Help with briggs carb jetting

[vintagebriggs5hp]

I have this old race kart I put a 5hp flathead briggs on. I am having an issue getting the carb dialed in with the mods I have done. so far I have done open air filter, 3/4" header, Shaved eyebrows, and ported intake and exhaust. I smoothed out all the casting on the intake since they stuck up bigtime and widened the sides and roof quite a big. On the exhaust side I just smoothed it out not enlarging it any. I think since it flows alot more I need to rejet but am not sure what the stock size is and what the next size up I would need to drill to. I adjust it to 1 and a half turns out and it wont even start. I then turn it another 1/2 turn and it starts. Seems as though it goes back and forth where it like to run. sometimes it runs better all the way out and sometimes it need to be turned it a little. I know though turned in a little it has to be lean cause it backfires like mad and fire comes out the exhaust. What size jet should I go to? and will the stock jet needle work w/ an oversized jet? thanks in advance

 

[briggsman]

As far as the jet size im not sure you would need to really get an assorment of jets, and just keep playing around with it till you get it right( all engines are different, what works great in my engine probably wont be that great in your engine), just after you choose a different jet just keep and eye on your spark plug, to see how the engine is running, and you will have to get a matching needle for the jet you are going to use. There is a place called cometkartsales.com you should give them a call and they have all the jets that you could ever need, they used to sell an assortment box of them(thats what I bought), that would be the way to go!