Fuel pressure issue?

[gull]

I remember some VWs around that era had idle timing that was retarded to a few degrees ATDC, to reduce idle emissions. Often these were cars that had dual-vacuum distributors, with one advance port and one retard port. The idea was the timing would jump to a more normal value as soon as the throttle opened and the manifold vacuum went down. I don't know if that's the case with your car, but it could be. If you have one like that it's particularly important that you pay attention to any hoses the instructions tell you to disconnect and plug while setting the timing.

I don't think a different camshaft would make a big difference in where you set the timing, but a different distributor would.

If you want to experiment you can try advancing the timing until the engine starts to ping under load, then backing off slightly until the pinging goes away. This might give you a little more performance but might also compromise your idle quality a bit. At least, that was the trade-off on my car. The stock setting is a tradeoff between performance at speed, idle emissions, and tolerance for lower-octane gas.

[rb0095]

If you want to experiment you can try advancing the timing until the engine starts to ping under load, then backing off slightly until the pinging goes away. This might give you a little more performance but might also compromise your idle quality a bit. At least, that was the trade-off on my car. The stock setting is a tradeoff between performance at speed, idle emissions, and tolerance for lower-octane gas.

When advancing the distributer should the idle go up, or down?

Yes, my car does have a dual vacuum distributer, one line is the vacuum advance (intake manifold vacuum line), then the other is the vacuum retard (it connects to the throttle body in front of the valves, that way when valves are closed at idle this line does almost nothing). Didn't think of that before...so if I had the other type of distributer (single vacuum line) then I would time the engine to 6 btdc? is that right?

[gull]

When advancing the distributer should the idle go up, or down?

Could go either way. There will be a point where the car idles best but this doesn't necessarily coincide with the point where it runs best overall, unfortunately.

Yes, my car does have a dual vacuum distributer, one line is the vacuum advance (intake manifold vacuum line), then the other is the vacuum retard (it connects to the throttle body in front of the valves, that way when valves are closed at idle this line does almost nothing).

Actually, it's the other way around. The retard side goes to the manifold. At idle, manifold vacuum is very high, but it gets lower the more the throttle opens, so this provides timing retard at idle.

The advance side goes to a vacuum fitting at the venturi (narrow point) of the throttle body. The air speeds up going through that narrow area, and its pressure drops due the Bernoulli effect. (This is the same effect that causes water to rise in a straw when you blow across the top of it.) The more quickly air flows through the venturi, the more vacuum appears at this fitting, so vacuum *increases* as the throttle opens and the engine draws more air. This provides more advance as you accelerate.

In most distributors there's also a centrifugal advance system that advances the timing as the RPMs increase, too.

If you rev the engine while you watch the timing marks with your timing light, you should see the distributor advance mechanism at work -- the timing marks will appear to gradually slide out of view as the engine speed increases.

Didn't think of that before...so if I had the other type of distributer (single vacuum line) then I would time the engine to 6 btdc? is that right?

That's right. You'd connect the single line to the fitting on the throttle body.

[rb0095]

Actually, it's the other way around. The retard side goes to the manifold. At idle, manifold vacuum is very high, but it gets lower the more the throttle opens, so this provides timing retard at idle.

Thanks for the correction.

Based on that information it seems that since my car has the dual vacuum distributer the amount of vacuum at idle is important. (for other reasons as well)

[rb0095]

Ok, so it's been a while since I've posted any updates for this issue. Just wanted to post one now.

The test plug for using a dwell/duty reading to adjust the fuel mixture is working. (I don't remember when/what I did to get it working)

I haven't done anything (that I can remember) to find and fix vacuum leaks or the fuel issue for a while. I currently need to do a valve adjustment and a tune up, but overall the vehicle runs well (it just doesn't start right).