I got to thinking about this reading through another thread on carb tuning and wanted to kick it around a bit on its own. I figure a lot of what people try to diagnose and fix as a carb problem may start with the ignition and timing. Since I've still got a couple of minor stumbles to tune out of the Bronco, I want to get the timing 100% dialed in before I fiddle with the carb anymore.

I'm going to get out the timing light and a tach to map the timing curve
but what I'm clueless on vs. a SBF is the timing curve for a 300. What is the maximum mechanical advance? At what rpm should max advance happen? How many degrees should the vacuum advance add?

Right now my truck drives best with base timing @ 16-18 degrees - just shy of pushing back on the starter when it's warmed up. That's nuts for a stock cam, Offy C, and EFI manifolds. And it pulls the idle to 800-900 rpm. Assuming there are no mechanical issues with the dizzy, I'm thinking having the mechanical advance come on sooner would let me dial back the base timing. But how soon and how much total is the question.

Any thoughts?

It seems like this would be an even bigger issues once you add a cam and some head work to the equation. Anyone had the distributor recurved when they built up their 300? I'm half-way to taking mine to sombody with a timing machine and having it dialed in just to be sure.

 

A recurve is usually in order on the DS2 as the stock unit is curved for emissions.

Check this out: Duraspark_distributor_recurve_instructions_index

 

Alrighty then, I finally had a chance to pull the dizzy and see what's what.

First off, the vacuum advance was cratered. Held no vacuum pressure on the diaphragm, it was just like sucking through a drinking straw. So in addition to the lack of vacuum advance action there was another below the carb boosters vacuum leak.

Oh, found another vacuum leak on the vacuum line to the A/C vent controller too. Which helps explain why I couldn't get the idle down where it needs to be, and why it wanted so much initial advance. The engine was pulling ~20" of vacuum before, I'm interested to see where it will be once I get all this stuff squared away.

Inside the dizzy, the mechanical advance is set in the 16 slot on the reluctor (32 degrees advance). The other side is 21 (42 degrees). No worries with a stock short block but when (if?) I build a new engine with compression around 9.5:1 it may need to be welded back to 24-26 degrees total advance.

I had another duraspark dizzy sitting around (from a 302) and swapped the heavier spring in the 300 dizzy for the lighter spring from the 302 dizzy. I'd like to see the advance all in between 3,200 -3,500 rpm. Anyone have any ideas on spring combos? I know I'm going to have to end up pulling the dizzy a half a dozen times to get it dialed in.

Of course, neither of the auto parts stores near my house had the advance canister in stock and aren't getting trucks today because it's a holiday. Oh well, plenty of other stuff to fool with until Monday...

 

Can't seem to find my spring chart to tell you the combo you're looking for. But, if you get the vacuum can with adjuster you can control how soon the vac kicks in. It will have an allen head adjustment screw in the spout. I'll keep looking. Crane was selling the canister and spring kit as a package, but I haven't looked for it for awhile. They are back up and running, but don't know if it's still offered.

 

I read a good article about how to recurve your dizzy. IIRC, it was on classicinlines.com, or the Pony Carbs site.

During my build I found a freelance dizzy guy who works out of his garage. I told him of the build, and set up my ds2 for $175. He set it up with 32 total, with 16 base. But the starter didn't like it there, so now I'm running 14 base. With my mild cam, 1.7 rockers, I get about 17 vac.

No pinging unless the outside temp is over 110. Then I have use higher grade or octane booster.

With an older engine it is possible the v.dampener is off, slipped on the rubber. If you read the info at Pony Carbs site, he swears to time with a v.gauge! Using a t.light on my engine scares the ... out of me, after getting bit. Now I nearly put on a wet suit when I'm going to use my light. I got to get one with batteries.

 

nevermind...

 

I wear rubber gloves and boots just to change flashlight batteries....and a vacuum gauge for timing and carb adjustments. A timing light satisfies one's curosity but arbitrary timing numbers don't necessarily mean a result the engine likes. JMO

 

If you're good, once you have the spring you like, you can rotate the engine until the tabs inside the distributor are accessible through the wire clip hole in the distributor. Grab the rubber grommet, give it a 90° turn, and pull it out. Put a flat bladed screw driver through the hole to bend the tab one way or another (careful to do it slowly). Saves having to pull the reluctor off every time which can be a huge hassle.

 

There's a downside to timing with a vacuum gauge - you can't see the mechanical advance curve. I'm 90% sure the last little bit of bog and laziness the truck's got at WOT is just a matter of timing.

Just out of curiosity, how would a vacuum leak at the ported vacuum affect the engine at driving speed/load?

For starters I've got the new vacuum can set up to start advancing at 8-9" and it's all in by 15".

 

a vacuum leak on the ported vac would result in a lean condition at anything other than idle, right? that may be contributing to bog/laziness.