I want to kick this around some so maybe I can get a better understanding as well as some of our resident lurkers that are interested.

I put a bigger roller cam in the Bronco than factory (351W). I have it set around 10° BTDC now.I may be hearing a little rattle during heavy acceleration still. I am considering going to about 8°. Also when it gets hot I still get a little pre ignition run on when shutting it off. So this makes me think there is still too much advance.

When changing to a bigger cam in my mind it seems like it would want the timing to be coming in "later" . IOW if factory specs call for 10° BTDC so 8° would be later. In my mind this makes sense because the cam is bigger and holding the valves open longer and opening them higher. So if it takes longer for the cam to do its job wouldn't I want the spark to wait just a little longer too?

Am I way off in left field with my theory of how it works?

 

You want to advance the timing. With the larger cam your engine is now suited to run better at higher rpms. This requires the timing to be more advanced. The cam pretty much kills the use of the engine at lower rpms.

This is also probably why it's running on when you turn it off. You are having to open the carb up and raise the idle speed up to keep it running with the rough cam. This introduces a lot of air to the engine, and all that air entering when you turn the ignition off can let the engine keep running if there is a hot spot inside the combustion chambers. And with these engines, there usually is a hot spot somewhere that will keep igniting the mixture.

To stop the run-on, turn the idle speed down and much as you dare and it still runs. If you can't get it low enough, I would go to the junkyard and get a A/C idle kicker solenoid and retro fit it to the carb and hook it up to ignition on power. Turn the key and tap the throttle, and the kicker will extend out and you can set the idle there. When you turn the engine off, the kicker will retract, restricting the air to the engine and it will not run-on like it does. The factory actually used this method on some of their configurations.

 

Remember that going from 10deg BTDC to 8 is retarding the timing. You want to advance timing (so spark occurs sooner in compression stroke) so you would go from 10 deg to 12 deg BTDC - to advance.

 

OK, that makes sense Dave.

If I advance it anymore from here it rattles under acceleration. Like I was saying there still may be a quiet rattle under acceleration.

Is the rattle I am hearing related in any way to how rich or lean the carb is?

 

The answer to your question is a qualified "yes". Enrichening the mix can help with detonation. But not dramatically. In any event, you do want to get your air/fuel ratio sorted out before finalizing your ignition timing.

So, on to the rattle under "acceleration". If that acceleration is full throttle then you probably have either too much initial advance or too much centrifugal advance dialed in. (I say "probably" because you still don't have the air/fuel mix sorted out and are running lean due to not having a PCV valve, so the rattle may go away when you get the AFR sorted.) But if the rattle is during part-throttle acceleration then the vacuum advance may still be providing some lead and you could adjust it. But you can't really tell unless you hook up that Snap-On vacuum gauge and figure out both under what vacuum you get how much advance, and under what throttle and rattle you have how much vacuum. In other words, you need to know how much advance the vacuum unit is giving when the rattle occurs, but the only way to measure that is to measure advance vs vacuum while stopped, and then vacuum while accelerating.

 

Sounds like I need to install the adjustable vacuum advance unit also before I get too carried away.

 

Yep. And the PCV valve. And settle on step-up springs in the carb as well as rods and jets since that dictates the AFR under acceleration.

 

I would disconnect it for now, and tune it without it hooked up. I think you will like the results, it will let you run the initial timing way up above 10 degrees. You will get a very responsive engine like this. Then once it's all worked out, you can buy an adjustable vacuum advance and tune it in later. It will just add a little bit on top of the timing you already have. You will probably have to re-adjust the initial once you install the adjustable advance, but you should already have that number in your head and you can just set it to that.

If this was some sort of mud truck or something like that, I would not even bother with the vacuum advance. That's why most racing distributors don't even have a place for them, a engine that is wide open or with a very heavy load on it most of the time will see no benefit from a vacuum advance unit, since there is hardley any vacuum on the engine in this condition anyway.

 

Good idea, Dave. Get the initial and centrifugal sorted and then add vacuum.