70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

[Donald A.]

OK, my new to me 70 LS-5 has been running rough since I got it a little over a month ago.

I did all the general maintenance items.

New R-43T plugs
New cap, rotor
Compression test (170-160 on all)
Rebuilt carb (professionally)
New correct plug wires from Lectric Limited
Set correct timing and dwell according to book
adjusted idle speed needles

Engine still running rough so I did the ole trick last night of running it in the dark.

All kinds of arcing coming from the plug wires and pops of ignition blue flame, especially on number two. So I made sure the boots were on the plugs properly and I still had arcing.

Another strange thing was that I could see blue luminescence at the point were the braided shielding was coming off from around the wire to where it gets grounded to the valve cover.

I can only think of two things so far.

1. Really bad set of wires

2. Something going on with the voltage of my coil causing way too much voltage and the wires can't insulate properly

A. I did not change the coil. It looks like an old standard black coil

B. I don't see a resister/ballast anywhere and don't know if it should have one.

I'm going to look in the books to see what I can read about but wanted to put it out for help.

One of my first thoughts for testing would be to go to the parts store and buy some modern wires and put them on to see how it will run then.

the Part B above has me wondering though.

Thanks for any help

[Edward B.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

"One of my first thoughts for testing would be to go to the parts store and buy some modern wires and put them on to see how it will run then."

Good idea!

[Tom L.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

OR put the old ones on if it ran OK with them, but a good place to start. I've heard of problems with LL's wires but have had none with mine and they're 6 or so years old. There is not a ballast resistor, there's a resistance wire as part of the harness. It has a braided shielding on it. You should be able to identify it where it attaches to the coil. Good luck!!

[Jim T.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

1967 Corvettes were the last year for a external ballast resistor. Later Corvettes used a resistor wire from the back of the fuse block to the coil for point type ignitions. The resistor wire has a cloth-like covering on the outside. My Corvettes are drivers and I use MOROSO spark plug wires on three of them. A 68, 70, and 85. The 68's have been used for over 25 years.

[Ken A.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Plugs are wrong, should be 45's or 46's or equivalent. Coil is also incorrect.

[Donald A.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Plugs are wrong, should be 45's or 46's or equivalent. Coil is also incorrect.

Ken, don't know what you mean by coil wrong? Also pretty sure book says R-43T plugs so that is what I got and gapped at .35

all. I got a new standard replacement coil. Verified that I saw the braided wire on the positive side of the coil and installed.

This morning I'm putting on a standard set of new wires and will see how it runs. Hope to report back in a bit

BTW I pulled the number 2 plug ( one that was doing most of the arching) and clearly it was not running well. Kind of fouled up. So pulled number 4 next to it and it looked perfect. I switched the two.


thanks

[Donald A.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

1967 Corvettes were the last year for a external ballast resistor. Later Corvettes used a resistor wire from the back of the fuse block to the coil for point type ignitions. The resistor wire has a cloth-like covering on the outside. My Corvettes are drivers and I use MOROSO spark plug wires on three of them. A 68, 70, and 85. The 68's have been used for over 25 years.

Ok

here is a pic of the coil wire you speak of. Hope this looks correct.

I put in new plug wires and coil.

Still running with a miss. About at wits end!!

my brother suggests to check the voltage on coil. Says if voltage regulator is bad I can get too high of voltage and cause this.

Im running out of options.

Anyone have ideas??

[Roger G.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Have you done a compression test?
You say you swapped plugs.
Does the same cylinder miss?

I have braided wires on my 70 LS5.
I may try starting mine up this evening see if there are any flashes.

[Thomas H.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Confirm that the arcing is gone with the new wires (another run it in the dark test). What brand / type of wires did you replace the braided ones with?

If there is no evidence of arcing I would look at / replace or adjust the following:

Double check you plug wire installation / firing order. Then check it again.

Dwell - verify setting
Timing - verify setting
Rotor - inspect for evidence of arcing - replace with, or try, new one
Dist cap - inspect terminals inside and out - replace or try new one
Point condenser - replace
All plugs - replace with new set - gap to specs (wouldn't be the first time I've run into a cracked insulator)
Verify voltage going to coil - should be around 9 volts when running (double check the 9 volt spec with a second source just to make sure I'm telling you the right voltage. going by memory.......)

Once you have verified that the ignition system is functioning properly and if you still have the miss, then you need to go down the compression test / valve train path.

Keep us posted with your findings. It is always good to let us know what the remedy ended up being.

Tom

[Donald A.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Confirm that the arcing is gone with the new wires (another run it in the dark test). What brand / type of wires did you replace the braided ones with?

