Intermittent misses - MSD? Distributor? Plugs?

[Orange911S]

This has been a bit of a saga and I put up a couple of other posts regarding earlier chapters of this story. The replies are very insightful and helpful.

I have been chasing ignition issues since I put in an MSD 6425 and MSD coil to replace the perma-tune CDI that went bad.
I have a '77 911S, stock engine with a little over 34,000 miles. The car came out of retirement last fall and had a good going over and ran well until the no start episode that turned out to be an ignition issue. Here is what I have done to chase the problem:

- Replaced the Perma-Tune and stock coil with MSD. Per the instructions in Wayne's article, I used the three plug connector from the Perma-Tune to attach the switched power and the trigger wire. I used spade connectors and wrapped the connections with electrical tape. I used the ground bolt on the chassis just forward from the electrical for the ground for the 6425. I have rechecked this a few times and it remains tight. The hot power is connected to the starter, I ran the wire between the engine and the firewall and down the hole above the starter near the right rear shock.

- I also replaced the cap and a new non-resistor rotor.

- I had serious backfire (so loud people grabbed their children and ran) and stalling issues a few times and replaced the MSD coil (once in a 7-11 parking lot and once at the gore point of an interstate on-ramp) with the original coil. I also replaced the coil to distributor wire. This "solved" the problem for only a short time. I don't know if I was getting interference from the trigger wire and that caused a failure in the coil, but twice when I put in the old Bosch that got it running.

- I also set the timing and the dwell. The engine was running fine before and the timing was not far off, but the pita that the points were caused me to decide on going the Pertronix route.

- I replaced the plug wires last weekend with Clewetts and regapped the plugs to .045 from the factory settings. The plugs were replaced 2500 miles ago and appeared to be in good shape. After this I was still getting misses at high RPMs so I was babying the car around on surface streets. Not the ideal way to enjoy a 911!

- Yesterday I reread the MSD instructions and figured that I needed to clean up the placement of the trigger wire. I had not shortened the original wire and pushed it to the back of the engine compartment on the left side. Yes, this was not the brightest thing to do, but I thought that I would get to it sooner than I did. The car died twice on the way home yesterday and "rearranging" the white wire each time allowed me to get it started and make it home.

- Last night I shortened the trigger wire and laid it on to the ledge under electrical panel and tucked the three wire plug up there as well.

- I have still not shortened the wire (orange and black) from the MSD to the coil. It is snaking around a bit, but I am trying to do only one thing at a time.

- I have a Pertronix Igniter II and Pertronix coil that I plan to install soon, but would prefer to have some of these other issues worked out.

OK, shortening the wires seemed to help, but I still got a couple of small backfires and it stalled at a light on the 5 mile drive to work today.

So here are my questions:
Does this seem to be an EMI issue, and if so, can I put some sort of shielding on the trigger wire?

Is there a better placement for the trigger wire?

Could the old three plug connector be the issue?

Is the chassis ground connection a potential problem?

Could the coil wire (orange and black from the MSD) be picking up EMI and causing this kind of problem?

Am I missing something that could do with the distributor?

Will switching to the Pertronix solve any of these types of problems?

What questions am I not asking?

Thanks again. This forum has been very helpful.

[Bob Kontak]

Recent post where the color of the trigger was discussed (at length) as a common error for install.

In short, the one that MSD says is "+" goes to the braided shield (trigger ground) and the one that says "- " goes to the actual and for true trigger.

Do you have a violet wire? Where is it connected?

[Orange911S]

I have a points set up and not the magnetic set up. According to the wiring diagram I followed the instructions. I am curious about the braided shield. I have braided shields with grounds from the spark plug wires I just removed... maybe those would help.

[dicklague]

I would start with power and ground. Where is the large black ground wire hooked up?

Where is the heavy red wire hooked up? Is it through the 14 pin connector or one of the fuses on the fuse block in the rear.

I had this problem with my CDI install. The 14 pin connector was not providing consistent power.

It is also recommended to install an electrolytic capacitor in the power line to the new CDI to smooth out spikes because the battery is so far away from the CDI.

The above are easy things to check and fix. they are a potential problem area.

[Orange911S]

The black ground is to the chassis ground in the engine compartment. I have not considered running it to the battery, but could try running it to the fan housing. Originally it was grounded to the back of the aluminum plate, but input from the forum said that wasn't very good.

The power is run to the starter. Honestly I have not fussed with this (yet) thinking that this connection was in good shape. I suppose I could run it to the fuse block. That would cut down a long run of wire.

[Orange911S]

Any suggestions on size and type of electrolytic capacitor, and how to attach it?

[tirwin]

Check the other end of the ground strap.

[Orange911S]

You mean the one going into the harness on the MSD?

[Orange911S]

I am tempted to switch the ground to the fan housing to see if that makes a difference.

[tirwin]

You mean the one going into the harness on the MSD?

The braided ground strap that goes from the driver side panel in the engine compartment. The other end. Could be loose, frayed or corroded.

