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By Melly - 3 Years Ago
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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.
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By 55blacktie - 3 Years Ago
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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.
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By Melly - 3 Years Ago
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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.
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By rponfick - 3 Years Ago
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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
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By 55blacktie - 3 Years Ago
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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.
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By Melly - 3 Years Ago
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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.
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By 55blacktie - 3 Years Ago
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I just reviewed Engine Masters episode. It's the Street Demon that has to be removed to change jets. Good news!
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By Ted - 3 Years Ago
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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.
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By DryLakesRacer - 3 Years Ago
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Only the top of these carburetors needs to be removed to change the jets. I believe they use Holley jets.
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By 62bigwindow - 3 Years Ago
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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.
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By Ted - 3 Years Ago
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As has already been mentioned, the Summit carbs use Holley jets, power valves, and needle seat assemblies. They are reasonably easy to rejet without removing the carburetor from the engine.
Something else to keep in mind is that most modern replacement carburetors are designed to be used in tandem with a PCV valve system. With this in mind, the idle circuits are calibrated to be richer than the earlier non PCV carburetors to compensate for the additional air flow at low rpm created by the PCV system. If not using a PCV valve with a modern carb, then it may be necessary to perform tweaks to the idle circuit if there is not enough mixture control when adjusting the idle mixture screws.
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By Melly - 3 Years Ago
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Ted
When you say "perform tweaks to the idle circuit if there is not enough mixture control when adjusting the idle mixture screws" could you please explain a little more. I ask please excuse my stupid. Try to learn as carbs have never been in my wheel house. Back in the day had a friend I relied on but that option is no longer on table. My location does not have much options. Thanks for your help!
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By Ted - 3 Years Ago
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It would be helpful to know more about your engine combination. PCV system? Fresh rebuild or lots of miles? Vacuum accessories (power brakes, etc.)? Camshaft specs? There is some variability in the stock camshafts depending upon the year model and/or a replacement.
Available from Summit are the idle feed restriction kits for the Summit branded 500, 600, & 750 carbs. While I haven’t needed one of those kits to fine tune the idle circuit, it is an option. Here’s the link to the idle feed restriction kit.
Summit Racing SUM-M08041 Summit Racing™ Idle Feed Restriction Tuning Kits | Summit Racing
The secondary side fuel float level is important as there is an idle circuit also on that side of the carburetor. That idle circuit is fixed and non-adjustable and is there so that the fuel in the secondary reservoir is constantly being refreshed and not becoming stagnant. But it does add to the total amount of fuel being delivered to the engine at idle with the idle circuit being calibrated to compensate accordingly.
If you can screw the idle mixture adjustment screws all the way in and the engine continues to run at idle speed, then there is too much fuel still going to the engine. This can be a result of not running a PCV system which automatically has the low-end fuel mixture running on the rich side in which to compensate. Another possibility is the primary throttle blades are open too much which in turn has the intermediate or power circuits also throwing fuel into the mix. If the primary blades are open too much, then that must be corrected to get the idle mixture screws back in the range in which they need to work within. This instance occurs predominantly when trying to run a carburetor that's too small on a larger engine but internal engine problems or camshaft timing can also be contributing factors.
With the carburetor removed from the engine, there should only be about 1/16” of the transition circuit slot exposed to the lower side of the carburetor with the primary throttle blades in the idle position. If there is more than this, then the potential is there for an excess of low-end idle fuel and hence, the idle mixture screws not being able to be closed enough to compensate for that. One fix for that is to open up the secondary blades some more so that the primary blades can be closed accordingly. Starting with a half turn on the secondary throttle blade opening adjustment is a starting point for this. They are other fixes for this but those are usually associated with camshafts that are much more rumpier than stock.
If the amount of exposed transition circuit looks to be okay at the idle position but screwing the idle mixture screws in all the way to the closed position will not kill the engine, then there is still too much fuel going into the engine. Assuming the float levels are correct for both ends of the carburetor, then the idle mixture air bleed hole needs to be increased in size. While a hand full of the high performance aftermarket carburetors will have air bleeds that can be changed out, many aftermarket carburetors have fixed air bleeds and those have to be modified by either making them smaller or larger depending upon the specific problem that’s being addressed.
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By Melly - 3 Years Ago
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Ted
I want to thank you for you taking the time to reply. I really don't know much about all the internals of motor but appear to be pretty stock. Has a 57 dizzy, pentomic ii inside. Power steering no PVC set up. It seems to be burning very
very r rich. And I am way above my pay grade. So maybe best to look for a shop that might help.

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By panel driver - 3 Years Ago
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Just a thought, are the basics good, compression, timing? Did it run ok before the carburetor replacement?
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By Melly - 3 Years Ago
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compression is very good, have change timing all over, seems to want a lot of time to run. ran good before switch, I am thinking that the floats are way too high and at 68 pri jets and 73 sec jets need to be downsized,
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By Ted - 3 Years Ago
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Here are some other considerations. Are those pictured spark plugs the result of a lot of idle time or is that after some cruising time?
Are you using direct vacuum or ported vacuum to the distributor? Ported vacuum will help to keep the spark plugs cleaner at idle. In the grand scheme of things, there should not be any idle speed or ignition timing change when hooking/unhooking the vacuum line going to the distributor. If there is, then the primary throttle blades are open too much at idle or the vacuum line is hooked up to direct vacuum. If there is a timing change at idle but the vacuum source is 'ported', that is indicative of the primary side fuel transition slots being exposed too much at idle. That's fixed in a number of ways with the simplest being to just open the secondary slots some more.
Does the engine tend to flood or is hard to start after it sits for a bit? This could be related to leaking power valves and/or fuel needle seats not holding after shutting off the engine.
Is there a fuel filter prior to going to the carburetor? No filtration can point you back to the fuel needle seats not doing holding the fuel back.
What does the fuel level look like when looking at the sight glasses on the sides of the bowls? Anything more than half-way up on those sight glasses would be on the high side. If allowing to engine to sit for at least an hour after shutting it off, does either the primary or secondary fuel levels drop down? If so, then that needs to be addressed with the power valves and/or their gaskets being something specific to look at.
Do the idle mixture screws have an effect on the idle? How many turns out on those screws when you get the idle characteristics you want? Does screwing those same screws in part way or all the way in kill the engine?
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