can't get plugs to spark

[pete29]

i am having trouble getting my 72 gmc 350 to spark. the points are producing a spark when you crank it, but not the plug Also could someone tell how many volts or whatever run thru the coil wire when you are turning it over.

[BLUE THUNDER]

Sounds like the coil is bad. I wld change it to HEI. I put an HEI dizzy in my 72 Blazer and it runs awesome, well at least the motor does. Good luck, Mario

[pete29]

Originally Posted by BLUE THUNDER

Sounds like the coil is bad. I wld change it to HEI. I put an HEI dizzy in my 72 Blazer and it runs awesome, well at least the motor does. Good luck, Mario

it has a brand new coil on it, but how hard and about how much does it cost to put on hei.

[Yukon Jack]

If you go to the FAQ forum there is an index and it will link you to at least a couple posts on going from points to HEI. It's pretty simple and can be done with a readily available junkyard distributor - new is nice, but definitely you will see improvements even with a good used one.

[jaros44sr]

Originally Posted by pete29

i am having trouble getting my 72 gmc 350 to spark. the points are producing a spark when you crank it, but not the plug Also could someone tell how many volts or whatever run thru the coil wire when you are turning it over.

About 60,000, do you have a ballast hooked in there? I don't think you want to run 12 volts to the coil, I'm in the same spot you are, so I guess we'll learn together, been along time since I played with an ignition system

[pete29]

The coil is brand new, i think i am getting to much juice to the coil, because it started to melt the points and the wire from the neg of the coil to distributor. The hot wire running to the pos coil has 12 volts and is running from the ignition switch thru the fuse box which has a 30 amp fuse. Any way to make it not have as much juice when turning it over.

[RustyParts]

I may be mistaken, but I think the original wire running to the coil was a resistive wire reducing the voltage to the coil to around 8 volts, and there was also a yellow wire running from the starter solenoid to boost the voltage back up to 12 volts when cranking the engine to aid in starting the engine and as soon as the solenoid disengaged the voltage was reduced back to the 8 or so volts.

You can install a ballast resistor inline your present ignition wire to reduce the running voltage and add the wire from the starter solenoid to aid in the starting voltage. Also if you are melting the points the capacitor, mouted on the side of the coil bracket, is probably bad. It's purpose is to suppress the arc across the points when they open.

The HEI idea works too!!

[truckdude239]

6-8 volts there is a yellow resistor wire the goes tot eh coil and then the neg side of the coil goes to the points. when the points close it completes the ground and allows the coil to wire. so baslicy there should be 0 volts on the wire going into points. at least that is how mine is set up and it runs

[69 K-5]

A junk yard HEI with the large cap is extremely hard to install in a 4x4. There are some clearance issues, or at least there were in my 69. I went with an Accel electronic distibutor. I've had it for about 10 years and have never had to mess with it. It's actually the only thing I didn't replace when I put the new engine in.

[jaros44sr]

Good info Guys, I knew y'all'd come thru...gotta check my wiring diagram to find this resistor wire

Nice to see ya back RustyParts

[Jtrux]

Okay so let's clear this up. You get spark at the points?? I'm not too sure what you're talking about there.

First to check the coil, take the wire that goes from the primary tower of the coil to the center of the distributor cap and hold it near a ground source (block or firewall, probably block since firewall will be painted not allowing a good circuit ) and have someone turn it over, if coil is good you will see spark, no sparky, coil is bad.

If you're getting spark there but nothing at the plug there could be several things wrong. First what are your points gapped at?? 15 minimum and 19 if you have a high voltage coil. Actually let me correct myself you will need to adjust the dwell, IIRC you want anywhere from 32 to 36 degrees of dwell. If you have a stock coil you can keep it lower if you have a higher voltage coil you want it to be higher. Stock coil is around 40K and aftermarket is up to 50K or so.

Quick explanation...The wire going into the positive side of the coil should be roughly 6 to 9 volts. When the points are closed the voltage is entering the coil and making it's way though the primary windings of the coil increaing voltage by several hundred volts, when the points open a magnetic field that was containing the voltage to the primary windings drops and voltage jumps to the secondary windings of the coil increasing to many thousands of volts and then exits through the primary tower and makes it's way to the dizzy, then to the plug. The higher the dwell number the longer the time the coil has to produce maximum voltage. If you have a high voltage coil with low dwell then you're shorting yourself on voltage and the same goes if you have a low voltage coil with a high dwell, the coil will reach maximum voltage then drop off, so you are still shorting yourself. There's a little science involved with it.

So if you still don't get spark i'd be willing to bet money that it's your capacitor.

Oh and as far as the stock wiring goes, there is probably a wire coming from your starter to the coil. The reason it's there is because when you're cranking the engine over the starter is sucking up all the juice so the resitor wire that's only allowing 6-9 volts to the coil is probably getting half that so inorder to help with starting a wire goes from the starter (12 volts) to the + only during start up to help during initial start up since the starter uses so much power. Then when you motor fires up and the starter is disengaged your coil gets its power from the resistor wire.

Hope this all makes sense.