That’s a ‘Yes’ on opening up the secondary blades more so that the primaries can be closed up a bit to get the primary side intermediate fuel circuit back where it belongs.  Follow the steps in Steve Metzger’s post. 
http://forums.y-blocksforever.com/FindPost124606.aspx
When all is well, you should be able to close the idle mixture screws and kill the engine.  When you can achieve that, you can open the idle mixture screws for the correct idle mixture amount.
 
You don’t have to have a back fire for a power valve malfunction.  A power valve with the wrong opening point can lead to a rich idle if the manifold vacuum is low enough.  A bad diaphragm or improperly installed gasket at the power valve can also allow for excess fuel at idle.

Lorena, Texas (South of Waco)

Here, half way down, in Greenbird's post.

http://forums.y-blocksforever.com/Topic124596.aspx

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Just to clarify. I need to open the secondaries slightly to allow the primaries to close and allow the idle circuits to do there job? I'm sure the power valve is not blown. Never had a backfire or any other problem. The engine just seems to run rich especially at idle.

Durham Missouri

It’s not necessarily about the carb size but how well the fuel is metered into the engine.  Smaller cfm carbs are at a disadvantage on larger cubic inch engines and especially where the camshafts are larger than stock.
 
The Summit 750 carb works well on many of the Y engine builds I do.  What helps to make that carb so adaptable are the annular fuel discharge nozzles which helps low rpm fuel delivery when the manifold vacuum has been compromised with a larger camshaft.  Here’s the article that was published in the Y-Block Magazine of a 315 HP 303 CID Y that uses the Summit 750 carb.
http://www.eatonbalancing.com/blog/2015/05/01/unported-iron-heads-can-still-make-over-a-hp-to-the-cubic-inch/

Lorena, Texas (South of Waco)

The motor has a pretty big cam so the idle is set a little high. What Charlie described sounds like what may be happening. Thanks for the advice. Looks like I have some more work to do.

Durham Missouri

The Summit carb is a copy of a Holley 4010, which is similar to an Autolite



Lawrenceville, GA

Charlie's answer is right on and missed by nearly everyone when the mixture adjustment does not work.
miker: the Summit is a copy of the Autolite. When set up correct are a very good street carb.

56 Vic, B'Ville 200 MPH Club Member, So Cal.

If it is an honest 300hp, a 650-750 is most likely what you want.

Typically, if the idle adjuster screws have no effect, then the power valve is blown or your primary throttle blades are opened beyond the idle transition slots and pulling fuel from the boosters.  The higher idle required for a 300hp 292 makes the latter very likely.  An easy way to verify that this is happening is to look down the primaries, using a mirror, while the engine is idling.  If you see fuel coming from the boosters, then you are into the primary circuit and not the idle circuit.

Many carburetors have an adjustment screw for the secondary throttle blades.  Usually, you have to remove the carburetor and flip it over to access that screw.  See if yours has that and crack the secondaries open a hair.  That will allow you to close the primaries to get a better position relative to the transfer slot and make the idle circuit work.  After each adjustment, and with the engine idling, use a mirror to look at the primary and secondary boosters.  If you see fuel coming from them, then it is not yet setup correctly.  It will take a little trial and error to get it to work and it may not at all, if your combo is to radical for the carb.



Lawrenceville, GA

Just out of curiosity, which 600 Summit carb?

I've had good luck with the Edelbrock/AFB clones, 1405 or 1406 as I recall. I've also been happy with the Holleys. There was a recent post on the 390 cfm's about setting the secondary throttle shaft. I think that applies to the 600's, or at least to the 1850 series.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

What would be the ideal cfm 4 barrel for a 300 hp 292? Right now I have a Summit 600 cfm with vacuum secondaries and the motors a little too rich. Also I'm not that impressed with the quality of the carb. The idle air mixture screws leak and can be backed all the way out with no change in the idle. I have a 600 cfm vacuum secondary Holley that I may jet down and try. I'd really just rather find the best size carb and be done with it.

Durham Missouri