390 pulsing problem- RE-HASH

blackagatha · Post by **blackagatha** » April 11, 2007, 11:02 pm

My 390 currently has a slight pulse, most pronounced at about 2000 rpm. The first time I recall having any related trouble was about 2 weeks ago, when I was driving back to school, after spring break. Middle of the night on wide open freeway, I was pissed because I got lost as hell on the wrong exit.... Phoenix's Happy Valley IS NOT A HAPPY PLACE... I got all mixed up on a freaky roundabout, and lost about 10 miles....

Anyways, I was going about 95, pretty much max throttle, when all of a sudden, she started hitching and backfiring. I figured I would limp it the 5 miles to the next exit, as it was still running, but would backfire every time I pushed the pedal much more than about 2500 rpm. But almost as quickly as it hit, whatever was happening quit. I romped on it a few times to verify, then drove it until 2 days ago like that.

The incident seemed similar to the time my points went shut... Except I had a Pertronix Ignitor installed when it happened. The only thing I can think is that the unit may have overheated? I don't have any idea.

I pulled the Pertronix out 2 days ago, and installed a set of fresh points to see if that was the problem. It seemed to lessen the condition, but it still pulses a little. So I tend to think that was not the problem... or at least not the only problem.

The pertronix was causing my tach to fluctuate about 500-800 rpm, when the engine rpm was relatively constant. It also seemed to read too high when the Pertronix was installled. With the points in, the tach needle is smooth and indicates a more approprate seeming rpm. The points don't seem to support as many HP, but that's expected... But not related to my trouble.

I got a fresh set of plug wires on her, as I accidentally ruined one of them a while back and got them replaced on lifetime guarantee at Autozone.

I put on a new cap and rotor about 3-4 days ago, because I noticed a bunch of spark-chips in the cap. Because of this, I strongly suspect the ignition for causing my trouble, because I have never found erosion in the cap- especially with the Pertronix.

I checked the plugs today, none of them were loose, none fouled, none with a bad gap.

Is there any way my coil is putting out the pulse? I suppose it will probably be the next thing to go...

It's less than 2 years old though... I wouldn't think it would be the culprit...

I should reiterate the fact that the engine is not hitching or running terrible. It's just thumping. When bent over the engine, it's not even noticable; but sitting in the cab, it's really obvious. It comes up through the seat and the steering wheel especially. It's like 4-5 throbbing pulses followed by a brief pause, then it repeats. It's really rhythmic, and always the same. It's definitely NOT a random pulse.

Thanks in advance for any help you can offer me...

Slick Fan · Post by **Slick Fan** » April 12, 2007, 11:41 am

Pull off your rotor & try to wiggle the top of the distributor shaft left to right, or front to back. I've had the upper end of the distributor housing wear out before, causing all sorts of erratic ignition behavior.
It wouldn't hurt to pull the distributor out & spin it, either. I've had the lower bushing bind up on a Mallory one before, that just about broke the drive gear.

Other than that, yes coils can go bad, they can also develop carbon tracks. Examine the tower where the coil wire plugs in very closely for carbon tracking (looks like a crack) leading to the negative terminal.
Was it a humid/rainy night when it started acting up?

eliminator46 · Post by **eliminator46** » April 12, 2007, 2:50 pm

Sounds "Harmonic". Like a balancing issue. My other thoughts are a plug wire going bad and arching off to the nearest metal source like exhaust manifold. I have had that happen even with the 8mm lifetime wires.

shawns fords · Post by **shawns fords** » April 12, 2007, 2:54 pm

I was thinking engine harmonics also, but kinda hard to tell the actual description online bring it over to colorado so I can take it for a drive

DocDave · Post by **DocDave** » April 12, 2007, 6:43 pm

Had it happen just as described. Turned out to be the fuel pump had to be replaced a week or two later. This was caused by a blocked up fuel filter.
Backfires every time you try to give it more gas.
That was my story, worth checking out the fuel pump pressure and filter
Dave

blackagatha · Post by **blackagatha** » April 12, 2007, 10:51 pm

doc dave... backfires... interesting... I should unscrew the filter canister and check it.

It wasn't backfiring until I put the points back in, but it blows shit out the carb every once in a while when I try to hurry it up.

