Spark Plug Wire Resistance

[Jeff B.]

I have installed a new set of dated LL plug wires on my 1972 LT-1. I am having issues with rough idle and an occasional backfire through the factory Holley (Powervalve is OK). Today I pulled the plugs and found all eight to be a tan color- most of them have a darker spot on one side for some reason. I checked the resistance of the wires and found them to be:

#1- 24,000 ohms
#2- 23,000 ohms
#3- 24,000 ohms
#4- 24,000 ohms
#5- 17,000 ohms
#6- 17,000 ohms
#7- 17,000 ohms
#8- 15,000 ohms

I am considering changing the LL Repop wires to the Delco 508N based on these ohm readings although none of them are over 25,000. Will I see any noticable difference in attempting to get the car to idle properly if i switch out the wires? Does anyone know what ohm numbers I will get with a new Delco 508N set?

Thanks in advance!
Jeff

[William C.]

Re: Spark Plug Wire Resistance

Should be about the same, about 4000 ohms per foot of length (3000-5000 was the spec as I recall). Plugs sound normal, and the '72 was a low compression engine package so the ignition system may well not be your problem. Did you do anything else at the time you installed the wires?

[Edward J.]

Re: Spark Plug Wire Resistance

Jeff did you check the normal things like timing, vaccum advance operation ( Should be hooked to direct vaccum for better performance).
Vaccum hoses for possible vaccum leak which can lean out fuel mixture and cause rough idle speed.

If these check out I would be looking at the carburator, possible accelarator pump problem. A quick look down the throat of carb. while engine is off and look for a squit of fuel when opening the throttle slowly.

[Jeff B.]

Re: Spark Plug Wire Resistance

Bill,

My problem is that I did everything at the same time over the past couple of years as I reassembled the car and have recently started the engine but haven't yet driven. I have the dwell at 30 and the timing set to 36 degrees total not counting the VAC B28 can running on full manifold vacuum (the TCS is bypassed). I also can't get the car to idle at all when this VAC is disconnected and hose plugged. I was hoping my problem was something simple such as the wires. Any other thoughts?

[Edward J.]

Re: Spark Plug Wire Resistance

Jeff sounds like a vaccum leak problem, if you start engine cold with choke closed, does it seem to run okay? Then as engine choke butterfly opens with a warm engine, Does the car idles roughly? This a symptom of a vaccum leak or a really lean fuel mixture.

[Jeff B.]

Re: Spark Plug Wire Resistance

The car runs OK at higher RPM on first start with the choke closed. With a vacuum leak would I be able to get a reading on my vacuum gauge? Is the power brake booster a possibility for a leak? The booster is one of the only things I didn't rebuild on my car.

Thanks for all the help!

[Duke W.]

Re: Spark Plug Wire Resistance

If you want some better advice, give us some data - like the actual manifold vacuum/idle speed you measured along with intial timing and total idle timing with the VAC connected.

Duke

[Edward J.]

Re: Spark Plug Wire Resistance

The car runs OK at higher RPM on first start with the choke closed. With a vacuum leak would I be able to get a reading on my vacuum gauge? Is the power brake booster a possibility for a leak? The booster is one of the only things I didn't rebuild on my car.

Thanks for all the help!

Jeff, To check the booster diafram, Just pinch the vacuum hose shut if car smooths out you found your vacuum leak. You can do this to the other vacuum hose fittings at the intake, since there are quite a few that run the headlamps actuators andd wiper door. and don't forget the PCV valve hose.

If you have a little carburator cleaner you can spray alittle around the base of carb. to check to make sure the carb. base gasket is sealed. Just note that it will eat at he engine paint so a rag under the area you spray will help. if there is a leak there you should hear a RPM increase when you spray the area. Just note that the throttle shaft areas some may bleed buy.

[William C.]

Re: Spark Plug Wire Resistance

Very possible the carb has acquired some issues during the time it was sitting. If it runs OK when choke is on, but not when choke is off, I'd start looking at things that are below the choke.

[Jeff B.]

Re: Spark Plug Wire Resistance

Thanks for the suggestions! I plan to start it back up this weekend for further diagnosis and will post my findings.
Jeff

[Jim T.]

Re: Spark Plug Wire Resistance

Jeff your LT-1's Holley carb may need a good metering block cleaning. Very easy to do with a can of Gumont carb cleaner using the plastic tube that comes with the spray can to directly spray into all the holes of the metering block. The brass insert valves in the top front of the throttle bore and also be cleaned with the direct srpay through the plastic tube. Protect all painted areas like fenders and cowling painted pieces when doing this.
I would consider installing a new accelerator pump with the fuel bowl removed and check the rubber valve that allows gas to come into the accelerator pump cavity and block it from going back into the fuel bowl.

Holley fuel bowl and metering block gaskets as well as power valves can be bought individually in blister packs.

If you ever strip the threads for the fuel bowl bolts, new threads are easy to have with a Heli-Core kit.

[Jeff B.]

Re: Spark Plug Wire Resistance

Thanks for all the help! My issue turned out to be a vacuum leak- in testing, the brake booster is OK but my vacuum leak is the original AIR pump diverter valve (it will not hold vacuum using a mityvac). I plugged the diverter hose with a golf tee and pulled the belt from the pump. Idle is much better than before- I was able to get a fairly steady 11.5 inches of vacuum at 920 RPM. Does this seem reasonable for a 1972 stock LT-1 engine? I still haven't driven the car but the throttle response seems somewhat sluggish at low RPMs. I went through the Holley (cleaned, new power valves, accel pump, gaskets etc.) before it went back on but might be the problem. At idle the exhaust is enough to make eyes burn if standing at the rear of the car. Is this in part due to the AIR pump not currently operating or carb adjustments?
Jeff

[Duke W.]

Re: Spark Plug Wire Resistance

That's about right, but if you convert the vacuum advance to full time and install a B28 VAC you should get 12" at about 900 and better fuel economy.

You can improve low end response with a quicker centrifugal curve and still use 87 PON unleaded. High overlap cams in combination with low compression are a BIG low end torque killler, but running as aggressive a centrifugal curve as the engine will tolerate without detonation will help a lot. Or you can go more aggressive and use mid-grade or premium.

Pre-emission engines idle with about 6 percent CO and several hundred ppm HC. Removing the AIR essentially brings it back to pre-emission values.

Duke

[William C.]

Re: Spark Plug Wire Resistance

If the eyes are burning at idle, the carb is malfunctioning

[Jeff B.]

Re: Spark Plug Wire Resistance

Thanks guys for the additional insight. I am running the B28 VAC supplied with full port vacuum. Distributor end play has been set to 0.005" but I haven't yet checked the advance curve (brass bushing is in place). Thus far all I have done is set the timing to 36 degrees with the springs removed and VAC disconnected. I do have original weights and springs as well as a Moroso spring kit (right now I have the medium strength springs from the kit in place with the GM weights).
I am thinking I should remove the 6239 Holley carb and re-check as there has to be something wrong with it - any thoughts of possible causes of the odor? I am assuming it could be numerous things.
Jeff