Yamaha G9 carb help

[chazdo2]

I got rid of the exhaust and put a straight pipe with a mower muffler on it. I check the plug and it is running too lean. I am assuming that the air box line that goes to the valve cover provides vaccum to the fuel pump. Does anyone have an explanation of this? Does it help the fuel pump provide gas to the carb? If that is so maybe I have an air leak in the air box so the fuel pump is not keeping up? What about a bigger carb? I have a few but will I need to get the vacuum hose hooked up so the pump will pump enough gas if I am assuming things right? I hope I dont sound crazy maybe an explanation of the fule system might help. Has anyone used a bigger carb, if so what size? Thanks for any help this site is great for us backyard nuts! Bad things happen when you turn a tech loose with a welder and a small bit of mechanicle knowledge!!

[sho305]

It will run lean if you change the airbox at all, or the exhaust. Has nothing to do with the pump it is because you changed the airflow into/out of the engine. Jet it richer or drill the jet/etc. The pump gets a signal from the crank low in front and that line tees to the pump, then goes up to a valve that lets certain amount of air through. You may have to clean it and don't put it in upside down I think it lubes the rockers with oil or the other line in back does I forget. The line from the top to the airbox is just for blowby like an EGR, if you overfill the engine with oil it will come out and fill the airbox. Not sure about larger carbs, mine with hogged airbox does not go faster the last 1/4 or throttle so I don't see how the carb could be too small. If you want power better off to go big block, you can't get much more from the stock engine barely enough to feel the difference. Nobody makes cams for them far as I know and that is what they need. Make sure the pump lines are not cracked, it should puke out squirts of fuel on the starter if you take the line off the carb, way more fuel than it needs.

[Mike Mac]

Search;jetting carberator. You need to rejet the carb main jet larger until the plug is tan or gray in color at wide open throttle for about 50' at modderatly high RPM. Shut off key switch before you lift off throttle, then shift to neutral rolling to stop. Than you can read the plug for color, waiting till the motor cools for awhile before you attempt the pull the plug. Don't want to gull the threads on aluminum head.

[chazdo2]

Thanks

[Mike Mac]

Please read all the threads that pertain to jetting. You can blow up a motor quickly running it lean! So its got to be done right.

PS. I use never seize on the plug threads (small amount) tho.