07 450 throttle response?

Hi I have a 07 yz450 and as I am at low rpms a lot of times when I go to grab more throttle it will bog out then hit hard. I'm not sure what's causing this. It will do it in every gear. I was thinking I could adjust screw on carb? I'm not sure any help would be great. It's all stock except 52 tooth in the rear

i had a very similar issue on my 08 yz450f. it ended up being the accelerator pump diaphram, it was extremely chewed up and deteriorated. here are a few things to check that might fix yours.

1. the bog is almost always caused by a LEAN condition in the Carburator. so first things first, make sure the air/ mixture screw at the front of the carb is adjusted right. you adjust it by bringing the engine up to operating temperature and then slowly turn it out or in until the rpm's increase. it will only be 20 to 50 rpm but you will notice a point in which it will run at its highest rpm.

2. if there is still a bog, take off the carb look down the barrel of the carburator (with safety glasses on. you dont want fuel in your eye.) and turn the throttle. you should see a stream of fuel shoot out from a port in the barrel. if you dont see a stream of fuel, you have either a blocked port in the accelerator pump circuit or a hole in the diaphram. if it does shoot fuel out of the port, still pull off the accelerator pump cover and dissasemble the accelerator pump anyway because the diaphram could still be chewed up (this is what happened with my bike, it squirted fuel, just not nearly enough fuel because there was some blowby through my deteriorated diaphram).

3. if # 1 & 2 check out, consider putting in a bigger, richer pilot jet ( also known as a SLOW JET). after installing the bigger pilot jet, Repeat step #1 to crispen up the idle.

Hope this helped.



mine has a bog off idle to when I bought it first,couple times it almost shut off when I hit the throttle hard.I used an o-ring on the pump arm and haven't had the bog since.You can find the thread in the stickies for O-ring mod,its an easy try and if it doesn't work then tear into the carb.

It sounds like a classic case of a wrong accelerator pump squirt volume. First verify you do have a squirt as noted previously. Then looking through the inlet of the carb time the duration of the AP squirt with a video camera or carefully with a stop watch. Ideally it should squirt between .4 to .8 seconds or basically .5 to 1 second. Yours may squirt as much as 2-3 seconds or barely anything at all. There is a sticky in the YZ/WR250F Forum on exactly how to do this. You can then tune your duration with a larger or smaller leak jet to get it into that desired range. Finally make sure your squirt timing is right and it barely misses the slide. If your squirt is way too far off stock and different size leak jets won't help, then there are 2 modifications you can make to your carb to either increase squirt or decrease it. Read up on that sticky and it will explain all. Mine had too little of a squirt and by simply swapping out a differnt leak jet cured it. Got it currently at a .6 second squirt. Timed it correctly and bog is gone.

There can be a number of different causes, including being too rich or having too large a pump shot (which was the case with the YZ400). But normally it's a lean stumble with the newer bikes.

Accelerator pump volume and timing should be set according to the manual, and the idle mixture set to the baseline. If the idle can't be trimmed right, it may be a varnished pilot jet that just needs a good cleaning, or it could be a size small. But also remember that it is not the primary task of the pilot jet to transition the carb from idle to a more open throttle, and if you try to make the pilot cover that move when the problem is actually somewhere else, you'll end up with a bigger mess. Once the pilot is correct, items to investigate are the accelerator pump and the upper diameter of the jet needle.

Then too, there's riding style. Properly jetted big thumpers are not ever going to be able to take an instant idle-to-WOT whacking like a two stroke can, and that's all there is to it. You have to learn to roll the throttle open a bit more smoothly. Not slowly, just not a snap.

When it does the bog, are you going from no throttle to 1/2 or full throttle quickly? Will it only do it when wacking the throttle quickly? If you roll on the throttle slowly- like over 2 seconds is it fine? That will isolate your accelerator pump, as its inactive at steady state cruising. only used for fast throttle transitions.

I picked up my LE 07' 450 about 6 months ago and only had about two hours on it if that. I too had the same bog issue when I hit the throttle coming off idle and then would abruptly take off! Took mine to our local pro Yamaha dealer and they rejetted the carb and suggested a Boyesen Quickshot 3 accelerator pump and fuel screw. The Quickshot 3 is great as you can dial in the leak jet and then adjust the fuel screw as necessary. I believe they went up 2 sizes on the pilot jet and 1 on the main.

Cost around $300 which included parts and labor which was not bad as parts cost was half that. The bike runs perfect now! No choke or throttle to start and starts 1st or 2nd kick every time. Absolutely no bog or hesitation what so ever in any situation.

Also just added the GYTR off-road flywheel (9.21oz.) and it made a huge difference in smoothing out the abruptness of 1st gear and it will motor along in 1st at idle without stalling. Still have every bit of power but much smoother and is great on the track and trails.

Had a CRF 250x before and absolutely have no regrets moving to the YZ!

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