Help - 1970 911 tach zeroes then engine tries to stall

[auto*legends]

Issue: Tach often doesn't register at all when starting the engine and it then won't register until the engine RPMs are increased -- estimate 2000 RPMs, and conversely sometimes it will start and idle just fine with 900+- RPM showing. But the worst part is that sometimes when starting to accelerate from a stop, the tach goes to zero and the engine tries to die. Pumping the gas will often overcome the stalling/dying engine.
Relatively stock 1970 911 T 2.2 -- running Weber 40 IDAs and a Bosch centrifical advance distributor. New items that didn’t fix the issue: plugs, points, cap, rotor, wires (non-shielded), valve adjustment, changed to new Bosch CD-type-short-coil (was having issue with old unit too). Also, cleaned the Bosch CDI unit 3-pin connector and pins along with the "D" connector terminal and connector using electronic contact cleaner. It didn't make any difference.
The car isn't driven much, but this issue seems to be getting worse. I can set the RPM (idle) higher (1200+ RPM or so) using the hand control which seems to help a little, but the tach still doesn't register when starting and will still sometimes go to zero and then the engine tries to stall. I don't know how a stock Bosch CDI unit acts when it's going. Is that the problem?

[Jonny H]

Assuming you have made all the normal checks on points etc...

The ’69 and some ’70 cars drive the tacho from the coil primary voltage (later cars drive from the distributor). If you have two blue/yellow wires on the coil, your car is the early type.

If you have the early type and the rev counter is not reading correctly, it does point to the CDI box. It could also be a fault with the Bosch tacho adapter. You could try disconnecting the blue/yellow wire from it. The tacho won't work but it could provide a clue if the engine runs better.

[auto*legends]

Thank you Jonny,
I checked and mine is definitely the early 2-blue/yellow variety. However, it doesn't have a Bosch Tachometer adaptor. The Bosch distributor with points is being used in place of the original Marellii distributor with points. Should it still have the tachometer adaptor?
The engine compartment relay board still has the original Bosch RPM sensor (small round aluminum relay) and the Bosch RPM Transducer (taller rectangle relay) that according to the service manual is used only on the carb-version 911 T models. This car came with Zenith carbs. The Bosch distributor doesn't have a condenser like the original Marelli did, but I think the only outcome of that will be shortened life of the points. I haven't sourced parts yet, but I know that it will take time to have the CDI unit rebuilt. I thought it might be prudent to try to find the relays first.
Your thoughts?

Originally Posted by Jonny H

Assuming you have made all the normal checks on points etc...

The ’69 and some ’70 cars drive the tacho from the coil primary voltage (later cars drive from the distributor). If you have two blue/yellow wires on the coil, your car is the early type.

If you have the early type and the rev counter is not reading correctly, it does point to the CDI box. It could also be a fault with the Bosch tacho adapter. You could try disconnecting the blue/yellow wire from it. The tacho won't work but it could provide a clue if the engine runs better.

[timmy2]

The '70 tach has a conditioned signal from the round 3 wire device (intermediate unit, item 10 on Factory '70 print SL35, with Black/Violet, Blue/Yellow and Brown wires, mounted on the fuse panel) that is using the signal from the coil to send to the tach.
It could be as simple as a failing coil, coil connection or wire causing the cdi to cut out.
Switched CDI Power from front fuse 7-8 and at rear fuse 1 could be cutting out as well. (switched power from the ignition switch Red/Black at 7 goes to fuse 8 behind the fuse panel via a linking bar that can come loose!)
Or, your CDI unit is failing, have it tested, (Ingo perhaps) or swap it out for a good used one if possible?

[Jonny H]

Originally Posted by auto*legends

Thank you Jonny,
I checked and mine is definitely the early 2-blue/yellow variety. However, it doesn't have a Bosch Tachometer adaptor. The Bosch distributor with points is being used in place of the original Marellii distributor with points. Should it still have the tachometer adaptor?
The engine compartment relay board still has the original Bosch RPM sensor (small round aluminum relay) and the Bosch RPM Transducer (taller rectangle relay) that according to the service manual is used only on the carb-version 911 T models. This car came with Zenith carbs. The Bosch distributor doesn't have a condenser like the original Marelli did, but I think the only outcome of that will be shortened life of the points. I haven't sourced parts yet, but I know that it will take time to have the CDI unit rebuilt. I thought it might be prudent to try to find the relays first.
Your thoughts?

Terminology. "Bosch tacho adapter" same thing as Bosch RPM sensor (small round aluminum relay)


I would pull the blue/yellow wire from the bottom of it and see if the car runs any better. Easy test to do. Tape it up though as there is 400V on that wire!

[auto*legends]

Ok, I removed the blue/yellow wire from the 1970 Bosch Tach adapter (Intermediate Unit) Bosch number 0 227 990 001, Porsche number 901.602.601.00 and it didn't seem to make much difference except as expected, the tach didn't work at all.
When taking off at low RPMs, the engine occasionally will start to stall, or it may on occasion just cut-out at idle. It may also idle and accelerate from a stop just fine -- probably about 60-percent of the time. The predecessor to the fault is always the tach going to zero -- first. The tach almost never registers when starting the engine nor when the engine is at idle. The car does however run like a scalded dog when accelerating above 2000 RPM, and it will highway-cruse just fine.
Also, I did look very closely at fuse panels I and II. They both seem to be reasonably clean. Fuse panel III is attached to the relay board and it also looks OK too, but I think it is mostly for a sportomatic car.
So with no improvement by removing the blue/yellow wire, does this mean that the CDI unit is probably failing? I checked some local Porsche friends and I located a good CDI that I can borrow next week for a test. However, even with all of my cars and spare parts, I don't have nor do any of my friends have the a spare Bosch Intermediate relay --- URG! -- It is on my list for the next swap meet though.