Back Firing

Ok, I have taken some of your advice and done some measuring on my coils and the spark plug wires. I know that having this kind of ohmic value on automotive wires would be a huge issue, but I will share the numbers anyway. Here comes my endless spew of numbers.

Spark plug wires – wire 1 = 9.5 K Ohms
Wire 2 = 6.4 K Ohms
Wire 3 = 5.1 K Ohms
Wire 4 = 8.5 K Ohms

Yes I know these are in Kilo Ohms

The low volt and high output sides of the coils are as follows:
Right Coil =

Low volt side is 2.9 ohms. According to the book 2.8 ohms is the cut off. The only issue is that it was dancing between 2.8 and 2.9 quite a bit. The high output side was hard to measure. Unless I was right on the tip, I couldn’t get a serious reading. Without the plug wires on I could get 12.3, but it was really hard to get contact and keep it at that point. I put wire 1 and wire 2 and tried to get a better reading. Since those wires have all those kilo ohms I would suspect we need to subtract the resistance from the plug wires. All together it came out to be 26 K Ohms. With those plug wires plugged in. Someone out there can tell me if those coils are still good. I suspect all of you will tell me those wires are crap.

Left Coil=

Low volt side ran about the same as right. It danced between 2.8 and 2.9 ohms here. The high output side was again hard to pin down. It took getting my tester points in just the right places on the coil, but I got everything from 12.8 (which I think is within limits) to 204 K Ohms. I guess again it was just due to my placement of the probes on the high output side of the coils. It seemed I could be touching the metal in some places, but still gets that high ohmic value. Using the same 1 and 2 wire, I got 26.3 K Ohms. Perhaps you guys out there could comment on these numbers. Let me know what you think the resistance should be on those wires. And also what they should be on those coils. What is something good that takes off corrosion? Especially on the inside of those high voltage end of coils.

I am all for someone a lot smarter than me to crunch these numbers and tell me what they think.

Do you have solid wires? If you do you should not be getting any resistance. The only resistance is in the cap for the spark plug. Did you check what kind of voltage at the coils yet?

If it backfires thru the carb(s) it could be; a cracked or loose intake manafold, timing, fuel too rich or too lean, valves not seating, blown cyl head gasket, secondary wires not installed properly.

If it backfires thru the mufflers it could be; a too lean mixture, fuel enricher stuck, carbs flooding, late valve timing, burned exhaust valve or a weak (or broken) exhaust valve.
one of your coils could have a hairline crack, that will cause backfiring, or old plug wires that are shorting out. If you get it running get it in a dark place and look for blue "lightning" dancing around the plug wires.

P.S. most of the above stuff can be ruled out, especially if it was running ok when all this started.
Although?, my engine was running fine untill it refused to start one day, turns out the intake valve clerance had slowly but surley closed down to "zero"

p.p.s. don't give up. It's cheaper to fix em than it is to buy a new one, or worse yet? a used one that has worse problems.

Post edited by: ~DW~, at: 2005/10/18 11:12

Well I did illiminate some of those. I made sure the cam timing was right on, but it seems to me that wires are a good place to start. That many thousands of ohms can't be good. I know on the automotive side, you want the least amount of resistance as possible. I just know those 2 and 3 plugs went black with it just running a little bit. I guess I can turn around and recheck the valve clearances. Thanks for the ideas! Any others would be helpfull as well. I am always open to something new .

Well I did illiminate some of those. I made sure the cam timing was right on, but it seems to me that wires are a good place to start. That many thousands of ohms can't be good. I know on the automotive side, you want the least amount of resistance as possible. I just know those 2 and 3 plugs went black with it just running a little bit. I guess I can turn around and recheck the valve clearances. Thanks for the ideas! Any others would be helpfull as well. I am always open to something new .

My SR reads 23k ohms between the plugs, which is right where Clymer says it should be, and 4 ohms between the wires on the low side.

You still haven't mentioned whether you see a good spark when you crank it over with the plug laying against the engine. Seeing a blue spark is worth hours of playing with a meter.

You can also switch the supply end of the coils, and swap the coils (wires 1/4 go to 2/3 and vice-versa). If running the 1/4 coil on 2/3 and the 2/3 coil on 1/4 (with the green and black leads to the coils switched) doesn't change anything, it's probably not a coil problem, and you can move on.