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Low RPM Bog, 07 YZ250f

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I have a 2007 YZ250f. When getting hard on the gas at a low RPM the bike coughs and stutters a bit. When I clutch it and raise the revs up, it runs perfect. If I wack the throttle at mid to high RPM it runs perfect. Its only at low RPMs I have the problem.

5000ft elevation

Stock needle, 3rd clip

172 Main

45 Leak

45 Pilot, 1.5 turns out

I tried the Oring mod, it seems to make it even worse.

The bike runs perfectly, except for this one problem. I put on a FMF Power Bomb, is it possible that it took away some low end and is the reason for the bog?

I'm thinking I may need to adjust the AP, but not sure which way to go with it.

Thanks for the help.

Cheers

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Tweak the AP timing screw. 1/2 turn at a time. Try clockwise the first time. Tweak, test ride (Ride, do not just rev standing still, ride normally). Keep notes.

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Turning the screw in about 1.5 turns seemed to help. I'll be going on a long ride tommorrow, and I'll report back if all is well or not.

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Its still bogging coming out of corners. I'm having a hard time clearing some of the jumps because of it. It seemed to get better as the day went on and ruts started forming in the corners, I could carry more corner speed and wasn't down so low in the RPM range.

I guess the next step is to take off the subframe and see when the squirt is happening, if its hitting the slide or not??

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Well, I would first see what size leak jet you have. And I'd take a peak at the squirt to ensure it is a strong.

By tweaking the timing screw, if it was getting better, that is more important than trying to visually set it. You can try turning it more and test riding. If you do not have the oring mod on there, put it on.

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I know the bike has a 45 leak jet, I also put the oring on while out at the track, it also seemed to help a bit. I'm going to check and make sure I'm getting a good squirt. I bought this carb used because the stock one got damaged with an aftermarket fuel screw. My bike has had this bog since I put this used carb on. I usually ride the desert, so the low speed bog didn't bother me too much. Now that I'm going to the track, its much harder to deal with.

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I'm ready to shoot this bike. I'm gettin so sick of working on it just to get rid of the bog. Is it possible that the header is causing this? It has a FMF Power Bomb.

To recap;

172 main

Stock needle, 3rd clip

These are the way the bike came stock, It didn't used to bog

O-Ring Mod

45 Leak Jet

Fuel screw 1 3/4 turns out

5000 ft elevation

When going full throttle at low speed, the bike bogs for a second, then picks up and runs great after that.

I checked the AP squirt. I timed it to just miss the slide, the squirt is nice and strong.

I have moved the AP screw two turns in both directions, 1/2 turn at a time, the bog still exists at all adjustments.

Is it possible that becasue of my altitude, the leak jet is too small? They are hard to change, so I've been reluctant to mess with it. It had this same bog when the stock 80 leak was in the carb.

I have a race in a week and would really like to get this taken care of. The bike is rideable, but I know it can be better.

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If it has a lowend bog,does the fuel screw have anything to do with it,I have a 2009 KX250F that has a bog and I raised the pilot from a #40 to a #42,seems like it helped but didn't completely take it out,has a #55 leak jet,I also adjusted the AP timing.

How reliable are those aftermarket fuel screws?

Can you get the bog out completely on the stand?

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Tiny bit but... the fuel screw and pilot jet are for idle. You set them up for that then leave them alone.

Did you do the oring mod or stiffer AP spring? You also may want to try a smaller leak jet like a #40.

You can elminate the bog when the bike is on the stand though I wonder why having it gone when mindlessly reving the bike in the garage is what you want when riding is more important. Not trying to be too much of a wise A** but realistically, the stand/garage performance means nothing. Test through normal riding. Do not ride it with the intention of making the bike bog.

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Question? When cracking the throttle from around 1/8-to-1/4 and then holding it there does the motor rev out and want to carry a higher RPM than it should for that throttle position?

Question William1, he stated the "bike coughs and stutters a bit at a low RPM" and also stated that the bike seem to run better later in the day (warmer I assume) than it did in the earlier part of the day (cooler I assume). That sounds lean to me. Cooler temp. (more air / less fuel) = more issue > then < warmer temp (less air / more fuel) = less issue.

If he tries to richen up the 1/8-to-1/4 throttle position with the leak jet alone will it richen the rest of the throttle to much and create issues then where they don't exist now? I would have thought to change the pilot jet first to get the "coughs and stutters" out then moved to the AP/leak jet for fine tunning to get any bogs out.

Just curious, still trying to figure these four-stroke carbs. out. Good talking to you again, William1!!

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If you are on flat land and hold a small throttle opening, you'll be surprised how fast the engine can run. Throttle position and RPM are load dependant. Come to a hill and the speed drops (applying load).

When setting up a carb, you must get the main jetting right first (pilot/fuel screw, needle and main). Then you work on the AP. These are completely seperate sections of the carb and must be adjusted as such.

Steady state running is supplied by the jetting, acceleration is assisted by the AP.

A cough/stutter could be rich jetting. If so, it would get worse as the engine and air heats up. A lean situation is a dry flat bog (typical issue with the AP). But you must ensure your base jetting is right before you mess with the AP. Many guys try to resolce an acceleration bog by screwing with the jetting only to have another issue appear.

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I didn't get to ride it Sunday, I'm riding tomorrow so I'll report back then. I've never tuned it on the stand, I always make a change, then ride it for awhile to see how it did. The bike is very rideable after adjusting the accelerator pump, I'm just trying to get it perfect now. My friends say I'm too picky. My next bike will have fuel injection, I understand that much better than carbs. This is the first carb I've messed with.

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I'm having a very similar problem except its at the whole power band. from inital throttle to WOT and cruising at any speed. it was there since the day i got the bike new but a little worse now.

07 yz450f

I'm at 2000 - 3000 ft. around 55-65 degrees here now. I first installed a twin air air filter. no change. installed 160 main (165 stock) and needle at same position from dynojet as per their instructions of having aftermarket exhaust. a little worse. i just put on my yoshimura rs-2 slip on. no change. Fuel screw just makes it pop a LOT more at about 1 turn. 2 turns out, same running condition just less popping. I'm thinking main is rich but it seems to be popping a little more than it should. it also seems like those coughs or bogs i'm experiencing is from a pop. so i dunno rich or lean? main jet? needle position? I just bought a #48 pilot jet (have a 45 stock) after reading on this issue a lot but now i'm not sure that will do anything for me. it gets pretty hot here in the summer months so i'm thinking it'll be a bit rich around that time

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Im at 5000ft also and I am having a heck of a time getting rid of low rpm bog on both my crf250x and crf450x. havent messed with the 250 much but the 450 is completely opened up, 170 main, 3rd pos needle, 48 pilot, 1 3/4 turns out

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