Box Stock

Modifying predator carb

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fatjet87

Member
I bought a new hemi headed predator about a year ago. Just wanting a little more power out of it. It's pretty much all stock except the block has been decked so piston is at zero when at top dead center, I milled the head .070
Engine runs very well like it is. I measured the throttle bore and its at .0750, I disassembled carb and ran a 47/64 drill through it followed by a .750 reamer. Doing this obviously busted through the thin wall casting of the carb body. I epoxied all the holes in and re-assembled carb and bolted back on engine. I'm trying to get the jetting dialed in at the moment. It doesn't want to idle for nothing even with the idle screw turned all the way in. Needing some ideas to try, thanks.
 
So you have a carb that is the same size all the way through, right? That's not a good idea in my opinion, a carb likes to have a smaller venture than the butterfly area (to some degree anyway).. I never go as big on the venture and the butterfly area. The first thing I would do in your application is remove the lower oring on the pilot jet..and see what it does.
 
Make sure you don't have a hole in the epoxy somewhere, like around the low speed circuit. Best thing you could do is go ahead and bore the throttle side out more and put a big butterfly in it if you haven't already. Did you change the exhaust or does it still have the stock muffler and airbox?
 
It still has stock muffler and air box on it. A little track down the road from me race every week. Been running in the stock appearing class. After closely looking at the carb I found a place around where the pilot jet assy goes in at that I had missed a spot. Put more epoxy on and honed it smooth. Put carb back on and she fired right up and idled like normal. Gonna try and run it around the yard today just to see how it feels. Why do you want the Venturi smaller than the throttle butterfly bore? I was thinking you would want them the same size. I'll admit I don't know much about these type carbs but I want to learn all that I can, and it's no biggie if I kill a carb during this learning process like I probably already have with this 1. Thanks everyone.
 
In the carburetor, the venturi is a streamlined restriction designed to cause air to momentarily increase its speed while passing through it. This creates a drop in air pressure, in the venturi, commonly called "vacuum." By locating the upper end of the fuel tube or discharge nozzle in the center of the venturi atmospheric pressure pushing down on top of the fuel in the fuel bowl will cause fuel to flow to this area of lowest pressure. This low pressure area increases or decreases in strength according to air flow and engine speed, thus controlling the amount fuel flow.
At engine idle speeds, there is insufficient air velocity through the venturi to cause fuel to be drawn into the air stream. Also, with a sudden change in engine operation - such as rapid acceleration - the venturi effect is momentarily lost. Therefore, other systems must be designed into the carburetor to compensate for this loss.
 
Thanks for sharing all that info. I'm an old school guy used to working on flat head Briggs. On those type carbs they respond well when you open the complete bore of the carb up to .750. I thought the same type mod would carry over on these newer style float bowl carbs. Oh well I'm probably gonna try and do what w5r suggested and increase the throttle blade more than the Venturi. And then just make a bigger butterfly to match it.
 
I'm not sure it works on these carbs, but flatty stock carbs used a shaped butterfly to help the venturi effect. May be worth a try on this carb, if you think you've gone too far.
Grab a stock carb to see what I'm talking about.
 
We shall see very soon. I'm not expecting a lot out of it, I hope it surprises me. I revved it up on the stand and it revved clean all the way to the top, of course that was only for a second. I'll keep you posted what it does out on the track.
 
fatjet87 said:
Thanks for sharing all that info. I'm an old school guy used to working on flat head Briggs. On those type carbs they respond well when you open the complete bore of the carb up to .750. I thought the same type mod would carry over on these newer style float bowl carbs. Oh well I'm probably gonna try and do what w5r suggested and increase the throttle blade more than the Venturi. And then just make a bigger butterfly to match it.
Click to expand...

I just copied and pasted the info.. LOL
 
Just wanted to give everyone an up date. My son took his kart around our little backyard track today. It has a stock predator carb that I drilled and reamed out to .750
Wow I can't believe how good this thing runs, it pulls like a freight train out of the turns and also runs very clean up top as well. I'm pleased with the results.
 
fatjet87 said:
Just wanted to give everyone an up date. My son took his kart around our little backyard track today. It has a stock predator carb that I drilled and reamed out to .750
Wow I can't believe how good this thing runs, it pulls like a freight train out of the turns and also runs very clean up top as well. I'm pleased with the results.
Click to expand...

that's awesome!!
 
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