My 8007 Holley 390 carb. came with #51 jets. How do I know it will be safe to install #50 or #49 jets? I don't want to run it too lean and burn the valves. The engine is a stock 1953 8BA flathead. My goal is to get better gas milage. Thanks.
Short answer, you don't ! But long before you have valve problems the engine will give you clues ,running hotter ,pinging,lack of power etc. With modern unleaded fuels reading plugs is a lot harder than it used to be. Nowdays with the correct mixture the plugs will look unused short term. Being you have 4 barrel carb you can down size the primaries and richen up the secondarys to correct for WFO if NEEDED. Try 2 sizes smaller to start and watch for running hotter and pinging ,all OK then try 1 or 2 smaller yet.
You cant hurt anything by trying different jets unless you let it ping for ever. Just drive it and check your plugs. The only way to truly know is to use a wideband O2 sensor.
X2 on using a wideband O2 sensor. If one has to ask the OPs question, he's going to need more direct feedback as to A/F ratios and jetting than we could give here. Weld in the bung and presto, no more guesswork.
another way is to mess with the power valve - it is a vacuum operated enrichment circuit. Read about how they operate, they are not an absolute either open or closed. A power valve begins closeing earlier and reaches complete close at the stated value. With a little experimenting you can get one to begin to close sooner at a particular vacuum which will lean the fuel mixture but when under a load it'll give you more fuel. You need a vacuum gage in the car - it'll help your driving habits too - and then you can tune right in on the power valve.
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