Help me upgrade my ignition!

[Swede]

For reference, here is a somewhat better representation of what I have running now. I have a hard time getting my brain to think on polarity and having a positive "ground", but the engine doesn't care.

 

[Lakc]

The only problem we encountered was that normal ignition points are not happy with very low currents and we had to increase the point current to 50mA (0.05A) to maintain good operation.

Thats the self cleaning function of the points (and, incidentally, relay contacs), tiny amount of spark wipes out the oxidation, too big of a spark and the current wont collapse fast enough across the coil to induce enough secondary energy for the plugs.

Pic is describing a capacitive discharge ignition system, which for most models will probably prove the better choice. I dont want to derail this thread into a debate on semantics, lets see what we can do with Swedes current setup without too many major engineering changes.

For reference, here is a somewhat better representation of what I have running now. I have a hard time getting my brain to think on polarity and having a positive "ground", but the engine doesn't care.

If you talk to the military guys, they have to learn electron flow where everything goes from negative to positive.

One small problem thats going on here, assuming thats a standard automotive ignition coil, is the secondary is tied to the ground terminal, but the current return path from the spark plugs has to go through the coil primary winding before it returns to the secondary tap. You should actually have the coil negative terminal attached to your block, and the coil positive terminal attached to the transistor. That may give you a small benefit.

The problem with using a positive ground engine block, is a spark jumps easier from a hotter area to a cooler one, and the electrode is designed to be the hottest part, so the ideal way to use automotive ignition coils is just like the way they are used on a car.

 

[Ken I]

Picclock - no offence taken at all - in the past I used CD systems - nice fat spark.

Lakc - agree - we should be helping Swede with his problem.

I still think a capacitor across the emmitter & collector will improve his spark - without any other changes - its simple enough to just try it before doing anything more complex.

Ken

 

[Lakc]

I still think a capacitor across the emmitter & collector will improve his spark - without any other changes - its simple enough to just try it before doing anything more complex.

Its quite simple and worth a try.

If there was a way to inslulate the points from the distributor, that system should work with a negative ground.

The one problem we face here, is without an accurate way to measure spark energy, or even a specific set of ingition system shortcomings, all the results will be purely subjective.

 

[Swede]

Thanks guys...

Lakc - on my engine, for the moment, the positive tap of the coil primary is tied to the engine block and thence to the positive terminal of the battery, so this is opposite of what you recommend. That will certainly be an easy fix. Just have to loosen the coil in its clamp, rotate 180 degrees, and re-attach the wires.

I selected this NPN device because I had already blown up more than one PNP transistor. With this thing being made and advertised for ignition systems, I figured it'd work well, and it does, but I'm at a loss as to how it'd be wired for a conventional negative ground scenario.

In my limited electronics brain, I look at the transistor like a switch, either between load and ground (NPN) or between power and load (PNP). I know it does more in terms of the spark and the coil primary, but for wiring purposes, that's how I see it.

So the next step to try is a capacitor across the emitter & collector? Does this need to be as close to the points set as possible? I've got very limited space inside the distributor. Can you guys recommend a capacitor type and value? If I understand correctly, you can't just use any old ceramic cap. Do they sell suitable capacitors for ignitions at lawn & garden stores, for example?

I also need to set it up so I can examine the spark, hopefully giving me a feel for how "fat and hot" it is.

I'm also going to add a 12V LED on the ignition box so I can get a visual clue that the points are closed. I dislike the fact that when I switch ON, I can be draining the battery and heating things up, even with the engine static.

f there was a way to inslulate the points from the distributor, that system should work with a negative ground.

Unfortunately they are not. :-\

 

[Lakc]

Lakc - on my engine, for the moment, the positive tap of the coil primary is tied to the engine block and thence to the positive terminal of the battery, so this is opposite of what you recommend. That will certainly be an easy fix. Just have to loosen the coil in its clamp, rotate 180 degrees, and re-attach the wires.

Yes, easy to change and change back if it doesnt help

So the next step to try is a capacitor across the emitter & collector? Does this need to be as close to the points set as possible? I've got very limited space inside the distributor. Can you guys recommend a capacitor type and value? If I understand correctly, you can't just use any old ceramic cap. Do they sell suitable capacitors for ignitions at lawn & garden stores, for example?

You can try that, just put it across the emittor and collector. Your points dont carry any ignition current to speak of, so its not a factor where they are in the circuit. You could try a condenser from any automotive points set, or lawnmower if you have to.

I also need to set it up so I can examine the spark, hopefully giving me a feel for how "fat and hot" it is.

Just beware, high voltage is tough to measure. Things like the color of the spark are just subjective. Spark energy, current x voltage over time (in milliseconds) is the true indicator.

 

[Swede]

Thanks! I'm going to give this all a try. I've also picked up a cool three-bladed prop (18") for it. The current prop is too small, a two-bladed 22" job. Not that the prop has anything to do with ignition, but the engine was running at too high an RPM, sometimes up near 5,000, which of course reduces dwell time. More load, lower RPM, should equal smoother and more efficient running.

I appreciate all of the help from everybody. If I can get the mechanical points to work properly, with all cylinders firing every time, then I'll probably not mess with hall devices again.

One of the things I've been kicking myself about is the spark plug holes. They are all tapped 1/4" - 32 for Rimfire plugs and similar, and if I had it to do over again, I'd press in some 3/8" thread stainless bushings, which would in turn accept adapters for 1/4" -32 plugs or any larger plugs I wanted to try. It'd have given me more options.