YZ250F carb problems

MXracn746

MXracn746

PR Addict
When I crack open the throttle, regardless of RPM, it boggs bad. Probably, to the point that if I didn't let off the throttle it would die. Slow increase in throttle no problem. Also, at idle the pipe glows orange at the header, for about 3 inches. Also, it seems to pop more than normal when decellerating. I think i got more than one adjustment out. Just want to know what direction to go. Tried raising the clip on the needle with no help. Also valves are all checked and adjusted. Also the fuel screw adjusted to 2 1/2 out. Idles good. Any ideas? Thanks guys!
 
You should start with a good cleaning of all the carb circuits. Pay close attention to the pilot jet, and its circuit. When carb is apart you should be able to spray carb cleaner into the pilot hole and have fluid come out of the air mixture screw hole. You should be able to hold the jet itself pointed at a light and see through it if not replace the pilot jet with one the same size or one size larger(should be a number 40-42 jet). Also pay attention to the jet in the float bowl and the passages that feed fuel to the accelerator pump. If after that bog and backfire are still there look for air leaks at the manifold mounted to the cylinder head.
 
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Maybe check where your pipe comes together also. MXA just ran this Tech Tip. Probably won't hurt to look at it?
 
Well, the pilot jet was actually clogged, couldn't even see through it. Got a new one, one size bigger (45 from a 42) putting it in later tonight, Gonna try it tomorrow. Thanks guys!
 
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Well, the bigger pilot didn't really help, so I put the cleaned up original 42 back in. Cleaned up the carb real good. Have the clip up one from original position on needle. Fuel screw out 2 1/2 turns. No more popping or glowing exhaust. Idles good, starts good. Up to 1/3 or so throttle is good. But, Try to twist it anywhere past 1/4~1/3 throttle, and it just falls flat on it's face, boggs regardless of RPM I'm at. In fact if I just hold the throttle there, the bike will die. went up from a 178, to a 180 Main - no help. Suggest Mabey either I didn't either go high enough on the Main, or need to go to a 175 or 170? Mabey drop the clip on the needle back down one to it's stock position. The intake boots are all good, no leaks. Can squirt WD40 on them at idle - no change. Any ideas?
 
Have you tried riding the bike yet? Does it bog under load(while riding)? I have seen bikes that bog while being revved real fast on the stand but are fine while under load. If an issue while riding I would look at the accelerator pump circuit of the carb maybe make a leak jet change? Did this issue pop up all of a sudden or did you notice it slowly? You can also check by where the throttle cables connect to the carb make sure that the plastic accelerator pump lever is moving the rod when the throttle is opened.
 
Oh, and yes, it does it under load while riding. Get into the upper RPMs though, and not as much as the low to mid range. It actually starts at about 1/4 throttle. Just starts dying, like someone is holding in the kill switch.
 
How old is your gas? Try running some vp, cam2 or something better than pump gas to see if it clears up.
 
if it was bad gas it wouldn't run at all I don't think. Sounds like a jetting nightmare, similar to what my 144 was giving me last year.
 
From everything I've read and gathered so far, I'm leaning towards the accelerator pump. Putting the jetting back to the way it was, and going in that direction tonight.
 
When you cleaned the carb, did you remove the pilot screw? If you did, did you keep track of the tiny flat washer and o-ring that seals the tip of the pilot screw? If that o-ring is deformed / torn / gone, the bike won't run worth a crap. Proper assembly order is screw / spring / washer / oring. If the washer and o-ring are reversed, the spring will chew the o-ring up.

Acc pump is a good idea. Mine died on an old 426. AP failures usually manifest at quick throttle openings though. One quick way to check is to pull the carb, reconnect the fuel line and throttle cables, then open the throttle quickly. A small but continuous stream of fuel should squirt out of the AP outlet into the carb body.
 
baxterj787 said:
When you cleaned the carb, did you remove the pilot screw? If you did, did you keep track of the tiny flat washer and o-ring that seals the tip of the pilot screw? If that o-ring is deformed / torn / gone, the bike won't run worth a crap. Proper assembly order is screw / spring / washer / oring. If the washer and o-ring are reversed, the spring will chew the o-ring up.

Acc pump is a good idea. Mine died on an old 426. AP failures usually manifest at quick throttle openings though. One quick way to check is to pull the carb, reconnect the fuel line and throttle cables, then open the throttle quickly. A small but continuous stream of fuel should squirt out of the AP outlet into the carb body.
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Yes I did check the oring, washer order, twice. I'll check again, got it off the bike. I'll check the squirt. Thanks Jason.
 
Another idea I had: how about the o-ring around the float seat? I fought my Yamaha Virago for over a month with similar symptoms (would die above 1/4 throttle) because fuel was leaking around the seat of the float valve.

Last ditch attempt: unplug the TPS sensor (White plastic connector with 3 pins) and see what happens. When the TPS dies the ignition goes to a really strange advance curve above idle. The TPS is a potentiometer that allows a DC voltage signal to change as throttle position changes. Sometimes the potentiometer develops a dead spot.

Good luck.
 
Well, I put all the jetting back to stock for an 09 YZ250F, went and did the O-ring mod on the accel pump arm, (all you do is put a #78 oring around the arm, and the AP timing screw, behind the screw, so they both turn together) and wa-la, and runs great. Researched this a lot, and there's a lot more you can do, depending on how far you want to get into it. Guys on CRFsonly.com had you filing down the rivet on the diaphragm, going to the Boyesen Quickshot, adjusting the timing screw, going to larger leak jets, among other stuff, but basically this worked and it was easy.
 
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