midrange

Had my engine rebuilt over the last 4 months, 455, 40 over, 10 to 1 Icon's, Erson roller lifters, Lunati cam (thanks to Mark, cutlassefi ) 2600 stall convertor, headers, Accel distributor, 2 1/2 exhaust with Dynomax Super Turbo's, and 3.73 posi.
After a break-in period with rough idling but smooth running, the biggest problem I'm having now is traction, but that's not really a big deal.
However....

I had a hell of a time getting the timing and idle set. Initial timing (vacuum advance disconnected and plugged) is at 25 degrees. I double checked TDC, and everything is where it should be.

It will not run with at all with anything under 15. It seems to love 25, good idle (950 +/- rpm's), 17 inches of vacuum, and excellent throttle response.
When I hit it with 3000 rpm's the timing is reading 36, and steady.
At this setting, when I hook the vacuum back up, it's bogs, chugs and wants to stall. Unhook it, and the engine picks back up and purrs.
I can't advance the distributor to the point that it gets hard to start, because the car won't idle with much more advance.


After talking to several engine/car guys, the consensus is that if it runs that good, no pinging under load, and such good throttle response, I should leave the vacuum advanced unhooked and see how it does.

What is causing me to have to run with 25 initial and not need vacuum advance?

Octania

set timing to a happy point WITH vacuum connected... see how that behaves. Manifold tap for vacuum.

MDchanic

It sounds like you have a good initial setting for the timing, fully advanced, at speed.

Now you have to work backward to get the best timing at idle, by varying the advance springs.

Once you have both full advance (just shy of pinging at full throttle, through the rev range, hot) and idle (starts easily, idles well) set, you can work to fill in the middle with the vacuum advance, looking for as much advance as possible at roughly 3000 RPM at different throttle positions. Drive with a vacuum gauge to see whether there is a specific vacuum level that tends to ping.

One step at a time.

- Eric

midrange

I did have it set to a manifold tap, but the idle never settled down and caused the timing to wander.

Do you think that the springs are causing me to have such a high initial timing? The distributor came with 3 additional sets of springs. On a scale of least stiff to stiffest, the springs that came installed on the dist are the third stiffest.

oldcutlass

Are you using an Accel HEI?

MDchanic

If you tighten up the springs, it will take longer (wider range of RPMs) to go from idle to maximum advance.

You can mix springs - one heavier and one lighter - to make 7 different levels of advance (4 pairs of springs, 3 intermediate combinations).

The problem with manifold vacuum and too-weak advance springs, is that the weak advance springs have a lot of variability at low RPMs, and varying timing tends to vary RPMs, and varying RPMs tends to vary vacuum, so the idle increased, that increased the advance, that increases the idle, that increases the vacuum that increases the idle, etc.

If you have the advance springs set up so that nothing happens around idle RPMs, and the advance diaphragm set up so that it does not start to relax until a few inches below your idle vacuum (ie: you have to be driving with the pedal down a bit before the vacuum advance starts to retard), then connecting the manifold vacuum to the advance should not cause variability.

- Eric

midrange

Yes it is.

Just to update my situation, I went over to the Dream Cruise yesterday and the car ran excellent. It took 1 1/2 hours to get there from home. Purred like a kitten doing speeds from 45 to 60. At the cruise, in stop and go traffic for 2 hours, the car never missed a beat. Temp never went over 192, and throttle response was still excellent.

I think I have a slight misfire or carb issue, as every now and again, while sitting in traffic, the idle seemed to bounce and drop a bit, but nothing a quick blip of the throttle wouldn't clear up.
Since I've never had a cam with any sort of lope to it before, maybe it's something as simple as bumping up the idle a bit more.
I have yet to pull the plugs today and see how they look, but as of now, I have to say I don't think I need to have the vacuum advance hooked up.

MDchanic

They all do that.


You don't need to, but if you do, you will probably see improved part-throttle running and fuel efficiency.

- Eric

oldcutlass

I think your distributor has 17 degrees of mechanical advance, so at 25 initial plus 17 mechanical it puts you at a total mechanical of 42. I would think that you are getting more advance at higher rpms unless you installed a mechanical stop. In addition make sure your vacuum advance is not sending your total over 49-50.

The surging your talking about is probably air/fuel mixture setting or too high of an idle speed.