So I recently learned that the later (fast burn) 4.9l heads don't play well with too much advance timing. I had selected the GM HEI distributor for ease of installation and a little extra voltage at the plugs. But as I understand it this distributors advance may be a little slow on the uptake and a little high on the overall limit. While I can change the springs to speed up the advance, I'm not sure of a good method to limit the mech advance. Would reducing the base advance to 6' or 8' help to reduce that total or produce problems @ idle or start up? Thanks for any advice you can give.

 

Setting the base timing will affect the total timing. You will have to retune the carb to your change in base timing, since it affects the idle speed more than anything. But I would not take too much stock in what I read on the internet. Most engines run their best and get the best power and fuel mileage when the timing is at the max with the grade of fuel you happen to be running without pinging.

That is how the modern fuel injected cars work. The computer advances the timing till it hears pinging, and then retards it. It then starts over advancing the timing again till it hears pinging again. It does this over and over as you are driving, keeping the timing as advanced as possible, no matter what type of fuel you happen to be using at the time.

The best way to set the timing is to advance it, take a test ride and give it a long pull on a hill. If it pings, set it back a little bit. If it doesn't advance it a little more and take another test drive. Get it as high as you can without it pinging. Then cut it off hot, let it sit for a minute, and then try to start it. If it struggles to turn over, turn the distributor back a little more till it cranks over easy.

The HEI is easy to change the timing curve, it's all on top. You can also change the total advance, by taking pliers and bending the tabs where the springs are. You should be able to look that up and see how to change the total amount the weights and springs move, and use a dial back timing light to get everything set if you want to modify it. I would leave it alone at first though.

 

When you retard the base timing it retards it across the board.
So if base is 15* BTDC and total is 55* BTDC and you back / retard the base to 10* BTDC the total will be 50* BTDC. just numbers I used to show across the board.
With out looking at the distributor I cant say how to limit total timing but leave the base at the 15* BTDC (just a number)?
If the weights have a stop they hit sometimes you can put a short piece of vacuum hose over the stop so the weights hit it and dont go as far.
If that is not enough you can put a larger hose over the first one but that will really retard the timing!

I have the base set to 12* BTDC with vacuum hooked up I get some pinging but if I back up fof the throttle a little it stops I just have not looked what it will take to limit either vacuum or total on the factory DSII distributor.
Dave ----

 

So if I set it at 6-8 it will not likely push up past 30-32' with mech advance? That's my hope anyway. I think I will set it to 6' and work it up slowly & choke down on the vacuum advance a bit until i am happy with it.

 

The only way to know is to put the timing light to it and run it from idle up till advance stops, 3000 RPM? without vacuum hooked up.

Now if you watch Engine Master's they are only trying for the most HP & TQ and not how it will run / act on the street and they DO NOT use any vacuum advance to get to the 32 - 36 BTDC mark.
With that said you want vacuum advance hooked up as it helps with MPG and how it run / drives on the street.

What I did was get the motor to start / run good with timing base set and that was think 12* BTDC, no vacuum and idle set to think 600 RPM.
I dont remember what my mechanical ended up at or the RPM, got notes somewhere just dont know ATT.
Once I got that where the motor liked it I then worked on the vacuum side. The adjustment only changes at what vacuum HG it starts to pull in it DOSE NOT adjust the max limit.
Judt for numbers it can say start vacuum at 5 HG or 10 HG but both may add 30 * of advance on top of whatever base & mechanical was.

When you hook up vacuum you may see it jump to 50*-60* BTDC
So if you had 36* BFTDC of base / mech. and add 30* BTDC of vacuum you now have 66* BTDC.
So get a note pad, dial back timing light, vacuum gauge and a tach and start tracking what the timing dose at each RPM, think I did it at 500 RPM from idle to 3000 RPM.
This way you know what you got before starting and with each change you need to run the full test / check again.
It is a lot of fun
Dave ----

 

I have a dial-back timing light, and plan to map it out fairly tight once i am thru with the cam break-in. (Tho as i understand it the dial back feature is designed for rides with computer controlled timing. And has to be read at zero with non pc rides because the built in delay will read off otherwise). I have stripped the ECM and all its wiring from my Truck along with the E G R. So i have some tuning to do!

 

I have never heard that about dial back timing lights. The timing light is triggered by the spark traveling through the wire, it doesn't know if that spark was created by points, ignition module, or a computerized system. When the spark hits the cylinder is all you are worried about, not what a computer might be calculating up the line.

From my experience, you will be happiest with the static timing as high as it will allow you to run it and still crank over well when hot. Your throttle response will be much improved and low end power also. I would do all this with the vacuum advance not connected.

Once you get it running good, with the timing most likely 12 BTDC or higher, you will then hook the vacuum advance up and it will ping like crazy. So then you start pulling the timing back, till you are where you said you might be, 6 or even lower to make it quit pinging, and then you will find you have ended up with a turd with no throttle response.

So I have found the best thing to do is disconnect the vacuum advance, turn the timing back up, and drive it like that till you figure out a plan on how to turn down the amount of advance the vacuum advance adds. I have done some rigging in this dept with limited success. There are rumors that some original Ford vacuum advance units have a allen screw up inside the vacuum nipple that you can adjust.

 

Yes sir, the plan was to set base fairly low to start and work it up slowly once i am thru with the cam break in. The vacuum advance is currently plugged off and i have an adjustable vacuum advance. When ready i will set it to minimal vacuum and work it up as well. The timing light thing came as a surprise to me to. Evidently they have a built in lag to compensate for the ECM when using the dial back feature. And several online podcasters refuse to use the feature with non ecm engines because of that. IDK myself, but it makes some sense.

 

Take podcasters for what you paid them and a grain of salt
How did find this "lag" with a non-dial back light? How do they know that non-dial back is not the lagging light? How big is this lag, milline seconds?
I have never heard of this with any timing light and like Dave said it triggers from the spark in the wire.
Also if there is a lag as long as you use the same light all the time what is the big deal? Again I dont see this as a thing.

The only thing with computers you want to turn it off from adjusting timing when checking like when you pull the SPROT? on the feed back / EFI trucks or the vacuum hose on older trucks.

For break is dont have the timing too far retarded as the spark will be happening too late and you start the valves & manifolds glowing red hot.
It helps to have other eyes when doing the break in as you cant watch everything.
Have cooling system filled with water and have a hose handing for top off, no cap on radiator. Motor & P/S fluid checked.
Timing light hooked up and timing marks marked with paint so you can see them.
Fire it up eyes looking for leaks, oil PSI, get the RPM up to 2500 - 3000 and check the timing. If you dont have vacuum hooked up set timing to 32* - 36* BTDC and let her run for for the 20 to 30 minutes going between the 2 RPM's.
Good luck
Dave ----