Twin Plug Intermittant Ignition Problems

[powderfinger911]

Hello All,

I have recently rebuilt my 69 MFI engine with a twin plug setup. 2x Retrofit CDI+ boxes, a twin plug distributor with 2x magnetic pickups and obviously 2x coils.

I have spent ages chasing issues with the ignition dropping out constantly on both banks. When I put a timing light on either bank you see the light flashing regularly for a few seconds then skipping flashes coinciding with the engine stumbling. Its basically constantly misfiring on both banks but it does fire enough for the engine to keep running, albeit not well.

I have no idea what could cause this same issue to both sides of a largely separate ignition system, does anyone have any ideas/experience of this?

Things I have checked
1) Both CDI+ boxes are new and getting constant 12V (I tested this), both earthed through the wire shielding strap on the distributor harness
2) All new HT leads and plugs
3) Solid battery connections and engine earth strap and starter .
4) both coils are original 70s units but I cant see them both having the same fault?
5) I have turned off one bank at a time and the other bank continues to drop out
6) Distributor cap and rotor, although used appear ok.

Any ideas of what to check for would be appreciated, all I can think is that there is a CDI issue that affects both bxes in the same way or a distributor issue that is somehow affecting both banks at once

[al lkosmal]

Is it possible that you are getting some EMI/RFI interference between the high energy ignition stuff (distributor, coils, spark plug wires) and the low voltage to the CDI+(s)/ microelectronics?

I would try moving the CDI grounds away from the distributor grounds and ground them to the ground stud located behind the fuel accumulator/fuel filter area.......check wiring to minimize high voltage wiring/devices being close to the low/control voltage stuff.

Maybe someone with an electrical Engineering degree can provide some insight.

regards,
al

regards,
al

[JoeMag]

Check to be sure distributor signal wires are as far as possible from plug wires. Had stumbling issue on initial dyno/tune about 10ths ago and that was issue.

[powderfinger911]

Thanks for the suggestions guys.

Spent all day trying to fix this yesterday. Without success. At times I managed to get one bank firing OK but for seemingly no reason it would go back to misfiring again.

I disconnected the CDI harness earth shielding at the box end and I earthed the CDI+ boxes to the point behind the fuel filter. Ran the engine - no difference.

I disconnected the earth shielding at the distributor end where both of the magnetic pickups earthed as well. At the same time I moved one of the magnetic pick up earths to a seperate earth point, thinking that maybe having them both earth at the same position was causing some intereference between them. I made sure the distributor harnesses were were tucked away up on the left hand frame rail away from any HT leads. At this point I have the upper bank firing properly so I thought I was getting somewhere. I swapped the coil leads over on the dizzy cap and the upper bank kept firing OK and the bottom bank kept misfiring. This told me that the issue must be the distributor onwards.

Although new, I have different HT leads top and bottom so I swapped over a pair of leads to see if that made a difference - it didnt.

I have different plugs top and bottom, BP7ES on the top and BCPR7ET (964plugs) on the lower bank. This is because the the lower plugs need to have a smaller Hex head on them to install them. I think this is pretty common arrangement. Unfortunately I cannot swap them over to test and see if there is a difference

I spent ages looking over and cleaning up all the contacts and points in the distributor cap and rotor arm. Looking at these cheapo repro Jag caps I dont have any confidence in their quality. I looked at Jag forums where there are many articles on issues with these crap quality caps that the unavailability of genuine Marelli caps. There are lots of issues particularly with the carbon brush in the top of the cap which was what the misfiring bottom bank was running through. I tried numerous things to try and improve contact in the carbon brush and through the rotor arm etc without success. Im not sure if this cap has the correct carbon brush in it as it seems loose in the slot it sits in and appears different to ones I have seen in photos.

After a lot of messing with the cap I started it for the fiftieth time and straightaway it sounded much better, I thought this could be it. It was firing on both banks as I was moving around my timing light on numerous leads I could see it was running properly for the first time, then for no reason after about 1 minute of running great the revs suddenly dropped and went lumpy and both banks had started misfiring again. All I had done was clip my timing light on a few HT leads without moving them from their natural position and not touched anything else.

