Duraspark Recurve Fun

[colnago]

For those of you who are into this kind of thing, I decided to try understanding mechanical advance, what my current setup is, and how to modify it.

First, I started with a Duraspark distributor that I got from RockAuto. Nothing special, just something to get away from points and my Pertronix unit. I spent some time under the hood yesterday mapping it out. Initial Timing was 11 BTDC. It looked flat until 1000RPM. At 1500RPM, it had started climbing. I thought it was all in at 4000RPM, and I had 35 degrees advance. Looked like my distro was set up for 24 degrees advance at the crank, or 12 degrees at the distro.

Today, I broke into it, and found that the opening was closer to 0.5, so it was 14 degrees at the distro (28 degrees at the crank), which meant that I still wasn't full in at 4000+. The other slot was close to 0.4, making it a 10L slot. So, I popped it out to swap slots.

I had purchased the Mr Gasket 925D spring set. After I swapped the slots, I put one of the new springs in for the lighter Duraspark spring. I noticed with this smaller slot, the heavy spring never came into play, so I decided to swap it out, too. Now, I don't have the "step" from the original setup.

When I remapped it, I set my initial timing to 16 degrees. Pretty flat to 1500RPM, and all in by 3000.

I also changed my vacuum advance from ported to manifold. So far, the truck likes it, and my fun-o-meter is almost pegged. I still have some tweaking, because my idle is a little high, and my vacuum is a little low (14-15hg). I did notice that with a bit of choke, vacuum was a solid 16hg. I also need to work on the vacuum advance (I have an adjustable can), so maybe next weekend I can try to make it ping.

Anybody see any potential gotchas or advice?

Joseph

[basketcase0302]

You can save a lot...of money on gas over even a year by tuning like your doing Joseph-kudos to you!
I'm not sure if I missed it or not but you are starting your initial mechanical spring changes and tuning with the vacuum advance unhooked aren't you? As that is always the preferred method to first get the mechanical tuned then hook up the vacuum advance. Another "trick" that Ford started adding in the middle 70's was to run your advance through a PVS, (ported vacuum switch) which is very simple to install and hook up:

http://repairguide.autozone.com/znetrgs ... 053c99.jpg

[colnago]

Definitely tuning with vacuum advance disconnected. The recurve was actually fun, in a sick, distorted way. The old girl has a bit of sass to her now! Best out of this is, now I have a better understanding of what is going on. Before, it was, "Turn the distributor until it pings, then back off by two degrees." Now, I know why you have to back it off; it's not just a number. Also, it was so easy to change the behavior of the engine. From start to finish, I was done in a weekend. To do the actual recurve, I think it was a total of two - three hours.

Interesting with the PVS. Looking at your pic, it makes sense. Right now, I have my vacuum advance hooked up to manifold vacuum. She's very responsive, but the idle is higher than I like. I have an adjustable vacuum can on the distro, so I have a lot of playing to do. Throw in your PVS, and you just doubled my fun!

Another good thing with swapping the mechanical advance, I'm getting really good with stabbing the distro back in!

This weekend, I'm rebuilding the carb, then doing some massive tweaking. More posts to come ...

Joseph

[basketcase0302]

Joseph,

The PVS was another one of those EPA "monster" things I believe mandated, (but I don't know if chebby's have ever had them) in the early 70's to increase gas mileage and decrease pollution. I loved running it because it gives your dizzy the true vacuum on the warm engine.
Believe I still have remnants of a recurve kit here with different springs and weights for an Autometer dizzy I still have.