I keep searching for a way to do ignition timing on my 1990 renix 4.0. I keep on hearing that it isn't adjustable, but my question is why does it have a timing mark? Also its about 16 degrees advanced, how do I fix this?

 

The best thing you can do is "index" the distributor.
The ECU adjusts ignition timing based on inputs from the CPS and other inputs.

Here's some stuff to help you understand.
CRUISER'S MOSTLY RENIX TIPS

DISTRIBUTOR INDEXING

OCTOBER 30, 2015 SALAD 103 COMMENTS EDIT

1. Remove the distributor cap and cut a “window” into the side of the distributor cap at the #1 spark plug wire post . The “window” should be large enough to allow easy visual inspection of the position of the distributor rotor at the #1 spark plug wire post. Reinstall the distributor cap.
2. Use Tip 12 first to guarantee you’re on #1 TDC.
3. Install a ¾” wrench or socket onto the vibration damper retaining bolt. Rotate the engine in a clockwise direction until the #1 cylinder is at top dead center. Align the timing mark on the vibration damper with the “0” degree mark on the front cover timing scale. The tip of the distributor rotor should be near the #1 spark plug wire post.
4. Disconnect the distributor electrical connection. Remove the distributor holddown clamp, holddown bolt and distributor. Remove the distributor cap and rotor.
5. Place the distributor housing upside down in a soft jaw vise. Scribe a line 1/2 inch from the end of the distributor locating tab. Cut the distributor locating tab at the scribed line with a saw.
6. Remove any burrs and metal filings from the distributor. Reinstall rotor.
7. If necessary, using a flat blade screwdriver, turn the oil pump gear drive shaft until the slot is slightly past the 11 o’clock position. The oil pump gear drive shaft is accessible through the distributor mounting bore in the engine block. A little tip here. Rather than use a dizzy gasket, use an o ring instead. NAPA #727-2024.
8. Visually align the modified locating tab area of the distributor housing with the holddown clamp bolt hole.
9. Turn the rotor to the 4 o’clock position.
10. Lower the distributor into the engine block until it seats. The rotor should now be very close to the 5 o’clock position.
11. Reinstall the distributor cap with the cutout “window”. Rotate the distributor housing until the trailing edge of the distributor rotor tip is just departing from the #1 spark plug wire post terminal .
12. Reinstall the distributor holddown clamp and bolt.. Reinspect the position of the rotor to the #1 spark plug wire post to insure that it has not moved.
13. Install the new distributor cap, reconnect the distributor electrical connections.

HERE’S WHY DISTRIBUTOR INDEXING IS SO IMPORTANT:

Distributor indexing explained:

For clarification though, that’s not a cam sensor inside the Renix dizzy. It’s there to fire the injectors sequentially with the firing order. You’ll never notice if it went bad because the ECU will try to “guess” where it is and does a heck of a job at it.

As for the “timing”, it is controlled by the ECU. Ever notice how wide the tip of the rotor is? Try and wrap your head around this:

When the ECU yells “Fire” to the ignition control module, where is the rotor in relationship to the dizzy terminal? Not to the terminal yet? Past the terminal too far?

What happens to the spark/secondary ignition strength when it has to jump the Grand Canyon in comparison to shooting from a rotor tip? Poor ignition performance, bucking, jerking, longer crank times.

The factory was aware of this and issued a Technical Service Bulletin on it. This Tip, #13, is a condensed version of that factory TSB. You wouldn’t believe how many we found out of whack when I worked at the dealership. Yours is probably messed up also. RARELY did we find one set accurately.

Use Tip 12 first to guarantee you’re on #1 TDC.