ATV Florida Forum

I just bought a 2007 YFZ 450. It was running strong, but had a very slight hesitation in the low end. It would spudder a tiny bit when dumping the throttle from an idle.

I took the carb off (disconnected the throtle cable, left everything else connected) and removed the float bowl. The bowl was loaded with varnish, had obviously been sitting arround a long time with bad gas. I clean the bowl with carb cleaner. I removed each of the jets and the fuel mixture screw and cleaned them. I replaced the jets tightened to snug, and set the fuel mixture screw to 2 turns out (yes, I didn't count 1st, silly me).

I put it all back together, started it up and it is running very poorly. It sounds fairly smooth on the idle, and if I slowly give it gas, it will accelerate. However, when I dump the throttle, it sputters horribly and just stalls out.

I took it apart again, and found the diaphram for the accelerator pump was dry-rotted some, so I replaced it. I took all the jets out again and cleaned them again. I also put in one of the aftermarket fuel mixture screws that can be thumb adjusted w/o removing anything from the bike (www.outlawracingproducts.com), set 2 turns out.

So I started it back up, same problem. Starts right up, idles ok, if giving it gas slowly it will run up and sound smooth (in nuetral). BUT if I dump the throttle, it's a spudder, choke out and die fest.

I'll take it apart again today to see if I missed anything. Any help or advice would be awsome.

make changes 1/8th at a time in an out. Also look for the air bleeds in the venturi, blow them out and run a fine wire through them.

is the accelerator pump working?
remove the airbox and look into the carb,pust the throttle and watch to see if any fuel comes out of the brass squirter.
ANY upward movement of the slide should spray fuel,if there is a hesatation,adjust the linkage.
if there is no pump shot at all,recheck the pump assembly,or for a missing leak jet in the bottom of the float bowl.

I had the carb off, the fuel on, pulled the throttle cable and fuel squirted from the accelerator pump like crazy (after replacing the worn-out diaphram). I will check it to see when exactly the fuel sprays. Thanks.

Tonight is the last night I am going to try to get it working. If it's not working tonight, it's going to Barney's in the morning.

 If you still have a stumble you might try installing a #40 leak jet at the bottom of the fuel bowl..

There are 2 things that control the bottom end, the pilot jet and the needle position. and as a Carb wears it will need different adjustments, also depending on engine mods. we run a #50 pilot and adjust the needle up, putting the clip down to bring the needle out of the jet holder until the hesitation is minimal. remember as the engine heats up the richer it get , and summer is just around the corner. if you still have trouble, give me a call. i will be glad to do it for you. Rick Todd @ RTR PERFORMANCE ENGINES. 863-512-6096.I an located in Bartow near the Bartow moto cross. PS as far as the fuel mixture screw settings any thing over 3 turns are not affected , as long as it idles fine  and the head pipe don't glow within the first 5 min that part is good. Ck us out @ rtrperformanceengines.com.