69 BB L68 timing on pump gas

[Richard B.]

Re: 69 BB L68 timing on pump gas

Duke

I'm looking at the AMA Spec for the L68. I'm not sure what it all means so I may need some help

Cent'fal adv. in crank degrees @ engine RPM

Start (rpm) 900
Intermed points deg @ rpm nothing in the block
Max deg @rpm 26@3800

Vacuum adv. in cranks degrees @ eng rpm


start (in Hg) 7
Intermed points deg @ rpm nothing in the block
Max deg @ in. 12@12

Timing 4BTC @750 rpm

Is this what you were looking for?

[Duke W.]

Re: 69 BB L68 timing on pump gas

The centrifugal curve is short and rather lazy. It's set up for emission control, not torque or fuel economy. With only 4 degrees initial timing, total WOT timing is only 30 degrees, which is well below the optimum range of 38-42.

The VAC corresponds to the "360" specs and will be okay for full time vacuum advance.

The first thing to do is verify that what you have meets spec or is something else.

If it checks out to spec you can start by advancing the initial timing to about 12. If it detonates, back it off in two degree increments until the detonation goes away.

Another strategy is to grind out the slot in the distributor cam assembly to increase the centrifugal to about 30-32, so you can run less initial, and then you can get a spring kit and quicken it.

The basic stategy is to get as much advance as possible for every operating condition without detontion. This will optimize torque and fuel economy.

Modern engine control systems with detonation sensors and high compression run at the ragged edge of detonation under most operating conditions, which is the most efficient operating point for a give fuel octane. That's why modern Corvette engines produce huge torque at low revs along with high specific power and deliver very good fuel economy. With the simple centrifugal and vacuum advance systems we can only approach such flexibility, but with some experimentation you can usually develop and fairly optimized spark advance map.

Converting the Holley to full time vacuum advance has been discussed many times, but before you do that, test whether the VAC actually works and meets specs. Most don't last 40-plus years.

Duke

[Dennis S.]

Re: 69 BB L68 timing on pump gas

With only 4 degrees initial timing, total WOT timing is only 30 degrees, which is well below the optimum range of 38-42.

Richard
Duke sure can explain things well. The factory timing setting is nothing short of anemic. If you just loosen the hold down clamp and rotate the distributor counter-clockwise...you will notice the engine rpm increase and the idle characteristics improve. That alone should tell you the engine is happier with more timing. Then back off the idle stop screw on your carb linkage. No more dieseling when you shut it off. Take it out for a spin and tell us about the difference. You can do it this way, then confirm with a timing light where you are at.

Equus/Innova makes a couple of affordable digital advance timing lights with an LCD tach readout right in the display. You can actual map your advance curve...just takes a little time under the hood, leaning over that blast of hot air from the fan as you are revving to various rpms. I personally feel a little uncomfortable at much over 3000 and don't go there, because my timing is "all in" by 2800.

[Richard B.]

Re: 69 BB L68 timing on pump gas

OK, I think I got it running OK, had a little issue getting the idle down to 800rpm. The secondaries were not closing all the way. I put a lubricant on the cleaves and bolt and I think I got it.

When I put the timing light on it the timing was set at 12 degrees BTC. No more dieseling at shutdown and it seems to be running OK, I will be taking it out tomorrow to a local car show so I should get a better fell for how it is running.

Thanks everyone for your help.