Summit Carburetor Modification

[CincoBravo]

Before I knew any better, I bought a 500 CFM Summit Street & Strip carburetor (part number SUM-M08500VS) for my stock/mild 302. Set it on top of a Weiand Street Warrior Intake Manifold (8124WND). It works flawlessly on the street, but even the slightest off-road activity results in stalling out (every time, without fail). I'm now much more informed on the shortcomings of some carbs off-road, but I already own this thing. Has anyone modified one of these Summit carbs to get better off-road performance? I've been told about spring loaded needle and seat assemblies, and I'll try that, but has anyone had luck with anything else?

 

[mrdrnac]

If you extend the vent tubes with a hose and connect them like a U and vent the top center of the hose it may help like a holley truck advenger carb.

 

[sanndmann3]

maybe lower the float levels some to prevent fuel sloshing?

 

[cs_88]

I'm running the 500 cfm Summit but I don't have any off road experience to share.
This spring loaded needle and seat may be worth trying. It has the same diameter as the needle and seat on the Summit carbs.

https://www.holley.com/products/fue...nd_components/needle_and_seats/parts/18-2SQFT

 

[spap]

Can you get an off road kit for the carb.
Can include a stiffer or include a spring for the float and needle valve
I got one years ago

 

[spap]

This is an off road kit for a Holley 2 barrel


this kit is the ultimate in fuel control for Holley 2 barrel carburetors used in off road, tough truck, derby, and other applications where proper fuel control is a must. this top quality kit contains everything needed to convert to off road use....1- .110" spring loaded off road needle & seat assy., 1-press in metering block vent, 1-extended bowl vent tube, 1-new non stick float bowl gasket, and 4 reusable nylon bowl screw washers. fits Holley model 2300 performance 2 barrel carburetors such as 4412, 7448, etc. and most similar AED, BG, Demon, Grant, and QFT carburetors. please see our store listings for a complete selection of hard to find parts & hardware for Holley carburetors.

 

[blubuckaroo]

Your not going to find carb that runs perfectly right out of the box. They’ll all take Some tuning. If you don’t, you’ll leave good performance on the table. If you look at the spec sheet in a carb rebuild kit, you’ll see a number of float settings for the same carb on different vehicles. That’s because of the engines attitude in the engine compartment.

 

[jckkys]

Any carburetor is going to have some fuel spill out of the booster venturis when the carb is shaken. This will kill the engine with the throttle closed. It's called bump stall. I've found that a high curb idle speed will supply enough air to keep this momentary over rich mixture from killing the engine. The anti-dieseling solenoids can be wired to a switch activated when you start going into rough terrain. I've played all the tricks with the bowl vents to no effect. Holley made an off road 600 CFM 4 bbl kit that included 2 side hung bowls, spring loaded needles, individual fuel feeds to both bowls, and vent whistles in the metering blocks. The side hung bowls were the best change. Center hung bowls were never good for off roading. They were designed for road racing. Another consideration is the size of the venturis and throttle bores. For off road smaller is better. The Holley List 4412 has all the wrong stuff. The 7448 is at least small enough to work on a stock 302, tho it would benefit form a side hung bowl. The Summit alias Holley 4010 has some good features, like annular discharge booster venturis, and select power valve channel restrictions in addition to the common Holley tuning parts. I would suggest lightly lowering the float level and increasing the curb idle speed.

 

[blubuckaroo]

Excessive fuel pressure can cause an off-road stall too.
When the fuel sloshes in the float bowl, high fuel pressure can easily unseat the needle valve and overfill the float bowl.
I use a fuel pressure regulator called a Cagle. It reduces the fuel pressure to 2 PSI at high engine vacuum (Idle /cruise) and allows full fuel pressure at low engine vacuum (acceleration). I use mine with an Edelbrock carb which is notoriously lousy off road. The regulator along with lowering the float level has made mine work just fine off road.
These Cagle regulators are out of production but I can't see why a ordinary fuel pressure regulator wouldn't work just fine for you.

 

[CincoBravo]

Thanks for all the input guys. I really appreciate it.

 

[cs_88]

May also want to consider a spring loaded needle and seat that has a smaller diameter than stock.
This one is .097 vs the .110 and should flow a little less.

https://www.summitracing.com/parts/sum-200403

 

[broncnaz]

I've always had more issues with spring loaded needle seats so I dont use them. But they work for some. One of the drawbacks of the summit carb is the center hung floats they tend to cause flooding on up and down hill when offroad. So as others suggested extend vent tubes and lower your float levels I go offroad quite a bit and set my fuel level below the sight window. I actually have to rock the bronco a little to see the fuel level. But I run holleys with side hung floats so its not as big of a issue on them. You may want to go even lower where you really have to rock the bronco to see the fuel splash.
You will just want to make sure you can still maintain hiway speeds when you do this then you wont need to mess with it anymore.