Any idea why the rotor and cap decided to go bad all of a sudden? I know they don't just go bad for the fun of it; something had to cause it... Something ignition related.

It was NOT wet the night it acted up on the freeway.

I really don't think it's a balancing issue. Up until now, she's always spun really smooth. (well, except when I kept having trouble with points before I got the Pertronix) I know what ignition demons feel/look like. I drove 211 miles between Prescott and Tucson to go home for xmas with bad ignition. She hitched all the way. Felt like cracked and potholed asphalt, even when I could look down and see that it was perfectly smooth, brand new pavement.

shawns fords · Post by **shawns fords** » April 12, 2007, 11:14 pm

something I thought of while reading the other replies, one thing to do is turn the crankshaft with a wrench while watching the rotor on distributor to see how much the crank turns before the rotor does, it there is any slack at all in timing chain youll notice it here, and if the drive gear is wearing out it could all be related

blackagatha · Post by **blackagatha** » April 12, 2007, 11:23 pm

ok I just did a little looking around on google. Trying to learn how exactly the coil works/ what condenser does etc. I found a few comments from some crappy forum talking about the voltage supplied to the coil...

About the time I put in the Pertronix ~15 months ago, I disconnected the 6 volt resistor wire, and ran a hot 12 volt wire to the coil from the ign switch. I believe this was because the Pertronix suggested it, although it said it would also run on 6.

This page talked about too much voltage frying the coil... getting real hot, etc. I have no idea if it's getting hot or not... my guess is: Probably...

I'll also check the distributor shaft wobble and all that...

shawns fords · Post by **shawns fords** » April 12, 2007, 11:28 pm

I run p\ertronix in mine also ignitor II and a blaster II coil, this is how pertronix said to run it, a coil either works or doesnt, when it does youll run fine when it dont work engine wont work at all, i have never seen a coil cause erratic engine behaviors, I usually have a spare in case one fails bc if it does youll be left stranded, you can try putting one on it but I would really be suprised if it fixes the problem

blackagatha · Post by **blackagatha** » April 13, 2007, 12:47 am

well, that's what I thought too, but the guys on that other forum thought otherwise I guess

Slick Fan · Post by **Slick Fan** » April 13, 2007, 3:14 pm

I screwed up my coil once when I was a kid. I was following another kid's hotwiring instructions & hooked a jumper from the battery positive to the ground terminal on the coil. I left it there a couple minutes while we tried to start it.
Naturally, it didn't start & we gave up. I went to leave & the truck ran really nasty...but it ran, so I drove it home. Bought a new coil & all was right with the world once again.

I'm not suggesting you've fried your coil in that manner, but I do know that coils can go bad & not be totally dead. I guess each bad coil will probably act a little differently.
As far as I know, most coils aren't designed to run with a full 12 volts, I've always heard it's somewhere between 6 & 9 volts, depending on the application. I could be wrong regarding some of the aftermarket ignition systems, though.

blackagatha · Post by **blackagatha** » April 13, 2007, 5:27 pm

well, it's just a stock-issue style coil... and I've probably been over-juicing it for the last 15 months...

When we put in the Pertronix, we debated about it, and my dad couldn't come up with any good reasons to NOT hook up 12v, didn't know why they hooked up 6V in the first place. Figured it would have more available energy if ran on 12 v. So we hooked it up.

It seems that it may be possible that the insulation inside the coil may have overheated and deteriorated? causing a bit of internal arcing?

I figure that nothing can just go from perfectly good to failed just like that, unless hooked up terribly wrong. Everything shows some sign of deterioration before failure, doesn't it? I figure that's what the coil is doing.

Uncle Skip · Post by **Uncle Skip** » April 13, 2007, 5:44 pm

Guys.
You always need a dropping resistor in front of the coil unless you have an internally resisted coil (usually brand X).
Nothing over 8 volts to the coil, ever.
Get a good aftermarked coil (they are almost always externally resisted), use the supplied resistor in the box in front of the + terminal on the coil and you're golden.
The only time you get 12 volts directly to the coil is a momentary shot from the starter solenoid. You know the little wire that never seems to have any purpose on the solenoid (the i terminal? Stands for ignition) Trace that wire and you'll see it is the second wire attached to the + terminal on the coil. When the starter energizes, it sends a temporary 12 v. shot of juice thru that second wire connected to the coil to help starting voltage. Turn off the key (start) and there will be only 8 volt reading on that wire with the ignition still on.
Constant 12 v. to the coil will ruin the secondary windings and cause the problem you're having.
Give it a try and see if it doesn't cure your problem.
I hope you didn't ruin the Pertronix too.

