I have a 70 Charger with a 440/493 motor, 727 with a 3000 stall converter and 4.10 gears. The Compression is 10.8 to one, I'm running the 509 MP cam straight up and I have a Barry Grant 850 vac secondary carb on an Edelbrock Performer RPM intake.
I thought that the power valve guideline was to use one that is about half of the idle vacuum..... Is this true? I've had a smoke problem with the car that has been hard to trace. The car had been tuned at a dyno shop several years ago. The man recurved the distributor, rejetted the carb and slipped in a 7.5 valve, which may have been okay with a stock 440 magnum cam, but the thumpy 509 puts out far less vacuum than stock.
On a second note: Can an electronic ignition system degrade over time? Is it possible to slowly lose effeciency to the point where the system is unable to ignite the fuel mixture that has been the same for years? The reason that I ask is that The smoke problem has been intermittent and getting worse the past year. I wonder if I should consider switching from the MP conversion to something hotter. Of course I dont want to spend the money for NO improvement. Thanks guys!

I'm gonna go way out on the limb & say slight intake vacuum leak. Blackish smoke?. How much vac at idle in gear

Well I can answer your power valve question I think. A few years ago I had to really learn how to set one up to get a 350 cfm carb to work right on a hot rodded slant six. The cool thing was that although I didn't have a fancy datalogger I did have a vac. gauge in the dash right next to a wideband a/f meter.

The short answer is that Holley says use 1/2 your idle vac reading to determine which p/v to use. Some have better results going just a couple under the idle vac.

In practice it is a little more complicated. For my little slant the goal was to be able to use the main jets for cruising and only use the p/v circuit for heavy acceleration. The pvcrs (power valve channel restrictions) are the holes behind the p/v that act as jet sizes to determine how much gas the p/v gives. The spring and vac# make it just an on off switch and different holleys might have different pvcr sizes depending on their application. Most are probably optimized for a chevy 350 with bolt ons, but close enough for most stuff.
Getting back to my slant- I ended up having to restrict (drop little bits of wire in the pvcr holes then screw the p/v over them) my pvcrs. No matter how lean I made the mains when the p/v opened it would go pig rich. Once I got the amount of gas getting added under control so it would drop the air fuel ratio by only a couple points I was able to lean out the mains so it would just creep into the high 14.x:1 range at 3/4 throttle. I noted the vac reading around that throttle position and set the p/v to open then- when the motor really needed it. You could feel the motor getting a rush of power when it opened and brought the afr from lean cruise to sweet acceleration.

Your application won't need the pvcrs leaned out but it's important to recognize that the p/v will only meter a set amount of fuel like an on off switch. Your goal is to make the wot mixture just lean enough to need the p/v to get best power. Make it come on at the right time- 1/2 idle vac for a steady idle, closer to the idle vac for a more rumpety idle.
Also watch out if you race- high rpm (top end high gear) can possibly raise the vac. high enough to turn off your p/v circuit and lean you out, even at wot. This could hurt your et also your motor.

Good luck with the ignition. I'm no computer guy but I think it's the coil that dies usually. The spark box usually works or doesn't. I've burnt a couple old mopar boxes and a couple msd 6a boxes.

Rdr

I dont have any sophisticated analyzing equipment. I wish that I did. I DO have a vacuum guage, and this weekend I have the time to work on the car. The Barry Grant model I have came with 80/85 jets and a 6.5 PV originally. Last month I dug in to get a look. The Dyno guy put in the fatter jets and PV, so I am trying to get this carb back to stock to establish a baseline #. I have an assortment of jets and power valves, along with a set of teardown gaskets. So far I changed the primary jets back to #80 and lowered the PV to a 3.5. It idled cleaner than usual.... ZERO smoke, and it revved nice. No street time yet though.
The smoking issue: Yes, it would smoke a grayish black. It wasnt always right as I accelerated, it was wierd. I could roll around on the primaries all day with hardly any smoke at all. Once I dipped into the secondaries I got a small blast of grayish/black smoke, then near the top of each gear the smoke would really blow out. I did find once that the oil pan bolts were loose and found oil streaks on each header. I tightened the bolts and the smoke was far less. Later I got under the car again and noticed oil residue caked on the exhaust crossover pipe. The Trans speedo housing had a bad O ring! I never noticed ATF on the floor before because I have a black carpet remnant that I park on. The ATF leaves a gray color when burned, compared to a whiter color of engine oil smoke. I have yet to have the time to run the car since sealing the trans leak or the carb mods, but this could be the weekend!

Poor man's ignition tester - gap a known good spark plug at 1/4". stick it on the end of a plug wire and crank the engine. If it won't jump that gap in open air,it certainly won't fire under compression.
On the power valve,if changing to a lower valve helped either the old one was leaking slightly or the idle speed screw was opened too far and the transition slots were overexposed. Despite what Holley says the power valve should have no effect at idle. it is just an aid for transition. On my Duster's 360 I'm using a 750 Holley with a 9.5 Power valve. To get the right transition slot opening I set the initial timing to 17 Degrees. It runs smoothly and revs freely. I need to lean out the idle a bit but it starts so well I hate to. DaveRS23 brought this to my attention. he runs a 500" Cuda that puts out 530 H.P. at the rear wheels and uses a 10.5 power valve and he has spent YEARS aquiring this knowledge.
The power valve channel restrictors,as has been said can be used to adress a mixture problem,usually just coming up with the same total area of the jets and restrictors as original will suffuce.
On an aside, I was watching Horsepower T.V. (yeah I know) do a dyno run and at high R.P.M. fuel was being pulled from the Accelerator pump nozzles. Maybe something to think about.
O.K. rest your eyes for a minute and go play. Those of us in cold country are jealous.P.M. or e-mail me if you need more.
Ron

Bad news.....

I had a friend follow me today to trace the origin of the smoke. It was White/Gray and straight out of the exhaust. Its rebuild time. The carb changes I made slowed the car down too. The engine has been an enigma. High oil consumption, dirty plugs BUT high and consistant compression numbers and decent power. NO smoke until I hit half throttle and above. What the heck, man!!?? I need to get this motor apart to get a look at whats wrong.

Before you freak out and start living on beans and rice to save money for a full rebuild pull the intake. It could be sucking in water if it isn't sealed right. It would also cause some drivability problems. When its off you should be able to see if the gasket failed pretty easily.
I had a 318 that was sucking oil and water in thru beat intake gaskets and was still fun to drive, so I'd imagine a 500 CID motor could be pretty strong even hurt.

What Radar said, I'd get a new bathtub gasket & mock it up to see if the sides have been milled the right amt for spot on port alignment & if good install it w a THIN/EVEN coat of permatex #2 on both sides of the 8 port holes in the tin and I think there are now several varieties of RTV that will holdup there also. Still bad, if you have access to shop air & can borrow a leakdown gauge set, see what you get but I'm bettin on the valley pan gasket EDIT read this (5 minutes). "breakin secrets" at www.mototuneusa.com