Can coil points be filed if not burned to badly?
I've never had a problem in doing so but you will need to reset the amps as well.
If all they need is removal of a light amount of oxidation, then you can carefully draw a piece of very fine (400, 600 or so) wet or dry paper though them. Tear a piece an inch or two long maybe an inch wide. Fold it lengthwise so you clean both contacts at once.
If they are really burnt, you can file, but I would just replace them. If it has the old condenser in it, it's time for a rebuild anyway.
A diamond hone works very well in cleaning up tungsten point contacts. The points must be removed from the coil to use it but it can refurbish well worn and pitted point contacts.
A fixture can be used to keep the hone square with the point contact surface.
Coil points will not burn if they are properly adjusted for equal firing time (2ms with 12V pulse) with good firing consistency. The internal capacitor must be the proper value (0.47uF 400V) and have a dv/dt rating of at least 600V/us
Good firing consistency is very important because poor firing consistency is indicative of point arcing which will burn and ware point contacts at an accelerated rate. Simply checking coils don't double spark is Not sufficient!
Not sure that is factually accurate. Case in point, points in distributors, high tension aircraft magnetos, etc. certainly all have equal dwell time as they rely on mechanical opening of the points. I assure you these points are often found burnt. The usual culprit is a bad condenser as you will most of the time have peaks and valleys with a bad condenser.
A good friend of mine who passed a few years ago was a tractor mechanic for many years. One day he pulled a distributor off of an 8N I think and when he got the points out he explained to me that he would never just replace a condenser just because he was he was putting in new points. He said if they were just wore out without peaks and valleys he wouldnít change the condenser as they are often not good when new. His point was, if you have a known good one, keep it.
Not sure the ECCT can cure that as you claim.
The ECCT checks the value of the condenser and the leakage, if any of the condenser. It allows you to check the time to fire at various RPM's. Great tool IMHO.
I am not sure why points on an aircraft might burn.
Garry, Even coils with modern capacitors of the correct type will not prevent point arcing if the coil points are not adjusted properly. By that, I mean the balance between point gap, cushion spring travel, cushion spring tension and vibrator spring tension are not balanced properly. In such a case, the point contacts open slowly (with respect to the capacitor discharge time) and draw an arc between the contacts as they open. This typically happens on an intermittent basis causing accelerated point ware due to arcing, heating, pitting and carbon deposits/burning. The average coil current can read the correct value on an HCCT Ammeter under these conditions without double sparks being produced giving the false impression all is well. A careful calibrated eye on the Ammeter may spot needle variation that is an indicator of point arcing and can be done, depending upon how severe the arcing is and how much meter damping there is. The HCCT can and has been used effectively to adjust coil points so it can definitely be done.
Intermittent coil arcing is easily detected by measuring and displaying the time it takes the coil to fire each of many (100) sparks. A quick glance at that firing time distribution will instantly tell you if the coil is firing consistently (without arcing) or not. You are correct, the ECCT can not cure coil arcing by itself but it does do exactly what I just described very well. Alerting the user the coil firing consistency is poor so they can cure the problem.
Here is the coil firing time of a properly adjusted coil. Coil current (A) is read on the vertical (left) axis and time is shown on the horizontal (bottom) axis. The coil current ramps up to 6A then stops abruptly, firing spark in 2.150ms and does so consistently.
Here is an example of how an improperly adjusted coil primary current behaves when the coil points arc during opening. Coil current ramps up to 6A in 2.150ms as usual, however, the points open slowly to allow an arc to develop. The coil current continues to flow at a decreasing value as time goes on. The arc ceases after 2.630ms causing the coil current to stop abruptly and fire spark. Note also that the coil current is now at a much lower value (4.64A). Not only does the coil arcing delay (Retard) the spark but it also produces a much weaker (cold) spark due to the lower coil current at the time of firing. We can quantify the impact in terms of timing error assuming the engine is running at 1500RPM which means it is rotating at the rate of 9000 crank shaft degrees a second. Coil arcing caused the spark to be retarded by 0.48ms so that retards the spark by 9000deg/sec * 0.00048s = 4.32 degrees. The spark energy depends upon the coil current flowing in the primary winding at the time the current abruptly stops to fire spark and can be calculated from the equation e=(L*I^2)/2 where L is the coil inductance (0.0033H for a Ford coil) and I is the coil primary current. So normal spark energy is e=(0.0033H*6A*6A)/2 = 59.4mJ. The spark energy of the arcing coil is e=(0.0033*4.64A*4.64A)/2 = 35.5mJ which is 40% lower (colder spark) compared with the properly adjusted, non-arcing coil points. The engine may run rough with less power because the coil(s) firing time(s) are intermittently varying with respect to piston position.
Yes, I know, cabin fever is definitely setting in; but hay. A little mental exercise is a whole lot better than some of the wrangling that's been going on in other threads lately. Still think this is all a bunch of hooey? Just take a ride with someone who adjusted their coils for equal firing time with excellent firing consistency. There are plenty of folks out there who do and for good reason.
Do you ever get up to the Lakes Region of Maine?
Maybe you could fine tune my coils for me.
Would love to have you visit!
Small arcing is not preventable even with the right capacitor. Iím just saying the dwell argument isnít valid as it does nothing to explain how points with peaks and valleys burned into them get that way when dwell is equal (being mechanically controlled). The small arcing will eventually make reliable contact inconsistent which would then cause a timing error but not the other way around (normal worn out point). The claim that the ECCT eliminates that means points would never wear out.
Gary, Yes I agree, all points do wear during use and will eventually ware out even with proper functioning capacitor and with points adjusted properly. I meant to say proper firing time and firing consistency, not equal firing time and consistency; thanks for catching that and calling me on it! The OPs mention of points that burn badly I took to mean heavy deposit of black carbon, deep pitting and metal migration. Not normal wear that can be restored with a diamond hone. I don't believe that type of destructive point contact wear will occur if the points are set for the proper dwell time to fire with good firing consistency (no excessive arcing). Setting all 4 coils for equal dwell time to fire will sure make the engine run very smooth but is not a factor in determining the rate of point wear, unless of course, that equal dwell time to fire spark is way too long.
Gordon, thanks for the invite; we have been visiting Saco for the last few Summers so will let you know the next time we are headed up your way, usually around August. I would welcome the visit too!