distributor problem again

Does the vacuum advance diaphragm hold vacuum? Is its return spring broken?

Does the advance unit's rod (connected to the distributor base plate internally) move when you suck on the hose?

Does it hold its vacuum when you manage to block off the tube with a finger while under vacuum?

Does the base plate return to its rest position when the vacuum on the diaghragm is released?

The original diaghrapm will be well past its use-by date. The base plate may be binding for whatever reason.

John

Another possibility:

While the distributor base plate should rotate in response to changes in vacuum, it shouldn't move up and down.

On some Bosch distributors, the base plate is held down by a leaf spring (retained by a screw). If this spring breaks the base plate can move up instead of rotating, allowing the points to stay closed so there's at best an intermittent spark.

Note that there will be greater vacuum at partial throttle than while accelerating.

Again, you can check the movement of the base plate by sucking on the vacuum hose with the engine stopped and the distributor cap and rotor removed.

John

John

Explain?

For a given RPM, the inlet manifold vacuum is greatest with the throttle butterfly closed (foot off the pedal) and least with the butterfly fully open (foot to the floor). That's because the butterfly directly closes and opens the air path into the manifold.

The vacuum tube to the distributor is taken from a point which reflects manifold vacuum. To get maximum power and economy for a given amount of fuel, you want maximum ignition timing advance without the risk of pre-ignition/pinging/pinking. And that means less advance at full throttle.

The alternative to vacuum advance is centrifugal advance, where the degree of advance is governed by engine speed alone. In that case, the advance curve is designed for full-throttle operation, and economy at partial throttle suffers slightly.

VW industrial (stationary) engines were always fitted with centrifugal-advance distributors, and these distributors were much sought after by performance tuners whose modifications were likely to upset the vacuum advance curve.

John

Understood, however since the vacuum advance port is above the butterfly It's the transition from high vacuum (intake manifold ) to atmospheric pressure which allows a momentary vacuum signal to the advance port in the throttle bore. The key to remember is that it's momentary, not sustained as would be the case using an actual intake manifold take-off (i.e. like the vacuum brake port on a later bus.) On a dual vacuum dist. the advance port is above the throttle and the retard port below the throttle valve.

SA type distributors are best suited to the industrial motors which tend to spend most of their time at constant RPM or competition engines which are usually in the high rpm range. This is why the 009 is best suited to competition use and can't provide the initial advance at low rpm need for smooth acceleration. Also, aggressive camshafts often don't produce enough vacuum for SVDA type distributors to properly function at low rpm.>

It's been so long, I've forgotten some of the details, like the exact plumbing details of the vacuum source. Thanks for the reminder.

I do remember those dual (advance and retard) distributors. And combinations with both vacuum and centrifugal advance.

I used a distributor from a 126 industrial engine on my rally beetle (1600 Mahle cylinders with domed pistons) back in about

John

John: Do you recall if the dist. from the Industrial engine was a version of the 009 or did it give more overall advance? Bill

No idea sorry. I've not had a 009 myself. And my brother-in-law thoughtfully threw out my 126-engine distributor when he did one of his clean-ups and I was away :(

John