Engine missing/running poorly

[Mark Jones]

Hi everyone. My 1980 Spitfire 1500's engine has been running poorly for months now. I’m to the point of giving up, but I thought I’ll ask the opinion of the group about my problem. It’s a long message but hopefully detailed enough that someone will be able to tell me what my problem is.

It all started a couple months back while driving along it all of a sudden it began to stubble as if it were only running on two or three cylinders. It would do this for a minute or two and then go away. I checked that the SU HS4 carburettors were getting fuel (they were), that the pump was working properly (it is) and I replaced the fuel filter.

I then moved to the ignition side. I found I had a bad plug wire so I put a new set of wires on and checked that they were all good. I checked the spark plugs and found all four to look okay; all a light brown colour. I checked the resistance across the coil and found 3 ohms on the low tension side and 5,000 ohms on the high tension side, which I believe are good readings. Also the cap and rotor are relatively new and appear to be in good condition.

During the time of checking the ignition system the car started acting even worse. Instead of just running bad once in a while for about 30 seconds, it now runs bad all the time once the car has been driven for about 15 minutes; missing, stumbling. So now I am thinking that it is the Lucas’ Opus electronic distributor. According to others my symptoms seems right for a failing unit; runs okay when cold but starts acting up when hot. So last weekend I replaced the Opus electronic ignition with a Pertronix unit. I start the car, adjust the timing and go for a drive. The car does start and idle better than ever. Out on a test drive I can still sense a hesitation or slight miss and after about 10 minutes it really acts up again for a couple minutes and then goes back to the slight hesitation.

So now I’m wondering: failing coil? Bad resistive wire (what should it’s resistance be)? Is there a simple way to by pass the resistive wire? It really seems to me that I am not getting consistent spark, but I don’t know how to check that. Any other suggestions?
Thanks for any help you can give.

Mark

 

[donbmw]

I have a 59 TR3 that I installed the Petronix in. I have had the same running problems that you are having. One night I had it start running on two cylinders and it would not quit. I check plugs, wires installed new cap and rotor but this did not fix it.
I reinstalled the points and found it running greats and think it was better with the points than Petronix. When I removed the pickup cam from the distributor I found that it had come unglued. I have reglued the pickup and reinstalled the Petronix and found that it will only fire two cylinders.

 

[Andy Blackley]

Have you checked the vacuum advance? These are prone to seizing.
FWIW a new or rebuilt distributor, even one with points (like mine) can do wonders.

 

[Mark Jones]

The vacuum advance is working as well as the mechanical advance. I could try to find a club member with a spare dizzy and see what happens.

 

[Geo Hahn]

I would certainly try another coil... they can test fine at home but begin to fail as they warm up. I always carry a 'known good' coil for on the road failures -- if you find it wasn't the coil then you'll have a spare one too.

 

[Mark Jones]

Hi Geo; Yup I guess the coil is the next thing on my list. I tested the pertronix ignition last night and it is working fine. If it is not the coil then I guess it could the resistive wire.

 

[dklawson]

"I checked the resistance across the coil and found 3 ohms "
"Is there a simple way to by pass the resistive wire? "

Do I understand your posts correctly Mark? Did you disconnect the low tension wires on the coil and measure 3 ohms across the coil terminals? Are you saying that you know your car DOES have a resistance wire to/on the coil? Or... are you talking about measuring the resistance of the spark plug wires?

IF you do have a resistance wire supplying the coil (or a ballast resistor)... you don't need/run a 3 ohm coil. 3 Ohm coils are for non-ballasted ignition systems and if you supply one using a resistance wire it will operate at very low voltage and give you a very cold spark. This could indeed be your problem.

Likewise, if you just installed a Pertronix system, remember that the Pertronix requires that the red wire (on negative ground cars) must get a full +12v supply... not the reduced voltage on the coil (+) terminal of a ballasted ignition system.

As for bypassing this ballast wire resistor, you can bypass it by connecting 12v directly to your coil and not using the ignition switch. However, I'd put a switch on this 12v supply wire so you can stop the car without having to open the bonnet and pull this wire off the coil.

EDIT: I neglected to mention that if you are running resistive spark plug wires, you don't run resistive spark plugs with them. If you're running solid core wires you're OK with resistor plugs. If you're running resistive wires, get non-resistor spark plugs.

 

[Mark Jones]

Doug;

I just rechecked and I am getting 1.8 ohms across the LT side of the coil.

With regards to the Pertronix, I connected the Red to the White wire (+ve)and the Black to the White with Slate wire (-ve) at the distributor.

According to the diagram that came with the unit, the +ve post on the Coil goes to the ballast resistor/wire and then to the red wire to the Ignitor unit and the ignition switch.

Mark

 

[dklawson]

1.8 Ohms is much better since you have a ballast resistor.

Not having your wiring diagram in front of me I can only offer general comments about the Pertronix red wire connection. Just make sure the Pertronix is not connected directly to the coil (+) since you have a ballasted ignition. Make sure you've connected to a switched supply that will give a full +12v to the red wire. If your red wire is connected before the ballast resistor (effectively connected directly to the ignition switch) you'll be OK.

Sorry I can't offer more information or suggestions. When I read your first post the numbers you measured made it sound like a weak spark situation. It appears to have been a red herring.

 

[Mark Jones]

Hi Doug;

Thanks for your input. Saturday I decided to purchase a new coil since it was the only piece of the ignition system that had not yet been replaced. With it installed the car did not faulter like it had previously. I went out for a twenty minute drive on Sunday and the engine worked well; the missing/hesitation seems to be gone. I would have been gone longer but I was running low on gas and without my walet. At least it seems to be working well before being put it away for the winter.

 

[dklawson]

I'm glad you had a good drive without problems. I'm sorry it cost you a coil though. Coils do go bad, but most troubleshooting articles I've read comment on the coil frequently being blamed for problems that lie elsewhere. I'm glad your new coil has fixed this for you.