2.7 / 3.0 conversion running poorly
 

Hi all

I'm doing a conversion here, '74 911;
getting the 2,7 replaced with an '82 3.0 engine. (Bone stock, but no cat. O2 sensor is there.)

Control unit box and and relays that go with it, plus the harness came from the same wrecked donor car as the engine.
CTRL unit has two leads going to the front of the car: Red and red/white. One was for a fused +12V source, one was intended for fuel pump relay terminal 30. The 74 has no fuel pump relay, so instead the wire is connected also to the fuse box for +12V power.


Ignition system:
Stock 3.0 distributor with magnetic pickup (no points).
MSD Blaster CDI box, single spark, no tach output.
Tach signal comes from the coil negative, tach sorta works but is erratic.
Coil is from the 2.7 engine which ran ok with the MSD, so those were a pair to begin with.
Had to cut one ground wire from the engine harness, which was grounding out coil negative. (no fire).

New 3.0 fuel pump mounted by the rear tire where the old 2.7 pump was. I know the 3.0 pump should be mounted by the gas tank, but there's not much room there and this setup should work too.
Filter and accumulator are from the donor car, came with this engine.
Engine ran ok before the car was wrecked.

Valves adjusted.
Injectors all seem to have adequate flow & pattern (tested one by one by hand priming from the air flow flap).



So... I have the MSD Blaster reading trigger signal from the distributor, via the MSD magnetic pickup leads.
MSD has control over the coil, leads going from MSD to coil positive and negative. Tach lead can be connected to coil negative or disconnected, no difference in how the engine runs.

Problem:

Engine starts great, and idles ok. At part throttle, very small throttle openings, the engine stumbles and hesitates, and I get a loud afterfire from the exhaust pipe, along with a cool flame.
If I give it a LOT more throttle, the engine revs happily.

On test drive (load), it is very hard to get going with part throttle. Once at speed, you can floor it and the engine responds and you better hold on to yer hat :)


I have played with the (what I think is) idle bypass screw on the right side of the throttle body, but it still won't take throttle well.

A 76 would have a fuel pump relay, and it would also power the warm-up regulator and aux air regulator. On my setup.. I'm not sure these two are operational. (Checking as I type)


What to do?

Jan

Replaced the MSD with an original 6-pin CDI unit from another car.
Works like a charm, no flat spots or hesitation.
Too bad I have to put the 6 pin box back where it came from :)

I also tested different coils, resistances ranging from 0.6 to 3.8 ohms. They all made the engine behave differently!

So apparently the MSD is not compatible with the later model fuel injection control unit. I could now get a tach signal converter from MSD ($60) and see if that makes it run right, or just get a 6 pin box and call it a day.

I'm leaning towards a new 6 pin box, that I know will solve this problem.

Oh yea, this was an older "MSD Blaster Ignition", single spark. P/N 5900.

Jan