3.0 vacuum advance distributor swap for 964 mechanical advance distributor

[91166 Carrera]

I have a 1976 Euro Carrera 3.0 with a 964 mechanical advance distributor. After the swap with the original 3.0 vacuum advance distributor I now have a cold start "hunt" problem that I can't seem to work out. I've tried multiple adjustments with the AFR and idle but can't get the hunting problem to resolve. With the original distributor the vacuum would advance the timing and the engine would not hunt but the 964 distributor does very little advance of the timing resulting with a "hunt" problem

Currently the timing is set at 8 degrees BTDC, the AFR at 13.5 (2.5%CO) and idle at 900rpm.

Does anybody know the proper set up for this conversion to compensate for the cold start hunt problem?

Harry
1976 Euro Carrera 3.0 targa (2023 best of show IRPCA)
2024 Macan 4 EV

[76FJ55]

I may be confused, but doesn't the 964 have a dual distributor for twin plugs? are you running a twin plug motor? Also wouldn't the timing advance on the 964 would be controlled by the Motronic ECU not a mechanical advance? Or am i thinking of the wrong 964?

[91166 Carrera]

Right. The engine is twin plugged with the 964 distributor, but no Motronic ECU. I did this swap 25 years ago. It does wonders for the performance of the engine.

But on cold start the engine hunts several times and about 1,000 rpm jumps. After that it runs at a smooth idle of 900rpm. It never hunts when the engine is warm.

I was told to lean out the AFR while the engine hunts and when it stops hunting is where the sweet spot is. But after the engine warms up the AFR is too lean.

So am I stuck with this hunting situation?

Harry

[Peter M]

Mention of using the mechanical advance in a 964 rang an alarm in my mind as these use ignition maps in the ECU to control the ignition advance curve. A quick Google got the answer:

https://forums.pelicanparts.com/1347948-post2.html

https://forums.pelicanparts.com/1348935-post4.html

So, first thing to do is install a proper ignition system as there is a chance you are over advancing at higher engine speeds and risking detonation and engine damage. If this isn't the case, you are likely not getting as much performance as you could.

[91166 Carrera]

Peter,

Thanks for your reply, sorry it took so long to get back to you.

The links you provided were right on the money.

So, I need a MSD ignition controller. The question is which one. After researching the MSD web site if found the MSD 6AL-2 (#6421) and the MSD Digital Programable 6AL-2 (#6530). Both are compact which is nice.

Since I don't track the car maybe the 6AL-2 would be best. Your thoughts?

The engine rebuild was completed in 1999. The engine has the 964 distributor with the Beru wires (8.0mm); the Mahle 9.8:1 Max Moritz piston-cylinder kit; 2 black coils-one for each distributor head; NKG BCPR7ET plugs; and the M & W ignition box (Australia)-Pro Drag series with single plug port. Maybe my problem here is that M & W box?

Further, the 964 distributor was modified (welded) with a hold-down flange that was enlarged with a elongated hole to allow for the distributor to be rotated for timing adjustment. With the MSD unit where does the distributor get set at? At TDC?

I'm in new territory here. Is the MSD 6AL-2 (#6421) pre-programmed (mapped) for the 964 distributor (mechanical advance) such that it is basically a plug-and-play? I believe I need the distributor to advance on cold start to resolve the cold start hunting problem.

Last month I had the WUR and FD rebuilt by Rarly8 Motorsports (Alabama) and is on the engine.

I have been chasing my tail for years now with the cold-start hunting and rough running problems when warm. I have high hopes with this MSD product. Any insight would be helpful.

Harry

[Peter M]

Hi Harry,
Unfortunately, I can't help with MSD's or other alternative ignitions as my experience is limited to standard Motronic setups plus more recently, a Motec running CoP's. However, I notice MSD's are a common modification to run 964/993 distributors and a quick search of this Forum should get you the answers you seek.

[Mahler9th]

Several of us in Norcal have used 993/964 distributors for twin plug applications using MSD ignitions.

These distributors have been modified by installing mag pickups, so they can send a mag signal to an MSD 6A or 6AL.

We have used these set ups with large displacement engines and aftermarket ECUs.

Our application has been racing, so our cams and tunes have been targeted on racing performance.

One of our forum members ran this configuration for years before he switched to coil on plug.

I will be listing a 993/964 distributor that is modified with a mag pickup in the classifieds soon.

Please feel free to PM me and we can discuss the wiring, et cetera off line. Happy to help you if I can.

[Mahler9th]

We used the simple 6a and 6AL products back in the day.

We wanted parts that would be cheap and easy to find at auto parts stored.

Carried a few spare MSD coils and ignition boxes... rarely needed to use any.

The builder also employed the MSD 8680 in at least one application for cockpit adjustable timing.