Issues with 78 911 SC engine missing

[stormcrow]

I have been having a problem with the engine missing when I first start up the P-Car. The Tach is all over the place but mostly fluctuates between 1000 - 3000 rpms.

Replaced the distributor cap, rotor and new plugs Bosch W8DC that are specified for that engine.

Still engine missing when cold. So I decided the next thing to do was check the spark. I purchased a spark plug lamp - plugs between the spark plug and the spark plug wire. Made an adapter for it so I can install it off the top of the distributor.

Started checking each terminal individually - install the spark plug lamp - turned the engine on and watched the light. Checked plug 1, 6, 5 and 3. Well when I got to plug wire 4 there wasn't an even flash - it was erratic and sometimes wouldn't flash at all.

Now I replaced all the spark plug wires after I did an engine rebuild about 70,000 miles ago so I know they aren't the problem - they are solid core and all the connectors are on tight.

It appears the problem is with the CDI unit not putting out an even spark. Here is a link to the video I did showing what's happening.

It seems that after the engine is warmed up the problem does go away but when I put the AC unit on and the compressor puts a load on the engine the problem starts all over again.

Another problem I had was when driving down the highway the engine would shut off completely - turn the key to the off position, engage the clutch, start the engine - loud bang and the engine is running again. Erratic - never know when it's going to happen - only at high speeds over 70 MPH.

https://www.youtube.com/watch?v=UA1Bekep2OA&feature=youtu.be

So the next question is do I replace the existing CDI unit with the exact replacement or go with the MSD 6AL series ignition unit.

Any input with your experience with the MSD 6AL series ignition unit would be appreciated with regard to installation and reliability.

[snbush67]

Check and/or replace the distributor rotor and cap, I don't think the cdi is your problem. Make sure the rotor is seated all the way down and it is not burnt or swelling where the resistor is. Check the studs inside the cap for the same.

[stormcrow]

Shane,

I did replace the rotor and cap with Bosh replacements and did check to make sure it's seated all the way down. One thing I did notice when I looked into the old rotor was that the marks left on the terminals by the rotor were on the edge. That was one reason why I checked to make sure the rotor cap was seated properly.

One other question is - could the coil be the problem? I don't know if there is a way of checking it but it is original.

[Alan Lindquist]

Just curious... is there any radial play in the distributor shaft? You should be able to pull and push it for a slight rise and fall. But any "wobbling" in a radial motion is worn bushings and it's time for a rebuild or replace.

[theiceman]

if you think logically what youtold us .....

the spark is supplied to all the cylinders b the same CDI box coil and main wire .. the only think that is changing is the distribution of said spark , so the distributor questions are valid ones . lso dont assume just because you replace a wire it is good.
i would be hunting down the distributor rebuild thread and checking the weights.

[scotricker]

Or you could search for Barry. He rebuilt my distributor with all new parts' weights' etc

[snbush67]

Originally Posted by stormcrow

Shane,

I did replace the rotor and cap with Bosh replacements and did check to make sure it's seated all the way down. One thing I did notice when I looked into the old rotor was that the marks left on the terminals by the rotor were on the edge. That was one reason why I checked to make sure the rotor cap was seated properly.

One other question is - could the coil be the problem? I don't know if there is a way of checking it but it is original.

Right, the the cap is seated, but check the rotor itself as well, and your mechanical advance may be stuck. If the springs and weights are gummed up it can cause issues.

You can check to see if it is working by removing the cap and twisting the rotor CCW, it should rotate about 12 degrees and then spring back when you release it.

If it sticks or doesn't move then you need to pull your distributor apart and clean it. Or, as someone to do it for you.

As mentioned above you may have a severely worn distributor that is causing the rotor and cap to wear prematurely and may even have phasing and arcing issues.

An adjustable digital timing light is a great tool and is a must if you are going to be doing your own maintanance.

[stormcrow]

Here are a couple of other things I dd. I pulled the distributor and checked for shaft play - none or very minimal.

I also checked the height of the rotor on the distributor and compared it to the cap. With the distributor in place the brass on the rotor just touches the bottom of the brass lugs on the inside of the distributor cap.

I removed the rotor and installed a copper bushing to raise the rotor about 3/32 - just enough to bring it a little higher.

Also checked the terminals on the distributor itself for corrosion - none noted. The advance mechanism works fine -

Reinstalled and set the timing - still misses - the only thing I haven't done is actually pull each spark plug wire and ohm them out. I will do that next week.

With regard to the coil - did some research and will check that out for possible defect. Other then the only thing left untouched will be the CDI unit.