All this tells me is that both banks CAN fire ok at times, which makes me think that the pickups, CDI boxes, coils, leads and plugs are all ok and that the issue must be related to interference or the distributor cap/rotor which I took on and off a dozen times.

Even though I think the quality of the cap is not good and that there could be an issue with the carbon brush, that should only affect one bank AFAIK. I cannot see how a bad cap could be causing misfiring on both banks but at the moment thats all I have to work on.

At this point I gave up, back at square one, two misfiring banks. I have managed to find a used but genuine Marelli Cap and rotor arm on eBay so I'll wait for them to arrive before trying again unless there is another brainwave from somewhere to try something else.

[r lane]

+Are both banks missfiring simultaneously. If so, what I discern from your post, it appears ignition not fuel. Something common to both ignition systems. I mention that as I fought a missfire, embarrassed to say for 2 weeks, MFI car. Felt initially it was ignition. Was all over the place, not constant. The pump rod was too long and the pump was preloaded. That was it. Exasperating, good hunting.
Bob

[mikedsilva]

I'd run the resistor plugs on BOTH banks. AND get two new coils as they can do strange things when they are on the way out....

[stownsen914]

Swapping things around to find the problem is a good strategy. Eventually you will isolate the problem. A couple thoughts:
Good point above about resistance for suppression. Hopefully you have resistor wires if not resistor plugs.
And as noted, make sure your plug wires and coils aren't near your CD boxes, it may make them unhappy.
Do you happen to have access to a single plug distributor to help in troubleshooting? Since you suspect your distributor cap, it would be interesting to confirm that the problem goes away with a different distributor.

[stownsen914]

Originally Posted by mikedsilva

I'd run the resistor plugs on BOTH banks. AND get two new coils as they can do strange things when they are on the way out....

It's a good idea to keep a spare coil around for troubleshooting. Coil issues can be intermittent when one is on its way out.

[Jonny H]

Is the the type of distributor where the rotor has a reversible offset brass tip that can be fitted to trail or lead the direction of travel? If it is, from memory, the distributor can be fitted either for CW or CCW rotation. This effects BOTH the tip fitment AND the order that the leads are fitted to the cap.

From memory, the offset tip always trails behind the direction of rotation which also gives a clue to the plug order.

I am remembering a twin plug car where the distributor came setup for one direction but the rotor for the other. Caused issues similar to yours.


Also, if this is a distributor that uses an MSD VR sensor (pickup), the sensor wires are labelled + and -. Unfortunately, MSD are the only CDI boxes that fire on the positive edge, ever other box, Porsche or Classic Retrofit fire on the negative edge. Most auto electricians would assume that the + is the signal and the - is ground. But no...

The - (minus) signal lead from the sensor needs to connect to pin 7 on a Bosch/CRF CDI. The + on the sensor connects to 31 (GND).

[powderfinger911]

I'll order some resistor plugs for the upper bank.

I do have a spare coil but I havent tried using it yet as I dont think the issue is coil related, but maybe its getting to that point of trying now...

Unfortunately I dont have another distributor to try...

[powderfinger911]

Originally Posted by Jonny H

Is the the type of distributor where the rotor has a reversible offset brass tip that can be fitted to trail or lead the direction of travel? If it is, from memory, the distributor can be fitted either for CW or CCW rotation. This effects BOTH the tip fitment AND the order that the leads are fitted to the cap.

From memory, the offset tip always trails behind the direction of rotation which also gives a clue to the plug order.

I am remembering a twin plug car where the distributor came setup for one direction but the rotor for the other. Caused issues similar to yours.


Also, if this is a distributor that uses an MSD VR sensor (pickup), the sensor wires are labelled + and -. Unfortunately, MSD are the only CDI boxes that fire on the positive edge, ever other box, Porsche or Classic Retrofit fire on the negative edge. Most auto electricians would assume that the + is the signal and the - is ground. But no...

