Thread: super advance running better

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I put a timing light to my engine today (BB chevy 402)
When I first started it up it was running good and fired right up (new carb), I had a fairly smooth 700 rpm or so idle, but I shot it with the light and it said it was almost 50 degrees advanced. I wouldn't have thought that would even run.
I retarded it until it settled to somewhere between 10 and 12, but it made it run much rougher and I had to turn the idle up to more like 900.
I have a 305 advertised hydraulic cam in it, 226 @ .050 single pattern.
Is there something I don't know, or does this sound normal.
(I had the dist. vacuum advance plugged)

does this sound normal.

No. Something doesn't sound right. Was that 50 degrees at idle?

it was at idle, I couldn't believe it was running.
I retarded it until it was about 12, but it was running better
the first time. I don't know what was up with it. The mechanical advance was working too.

[QUOTE]Originally posted by tcodi
[B]it was at idle, I couldn't believe it was running.


Not doubting you but I would shoot the light again. Hard to believe an engine would run at idel at 50 degrees advance

You sure you have the light on the #1 cylinder plug wire? Left front. Stranger things have happened.

I'm also assuming you have the original balancer - or at least one for a 396 - 427, not a 454.

You might want to get a piston stop and see if your balancer is off. Summit part #PRO-66783. $7.88.

Could the balancer slipped?

the balancer has a new key in it. I had the balancer off when I swapped the cam out. Maybe it wasn't 50, but it was way up there at 40 at least. I now have it right at 10,(I just ran it again and checked again), but it was way up there initially.

The only thing that may have happened was some interference with some other plug wires on the timing light. At first I had the clamp on the wire right up at the distributor, but I noticed the timing was jumping all around so I thought other wires may have been triggering it. I then moved the clamp right down in front of the plug.
This was after that initial reading and an adjustment on the cap.
I did however have the cap location marked and the mark is now way off from where I have it now at 10 degrees.

I'm positive my balancer and timing tabs are correct because I remember from way back when I had the heads off I checked it to make sure it was really TDC when it read TDC.

I don't know, but it seems to be running ok now, I have about an 800rpm idle, it is a little rough when it's cold but smooths out a bit after warming up. It sure shakes a lot though at idle.

The balancer can still slip, there is a rubber section between the main body and the outer ring, and the ring will sometimes slip. The only way to check that is with the piston stop, find true TDC and check that your timing mark is lined up with "0". I think 10 degrees is not enough, my big blocks have always run best at 14 or even 16 in some cases, but every engine is different.

I guess I may have to investigate that, don't you need a degree wheel for the stop method though? I don't have a degree wheel.
for now I think I'll advance the timing up to around 15 or so, it was definitely running smoother before I retarded it so much.

I checked my TDC mark and it is correct. I now have the timing at 15 degrees at idle, it is running fairly well.
One question: What is the deal with vacuum advance? I have it plugged now, how is the advance going to change when I hook it up?
Lastly, my carb has a vacuum port for a "timed vacuum" and a different one for "full vacuum."
What is the difference and how do I know which one to use?

pirated from netwelding.com....

The Chevy instructions also say to use "Ported Vacuum" to connect to the distributor vacuum advance diaphragm. This means there will be no vacuum present at idle. This port comes from above the butterfly's in the carb. Therefore until the throttle is open, there is essentially no vacuum in this port to activate the distributor advance mechanism. However and old Peterson publication entitled "Basic Ignition and Electrical Systems" reviews the history of "Manifold" versus "Ported" vacuum. As they indicate, "Ported Vacuum" came about during the start of pollution controlled vehicles to reduce nitrous oxide emissions with lean air/fuel idle mixtures. They state that by using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system. In fact, on this 502/502 motor an additional ~ 10 degrees advance occurs at idle when the "Manifold Vacuum" port is connected to the distributor vacuum advance. This provides about 25 degrees advance in total when idling or when cranking! The engine idles considerably faster compared to when the "Ported Vacuum" port is employed. The throttle butterfly's have to be closed about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed. All else being equal, less butterfly opening means less fuel consumed which equates to less total heat to transfer to the coolant.

pirated from carcraft.com...

Where’s the right vacuum port to connect to the vacuum advance, you ask? Most street carbs are equipped with both ported vacuum ports and manifold vacuum ports. Vacuum is generated by the engine’s pistons traveling to the bottom of the cylinders—air is sucked in to fill the voids, which in turn creates a signal to the carburetor to meter fuel. Manifold vacuum is present anytime the throttle blades aren’t fully open, including idle when they’re nearly closed. Ported vacuum isn’t “seen” at all when the engine is idling, because it’s taken from a port above the throttle blades.

Some gearheads have near-religious views on ported versus manifold vacuum. Patterson connects the vacuum advance to either source, and chooses the “right” port after test-driving both hookups. He does say that full manifold vacuum should help tip-in response, and this setting made Smith’s Mustang feel crisper off-idle. But if your car idles shaky and “nervously” with manifold vacuum advance, it’s a sign that there’s too much initial advance. So give the ported vacuum a try.

thanks for that article.
that clears up alot