pcolt94

I was going to write this up with some questions as I was all over the map for a while. However I now have some answers and thought I would pass my experience along. These are my highlights.

I am writing this mainly because the HEI distributor is used in the Corvette (and lots of other GM engines).Starting the 350 engine has only been a trick and to be careful not to flood it but when it starts it ran good. Lately starting has been extremely difficult and just lucky, so something had to be done.

Basically there is fuel, spark, plugs good and been cleaned, wires OK, cap cleaned up and mechanically everything looks OK. Been pretty trouble free for years. My feeling was weak spark. I am using a spark tester and it does light up but I didn't think it was as bright as it should have been (but was not sure).

Had to come up to speed on the HEI, did a lot of reading on the internet & YouTube. I got every opinion under the sun of what the problem is. There really is not a lot of parts in the distributor. One opinion was the coil does not go bad.

The first things to check were battery grounds. All were cleaned and tightened and measured. Next to check 12 volts to the distributor, was there. During cranking it drops to 9.5 or 10 volts which is acceptable. I also made a continuity check from the plus of the battery to the 12 input of the distributor thru the ignition switch in run and start position, good. Just to be ****, I ran an independent voltage supply to the distributor to keep it at 13 volts while cranking, no start & no change. (I was beating myself up pretty well).

The main focus then was on the ignition module in the distributor, everybody replaces this (for no spark) as it pulses the primary of the coil like a switch. But I did have spark and had a hard time believing it was bad at this point. The magnetic lobes can wear out as there was some metallic dust in the bottom of the distributor but it's got some real age on it and I did have spark.

So before I thru any money at it and replaced the ignition module, another few YouTubes and I was going to check the coil and got it out of the cap. The primary check OK at 0.4 ohm (under 1 is good).The secondary check was to be 3000 to 30000 ohms. Everybody on the internet measured about 8.0 K ohms. As I read under the carbon button I got a high resistance of meg ohms. But once I got the spring out from being stuck it read correct. I cleaned it all up and reassembled it. Did a few other things in the mean time getting ready for the cap thinking I'm fixed and one more measurement before I install. (you can measure thru the carbon button in the cap, just subtract 7K ohms from the reading).But now I'm reading like 7 meg ohms. Back to the house, take it all apart and now it measures 6 meg ohms. I cooled it down it the frig, and warmed it in the sun and the reading went totally open both times. Now it's back to 9 meg ohm sitting in the house.

So I think I have found my weak spark problem. When the mixture was just right it would fire but don’t really know if the coil got worse lately. Anyway the part is on order and will be here next week. I will complete the thread if I have success or not but I feel pretty good about the coil being bad.

One last thing, I did take my in line spark checker and installed it on my 98 jeep and cranked it. The spark was 3 times as bright. This is why I stayed on the course of weak spark as being my problem. The coil was the one item that was not talked about much as being bad.

pcolt94

My coil came in early so to work I went. I first checked the secondary resistance and had 9K ohms, good to go.

Part looked good for a aftermarket part all dimensions look OK at first glance except the coil did not float in the core laminate as on the original. When I did the install I found the ground connections did not make contact as the ground strap was lose not contacting the core laminate firmly. Upon making exacting measurements between the two I could see a 3/16 difference from bottom of coil to contact on laminate. So I had to add 3 washers to make up the difference. Also the core laminate was all painted black. I had to clean all contacting surfaces. After that the ground contacts were good and it did fit in OK. I think the coil not floating was the difference in measurement as the new unit all was rigid.

This ignition coil is not made any more like the original GM. You have to buy a used one, NOS or an aftermarket unit.

Installed the cap back on the distributor and connected everything as normal. I wanted to monitor the input voltage while cranking and watch the spark tester for a few seconds. I also had a remote start switch so I could crank from in front of the car as I had been.

As I hit the starter switch surprised the crap out of me. It started instantly and had no time to look at anything as I wanted to. But did this a few times and all looked normal as I was more ready to look quicker. It does not crank anymore, it just starts. I can't actually believe the engine ran for all that time with a bad coil. But probably got real bad in the last week, who knows. Looks like I got a good fix.

If you read all this lengthy stuff I wrote I'm impressed. Hope you can use this knowledge or at least was entertained.

Tom400CFI

I read it all...good diagnostic process and resolution.

Good read, too. Not many people are thorough like you were...your efforts translated into a good and informative account for anyone who reads it. IDK who thinks "coils don't go bad". They don't go bad a lot...but the definitely can and do fail.

383vett

I didn't understand where the "quadrajet" comes in.

rattlec4n

Glad I'm not the only one...

Good write-up otherwise.

pcolt94

OK,
I was only try to give incite that this was not current fuel injected engine but an engine that was older with some real decades on it.

Obviously the carburetor has nothing directly to do with the distributor write up. Sorry for the confusion.