The - (minus) signal lead from the sensor needs to connect to pin 7 on a Bosch/CRF CDI. The + on the sensor connects to 31 (GND).

Hi Johnny.

Yes it is that type of distributor. It is marked Clockwise and it is grooved so the tip fitment can only be in one position. I cant remember now if it trails or is in front of the direction of rotation. I wouldnt have thought this would matter as long as the plug laeds are set up correctly. I have set up the wire positions correctly to the offset position of the tip.

Yes the distributor uses MSD pickups and it is wired accordingly. You might have hit the nail on the head with this suggestion, I certainly hope so. I will swap the wires over and try that. +ve to ground??? who would think that is a good idea!

[powderfinger911]

Well done Johnny you are the winner! When you know, you know. Got home from work today swapped the pickup wires so that the +ve orange went to ground, obviously :/, and there it is, firing perfectly straight away!

So there we go, if you are running an aftermarket twin plug distributor with MSD pickups (which I think most of them use) and you are trying to fire CDI boxes with them you need to flip the wires vs the MSD diagram.

I could have been weeks trying to fix this and not found this. Thanks so much to you johnny!

Took it out for a 20 min drive and its smooth as silk now! However, now that my engine is firing properly it is idling much higher so I now need to reset the whole idle speed, which on a MFI car is a PITA, but its much more fun that electrical problems...

[al lkosmal]

That was a great catch....it's nice when it turns out to be something simple.

regards,
al

[rsscotty]

It is important to recheck your ignition timing.

[Jonny H]

Originally Posted by powderfinger911

Well done Johnny you are the winner! When you know, you know. Got home from work today swapped the pickup wires so that the +ve orange went to ground, obviously :/, and there it is, firing perfectly straight away!

So there we go, if you are running an aftermarket twin plug distributor with MSD pickups (which I think most of them use) and you are trying to fire CDI boxes with them you need to flip the wires vs the MSD diagram.

I could have been weeks trying to fix this and not found this. Thanks so much to you johnny!

Took it out for a 20 min drive and its smooth as silk now! However, now that my engine is firing properly it is idling much higher so I now need to reset the whole idle speed, which on a MFI car is a PITA, but its much more fun that electrical problems...

Good to hear!

With the signal reversed, those poor boxes would have been firing with the rotor tip half way between two posts (out of phase). The spark would have had to jump that gap, then gone down the lead and jumped the spark plug gap as well! It's pretty amazing that it ran at all.

You will now find your timing is about 15 degrees out since you would have been timing out of phase with the rotor. It's a bit like clapping along to a song on the off-beat!

[powderfinger911]

Good point fellas, yes I will re-do my timing first.

[powderfinger911]

Originally Posted by Jonny H

Good to hear!

With the signal reversed, those poor boxes would have been firing with the rotor tip half way between two posts (out of phase). The spark would have had to jump that gap, then gone down the lead and jumped the spark plug gap as well! It's pretty amazing that it ran at all.

You will now find your timing is about 15 degrees out since you would have been timing out of phase with the rotor. It's a bit like clapping along to a song on the off-beat!

It would have been much easier to fix if it never ran at all! as the pickup would have been the obvious culprit.

While you are here, what plug gap should be run with your CDI+ boxes? and are resistor plugs or non resistor plugs best? I have ended up with a mixture somehow...

[Jonny H]

Originally Posted by powderfinger911

It would have been much easier to fix if it never ran at all! as the pickup would have been the obvious culprit.

While you are here, what plug gap should be run with your CDI+ boxes? and are resistor plugs or non resistor plugs best? I have ended up with a mixture somehow...

Non-resistor if you can find them but resistor is ok. 0.7mm gap is standard but you can go bigger (say 1mm) but that would depend on the cylinder pressure.

[powderfinger911]

Originally Posted by Jonny H

Non-resistor if you can find them but resistor is ok. 0.7mm gap is standard but you can go bigger (say 1mm) but that would depend on the cylinder pressure.

Nice one thanks johnny!