Cold start issues that defies logic

[91166 Carrera]

Twenty years ago I entered this rabbit hole and I'm still trying to get out.

I'm trying to resolve a pesky cold start issue.

I have a '76 Euro Carrera 3.0 (CIS) and when I rebuilt the engine I installed a 3.2 Max Moritz 9.8:1 Mahle PC kit, twin plugged the heads and converted a 964 distributor to a SC type with a "green" co-axial wire. No ECU. Sounds crazy right?

Back to the distributor. The internals (Hall sensor) of the 964 was gutted and the internals of an SC (magnetic sensor) was inserted. It went from a points vacuum advance to a mechanical advance. The distributor mounting flange was cut off and a custom flange with an elongated hole was welded on so the distributor could rotate for timing adjustment. There are two coils, one for each distributor head. This eliminated the need for an ECU.

The CDI is a M&W unit that is capable of firing two-coils, although the single red (12v) wire has been split to two, one to each coil.

The auxiliary air valve has been replaced, all the hoses (air and vacuum) have been replaced, replaced the WUR and FD, compression is good, fuel system and control pressure is good, replaced fuel pump, replaced fuel filter, and the AFR is at 13.5+/- (2.5CO), and the cold start injector mount (red) has been replaced. A smoke check indicates no vacuum leaks.

With this modification I lost the cold start advance and the engine hunts. After one minute or so the engine idle settles down to 900rpm and runs steady when warm. It doesn't hunt on warm start.

The distributor's timing is set a 8BTDC at 900rpm and advances 28BTDC at 4,500rpm. It makes 20 degrees when on the engine, although when put on a distributor machine it will make 25 degrees advance. I haven't figured that one out yet?

Does anybody else have this kind of engine modification and how did you fix the cold start hunting problem? Is this a cold start injection issue, a CDI issue or a non-ECU distributor issue?

Would eliminating the cold start injector altogether resolve the problem. There is a hand throttle that could be used during cold start.

I'm thinking of remodeling my rabbit hole for more space, I may be down here for a while.

Harry
1976 Euro Carrera 3.0 targa (2023 Best of Show, IRPCA)
2024 Macan 4 EV

[SCinNC]

My '79 started doing the same thing after airbox replacement and a couple of other repairs. The shop kind of shrugged their shoulders and said they didn't know what to do. I'd love to know what they did to cause it.

[PeteKz]

Just seeing this, since I was traveling the past two weeks.

My 1973 has a hand throttle, so I removed most of the cold start stuff and plumbing--AAV, ARV, pipes and hoses. I just have the CSV and TTS now, and the vacuum thermo valve to the WUR. I use the hand throttle to keep it idling for a couple minutes. It also works as a poor man's cruise control.

I have found that the tendency for RPM to hunt at idle often results from slightly too much timing advance at idle. Try reducing the timing advance a couple degrees at idle and see if it still hunts. If so, then focus on getting the advance curves (mechanical and vacuum) working to give you less advance at idle.

BTW, per others here, 28 degrees of mechanical advance is a lot for a twin-plug engine. Try 25. I'll bet if you just retard the dizzy 3* to get 25* advance, and that also reduces the idle advance by 3*, your hunting will reduce or go away.

[al lkosmal]

I would suspect the Auxiliary Air valve may not be opening when cold. If this is the case, combined with the Cold Start Valve adding fuel, it may result in an overly rich AFR.....causing the engine to hunt until it burns the CSV fuel and warms up......