86'Bowtie

Would a blown power valve on a Holley 750 DP cause the engine to blacken the sparkplugs and smoke from the exhaust?

Reason being is my car never smoked, until just recently I adjusted my carb and found that the idle mixture screws were adjusted unevenly. So, I turned all 4 out 2 turns and now my car smokes. My plugs are also charcoal black and they are 2 days old.

I know this is a rich condition, but would it be because of a blown power valve?

BTW the smoke is a whiteish tint and smells like rich exhaust.
Could my rings be worn? Valve seals?Also is my carb adjusted properly? or is there a different alternative? Thank you DUECE

Damon

For sure a blown power valve will give a VERY rich mix. However, given that it didn't have problems until you played with the mixture screws I'd start there first. Adjust them evenly leaner until it's obvious you went too far and then back the all out a bit from there. Do this on a fully warmed up engine, not on a cold one or the adjustement will be off once the engine does warm up.

sqzbox

Where did you get a 750 Dp with 4 idle adj. screws? There should only be 2 in the primary metering plate (one on each side). Did someone put two primary metering plates on your carb? Anyway, the best way to adj. your idle mix is with a vacuum gauge hooked up to a vac. port on the base plate (full port vacuum). Screw your air-fuel ratio screws in till the engine rpm starts to drop and then back them out till you get your highest vac. reading. This is called "lean best idle" and if you have a hesitation with this setting you need to adj. your accelerator pump. It sounds like your really dumping to much fuel. You might also check your float levels. To high a setting will also cause a rich condition.

86'Bowtie

I had a technican look at the carb and immediately pointed out a blown power valve. I'm going to adjust the carb w/ a vaccum guage and see how it turns out. Just don't wanna spend extra cash on something I wouldn't need now.

Also, if the rings were to burn out, the odor would smell like burnt oil right. Just checking, b/c my exhaust smells rich and raw.

sqzbox

If you didn't have a stinky problem before I doubt your rings would wear out that fast. Burning oil smell is different than rich.
Burning oil would be caused by bad rings, bad valve guides, and a stuck or bad PCV valve. When you get your carb fixed, be sure to change your oil. All that fuel you've been dumping down the intake has probably contaminated your oil.

88IROC350TPI

Its highly unlikely a powervalve would blow out for no reason - Unless youre carb has had a decent backfire or two. If your motor was fine before you touched the idle screws then thats obviously your problem. 4 corner idle carbs can be pretty sensitive to mixture adjustments. Although I have a Demon, if I turn all 4 screws out 2 full turns my car is just as bad as you describe, if not worse.

Let the motor warm up and start turning them all in 1/8th a turn at a time - Make sure you keep them turned out all evenly too.

sqzbox

Best result's are achieved with the vacuum gauge!

86'Bowtie

Thanks guys, I'm going to do as 88IROC and adjust them again.
What point do you guys start at? From fully turned in, or 1 1/2 out then start to adjust. I turned each screw in all the way then backed each out 2 full rotations evenly. So should I just turn them
1 1/2 turns out then fine tune from there?

Also recently I did have a slight backfire, that's why the guy told me I had a blown power valve. Any way to assure this problem?

sqzbox

The mech said power valve because you said backfire! That's an assumption! After you take it out, take it to him and ask to have it tested. See if he can do that, or just replace it and hope! Like I said, check your float levels too!