I just bought a 1985 f150 with a 351w. It came with receipts from reputable shop in town from 2010 that show it was rebuilt .040 over 30k miles ago. I checked the timing on it and I don't know what to make of the results. With the vacuum advance disconnected it has 18° at idle and advances to ≈38° at 3000 rpm. With the vacuum connected the numbers don't change and I tried ported and manifold vacuum. This is my first dive into timing but from what i've read isn't 18 way too much at idle? I've heard 10 is stock but up to 15 is okay if it doesn't ping. Mine isn't pinging at idle even with the 18 though so does that need brought down a few degrees or is it okay there? And what's going on with the vacuum advance? Like I said I'm new to this but shouldn't timing change connected to vacuum? It looks like a OE replacement distributor and it came hooked to manifold when I got it. Can anyone help me out here?


And on a second subject, I did a compression test and all cylinders came back at 115-120 with a slightly warm engine and oil didn't improve the numbers. I know it's good that they are all almost exactly the same but shouldn't they be higher even with a low compression smog motor?

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I'm going to try and verify TDC tomorrow. I'm gonna try and find a piston stop to measure it but if I can't find one in a store near me could I just put a long screwdriver into the spark plug hole and try to stop it when the handle is the highest on the compression stroke? Or is this a bad idea waiting to happen?

 

The initial timing # isn't really that important, verifying what it tops out at on the high side is. If you were to back off the initial timing then there won't be enough timing at high RPM. And 38° sounds about right. So long as it starts OK when warm or in hot weather, some engines will have starter kickback. At idle there is no load on the engine, can have lots of timing with no problem. Not a cause for concern.

As far as the vacuum advance, it sounds a little like the diaphragm might be lunched, so it's not pulling in any extra timing in. Apply vacuum to the distributor and verify whether the advance plate is moving or not.

 

To check the vacuum advance get a section of vacuum hose, attach it to the vacuum advance can on the side of the distributor, and suck on it. If you can draw any air through the hose the vacuum advance diaphragm is bad and the can needs to be replaced. A failed vacuum advance diaphragm would certainly explain the lack of any vac advance.

If you can't draw any air through the hose the next thing to do is pop off the distributor cap and see if the pick up plate moves inside the distributor when vacuum is applied. If it doesn't move it might need some lube and for you to work it back and forth a bit to loosen it up and work the lube in.

I think the compression numbers sound good and I agree with what Tedster says. On these old trucks it's really more about start-ability and drive-ability and if the battery is good and it doesn't kick back on the starter, doesn't ping, idles, runs and drives good then the timing must be about right. I think the factory timing was conservative (retarded) on these trucks primarily for emissions.

FWIW base timing is 12* on my 460 and I run manifold vac advance on top of that so I must be around 28* or so combined advance at idle. Works great.

Also, what's your altitude? The higher your altitude the more advance you should / can run.

 

Base timing plus centrifugal timing is your total timing. Vacuum advance timing is added to that. 38 is in the ballpark, with vacuum advance added in you can be close to 50 degrees advanced which is normal.

 

Well the canister must be bad because I could suck through it. I also hooked up a vacuum pump to it with the cap off and it read 0 and nothing moved. Is it simple to replace just the can?

 

It looks like that snap ring needs to come off in order to get the vacuum advance can off. I'm pretty sure I'll drop that somewhere with my luck so any advice on how to get it out?

 

You should be able to slide the little keeper off with a pointy little pick.

It should slide off (and back on) pretty easily, and you can put a little bit of tape over it, or a finger, to keep it from flying away or falling down inside your distributor.

Once you've got it removed apply a little Powerlube or similar to lube the pickup plate so it rotates freely.

FWIW, I got a new dizzy from NAPA and the brand new vacuum advance diaphragm failed in 1,400 miles, so I swapped in the 30 year old original and it's still working great. These days, new isn't always better...

And while you're in there, pull the felt puck out of the center shaft of the dizzy and put a drop of two of oil down the center of the shaft to lube the mechanical advance. Then put the rotor back on and work it back and forth against the mechanical advance springs to work the lube in.

 

I couldn't find an advance for any 351w so I ended up getting one for a 1983 302 instead. It looks like everything is the same physically but will it put out similar advance?

 

Post which brand can you have or better, the specs. Aftermarket advance cans are usually adjustable with a 3/32" allen wrench.

The split ring isn't really necessary, btw. Bend the new arm just so, and it won't go anywhere and it isn't needed. The vacuum advance as mentioned just adds another 10° or so under certain conditions - steady level cruise when engine vacuum comes back up, and part throttle acceleration from same. Don't confuse it with mechanical advance.

 

It's a duralast #FV2038. It's non-adjustable, that's why I asked if it should put out similar. I've read that it could vary from 6° to almost 20° depending on stock application..

 

I just finished replacing the vacuum advance but it's now at 30° at idle without the vacuum connected. How could this happen if I didn't remove the distributor? I think it pay be pinging at idle now but i've never heard it in person so idk if it is pinging.. how loud should it ping? It's almost inaudible and only sounds like it's coming from the driver's side

 

The sound seems to have gone away but the vacuum being connected still isn't changing anything. I can't suck air through this one and it holds a vacuum so what am I missing here?
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Now would be a good time to pop off the distributor cap and play with your fancy vacuum pump. Apply vacuum to the advance and release the vacuum and see if the pick up plate moves When vacuum is applied and returns when vacuum is removed.

Don't forget you just changed a part and maybe the arm on the new vacuum advance doesn't stick out as much as the old one. That would change the position of the pick up plate and thus change your timing. You might compare the two advances to see if there's a difference in the arm length.

 

Did you lube and check to see if the pick up plate rotates freely? If it's all gummed up and stiff the vac advance unit may not be powerful enough to overcome the resistance. Make sure everything moves freely.