[Orange911S]

Nothing like a little field experience to help diagnose things.
On the drive home from work I noticed some missing again and it stalled when I was in a right turn lane. Plenty of people that I had to wave around and one nice pedestrian who helped push me into the CVS lot.
When I turned the key it was cranking but no start. I tried my "move the trigger wires around" method from last night and that didn't do the trick. I checked the connections on the trigger wire and found that the spade connector coming out of the old three wire plug was loose. I tightened that up and got it started. It still had some misses and backfires the rest of the way, but I got it here.

I am going to bypass the old plug for trigger and switched power and hope that solves it.

[Orange911S]

I just bypassed the old three wire plug and the car started up and idled OK, then I gas it some gas up to 5000 RPMs and it died and won't restart.

I am puzzled.

Now I am thinking ground and power. I will swap the black ground on the MSD to the fan housing and see how that goes. I am hoping that I don't have to chase ground issues through the car, but if I do I am guessing that I will just run the black and red up to the battery.

[Orange911S]

OK. I am still puzzled. I took the ground from the MSD off of the chassis bolt and moved it to the an housing. I upgraded the connector to one that was thicker and would put more surface area under the washer and bolt.

I started it up and got the same behavior as before. It started with a little hesitation then ran and when I hit the gas it went to 5000 and then died.

Next stop is power. I am connected to the starter, and have a couple of connectors in the line so that may be the issue.

I have read about many connections to the rear fuse box and would like to understand the pros and cons.

No more wrenching tonight, but will be searching the forums for more clues.

[tirwin]

Could this issue have another underlying problem like a vac leak? You mentioned backfires so that's what got me thinking. What is your fuel system? Stock CIS?

I might also try backing off the plug gap. Start at .028" and if the problem goes away then increase by 0.005" - .033", .038", etc. - until it comes back and then back off to the previous.

What kind of plugs are you using?

[dicklague]

Originally Posted by Orange911S

OK. I am still puzzled. I took the ground from the MSD off of the chassis bolt and moved it to the an housing. I upgraded the connector to one that was thicker and would put more surface area under the washer and bolt.

I started it up and got the same behavior as before. It started with a little hesitation then ran and when I hit the gas it went to 5000 and then died.

Next stop is power. I am connected to the starter, and have a couple of connectors in the line so that may be the issue.

I have read about many connections to the rear fuse box and would like to understand the pros and cons.

No more wrenching tonight, but will be searching the forums for more clues.

the capacitor should be a 10,000 UF 25V electrolytic capacitor. the - connector goes to ground. the "+" side to the power wire going to the MSD.

When you say connected to the starter are you going through the 14 pin connector in the rear? Or are you connected to the heavy red wire in the harness that goes to the 14 pin connector?

[mysocal911]

Originally Posted by Bob Kontak

Recent post where the color of the trigger was discussed (at length) as a common error for install.

In short, the one that MSD says is "+" goes to the braided shield (trigger ground) and the one that says "- " goes to the actual and for true trigger.

Do you have a violet wire? Where is it connected?

This is not the same issue as was discussed in the other thread as the OP has points
and not a magnetic pickup type distributor:

"- I also set the timing and the dwell. The engine was running fine before and the timing was not far off, but the pita that the points were caused me to decide on going the Pertronix route."

"When you say connected to the starter are you going through the 14 pin connector in the rear? Or are you connected to the heavy red wire in the harness that goes to the 14 pin connector?"

This is one of the problems, i.e. the peak current spikes being drawn, of the MSD
and its multiple sparks causing voltage variations affecting the MSD's triggering.
The main power line should always be connected directly to the starter which
goes directly to the battery. The MSD ground should go directly to the chassis.
Neither line should go thru any connector.

[Orange911S]

The heavy red wire to the MSD runs to the starter, not the rear panel. I spliced in a length of wire to make it reach. I was confident in the connections when I put them in, but now I am doubting everything. Could this be picking up interference as it is routed behind the engine?

Am I off base if I decide just to run wires to the front of the car and attach them at the battery? It might seem like more work, but I am really trying to solve this one.

Tirwin, I am strongly suspicious that I am fighting electrical demons on this one. When we reset the timing we checked the vacuum and it is strong and when it runs I am getting good power.

Opinions on running wires up to the battery?

[wwest]

Disconnect the VR and go for a run....

I have long suspected that the bouncing tach issue is the result of intermittent shorts, shorting inside the alternator on a bumpy roadbed.

Tach bounces, rev-limiter locks up, car will not re-start absent an ignition off, fully off, for a slow count to 10.

EMI "spikes" due to shorting couples into the nearby ignition trigger.

[Orange911S]

wwest, VR = vacuum release?

I am showing my ignorance here so please explain about the VR.

So you suspect power and ground may be OK?

[tirwin]

He means Voltage Regulator. That's assuming you have the external voltage regulator type and no one switched you to the internal style that piggybacks onto the alternator.