YZ400F 99. Pops when throttle is chopped

[Xtreme_racer]

Hey Guys,

Hope you can help.

I've got a 99 YZ400F. When I chop the thottle coming into a corner it pops and bangs. I know that there's always going to be some popping because of the nature of the bike, but it seems to do it a lot more than my old 98 model did.

I'm currently running a doma race can, and have upped the main jet to a 180. I've checked the usuall places for air leaks (front pipe to cylinder joint, silencer to front pipe joint, i've checked the valve clearances, i've checked the carb to engine mount, and carb to airbox mount, you name it I've checked it), but still the problem continues. Am I just being paranoid and this is normal, or has somebody had this and got a fix for it.

Any suggestions?

 

[Thump]

It is normal for it to pop... Mine did it all the time. You can try moving the needle one clip position richer. I believe that is what RR told me to do.

 

[NVR FNSH]

Have you tried richening the fuel screw?

Brian

 

[Xtreme_racer]

Thanks for the info. I know their supposed to pop a little bit, and maybe I've just been riding the pocket road rocket to much recently, but it just seems a little to much compared to the last few I've had.

I've not tryed adjusting the fuel screw before. It was one of those things the dealer told me not to fiddle with. I know that's not reason to do so, but until now all my YZF's never seemed to need it.

I'm assuming the fuel screw is the one underneath the float chamber, which way should I be looking to turn it, and to what degree? 1/4 turn?

Thanks for your advice guys, this has really been bugging me.

 

[NVR FNSH]

I think you've got the correct screw in mind - it's bitch to get to. I use a screwdriver bit from my 6way(?) craftsman screwdriver. Turning the screw ccw (out) will richen the mixture. I'd go 1/8 turns at a time.

Brian

 

[Xtreme_racer]

Thanks for the info Brian.

I've got the UK equivelant of your GNCC events to do at the weekend (although on a much smaller scale. 3 hours over a 6 - 8 mile circuit. We haven't got the land to do it like you guys do!! :ugg: )

I'll give it a tweak, and see if that improves things.

Loz

 

[J.B.426]

The popping on decel is caused by a lean condition at closed to about 1/4 throttle. This is adjusted by, as mentioned above, turning the fuel screw out to richen and in to lean the mixture, and also by changing the pilot jet, which is on the bottom of the carb. If you have to turn the fuel screw past about 2 1/2 to 2 3/4 turns out you may need to go up in size on the pilot jet.

Now with all that said, my bike has always seemed to run the best when there is some popping on deceleration, so I wouldn't worry too much unless the popping seems excessive, and a fuel screw adjustment would probably cure that right up.

 

[Shawn Mc]

From every FCR tuning manul Ive read, and there have been a few, if the low speed fuel screw is adjusted out more than 2 full turns from bottom, you need a bigger pilot. The problem with backing the screw farther out than 2 is, for (1) its completely out of the circuit (2) It might fall out of the carb!

IF the bike just started doing this outa the blue, the carb is probably in need of cleaning.
This is just my 2 cents. Carb tuning is like opinions, lots of different ways.

 

[DPW]

Try this tool for adjusting the fuel screw, makes adjusting a breeze.

Kouba Link