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Anemic spark - Bad coil?

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Hi PiMan , what is this Positive Earth coil bit I be hearing about?---Keoke- /ubbthreads/images/graemlins/grin.gif

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OK, most coils are marked with a (+) and a (-) post for the primary wire connections. Convention has it that whatever the ground polarity of the car is, then that is the polarity that you hook up the wire going to the distributor (points). In other words...negative ground on car, negative post on coil goes to points. Positive ground on car, positive post on coil goes to points. This affects the polarity of the secondary ignition at the spark plug.

The coil will work either way, but it's been shown that with the correct polarity on the plugs, the voltage at the plugs is increased by 10-15%, or several thousand volts. Worth getting right.

/ubbthreads/images/graemlins/cheers.gif
 
if the coil is marked + and - then you can't really go wrong it's a problem when it is marked cb and sw that's when you need to know whether it's from a negative or positive earth car. Should fire which ever way you wire it BUT the stronger spark and better starting comes from wiring it right!
 
Naw to both of you. Whatcha gonna do when you get one marked "SW" an "CB",and the ground system has been turned the right way around?? throw it away!---Keoke- /ubbthreads/images/graemlins/grin.gif

Maintaining the correct polarity at the plugs is essential and only requires that you connect the coil up properly to yield a negative spark at the plug. For example a coil marked +/- used in a positive ground car will have the + terminal connected to the Dizzy lucar terminal which terminates at the points. The polarity at the plug can be verified using a standard lead pencil.
 
The real test is to hold the spark plug wire away from a ground and put a pencil point, the lead, in the gap. If the coil is hooked up properly the blue spark will be between the spark plug wire and the pencil and white feathers along with a blue spark will be between the pencil and the ground. The ground can be the spark plug and the engine must be running. Dim light helps. Make sure you are wearing rubber gloves as the clay/graphite core (lead) is conductive and is the full length of the pencil. Phil
 
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... The polarity at the plug can be verified using a standard lead pencil.

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You're right, but since you didn't tell anyone how that's done, I will...

Disconnect a plug lead with the engine running and form an arc between the plug terminal and the end of the cable. If the point of a soft lead pencil is placed in the path of the arc then a shower at sparks will be created, either on the plug side (if polarity is correct) or on the cable side (if the polarity is incorrect).

Just don't pull off that high tension wire unless you have your "third rail" gloves on. /ubbthreads/images/graemlins/jester.gif
 
Alec wrote:
"Please go back and ensure that the voltage at the coil terminals are as I said in my first post. Points open = 12 V, points closed = 0.2 - 0.4. If both are correct then the points are OK. Now double check your gap or dwell; dwell, especially can be in degrees or percentage so be sure what the spec is and what you are looking at, i.e if the spec is degrees, and dwell meter reads percentage, that will throw it way off. Check the spark at the coil post with a HT lead to the body, if it is still weak and the check outlined are right then it is either the coil or the condensor."

Got the 'points open 12volts' figured out today in a long session with my TR. I was getting that low voltage .03-.04 because my points were closed. Forgot about that. Did have some problems with the connection of the condenser wire and the lead from the distributor at the points post. Got that straightened out. Tried a bunch of things, suffice to say that every part of that ignition that I am aware of has been
swapped in and out. I won't bore you with a list. Finally,
and I am sad to say I do not know exactly why, I am now seeing what appears to me to be a decent 'jumping of the gap' at the plugs. One of my main problems is that I am not
really qualified, ok, not at all qualified, to judge the strength of a spark. I definitely have a spark that crosses that plug gap as I hold it within half an inch of the valve cover. Also I would say that from the HT wire it will jump at least half an inch with a blue line to the metal of the valve cover. For some reason I remember in another lifetime perhaps, seeing a larger spark and hearing it 'pop'. I obviously need to show it to someone who knows what a healthy spark looks like.

I am as confident as my rudimentary skills allow me to be that I am getting plenty of fuel. If I detach either one of the final feeds to either carb and the fuel pump is running it is coming out of there quite fast. I also notice that if
I have new dry plugs in there and remove them before idling or driving the car, they definitely are a little glossy with
gas and I can actually smell it on the plugs. I was able to start the car by retarding the timing using the static method where the pulley mark is 3/8" to the left of the pointer and tightening the distributor just as the light of the points opening comes on. That was a good feeling and the first time I had not had to start it three times with starting fluid to get it going on its own. I still can feel and hear a serious bog even sitting in the garage pumping the accelerator by hand. Just like when I drive it if you move the throttle shaft and butterflys too fast it is like something is not keeping up and you can easily stall it especially when it is not completely warmed up. If you keep the revs high it doesn't seem to have the problem. Another odd thing that might be related is that it will not bog in first gear when I first pull away. It will bog though right
after I shift into second or third if I do not keep the revs high enough.

I am now thinking that it was not starting without three rounds of starting fluid before because of a combination of two conditions. One, a weak spark or possibly no spark, for whatever reason which has at least turned into a stronger more consistent spark, why I don't know. The other reason might have been I had the timing too far advanced. It needed the much more volatile starting fluid to get going with less spark and too much advance. It would run three times or so only on the starting fluid and then I am guessing that those three brief running occurences allowed the conditions in the cylinder to finally combust regular old gas from the carb. I am still not very confident about the ignition and specifically the spark being as strong as it should be but I am going to try to lean out the carbs a flat at a time to see if too much fuel maybe causing the bog. Also the dwell ANGLE not percentage or anything else is right on 45 degrees which I would think would be spot on for a 4 cylinder engine. I have become very familiar with my ignition system swapping parts in and out. I don't know of anything else I can do there without a more practiced eye.

Thanks very much for hanging with me.

Jim Lee
 
Whoops....forgot coil resistance

Almost forgot. The zero coil resistance that I thought I had was an illusion. I did not have the ohm meter set low enough.
All three, now four, of my coils have a reasonable amount of resistance from post to post. If I remember correctly about 4.2 ohms.

Thanks,
Jim Lee
 
Re: Whoops....forgot coil resistance

The dwell angle should be 60 degrees.
And, 1/2" of blue spark should be plenty of oomph.
Jeff
 
Re: Whoops....forgot coil resistance

Jim,

I think you will find that leaning the carbs out to get the plugs to a tannish brown colour will improve acceleration conditions. It should also improve spark because the spark won't have to fight through all that black stuff on the spark plug. While your leaning the carbs you will also want to check the choke adjustment settings. Do you have a manual for the SU carb adjustments? The continued use of Starting fluid is really not good for the engine.
 
Re: Whoops....forgot coil resistance

Hello Jim,

you mention "old gas", just how old is it?, when a car stands for some time teh best idea is to drain any 'old gas' and refill with fresh. Fuel deteriorates with standing and is much harder to ignite.
To explain further, the voltage checks I outlined do two things, open points just ensures the correct voltage at the distributor and closed points checks for the overall resistance of the points, too high indicates dirty\worn points or poor connections and zero could indicate a short circuit, from, for example, incorrectly connected points. If that were the case the first open points check would also read zero not twelve. (And if left with the ignition on will overheat and possibly ruin the coil.)

Keoke "Hi PiMan , what is this Positive Earth coil bit I be hearing about?---Keoke-",
Why ask me, I've never heard of one either?

Coils themselves are not polarised, only the connections to suit the vehicle polarity.

Alec
 
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