I was fixing a volvo p1800.
This is a 2.0 liter motor with Djet (1973)

the only major difference is the TPS is two circuits instead of three, the rest is the same...the MPS is calibrated for the volvo...but its still an MPS. (the plenum is alot bigger than ours)

So, I was getting a weak and very erratic spark...it would work then it would not...it was driving me crazy.

The final test was this.
Take a spare plug. Using a cable ground the plug and use a plug wire to connect this plug to the coil directly.
With a screw driver, open and close the point (basically the screw driver IS the points in this test...just short out the two sections of the points till it sparks)

So, this was providing me with a random spark...hmmm
I found that the ground lead, or ground strap that ties the advance plates together was faulty..
Even though it looked visually ok...it was weak and somehow broken.
I took it out and pulled on it..its still attached...but its not working correctly.

I happen to have another set of advance plates...plopped those in and everything was fine.

The reason I share is this took me a long time to track down...
It would work, then stop..then run on two cylinders etc.
I checked and replaced everything..relays, FI points, you name it...

It was a lousy ground strap inside the distributor that was at fault..

Rich

That is a good problem!

It doesn't matter if it electronics or automotive, ground problems are difficult to find (at least by measuring). We often speak of checking grounds here but that is one I think most all of us would have missed. Thanks for sharing.

Good job diagnosing the prob.

I have a 2.0 djet. Good info to know.

I had the same issue. Started running like crap then wouldn't run at all, but would try to fire now and then. Checked spark... Was weak / yellow at plugs. Was strong / bright blue at coil. That quickly narrowed it to the distributor.

Sure enough, that little braided ground came loose on one of the plates. To get home, I ran a temporary ground from a plate to the dizzy body (one of the plates is already grounded to dizzy body, other gets ground via wire - so ran wire from that plate). Fired right up.

Not a bad idea to carry some wire, connectors, and a crimp tool in your onboard toolkit.

I never would have thought of that...

Pretty common problem. I used to stock the plates (if the strap is worn out, so's the plate). NLA. The Cap'n

Question, Cap'n...

I am curious, what wears out on the plates themselves? Surface wear, inhibiting smooth movement? What if they have been kept clean and lubed? Still?

Thanks.

They get sloppy, causing substantial dwell variation. The Cap'n

just added to my list of the top things to look for now...lol

Rich