Anyone out there used a Summit MO8600VS - 600 CFM carb on their 292 or 312 y- block?  I purchased this and appeared to maybe too much for me set up   I have a pretty basic set 292 with a 57 dizzy, pterionic II, Runs and Idel's good but at WOT stumbles. Pull plugs all show black running to rich.  So, thinking I should go to lower jets.  Looking for some direction.  Not a Carb. guy.

You're not the first who has said the Summit 600 cfm runs rich. You will have to re-jet. For a basically stock engine, the Summit 500 cfm might have been the better choice.

at this point i tend to agree.  But I already have the 600 to deal with.  Thanks! Have ordered jets will have to figure it out.  Hoping to here from someone that's delt with this problem for more insight, as I am not a carb. person.

What altitude do you live at?  I am just lowering my jets two sizes in my stock 1273, but I live at Denver altitude.  

Ralph

Melly, if you're at/near sea level, I would go down 4 sizes on both primary and secondary jets. According to Summit, the 600 cfm uses 68 primary/73 secondary. Their 500 cfm carburetor uses 62 primary/68 secondary. Going down four sizes will give you 64/69. What I can't remember is if the Summit has to be removed to change jets. I could be thinking of the Street Demon.

i am in Minnesota.     I am thinking that the lowering is what I am going to try and the 64/69 sounds like a good starting point.  Wish me luck as I am learning in area never had to be in..  Thanks for your in put.

I just reviewed Engine Masters episode. It's the Street Demon that has to be removed to change jets. Good news!

I have installed several of the Summit carbs on Ford Y engines with the majority of them being the 750 version.  Have installed a few of the 600 cfm versions with no obvious out of the box issues with those.  Both the 600 and 750 versions are jetted reasonably close for 700 feet above sea level but I can envision leaning the jetting either for higher altitude or extremely hot weather.  If the engine idles well and the idle mixture screws have good control at idle, then I suspect there is another issue going on.  I would first check the rear float level in the carb as that level could be either too high or too low creating an issue when going to full throttle.  A possible scenario is the rear fuel reservoir not getting much or any gasoline due to either a fuel inlet stoppage or a problem with the needle seat and/or carb float.  Spark plugs being too cold a heat range can also create some issues with what would appear to be a richness issue.

As a general rule, it’s hard to go too big with a vacuum secondary carburetor.  The secondaries open only the amount demanded by the engine assuming a stock secondary spring is being used.  The 750 version when running with just the primary side is like running a typical 350 cfm 2V carburetor.  Any air flow sizing of the carburetor above what the engine requires will simply have the secondary side not opening fully in which to compensate.

Lorena, Texas (South of Waco)

Only the top of these carburetors needs to be removed to change the jets. I believe they use Holley jets. 

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

The Summit carb is fairly simple to change jets on. I ran one on my car for a while. Other than a rich issue at idle it was a good carb. Summit has great tech videos on how to adjust these. Also the tech support is pretty good.

Durham Missouri