i have a 1975 straight six 300 motor inside the body of a 1966 F250. All my manuals are for the 1966. does anybody know what the initial timing degree setting is for this motor? my 1966 manual says 6 degrees BTDC. Is it same for the 1975?

thanks

 

Your initial setting is the least important one in your timing curve. I never care what it is as long as the engine starts good.

set your mechanical advance at 30-34 all in with the vacuum advance unhooked. then again with it hooked up just to make sure you're in the proper range. then check your initial so you have a reference point.

If it starts good ,runs good and doesn't ping under load your timing is correct.

 

IIRC 10-12 degrees BTC is where you usually start. <- well that's for a FE360, don't know if a straight 6 will differ much.

But heck 6-10 starting somewhere in that range will probably be good enough to get you started and idling without it dying.

 

440Sixpack - if i am setting my initial timing at 6 degrees BTDC, how am i getting to 30 or 40 as you suggest?? how do i go from 6 to 30? sorry, i am not a pro at this and don't know the various nuances of this process very well. i thought all i had to worry about was getting the initial timing right. what am i missing here?

 

You start at your 6, but the distributor has a "mechanical advance" built in that will increase the timing as engine revs up. So for example you rev up to 3K rpm, then see what your timing is (shine timing light down on to marks on the harmonic balancer). A mechanical advance in the distributor has weights on the end of it, as it spins, it advances the timing. That's how, at high RPM you end up with up to 30 to 40 degrees advance. Spark has to fire sooner because engine is turning faster, have to set the fire sooner so the fuel has time to burn before the piston comes all the way up.

The mechanical advance works by engine RPM, and Vacuum advance works with engine vacuum /load. They sort of "offset" each other to give you timing that adjusts properly for any engine RPM or load. They work together. but to check mechanical advance you have to unplug the Vacuum advance (so it's not influencing things) and then rev up the engine (reach over to the throttle and manually increase it) and watch your timing marks. I'm not sure the max RPM to look for max advance, i think some places it's around 3,500 RPM some it's 3,000 rpm

You can adjust mechanical advance by using different weights on it. I have never done so, or know many people that have. This is known as "recurving" the advance. You can also sometimes adjust vacuum advance, some vacuum advance units have a small screw inside you can reach through their vacuum hose port, and turning that screw adjusts how much they move / range of motion for the vacuum advance.

Obviously, also, turning the distributor will adjust the advance too. So if you set your initial at 6, and you find that your max at 3K is only like 20, that may not be enough.

Someone else will probably chime in here about what RPM to rev up to in order to check mechanical advance limits

It was my understanding that with mechanical and vacuum together, at highway speed you can even end up around 54 degrees advance total based on load.

 

Thanks Niko. very helpful. would any of what you said change if i had a pertronix set up in the distributor? i swapped my original points out in favor of the pertronix unit.

 

A lot of people don't know how to check their timing , even those who think they're pros. initial timing only means anything if your distributor curve is set perfectly , but it seldom will be. what effects power, fuel economy and engine life is happening at operating speed and your initial is meaningless now.

you need a dial back light , you can get timing tape for your engine but in my opinion this is a dumb alternative to a dial back since the dial back works on any engine all the time.

The first thing you need to understand is what you're doing and why. as the engine RPM increases so must your timing advance, but if it advances too much too fast you'll hear pinging, which is detonation. if your advance is too slow your power will suffer and your EGT's will rise. all this is accomplished with weights and springs in your distributor. after that your vacuum advance kicks in, under lighter loads your vacuum rises as will your vacuum advance. under more throttle your vacuum drops as will your vac advance therefore protecting you from detonation.

So to set your timing unhook your vac advance line and rev your engine to 2200-2500 RPM. turn your dial back light back until it's on the timing indicator at TDC, the number you read will be your actual timing at operating RPM, this is called " all in " meaning your mechanical advance is as much as it's capacity allows. normally this number should be 30 -34 BTDC.

now hook your vac advance up and rev it up again. it should be 50-60 BTDC , this number can vary quite a bit from vehicle to vehicle you'll just have to try it and see how it works.

Now check your initial to see if it's somewhere near normal, if it's a little low who cares, if it's too high you may notice your starter is being kicked when you try to start the engine . this means you need to work on your mechanical advance .

So if it starts okay and you have you numbers somewhere close to where they need to be go for a drive. hopefully under hard acceleration you don't hear any pinging and at cruise speed you don't have any surging. if you do back your timing of a tiny bit and see if it cures it. if not you have work to do on your curve.

This is a simplified version of the process, in a perfect world we'd all have access to a distributor machine to get it all set perfect but those seem to be history. so if this works great, if not it can be fixed but it will be somewhat trial and error .

 

That's a great description! The innards of the distributor are adjustable, come in many configurations though most people don't want to get in to that.

So consequently the "correct" initial timing needs to be determined another way. What complicates things, the shop manual might say 6° but who is to say what distributor is installed after 40 or 50 years, it may be the wrong advance curve. It also could be defective, this needs checked. There are a gazillion different possible combinations inside a distributor, depending on engine size, gearing, application. So it's important to determine what you have, at minimum, before trying to tune. Make sure it's a suitable advance curve for your purposes.

By pegging the RPM for a second and using a timing light we can see what the springs & weights are doing. Since it is a fixed advance inside the best we can do unless we want to modify or adjust the distributor internal advance is set the initial timing roughly wherever it needs to be in order to reach the target on the far end 36° or 34° or whatever number you choose, on the high side. So long as there's no starter kickback, anything from 2° to 20° or more can be made to work.

When you want to change it or verify this will take a few test drives. Bring a 1/2" box wrench with you. Try some full throttle accelerations and advancing the distributor slightly and try again, listening for engine ignition knock or "ping". The best point of operation will depend on fuel quality, altitude, temp. etc but this will get in the ballpark. This is without vacuum advance.

When vacuum advance is re-connected, it will be observed with a timing light that the advance at high RPM will be - say 50° BTC but remember when the engine is in neutral, there's absolutely no load on the engine. Vacuum advance is based solely on engine load. While the manifold vacuum is very high when cruising on the highway it's not that high. Don't worry about those numbers just yet.

The general rule is setup and tune the mechanical distributor advance first, it should run well. Then for a street driven motor re-connect vacuum advance and tune the canister itself. More test drive. Adjust the vacuum canister to pull in as much extra advance as it likes when cruising on level ground at part throttle in high gear. Most of them use a 3/32" or 2.5mm Allen wrench for adjustment.

 

thanks guys. lots to take in. will go back at it this weekend and see how far i can get with all that you gave me here. thanks a lot.

 

And no, adding the Pertronix Ignitor to take the place of the old points does not change the way the distributor reaches total advance, or how much advance it sees.
It does change some things, simply by (in theory at least) giving you a better and more stable spark at the plugs, so your engine's timing needs may change, but you're still setting the timing the same way. Whether just using the initial setting of your choice and running with it, or running down the optimum settings based on what the others have been describing.

You still want to know all that stuff. Just that putting an electronic ignition trigger into the mix does not, in itself, change those other parameters.

Paul