U@ss

blackagatha · Post by **blackagatha** » April 13, 2007, 6:01 pm

Thanks Skip!!!

I assume I haven't killed the Pertronix too, because it said it could run fine/ better on 12v. Also, when I called Pertronix regarding my issues, the guy said to hook a jumper direct to the coil from the battery, to bypass all my electrical system and possible demons. I didn't do this, because I know my electrical demons are long gone.

Now I know that a coil must not have more than 12v. Why is this though? Is it just that the high voltage transformer windings can't take all the energy produced? just seems that the resultant voltage would be higher, and make a more powerful spark...

I assume you probably also know what the 6 volt resistor wire is ALSO connected to...
When I hooked up the 12v wire, I unplugged the 6v wire from the ign switch, and started her up. The solenoid made a really wierd clunk and the started dragged, as if the battery was almost totally dead.... It made no sense; being as the pink resistor wire disappeared into the 40 year old factory electrical tape loom, then re-emerged out at the coil...

Why did it act like this? what else is it hooked to? not that it matters, but it just seemed extremely strange...

So, I re-connected the pink resistor wire, put a big wad of tape on the hot end of it, and tucked it into my wire loom tube.

shawns fords · Post by **shawns fords** » April 13, 2007, 6:41 pm

actually with the pertronix and a blaster II they tell you run 12 volts directly, even though you can buy a ballast if you want, mine is run as the destructions say to, havent ran a ballast since i got rid of points

Uncle Skip · Post by **Uncle Skip** » April 13, 2007, 7:15 pm

Shawn.
I thought the Blaster is supposed to be an internally resisted coil.
But even if its not, it is a special case.
Normal coils, even the HEI's have to step down the voltage to the primary windings........ one way or another.
If you run an MSD or other ignition box, the resistor is internal in the box for you.
U@ss

shawns fords · Post by **shawns fords** » April 13, 2007, 7:20 pm

well if its in the coil itself nobody would know that, lol when you buy this particular coil they have one that has external ballast for points ignitions if you are just updating the coil itself.

http://www.msdignition.com/coil_blaster ... 2_8223.htm

Uncle Skip · Post by **Uncle Skip** » April 13, 2007, 8:48 pm

If you've removed the resistor wire from your ignition harness, use the supplied resistor..... always.
U@ss

shawns fords · Post by **shawns fords** » April 13, 2007, 9:13 pm

I dont see why you think that but ok if you think so

blackagatha · Post by **blackagatha** » April 14, 2007, 12:42 am

well, I put in a new coil. Accel 12v. Immediately, it actually made it run worse. I then realized/ remembered that the trigger point for the points is different than that for the Pertronix. That would explain why she's been blowing smoke out the carburetor since I swapped back. I twisted the distributor around till it ran the best. Apparently a (bit) too far as she pings a little now. I need to back it off a bit tomorrow. But all in all, the vibration demon is still there. I think I need to swap out my fuel pump. I've been reccomended to do that by 3 people now. And I think I got a lifetime warranty waiting for me at Autozone. I love their warranty- no questions asked... ever.

Hopefully that will solve my dillema. If not, then Damn!

If it doesn't fix it, I'll check out the distributor real close and run a compression test with crossed fingers.... I REALLY hope I don't have internal trouble.

Is there any possibility that a shitty alternator could make the trouble??? My alternator makes a little bit of funky noise. See, when I was building my engine, I hung it from a nail on a rafter in the garage. It stayed there for a while, then fell about 9 feet and bounced on the concrete when the nail let go. I looked it over, found a small chip in a non-critical spot, and hung it on the engine. It's always made a little noise but works fine- almost 2 years now since the "incident". I think it has started making a bit more noise recently, though.
I got a good battery, good electronic voltage regulator, and assume that would steady down any crazy fluctuations. But maybe not? Volt meter reads real steady...