If there is no evidence of arcing I would look at / replace or adjust the following:

Double check you plug wire installation / firing order. Then check it again.

Dwell - verify setting
Timing - verify setting
Rotor - inspect for evidence of arcing - replace with, or try, new one
Dist cap - inspect terminals inside and out - replace or try new one
Point condenser - replace
All plugs - replace with new set - gap to specs (wouldn't be the first time I've run into a cracked insulator)
Verify voltage going to coil - should be around 9 volts when running (double check the 9 volt spec with a second source just to make sure I'm telling you the right voltage. going by memory.......)

Once you have verified that the ignition system is functioning properly and if you still have the miss, then you need to go down the compression test / valve train path.

Keep us posted with your findings. It is always good to let us know what the remedy ended up being.

Tom

Tom

thanks for for the note.

Take a look at the below and see if anything jumps out at you

compression test had all cylinders between 160 to 170

Measured voltage.


Car not running
At Alt is 12.86 volts
On positive side of coil is 7.10 volts with key on


Car running
At alt is 14.20 volts
At coil is 11.10. ( does this look OK?)

dwell re-verified at 29

timing re-verified at 6 degrees

new Accel 8mm wires put on

new cap and rotor and points and coil

new springs in the distributor per Lars paper so that weights would be sure to be all out by 2800 RPM or so.

[Duke W.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

As stated the R43-T plugs are way too cold regardless of what the "book" says. Those of us with decades of experience with these cars will tell you that AC heat range 5 or equivalent in another brand are best for normal road and highway driving.

You may simply have a defective plug on the #2 cylinder, which from what you say is now on #4,

Applying a film of silicone dielectric grease on spark plug and distributor boots will help with insulating and make it easier to remove the wires in the future. Also make absolutely sure that the terminals have good contact with the plugs and distributor connections, especially on those connections that are showing evidence of arcing.

There are many complaints about reproduction spark plug wires, so you might also consider pulling he boots back and checking the terminal crimp to the wire. This is also a good time to check the terminal-plug contact, and the crimps and/or connectors may need a little persuasion with a crimping tool or small pliers. Use silicone spray lube to pull the boots back over the crimp. Check wire resistance with an ohmmeter while wiggling/twisting the wire. The reading should be steady and no more than 10K ohms per foot of wire.

Duke

[Donald A.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Duke;

Thanks for the message. Sorry I'm not as educated but I don't know what you mean by "AC heat range 5". Could you provide the actual name and number of the plug you recommend?

Also, on my post about do my voltage checks look OK?

Thanks

[Duke W.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

In the AC numbering system "R" means resistor type spark plug, and "4" means 14mm thread. The "5" is the heat range, which ranges from 1 to 6, and the higher the number, the hotter the plug. "T" stands for tapered seat.

So I recommend as a starting point AC R45T, which would be the most correct for judging. All major plug manufacturers have online catalogs and cross reference tables, which you can use to cross the R45T to any other major brand if you like.

Most small blocks were originally equipped with heat range 4 and big blocks heat range 3, but some of us came to the conclusion decades ago that these heat ranges are too cold for normal road driving, and heat range 5 lasts a lot longer before they foul out and don't overheat in normal use. I used AC 43 back when I hot lapped race tracks with my SWC and 45 for normal road use, and those heat ranges were spot on for the type of service.

Spark plug heat range should be determined by the average load that the engine sees, not how much peak horsepower the engine produces. Corvettes are small, light weight cars, with powerful engines, but most of the time in road driving they are operating at relatively low load. High load can only be used for a few seconds at a time before you reach insane speeds, so a plug that runs hot enough to burn off deposits with average low load is necessary. Race track hot lapping requires average load of nearly 100 percent, so a cold plug - one that transfers heat to the cooling jacket more rapidly is necessary to keep the plug tips from getting too hot, which can lead to preignition, and preigntion can lead to destructive detonation.

All of your measured data looks okay to me. I believe your problem(s) are in the secondary voltage distribution system, somewhere between the cap/rotor and spark plugs, so that's the area where you should concentrate your inspection and analysis.

Duke

[Donald A.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Thanks for the education. I picked up a set of R45T plugs to install.

[John H.]

Re: 70 LS-5 (new to me) lights up at night under the hood. Running rough!!!

Thanks for the education. I picked up a set of R45T plugs to install.

Don -

I don't have my charts handy, but aren't R45T plugs for a small-block? I think big-blocks use a different plug, with a longer (3/4") reach dimension (for the previous gasketed plug, it was an R45XLS).