I have a 1968 F100 with what the VIN calls a 360, but when I bought it, I was told the engine was made into a 390. (I also don't know if it has a different cam etc.) Also on it when I got it was long tube headers, an Edelbrock performer 390 aluminum intake with Holley 4160 vacuum secondary 750 cfm 4 barrel carb. Also a Mallory Unilite distributor, and Mallory electronic coil. I never messed with the engine for about the 10 years I've had the truck. I pulled the engine to replace the headers with the same type. I also replaced the dual exhaust setup on the truck, and installed 1965 Shelby type exhaust. (glasspacks and 90 degree pipes exiting in front of the rear wheels). I was trying to adjust the carb, and while turning the idle screws I found one was 1/2 turn out, and the other was about 1.5 turns out. I put them both 1.5 out to start with. while adjusting, I found the truck to have only 16 in of vac. The markings on the vac guage face suggest "late ignition timing". I checked the timing with a light and the vac advance hose plugged. It was set to 12 BTDC where I put it when the dist. was reinstalled after removing the engine for headers a few months back. However, if I move the distributor and watch the vac. guage, I got up to 18 in of vacuum. and after fine tuning the idle screws to about 1.25 turns from fully seated. the truck has 19 in. of vac. and idles and sounds a lot better. Just turning the dist. by sound and feel everything seems ok. The timing light says just over 30 degrees BTDC?!?!? if I hook up the advance hose and rev up the engine, the timing advances further like it should. I figured the timing marks on the dampener may have moved due to the rubber inside deteriorating, but if I take out sparkplug #1 and move the crank pulley back and forth across the TDC mark, I can feel that the piston is at the upper part of it's travel at the same time. The truck runs so much better and I hear no pinging/ detonation. Any ideas why the base timing is showing 30 BTDC at idle??
I put vacuum to the distributor advance, and it moves, and stays there until I release the vacuum.

 

Does it feel like any play in the timing chain? possibly it got stretched out if it's not holding timing. Also the dampener could have spun but if you checked it and confirmed the piston is where the mark says it is in travel then the dampener isn't throwing off your setting.

 

The FEs love lots of timing, I ran mine at 16* without issue, it would have run fine with more but it was usually heavily loaded towing.

 

Yes, but 16 is not 30; that much initial would choke the starter, wouldn't it? What is the idle RPM when checking with the light?

 

Good point on the starter. How is it starting? Is the engine being spun by the starter correctly?

 

I just went through this exact issue with a car that I am building. I thought I had low vacuum at idle too.
Long story short..... Set your initial timing where you want it, 12 like you had it, or 16 like has been suggested.
Then, hook up your vacuum advance on your distributor, and then adjust your idle mixture screws to get the best vacuum. You'll also have to adjust your idle speed too while making these adjustments. Hope this helps.

 

Seems to me this is more of a curiosity kind of thing, though - why is the timing light indicating an initial crank timing of 30 deg. BTDC. This is unlikely, without a highly modified distributor installed, and the starter would have difficulty turning over the engine... So it's a bogus indication. However, it runs a lot better, not surprising.

This is one reason a vacuum gauge is so useful for setting timing, because it cuts through all the variables that can sometimes make using a light problematic. In general terms there's about 50 degrees of advance in play with a typical engine depending on RPM, load, vacuum, intended use, etc. The engine tuner can adjust when and how much this advance comes in, but even if the engine would start with 30 degrees initial, it would ping very badly with a typical stock advance curved distributor. There's just no room for any additional advance at higher RPM via mechanical and vacuum advance if the crank timing were actually 30 deg. BTDC.

 

I had a 1972 with a 302 when I was younger that I installed a Mallory unilite dizzy on and my timing ran around 24 Degrees BTDC. Like suggested earlier, I would leave it there and fine tune everything else and see what happens. As long as your not "pinging" at open throttle and under a load......Id run with it.

 

I agree - it's just tough to believe there's really 30 degrees crank advance going on here. 24 ain't 30, either, 6 degrees is a hell of a lot at the margin. Again, maybe with a racing distributor, but still question whether the starter would handle that.

 

I faced this same issue last week. Put the light on my truck for the first time and hooked up a vacuum gauge at the same time. With timing right vacuum gauge showed late timing. Moving dizzy to hit vacuum had me at 30* also. I left it at vacuum setting to drive it for the week to check it out. I thought maybe at some point someone had pulled the dizzy and clocked it wrong.
She is running pretty good right now.

 

Man that sounds like a spun vibration damper. I don't mean to insult you but are you sure you checked that damper mark with "#1" at TDC? At 30 BTDC I can't imagine the starter spinning the engine over smoothly.

 

Have pondered this, and how it would throw timing light indications off. On a Y Block - 15486372 - each cylinder TDC being 1/4 turn of the crank of course but - one tooth off clock index either way of the distributor doesn't equal one cylinder, tho it would throw the timing light off enough to be a head scratcher, I guess.

That's why vacuum gauges are nice. My 64 at first - had the distributor cap clocked 180 out, plus the damper slipped almost 10 degrees, and at that time had hardly a clue what engine timing was about. Figured out #5 was my #1, but never had it running right with a light. Until I got all that straightened out, just timed it for max vacuum at idle -1 in. And that was about perfect. Replaced the damper, and worn out distributor, runs a lot smoother and takes the guesswork out. Now I could time it by ear! Runs about 14-16 deg. initial at max vacuum iirc, 24 mechanical, all in by 3000, runs a lot better.

 

Isn't the firing order for a 390 1 5 4 2 6 3 7 8 ????

 

Could be, have to check.

 

I was asking because I thought maybe you typed it wrong. The firing order for a 360/390 that you say you have is 1 5 4 2 6 3 7 8.