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2009 YZ250F Accelerator Pump Problem?

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Hey guys,

I have a question about how accelerator pumps work and should act/respond on a 2009 YZ250F.

I have had the bike for just under two years now, and have had problems with the carburetor since the day I got it.

I just got it back from the local shop today for a hesitation/bog and high idle speeds.

When I got it back the Invoice said "Disassembled and cleaned carburetor, found gelatin like substance on pilot jet, cleaned and ran great on test run."

Sounds perfect. Like a dream come true. But when I got it home it was a different story.

I bought a bottle of Yamalube Fuel Stabilizer just to make sure I keep the carb clean this time, and because I am about to have a surgery that will keep me off my bike for two months. Anyways, I put about two ounces of fuel stabilizer in, shook it around, kicked it over and warmed it up and it sounded beautiful. Idled like it should, purred like a ferocious beast cat. I warmed it up for about 1-2 minutes, then took it for about 5-10 minutes of laps around the house and a couple runs up and down the road to test the higher RPM range. It ran amazingly. It pulled the front wheel off the ground through each gear.

Now, when I'm running it from an idle, just idling around the yard, and I go from Idle to Mid/Full throttle, it will hesitate and/or die if I don't feather the clutch through it. Is this normal? I called my dad who has been a mechanic for over 25 years and he was going on and on about how the bikes are running so lean that it's hard to get enough fuel in there for that amount of air at wide open and I understand all of that. But when I told him that it has an accelerator pump he was stumped too. I am no mechanic but I had always understood an accelerator pump to fill in that gap in fuel when you're accelerating. Shouldn't it have an instant throttle to rear wheel power when I twist the throttle?

I called the shop back and of course all of their service techs were busy and I talked to the Parts store clerk who was nice and helpful, but didn't really have any information that I need.

I have seen some after market accelerator pumps in magazines... Is this what I need to get that bottom end power?

Thanks for the help guys.


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Typical story.

1) Install a leak jet sized between a #40 and #60.

2) Oring mod or put a stiffer spring on the AP linkage.

3) Adjust the AP timing screw CCW in 1/2 turn increments, testing after each adjustment with a fully heated bike during normal riding. A YZ is a race bike and no racer putts about at idle and nails the gas. Test during normal riding.

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I guess what I was mostly trying to get at with that is:

1) Is it normal for it to do that?

2) Does anyone else's 09 four stroke do the same thing?

I'm not necessarily trying to TELL you that there IS a problem with my bike. More I'm ASKING you IF there is a problem.

Thanks for your suggestions.

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Ah, you need to be more careful in your choices of words.

A YZ is a race bike. As such, it is intended to be run fast, not putted about and is jetted from the factory accordingly. Thing is, most purchasers of these are not A class racers, many buy then for woods riding, so they have the issue. All race bikes behave this way.

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  • 10 months later...

I realize this is a year old post, but I'm having a similar issue, different story though. Bike is running like a top, a little tough to start hot, but not extremely so.

Problem is, as mentioned, when moving from closed throttle to open throttle after the bike has been at idle speeds for more than a second or two it falls flat on it's face, complete stall.

Now before anyone goes into the YZ is a race bike rant, let me explain how I race my YX... Hare Scrambles...

Situation: I'm hauling butt for 15 minutes then come up behind a slower rider. I let off the gas pull in the clutch and coast for a bit as I see a passing opening approaching at this point I open the throttle and drop the clutch and head over the handlebars because the bike stalled when I opened the throttle.

Another situation: traffic jam at the bottom of a hill climb, bunch of hosers stuck on sissy hill with a few rocks. I'm waiting at the bottom and see an alternate line. I go for it, but since the bike was just idling it falls flat again and I have to resort to kicking to start it up.

These ARE race situations and a bike that cannot transition from idle to throttle without stalling, thoroughbred or not is not a working bike.

Soooo, what is the o-ring mod and is that documented here somewhere (I'll do a search) and is the AP screw available from outside the carb? I'm am no fuel delivery expert, so I can't stand these problems. It scares me to death to fiddle with the carb.


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Oring mod is a sticky at the top of the jetting forum

Remove the cover over the throttle wheel (where the cable attach) to get at the timing screw. Keep VERY good notes about any AP timing screw change (how far and what direction)

Leak jet is in the float bowl, you must remove it to change the jet.

That all said, you cannot expect the engine to operate from IDLE to WFO without a bog. You can jet it to not bog at all but you do so at the expense of throttle sensitivity and fuel economy. Some riders simply turn up the idle speed a little as that makes a large difference.

If you are unsure what you are doing, get help or have a pro do it. Not worth screwing up and making things worse.

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Thanks William1.

We are not talking WTFO, we are talking 1/16 to 1/8 of a turn on the throttle, just enough to bring it off idle in normal circumstances... then puff, engine out. However, so long as I keep blipping the throttle constantly it has no problem and it will idle fine. It's a strange sequence of events where I'm riding, stop for a few seconds and then open the throttle, even the tiniest bit it stalls.

This happened yesterday in a big g-out ravine. I rolled to the top of the bowl pulled the clutch coasted into the ravine and rolled on the throttle as I came to the bottom to climb out. The bike died. Fortunately I had enough momentum to drop the clutch and bump start it. so I didn't just roll to a stop on the up-hill side.

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  • 4 weeks later...

Well, so far so good on the cheap fix...

I've run about .25 pint of Seafoam through the engine now. The problem persisted through about half a tank (WR tank so about 2.5 gallons) then went away. On the 2nd tank now still using Seafoam, when this tank is done I'll have put .5 pint through the engine so far so good, I guess the accelerator pump was a bit gummed up from all this high quality ethanol based gasoline we get these days.

I love cheap fixes, I hope this clears it up permanently, perhaps a pint of Seafoam annually to keep things running clean.

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