c3-lt1 -spark plug intrepretation

[Peter M.]

To all,

Need your knowledge as I am less than amateur. Have 71 Lt1. Pulled spark plugs (ac deco 44ts) to replace with same. Noticed some plugs were perfectly tan with no oil on threads while other plugs had oil on threads and were darker (hard to tell if tip was black from oil or gas).

Note reason for pulling plugs (besides not changing in 2 years) was car starting to run rough and also hesitates when first taking off(has factory 4:11 rear) in each gear.

I have Holley recently rebuilt by J McNeish and I put on car, found some vacuum leaks, pulled carb, tightened bolts on underside, and vacuum leaks went away. Adjusted fuel(turned Holley all the way in then out 2 1/2 turns on both sides. Car ran better but still hesitates takling off. Thus reason for trying plugs. Keep in mind I am not mechanical at all so this may be all the wrong things to do.

Help with suggestions please!! Please be explicit and don't take for granted I know simple stuff because I don't. Love the car and am trying to make it work. Thanks for any help you can give me.

[Jim T.]

Re: c3-lt1 -spark plug intrepretation

Has your LT-1 ever backfired during starting or taking off with the hesitation? If it has you may have ruptured a power valve. A ruptured power valve is a commmon replacement item when older Holley's do this. Later model Holley's have a modification to help prevent the rupture of the power valve. Easy replacement of the power valve for a person not experienced in Holley carb work. You Holley may have been rebuilt with the new blue color fuel bowl and metering block gaskets which can be reused if you are careful in the disassembly. Power valves can be purchsed at auto stores like Auto Zone in a blister pack of one. They used to be about $5 but now the price is about $8. Your current one will have numbering on it that will tell you which one you need to purchase. The numbering related to inches of vacuum. A 6.5 power valve will open to richen the fuel mixture when vacuum falls below 6.5 inches. A quick test of a ruptured power valve with the car running, first remove the air cleaner, then put a finger over the front vertical vent. If in a few seconds the car dies the power valve is ruptured and not holding vacuum. If the power valve is good, the engine will eventually stumble and die, but it takes over 30-40 seconds depending on how fast your engine is running with the fuel bowl vent plugged.

[Doug C.]

Re: c3-lt1 -spark plug intrepretation

Jim, On a 4160 Holley do you only cover the primary fuel bowl vent and not both the primary and secondary? Also how does a ruptured PV cause it to stall? Thanks - Doug

[John H.]

Re: c3-lt1 -spark plug intrepretation

Setting idle mixture at 2-1/2 turns out is pretty rich; use a vacuum gauge connected to a full manifold vacuum source and adjust the idle mixture screws for highest steady vacuum at idle.

[Peter M.]

Re: c3-lt1 -spark plug intrepretation

John,

Thanks for the tip. I don't have a gauge, so until I get one, do you recommend trying to turn it back a 1/2 to 2 or 1 back to 1 1/2? Know this is not exact but would appreciate your thoughts.

Also, any thoughts on why some plugs have oil on threads and others not or is that normal?

Thanks

[Terry M.]

Re: c3-lt1 -spark plug intrepretation

In my experience the oil on spark plug threads comes from external sources -- usually a valve cover leak -- and ordinarily on the rear most plugs on small blocks.

[Jim T.]

Re: c3-lt1 -spark plug intrepretation

Doug I am not a carb technition. The rear vent will not be active like the front fuel bowl vent. Fuel is being removed from the front bowl into the idle circuit. When the throttle is opened the fuel moves through the main metering circuit. The power valve when ruptured is open and supplies additional fuel to the main metering circuit and when it is not ruptured it will supply additonal fuel to the main metering circuit when the throttle is opened and the vacuum falls below the rating of the power valve. Technically how a ruptured power valve and putting the finger over the fuel bowl vent on a engine that is at idle RPM using fuel mixture from the idle circuit will cause it to stop running, I do not know. Over the years this test has always worked for me when I knew my engine was running to rich and the power valve was suspect. Clem might have the answer you asked for.

[Doug C.]

Re: c3-lt1 -spark plug intrepretation

Thanks Jim, The reason why I asked about front vs. both is that when I cover the front only the engine continues to run and when I cover both it dies almost instantly. The other question was just to increase my knowledge and understanding. Thanks - Doug

[Jim T.]

Re: c3-lt1 -spark plug intrepretation

Doug I have a Holley double pumper 650 on my 70, square bore type. I have recently had it apart. I will give it the test of covering both and see what happens tomorrow. Need to hook up a vacuum guage and adjust the carb anyway.