Question about points and MFI

[Jay Auskin]

I've followed most every post on MFI. I rebuilt my '72 2.4L T engine last year, and it runs great...for the most part. I've been getting ready for the "warm season" by reading up on how to properly tune, and diagnose any fuel/ignition systems on my car.

One issue I had last year was the tach would bounce. Also, letting off the gas at high RPM would cut fuel (guessing the microswitch is working), but it usually wouldn't recover. If I didn't keep the hand throttle up a bit so it would idle a little high, this would usually kill the engine.

So, I need to test:
-resitance in coil to check if coil is OK
-properly set throttle linkage to activate microswitch
-Adjust dwell...more on this:

I put new points in my car, and set the gap, then the timing, but I never checked the dwell. Hey, what can I say, winter was approaching.

So my question, is setting the points gap just a starting point, and then it is fine tuned by the dwell setting?? Because dwell is adjusted by the points gap, right??

Will this cure my bouncing tach if the coil checks out OK?? Then, will the speed switch properly read the RPM, and allow for fuel to flow again below 1300RPM?? All this assuming the speed switch works...man I hope so, those things ain't cheap!!

Sorry for the long post, and thanks in advance!! (no, that's not an ignition joke)

[Shuie]

bouncing tach is from the points being off.

So my question, is setting the points gap just a starting point, and then it is fine tuned by the dwell setting?? Because dwell is adjusted by the points gap, right??

Yes

Points have left me stranded twice. I went to a Crane breakerless ignition trigger on my car and have never had timing issues again.

[lateapex911]

yes...it is one of those "futz" with it deals...

I too have a Crane optical trigger points replacement set up. Thankfully!
Now if I can get the Webers running right....

[Jay Auskin]

Thanks guys. Now it will actually become Spring here in Chicago I can take the darn cover off the car, and hook up the battery!! Man it's been cold!

[Grady Clay]

Jay,

As Sherman said, the dwell is changed by the points gap.

The CDI is much less affected by dwell than the original Kettering ignition (points, coil, condenser) because the CDI senses points opening, fires the SCR and dumps the capacitor through the ignition transformer (which we incorrectly call the “coil.”).

The two usual circumstances where the points cause a problem is if the rubbing block wears down, the points tend to close up. Too close points cause poor spark timing and the tach to bounce around. The other problem is the electrical contact points need to be perfectly clean however right next to them is the rubbing block and distributor cam whizzing around that need lubricant. A little lube on the points causes the tach to bounce around and poor spark.

One of the best ways to maintain a distributor is to set the engine to TDC #1 cylinder and remove the distributor. Take it to a shop that has the machine that can bench run the distributor after servicing it. This way you can be sure the spark timing is consistent, the advance-retard is working properly, and it gets some run-in time before being reinstalled in the 911 and the timing set.

Of course there are other ignition system service items. Rotor, cap, wires, plug connectors, and spark plugs all have to be near perfect and are regular service replacement parts.

The condition of the electrical connections in and around the CDI box, ignition transformer, and connecting wiring is important for our old cars. That includes the ground connections.

The ignition system should be in perfect, like new condition before doing anything with the MFI.

Best,
Grady

[Early_S_Man]

Jay,

Don't worry! If you set the gap to 0.014" with new points ... the dwell will be perfect!

Here is a thread with all of the pertinent data for the distributor in your engine:

distributor lube...

And, here is the complete data and procedures for testing your CDI system:

Perma-tune CD or Bosch CD?