Spark to distributor but not to plugs - Bad Riddle (Page 2/2)
greenturnedblue JUN 22, 02:52 PM

quote
Originally posted by Patrick:

You need to determine if the spark is able to jump a 1/4" gap.



Maybe the ignition coil is generating a weak spark not able to jump from the rotor to the contacts on the inside of the distributor cap.

Yellow spark is weak and white or blue spark is strong. Weak spark may not be able to make the jump
Patrick JUN 22, 03:08 PM

quote
Originally posted by Lobster:

I'm not sure how to measure that spark off the coil...



It's simple enough to do. Pull the fuel pump fuse. Plug one of the longer spark plug leads into the coil's center terminal. Then select a screwdriver with a head that'll fit into the end that normally clips onto a plug. (I use a Phillips screwdriver with an end about the same diameter as the top of a spark plug.) While griping the insulated end of the screwdriver (fingers well away from the metal shaft), hold the metal shaft against something metal on the engine. Have someone crank the engine and slowly move the metal part of the screwdriver away from the metal ground to a maximum distance of 1/4". The spark should effortlessly snap across that 1/4" gap.

Remember to replace the fuel pump fuse afterwards.

By the way, have you measured the resistance of all the spark plug (and coil) leads?


quote
Originally posted by greenturnedblue:

Weak spark may not be able to make the jump.



Quite right. A lot of people don't realize that it's a lot more difficult for spark to jump a gap inside the combustion chamber while under pressure than it is while under atmospheric pressure. Jumping a .045" gap for example outside of the combustion chamber means nothing... it's got to be able to do that while under pressure. And that's why it's necessary to test the strength of a spark with a greater gap than what is normally used with a spark plug.

[This message has been edited by Patrick (edited 06-22-2022).]

Lobster JUN 22, 03:51 PM

quote
Originally posted by Patrick:

Quite right. A lot of people don't realize that it's a lot more difficult for spark to jump a gap inside the combustion chamber while under pressure than it is while under atmospheric pressure. Jumping a .045" gap for example outside of the combustion chamber means nothing... it's got to be able to do that while under pressure. And that's why it's necessary to test the strength of a spark with a greater gap than what is normally used with a spark plug.




Thanks, we just checked it and you might be right, the spark stops after about an eighth of an inch so my son is just on his way down to get a new coil. I'll let you know how it goes.
greenturnedblue JUN 22, 04:08 PM
When you get it running you should also adjust the timing since you replaced the distributor. even putting the new one in exactly the same position as the old one is likely to result in a few degrees difference. I have a timing light and could show you how if you'd like, it should just take 5 minutes. Are you in Metro Van?
Lobster JUN 22, 07:14 PM

quote
Originally posted by greenturnedblue:

When you get it running you should also adjust the timing since you replaced the distributor. even putting the new one in exactly the same position as the old one is likely to result in a few degrees difference. I have a timing light and could show you how if you'd like, it should just take 5 minutes. Are you in Metro Van?



Thanks to greenturnedblue, you win!! I thought I had good spark coming off the coil (it certainly looked impressive) but when I tried the screwdriver method greenturnedblue suggested, I could only get the screwdriver about an 1/8th of an inch away from the manifold before I lost spark. We picked up a new coil and the car runs like a champ!

I hope this helps others who find themselves in this situation down the road. We did bench test the coil after we had nothing else to check and it passed all of the ohm tests. Anyway, thanks again to all and I hope this thread helps someone else down the road

It drives nice but we will be checking the timing later tonight, thanks again!
Patrick JUN 22, 07:24 PM

Oh how soon they forget.
greenturnedblue JUN 22, 10:53 PM
Haha, thanks to Patrick for the screwdriver test you mean 😜 testing the resistance can check if it's shorting internally and therefore not sparking at all, but doesn't really tell you much in terms of how strong the spark is

Glad you got it running, I remember doing the exact same test when I was trying to figure out why my car wouldn't start. Turns out my issue was a lot more ..severe

All the best