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Can meaningful spark plug readings(for timing) be performed thru typical street driving or can they only be made at the track? I realize this is only one plug from eight but the marks are similar on all. Total timing on my 9 to 1 compression(bumped slightly from 8.4 to 1) 455 Pontiac is 36 degrees all in by 2800. For full street driving transparency, vacuum advance is 8 degrees at 15"Hg. The total timing mark is past the 90 degree bend on the strap. Does it look too far advanced? In regards to the black spots on insulator no detonation can be heard. I could be wrong but I believe this to be from oil consumption due to my factory forged pistons. (I get piston slap at cold start-up.) My car, SD-455, came from the factory with the assertion that oil consumption would be higher because of this. Am I ok or do you think I should back off 2 degrees or leave it alone since it is running great? Thank you.
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Race it, Don't rice it!
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Yes and No.
Timing marks on the strap can give you and idea on the total timing but you have to remember it's generic in nature.
The colors are even harder because different pump fuels are formulated slightly different so that changes the coloring by itself.
Plug are useful to look at for generic tune up purposes or just to get an idea of the averages but it's NOT doing you too much good to use for fine tuning.
You can add 2 more degrees of total timing in that one.
 

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A vacume gauge is my go to when I dont have a computer to go off.
If you can get a air fuel gauge in the exhaust it really helps me.

Tuning a carb is much harder then tuning fuel injection. Fuel injection is adaptive within milaseconds to different conditions such as temperature, altitude, humidity, octane/ethanol content, cylinder misfire, spark deactivation(several situations this can save the engine during tuning), and because they all have a return system have less chance of the fuel lines being affected by heat.

With a carb you set it for generic conditions(leaving power on the table to keep things "safe" ) and then need to retune when those conditions change.

It sounds like I am badmouthing carbs. I respect them for the simplistic nature. But tuning is much more labor intesive with a carb.

First thing I would do if I thought oil was getting past the rings would be a compression(and coolant pressure) test. Then pull the valve cover and find true top dead center checking for any pushrod or rocker damage while the covers were off. Pull the plugs and run my hand along the length of the wires checking for any bumps or recesses. Mark the relationship of the distributior to the intake with a soap stone mark(temporarily). Check the condition of the cap, rotor and all wiring going to the distributior for any loose pins. Loose pins can cause all kinds of issues on older cars. Almost as bad as corroded blade terminals in a through firewall connection.

A (o2) air fuel gauge in the exhaust, temporary tach set on the cowl, and a temporary fuel pressure gauge T'd in the fuel line, or a permant shrader valve with a fuel pressure gauge screwed in would be also done by me to aid in tuning.

Once all of that is all checked out I would start a tune. The above is maybe 2 to 4 hours of work on a bad day. But eliminates several "it might be's" while your tuning.
You can easily plug the o2 bung port, pull the temporarily tach plugging in the dash tach, and unscrew a fuel pressure gauge to let you easily change your tune in a parking lot if need be to meet the changing conditions.
 
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