vertical hit and miss engine

[don]

is it a normal thing to have the points bonce and spark? I'm using a auto coil and points,spark not real strong but blue, it did start several times but so far
only briefly, cylinder did get warmed up then nothing, spark gets weaker the more I try to start it. Do I need a different coil? Thanks don

 

[Brian Rupnow]

You need to run a condenser with a auto parts point set. If you don't, then it behaves exactly as you have described.

 

[don]

Brian, I do have a condenser (just forgot about it) but don't know if it is working, apparently not well. thanks don

 

[Brian Rupnow]

The points shouldn't bounce. The ignition cams I make are perfectly round, with a flat on one side only. the points stay open thru about 250 degrees. Then when the flat part of the cam gets to the rubbing block, they close briefly, then open again as soon as the flat has gone past.

 

[teeleevs]

The points shouldn't bounce. The ignition cams I make are perfectly round, with a flat on one side only. the points stay open thru about 250 degrees. Then when the flat part of the cam gets to the rubbing block, they close briefly, then open again as soon as the flat has gone past.

What happens with a model engine, it gets mounted on a wooden box, then a coil as large as the engine itself is hidden in the box. I know the coil will work without being EARTHED to the engine block but it should be! Battery or power supply NEG to engine block, POS through switch to + pole on coil, then - pole to the contact points, when points close (as Brian has indicated above ) a ciruit is completed to the engine block, a secondary coil (high tension coil within the same container ) is charged, then when the points open a very high voltage spark dives across the spark plug gap to the engine block but it must return to the secondary coil, if the coil case or lamingtons as the case may be is not EARTHED, then the only option left is for the spark jump back across the open points, this spark has a very small amperage so will do little damage to the points except for burning any impurities in the air until the points can no longer make contact. Does this make sense? Ted from down under

 

[Steamchick]

Also, (I am not an expert, so please check with others) the DWELL when contacts are closed is important for the coil to get sufficient magnetic charge to achieve the required voltage when they OPEN to discharge across the Spark-plug (or other weakest point of resistance).
Detail:
The coil is an inductor: (Sorry if I am teaching the experts my lack of understanding?) so Inductors start at high voltage when the contacts close and 90degrees out-of-phase they are at "zero" current, but rising rapidly (Held back by the back-emf of the inductance of the coil). The energy stored in the coil is proportional to current, so the contacts need to close for long enough to charge the coil. Then when the points open, the coil wants to dump all that energy - and does so across the easiest path - which is the spark-plug gap if all other resistances are big enough. But if the contact gap is too small, then the energy may arc across this gap as the resistance of the air can't resist the voltage of the primary coil. - Or if you put a big capacitor across the points it will rush into the capacitor instead! - So insufficient spark to ignite the fuel-air mixture. Incidentally the fuel air mixture at 10:1 compression has effectively 10 times the resistance of the same gap in ambient air. Hence the need for the coil to generate a few thousand volts! (to jump a 25 thou gap) compared to the primary voltage being 6 ~ 12V. (across the 15 thou points gap). At the points, the arc starts when the gap is just forming - less than 1/10th of a thou - and the ionised gases then conduct the arc as it is drawn-out with the points gap opening. If the gap opens quickly, it will cool the arc quickly as it is drawn-out, and naturally will quench as the incoming gas is resistive, as opposed to the conductance of the ions from the arc. But if the spark cannot strike on the spark-plug, instead of most of the coil's energy being dissipated across the spark plug, it all dissipates across the points - burning the contact points rapidly.
Or at least I think that is what is happening?
So you may be able to get a "Dwell meter" to check what your points are doing? (Motor Racing tuners usually need them).
And please check the capacitor across contacts is a correct size/type? (I had one partly fail on a Honda, and blew the exhaust silencer open with the mis-fire!).
And check points gap versus coil type and compression ratio? - Are the points showing signs of arcing? (Use a magnifying glass).
Finally, check the spark plug type and gap is appropriate. (A wrong plug type stopped my mates 2-stroke motorbike from running, but it always started easily.).
If you have "suppressing" HT leads, or plug cap. then that may also be the problem. (I had a mis-fire on my motorbike due to a suppressing plug cap being over 15KOhm, not the designed 10KOhm.).
Maybe the battery can't sustain the current required? - I have a digital radio that takes disposable batteries - then eats them - but hardly works at all with rechargables, as they can't sustain the voltage (which is 25% lower anyway). So maybe you need NiCads, not Nickel-metal hydrides?
K

 

[don]

I am very pleased to say, I solved my problem or problems, and the engine runs as it's suppose to, Hit, Miss the whole works. the sparking I believe to be oil on the points. weak sparks, weak battery now have a new lawn mower battery, and the real problem I believe was the needle valve had a bad solder job It was detached from the adjusting screw stuck in the fuel rail. How I found this was some how it started, and when the fuel was low in the line it seemed ok but when the line filled with fuel it was too rich and I had it closed so I thought. when I unscrewed the needle valve the needle was stuck in the valve. removed and resoldered restarted the engine and it runs excellent.

 

[Steamchick]

Hi Don,
Glad you could fix your engine with "basic mechanicing". The lesson I am struggling to learn is that in modelling, "perfection is almost a necessity, not a luxury". I'm still struggling to overcome my basic impatience - which is the cause of most of my projects that don't work. But glad you reached that Eureka moment when you cried "SUCCESS!". - Any chance of a one minute video of it running?
Again. "WELL DONE!".
K