'75S No Start with Backfires

[sketchers356]

Hi Everyone,

I am having trouble getting my '75S started this year. I replaced the wires, cap, and rotor earlier this year and the car started fine. I then started work on a heater backdate project. During the project when removing the old heater tin, one of the spark plug wires (Beru) got the slightest pull on it and disconnected. I have since replaced with a set of magnecores. I can hear my CDI box whining after I turn the car off. The car backfires but does not sound as if it is catching at all.

I know the most obvious thing is to check for spark, but I am too frustrated to wait for an assistant to help me with that, and was wondering if there is anything else I can look for.

I have checked all of the wires and everything seems in order. I did have to pull on the big wiring harness a bit as well to get the duct removed.

Is there a way to make sure there is continuity from the coil to the coil wire? Anything else to look for? I don't see any airbox shrapnel and I had a pop off valve installed.

Thanks for all of the help guys!!

[sketchers356]

Oh, I have a very old set of points currently installed. I have a pertronix kit that I was going to install but then the no start happened.

[Targalid]

Check for spark and fuel. An inductive timing light is easiest way to check for spark. Make sure your point gap is correct. Motor will not start if the points are too tight. Should be .012 inch. Then lift the air sensor plate with ignition switch turned to run to check for fuel delivery. Backfiring suggests your mixture is not correct, perhaps too lean.

[Bob Kontak]

Originally Posted by Targalid

Backfiring suggests your mixture is not correct, perhaps too lean.

Well, Alexander has some spark at some point in time or no backfires would occur.

I like the inductive timing light suggestion. Get a pal, get a kid from the neighborhood and have him crank the key. If the wires are long enough you can point the gun at the rear fender while reaching in the cabin and turning the key. Hell, tape the button closed an lay the gun on the floor. Put the spark issue to bed.

Next confirm timing.

Intake backfire. Too lean. Big intake backfire = big leak.

[BK911]

Plug wires crossed is my guess.

[Bob Kontak]

Originally Posted by BK911

Plug wires crossed is my guess.

Unless a complete cluster eff, the engine would run. Poorly. But would run.

Could crossed wires be coupled with another silly mistake? Hell yes.

[DanielDudley]

Cracked distributor cap will do that, or badly deteriorated plug wires if they are not shielded.

Start with a full tune up, and go from there.

[T77911S]

if it backfires you are getting a spark. its just at the wrong time, either due to plug wires crossed or dist 180 out. (or plug wires 180 out).


outside chance of too lean. usually that is noticeable after it starts, lots of backfiring.
air leak or CCP too hi.

[timmy2]

Check firing order to ensure new plug wires run to correct locations. Also check the new plug wires are fully inserted in the distributor cap and coil. I had the center wire back out on my magnecors when new. (At the coil)

You mentioned jostling the main harness. Check the coil negative wire has continuity to the coil case mount ground wire.
One weak point in the factory harness is where the brown wire to the coil mount is connected to the shield wire going to the coil negative wire.
Also check the wire to the points is secure.
Make sure your points are clean and gapped correctly.

[obscene]

this is really silly, but the last time my car behaved that way it was due to the rotor not being pressed down fully. certainly worth the 5 seconds to check...