Th0m4s

Sooo like the title suggests I've got problems setting the timing on my new old 12a. If I set the timing to the marks on the pulley the engine runs like crap and wants to stall if it gets the slightest load while idling (like headlights on). Here's the funny thing: if I set the trailing timing right on the leading mark (yellow) the engine is running like new without any hesitation.

I still can see the mark of the retaining nut on the dizzy from my old engine and it's exactly where the engine is running best.
The new engine is a bit of an unknown. I bought it from a wrecker and I found out it was rebuilt at some point because the housings don't match the plates. Could it be that the eccentric shaft is from some other engine, too and has a slightly offset groove for the pulley?

What could I do to find out what's going on here? Finding tdc is a bit difficult without timing marks I could trust🤔

Joekaistoe

If changing your trailing timing is causing significant running issues, there is something wrong. You should be able to run with trailing disconnected and see minimal change in idle quality.

My guess is that either your spark plug wires are mixed up, with your leading wires hooked up to the trailing terminals, or your leading ignition is dead and your car is trying to run solely on the trailing.

For the first possibility, double check that all the coil and spark plug wires are plugged in to the right places. If they are, double check that the coils are connected to the correct ignition modules.

For the second possibility, check the condition of the plugs, wires, coil, and ignition module.

j_tso

iTrader: (2)

Are you using a stock pulley? if so, the pulley has holes drilled every 90 degrees so it can be installed incorrectly 3 ways.

Rotate the engine so the face of the front rotor is like the picture below, use something to poke into the spark plug holes to check if it feels even.
Then bolt the pulley back on and the timing mark should be pretty close. From there you can go about checking ignition timing and adjusting the distributor.

Th0m4s

The ignition system is working like it should and the pulley is installed correctly buuuut by poking through the plug holes I found out that the marks on the pulley are off by about 20 degrees. I just wonder how this is even possible.
Now I need to find the exact tdc of the front rotor and I wonder how I should do this. Poking at the rotor surface through the plug holes gets me in the right direction but that can't be 100% exact.

Maxwedge

iTrader: (1)

Did you accidentally install the dizzy a tooth or two off?

Th0m4s

No I didn't. I can match the timing marks. The engine just doesn't like to run there.

Benjamin4456

Sometimes people paint the wrong divots on the pulley. Once they're this old often new "divots" will appear simply from rust and sometimes folks will paint them by mistake instead. Perhaps this happened with the previous owner, who knows.

My only other thought is if you were running some sort of custom igniter setup. Accidentally reversing the polarity of the pickup wires will retard the ignition events by a bit.

The way I locate TDC is by looking at the flywheel through an inspection plate. When the flat side is on the exhaust side of the engine you're at TDC. Unfortunately it assumes you have a stock flywheel and an accurate pulley since it doesn't give you much precision beyond 90 degree increments.