for you, that's OK. 20° initial is a lot, and may cause a little starter kickback when hot. Don't misunderstand, if the engine will tolerate that, it's absolutely fine. Make sure your cables and grounds are good.

With 20° initial and vacuum advance connected to manifold, that will be a lot of additional timing at idle. That's OK too, there is absolutely no load on the engine at idle. It will tend to to run cooler at idle this way, important in stop and go driving, or for people living in Arid Zone A, etc. Don't try to pass a smog test this way, it's waaay too efficient and will spike NOX emissions relative to HC and CO (as I understand it, don't quote me on that)

I'm wondering about the slot settings in the distributor though, the innards.

Shouldn't be necessary to set the initial timing all the way to 20°, to achieve 36° "tach'd out", it might be interesting to know what the slots are in that thing, or maybe it has super heavy springs and it isn't really done advancing all the way when you think it is. Basically what you're doing is using the engine itself as a distributor machine, and "curving" the distributor. Pain in the ***, but the alternative is to pay someone to do it. Usually V8 like 10° to 12° right in there as a generalization, and maybe 34° total coming in fast, by lighter springs and/or loosening the arm the springs connect to. The shop manual lists how much mechanical advance there should be at different (distributor) RPMs, it can be confusing because the distributor turns at half the engine RPM, and the degrees they list are half of what shows up on the balancer timing marks. The manual assumes the distributor is being checked or serviced on a distributor machine.