[snbush67]

You can just switch out the 4 and 5 plug wires and see if the symptom follows the wire. I know you know this but I'm saying this for the sharpshooters: change both ends.

[stormcrow]

Thanks for the reply Shane - I will also check that - your pretty smart - I never thought about doing it but I have seen it done in the past. Will keep you posted after I check it out.

[Walt Fricke]

Crow - no way the CDI can fire five plugs, but not the sixth.

Got to be something between the coil input on the distributor cap, and the plug gap, which is causing a problem on just one. Have you swapped plugs? Perhaps in conjunction with swapping wires - including their all important end connectors.

The connector has a graphite cylinder inside, looks like an oversized pencil lead, and it has a spring pushing on one end so it always makes, or is supposed to make, good contact at both ends. Nothing you can do if one of these connectors is bad except replace, for all practical purposes. The innards are epoxied in.

But you can test the connector with an ohmmeter. I forget the range of resistances, but you just disconnect a couple of them and compare. For that matter, you can stick the probe up into the end of the connector, and into the end at the distributor. If there is an open, or a very high resistance, you'll spot it. And this could possibly be heat related? Cold there is an open, things heat up and there is a tenuous connection, but under heavy load it takes more spark to fire the mixture?

My guesses, anyway.

[TCracingCA]

If the car missing after warmed up and then shutting off and restarting? By chance is your sprinkler system or damp moisture night getting into a plug boot. The test was netting bad results! Ok do this put car in a very dark garage and start it up and watch for fireworks show! If a wire is grounding out or the spark is jumping somewhere it would be easy to see in a very dark garage! Get a little pen light so you can examine whatever you find without turning lights back on!

[stormcrow]

Here's an update on the P-Car.

After reading all the comments I went back and checked several things. Shane recommended switching plug wires which I did and it appeared that the #4 wire was the problem. Changed out both boots on the wires and it seemed to help some. Still had a problem with the tach jumping until warm up.

Well after doing some extensive research on the SC 911 Distributor I decided to remove it and check it out. There are several links on Pelicanparts which were very helpful in doing this.

What I found out was the mechanical advanced weights were not moving. In addition to that everything was corroded along with one other problem. If I took the distributor and spin it by hand I would get a squealing noise. Upon further investigation what I found was where the terminal plate on the shaft was actually wearing against the plastic coil cover. Now if I push up the shaft on the distributor it wouldn't happen.

Well I completely disassembled the distributor, cleaned it up, placed a shim on the shaft between the housing and the mechanical advanced mechanism and that solved the problem. I also noticed when I took the distributor apart the mechanical advanced mechanism was rubbing against the distributor housing.

One other thing I found out was that there is supposed to be a screw in the center post where the distributor rotor is mounted. Apparently the shaft down to the advanced mechanical mechanism base can be removed. The purpose of the screw is to keep things together.

Anyway cleaned everything up, lubricated the shaft, mechanical advanced arms and put everything together and installed the distributor.

Started the engine and walla - no miss - the rpm indicator was solid on 1200 rpms and the miss was completely gone. The engine didn't sound this great since I rebuilt it.

I am now a happy camper - It was a job and took some time to do but it paid off. And I don't even need to replace the plug wires. Joy, Joy, Joy.

Here are a couple links should you decide to take the distributor apart - which I would recommend if you haven't done it especially on the SC 911 - not as bad as you would think.

Here are a couple links that gave me great information:
DIY Distributor Repair

Distributor service (Clean and lube) real easy without removing the pinion gear!


SteveKJR

[stormcrow]

OH I forgot to add - thanks for all the input - greatly appreciated - that's what makes this site such a great place.

[wayner]

Thanks for updating the thread with your findings.

Those are great links for my next winter project :-)

P.S.
Can anyone comment on what the available options for updating the distributor with something like an optical unit or something more modern?

[stormcrow]

Took the P-car out for a drive all I have to say big big difference not only in operation but gains on power. I would definitely recommend doing the distributor tear down, cleaning and oiling especially if it hasn't been done since the car was new

The best thing to use for cleaning is mineral spirits and a small stiff brush. You can pour it all over everything and it won't dissolve the varnish on the coil windings. Just make sure it dries out thoroughly like overnight and use compressed air to help.

[snbush67]

Originally Posted by wayner

Thanks for updating the thread with your findings.

Those are great links for my next winter project :-)

P.S.
Can anyone comment on what the available options for updating the distributor with something like an optical unit or something more modern?

Lock it out and program your optimal curve;

MSD 6AL-2 Digital Programable Install and Distributor Lockout