Ron,

     The answer depends on which 390 you're messing with. Probably the passage. There were some power valves made that had "picture window" openings to pass enough fuel on large carb applications. Most have a series of holes drilled, while those had larger square passages, thus the name. I would really be surprised if you needed to enlarge the PV passages but I've been wrong before! The fact that no one else has had to do this makes me wonder if there's not another issue. For what it's worth, almost all Holleys come from the factory with slightly rich jetting, on purpose. That includes the power valve circuit.     

Frank/Rebop

Bristol, In ( by Elkhart) 

Ummm.... not really sure what calls for the attitude, but at least get your facts straight and read the issue and other parts of the thread before you get on your high horse.  My engine is a 292, not a 312.  I never cleaimed that it was "built" for a high performance.  If you do the math, the 390 CFM should be right for an engine of that size at 5500 revs, assuming 85% VE, (which is optimistic).... Ill do the math for you; it calculates out to needing 394 CFM based on those variables.

MOREOVER - if you read the beginning of the thread, I have an edelbrock 500 cfm & a holley 390 (do you approve of the edelbrock?).  the POINT of this thread, as stated, was that I wanted to see how good I could get it to run with the 390, as that SHOULD be enough carb and give a little better throttle response.  Once i got it running as good as possible on the 390, I am going to work on the edelbrock a bit more and from there figure out which one I like better. 

And also FOR THE RECORD, the FOM doesnt ALWAYS start in 2nd gear... if you go to WOT from a dead stop, it will drop into 1st and function like a regular 3 speed auto.

And as for cam recommendation, it was after several long conversations with John Mummert, whom I trust.  he seems to know a *little bit* about these engines.

BACK TO THE POINT - I would like again to thank those who have contributed their knowledge & expertise to help me trouble shoot a carb problem that I never would have found.  Ted, regarding your power valve restriction comments - is that a matter of opening the port on the PV itself, or does it involve opening a passage in the carb body?

THANKS AGAIN to those who have been very helpful in sorting out the 390 carb!

Ron

Ron Grove

Wauconda, IL

Pete, this is not to argue or anything like that but this informatiom is from holley carb corp themselves. Holley 390 cfm carbs were used on ford 390 cu in hp engines  dodge 440 cu in and big block chevs back in the early 70s. I've run the 224@ .050 270 cams and they do some better than stock but not a whole lot because they are not a high performance cam compared to alot of other grinds avaliable today. I tried my 390 cfm on my 56 t-bird to see how it would perform on a highperformance 312 poked and stroked engine and it did fine, it just wouldn't do as well in the higher rpms as say my 600 or 670 holleys. I've used alot of other cam grinds and different holleys as well and learned of just how expensive this can be in finding the right combo.

-Gary Burnette-

OK, this is about a "built" 312 and then the owner puts a holley 390 on it for whatever reason. The cam pulls 11.5 inches of vacuum in drive at idle. WHAT IS WRONG WITH THIS PICTURE? You have a "Blue thunder manifold" and then whimp out on the carb? Now you try to tune the carb to get it to work with a FOM? For the record a FOM ALWAYS STARTS IN 2nd GEAR. Lots of things that should be changed, IMHO but the first is the camshaft. What were you thinking when you chose that cam? If you wanted RUMP RUMP there are better choices. You got some bad advice. It is NOT THE CARB. Only my opinion and not the popular one at that. Pete

As Gary mentions, the full throttle performance will not change with the different size power valves.  But there is another tuning opportunity that exists in this area and it’s in the size of the PVCR (power valve channel restriction).  If the engine likes more fuel under full thottle but fuel mileage is suffering with the larger jet sizes to get that better full throttle performance, then the PVCR can be enlarged appropriately so that the jet size can be kept smaller, but full throttle fuel mixture is increased.  This allows the smaller jet sizes to be used for cruising while a larger PVCR will allow for additional fuel in those situations where the vacuum drops below the power valve rating.  Just keep in mind that making the PVCR larger is easy but backing back up to the smaller sizes is considerably more difficult.

Lorena, Texas (South of Waco)

I don't believe you will pick up any more power at wot but you will probably get rid of the stumble you're referring too. Just try the 6.5 and see how it does, thats the best all around pv on the planet.

-Gary Burnette-

Is the only downside to too high of a power valve decreased mileage (assuming it isnt to the point of opening at or right off of idle)?  Could I possibly pick up a little more power at WOT with a higher PV?

Ron Grove

Wauconda, IL

If you feel a slight hesitation or stumble during part throttle acceleration where the power valve is still closed but the engine is wanting the additional fuel, then a quicker opening power valve can sometimes help.  If you have 11½ inches of vacuum with the tranny in drive, then the factory supplied 6½” power valve is typically more than adequate.  You can potentially jump the power valve up to a #85 (8½” vacumm opening) and have it come in slightly sooner but you’ll also see a slight decrease in fuel economy to go with the slightly earlier opening 8½” power valve but idling characteristics will remain the same as with the #65 power valve.

Lorena, Texas (South of Waco)

That is correct. In the center of the spoke which is approx an inch wide is right at 12 degrees. If the damper hasn't moved much the spoke nearest or right under  the damper timing marks is the one to mark. As Frank said when setting the timing     unplug the vacuum advance. Usually when using a small carb decreasing the size of the power valve will cause a lean stumble when accelerating.

-Gary Burnette-

Thanks again for all of the guidance everyone!  And thanks for the quick tip on timing!  Im assuming the spoke should line up with the pointer, correct?

What are the symptoms of a too small power valve?

Ron Grove

Wauconda, IL