Timing troubles

[Dave 56]

I am the person who posted a question about changing my distributor from the Bosch 009 to the stock vacuum advance unit. I am running the standard L-Jet FI. I installed a stock unit, left the vacuum connections disconnected, and timed the ignition to the recommended 7.5 degrees BTDC at 900 RPM. I then ran a hose from the throttle body to the "backside" of the vacuum advance unit (the larger stub). I had the engine running and when I connected the line, the engine backfired immediately. When I pulled it off, it idled fine. What did I do wrong?? Also, I have looked at the hose diagrams on this site and it looks like everything else is correct. When I put my finger on the end of the hose coming from the throttle body, there appears to be a lot of vacuum at 900 RPM. Is this normal??

[Dave at Pelican Parts]

The fitting that points back toward the distributor body should be vacuum retard. This should be hooked to a line that gets a vacuum when the throttle is closed. Sounds like it is, which is good.

Does the diaphragm leak? Pull a vacuum on the retard port and see if it holds. Use a mity-vac if you can get your hands on one, or other good source of vacuum. Sucking on the hose might find a really bad leak...

See what happens to the timing with and without the hose connected. Does it retard? By a lot or a little? You'll need to use your timing light for this test.

--DD