Yfz450 issues

[Yzhabs]

Hey all, i have an 07 yfz450 (carburated), it's popping with quick throttle bursts when in neutral. Also Seems to do it when going slow(1st gear) and when slowing down, also pipe gets really hot. Anyone know why or have any suggestions. Appreciate any ideas to help correct? Thanks!

[Greenwrench]

Sounds like your running lean and getting a lean pop . You might want to give the fuel screw a half turn out or so . I would pull the carb and clean it just to make sure your not clogged on your pilot .

[kurbe397]

Definitely agree with greenwrench on the lean pop and lean air/fuel mix.  The left over oxygen from the lean mixture igniting in your head pipe when you let off the throttle is what causes the pop and part of what makes the pipe hotter.  When you pull the carb to clean it, it's good to remove the main and pilot jets and blow them both out with compressed air. If you don't have a compressor you can get away with using just carb cleaner if you poke a piece of safety wire through the holes first to make sure they're clear.  While you have the jets out write down the number stamped on the side of each one.  That way if just adjusting the air screw isn't enough you don't have to open up the carb again to know what size jets you have.  Then it's just a matter of ordering/getting jets a size or two richer.

The extremely hot pipe is also due to the engine running hotter.  With an abundance of oxygen in the air/fuel mixture the combustion is a quicker(terms of milliseconds) and more violent burn causing higher cylinder temps.  Think in terms of an "explosion" going off in your cylinder with all the oxygen igniting almost instantaneously.  A richer air/fuel mixture will burn cooler with a lower combustion temperature.  This is due to the air/fuel burn being slower and more controlled.  Since gasoline is the catalyst but the oxygen is actually what burns, too little gas allows combustion to be wild and out of control.  The perfect amount of fuel in the air/fuel mixture allows the burn to ignite at the spark plug and have a smooth solid flame front that burns through the combustion chamber and ends at the cylinder walls and piston.  It all comes down to how fast the gases expand inside the cylinder because this expansion is what pushes the piston.  A smooth burn puts more force down on the piston instead of out on the cylinder walls and head.  Of course this all happens in a matter of milliseconds but that's the science behind whats going on.

Lean is mean until it's not because your engine melts or blows apart.  As a general rule carbureted race engines that are run at the limits of performance are rebuilt every race so they lean those suckers out to make 'em scream.  There is a huge difference between something built for reliable fun and something built for $15k - $150k to last one day or one moto running like a beast. 

"Lean is mean and rich is cool" 

I'd say this was a tad on the lean side. ?

[Yzhabs]

Thanks for the info guys. This may sound messed up but i cant find the air screw. Shows in the manual where its suppose to be, but not there. Took carb of to clean, the only external jet i found is the the one on the engine side that goes ip from bottom. Can anyone shed some light on this?

[Greenwrench]

Here's one I'm working on this big black knob is after market and the screw your looking for if is factory it will be a lil brass screw in same hole . 

[Yzhabs]

Thanks greenwrench. I seen that one but i was looking for a screw on the air box side. Isn't  that screw for fuel? Mine is turned to 4 turns out. Is that too much. What do you suggest. 

[Greenwrench]

Ya go all the way in with it and back it out to 1.5-2 turns out and see how it runs I start there and maybe slowly come out a lil bit but shouldn't be more than 2.5 turns out or else you need to get into pilot jet . That's your only outside adjustment besides idle .

[Yzhabs]

Ok, will try that. Thanks alot