I've recently done the duraspark mod for my truck and removed all of the computer crap.

For a while, It was running really good at 10-12 degrees BTDC and I have had issues with my power valve gasket leaking and flooding the motor out. I've gotten that fixed. It's not leaking but yesterday. Before I left the house I discovered that the gasket had leaked again (I fixed it after this last time) and now I'm not getting a decent intake/manifold vacuum at 10-12 BTDC. I normally get around 19-21 inches of vacuum and now it hovers around 17-19 inches.

I decided to mess around with the idle mix screws and I couldn't seem to get it to go back up. Assuming I had messed something up when I had to remove the carb to fix the PV leak. I'm also thinking it might have something to do with my carb mount gaskets (which I bought a bunch which will come in tomorrow)

I did fix the intake vacuum by advancing it to around 13-14 degrees of advance (Base timing) and then I tested my total mechanical advance and it sits around 28-29 degrees on the balancer. The vacuum advance is really what's worrying me. Mainly for the fact that it goes WAY off the timing marks on the wheel. So I can't

This is a photo of the balancer.
really tell what it's sitting at. I did buy an adjustable vac advance that can hopefully it fit and works.

 

Vacuum advance is a little tricky to measure, because it is an engine load based system.

Without using a distributor machine, the only way it is measured by a DIYer is with the engine in neutral ...without any engine load. So the timing light will indicate a very high level of ignition advance when measured with a timing light in the garage. On the highway at a steady level cruise there is hardly any load on the engine but there is some. Most factory stock engines cruise at around 50° BTDC or more.

As a practical matter it isn't too much of an issue if the vacuum advance can't be measured accurately.

Disconnect and plug the vacuum advance port. See if you can't get a few more mechanical advance degrees without engine complaint or engine knock on hard full throttle acceleration. This may take some test drives. Give it what she wants. Then back off just slightly from that. Leave some on the table for a possible bad tank of juice. This should be the same number of degrees that is listed in the factory shop manual. Keep in mind the shop manual lists advance degrees in terms of distributor degrees, which spins at half speed to the crankshaft. So double the numbers. I'm sure it's well north of 30 however.

Then tighten the distributor hold down bolt and don't mess with it again. Re-connect vacuum advance, and go for another test drive. Here listen for part throttle and steady cruise rattle or engine knock and surging. It appears under a completely different set of conditions than the mechanical advance mechanism. If there is engine knock, adjust the vacuum advance only. Find out that precise point where "too much" is on the vacuum can, and back off slightly from there.

 

Thanks for the quick reply. I thought I'd get a reply tomorrow from how late it was when I posted this. EST

So having above 30 or so BTDC isn't bad but the motor will tell me what she likes?

The only issue with my motor is that it got the famous 2.8L lifter tap dancing sound. It's hard to hear spark knock let alone, I haven't even heard it before. YouTube isn't much help since its people asking for help if they have spark knock haha.

I will do what you said, I've got work tomorrow at 4 and I'm hopefully gonna be getting my carb mount gaskets before then. Hopefully, that'll fix something as the gaskets I've got are torn up and wore out.

 

No, most OHV engines will be spec'd well north of 30+ degrees of ignition timing at certain RPMs. This would be without vacuum advance connected. In operation they work together but they are measured and adjusted separately.

The exact number distributor timing varies depending on fuel quality, elevation, weight, gearing, compression ratio etc. The best thing to do is find out what the shop manual lists and start there. Too much timing is always a problem, but so is too little. Any engine runs optimally with the ignition timing advanced to the point just short of engine knock at any given point in the RPM band or load point. It's all done by sensors and ECUs now, weights and springs and vacuum diaphragms were the only way for a long time.

 

The key thing with manifold vacuum is that it is steady, not a specific number. Although the number is important too. If normally the engine pulls 20" at local elevation, and now suddenly only pulls 18" that is a defect that needs investigating. A stock factory equipped motor should pull 19" to 21" at factory spec ignition timing and idle, assuming sea level altitude. Check carefully for some kind of vacuum leak. Could even be the vacuum advance diaphragm.