Not your average bog.....

[YZ426FPilot]

Ever since I got my 02 YZ426 there has been a pesky off idle bog that I cant seem to get rid of. Now before you write this off as another one of those old Yamabog issues, hear me out.

First of all here are the specs:

Stock motor & headpipe

ESeries S bend

Unifilter

Stock Needle, clip in middle

168 main

42 pilot

Zipty Fuel Screw

BK mod

the "NEW" Powernow

everything else on the carb is stock

Conditions:

70-90 degrees/ 1000-3500ft elv

Heres the story:

I rode it a few times before the powernow and it was never crisp as you first cracked the throttle. I added the powernow and the condition seemed get worse. If I just went from closed throttle to over 1/4 swiftly the bog was hardly noticeable. But when I got on tight trails and had to work the clutch & throttle (just barely cracking it rapidly, just to keep it from stalling) the engine would seem to load up or gasp and try to die. As soon as I stopped it would be fine and continue running. It will do this in neutral as well as lugging at low RPMs in gear. Bumping the idle up would help, just because revvs would be higher as I chopped the throttle & got back on it again. The condition seems to worsen as the engine gets hotter. This is driving me nuts, nothing has worked. Ive done the BK mod, AC pump works fine, gone from a 40PJ up to a 48 in all the condtions I listed above, still no change. I adjust the fuel screw correctly (Idle high & turn for peak RPM) but nothing can get rid of that bog. It runs great everywhere else. Yes my filter is always clean. All this usually happens after you let the bike warm up and ride it around for about 10 minutes.

Has anyone else had this problem?

[sirthumpalot]

Have you cleaned the carb out yet?

[TonyDeCoster]

Make sure the carb is spotless. Inspect the float valve O-ring while you're in there. It's a good idea to replace it as they do tend to lose some sealing ability once they get older. Set the float height correctly after reinstalling the float valve. Make sure the air jets and passages are clean in the throat of the carb. They affect the running big time if they're even partially plugged. Verify the AP circuit is working and is spotless. Remove the colars that prevent the carb clamps from being overtightened, two on the intake manifold, one on the airboot to eliminate airleaks in those areas. Pull the petcock while you're at it and check for proper flow, dirt, etc. Make sure the carb vents are open, not pinched anywhere. What is the duration of the AP spray? You might want to recheck that. And make sure the pump diaphram is good, no pinholes etc. Have you checked the valve clearence recently? They tend to tighten up. As for spark-related issues, make sure you have a good resistor-type plug. Iridiums work well in the 426. Unplug the neutral switch harness under the tank and tape the wires off for better spark when starting. Check the plug cap with an ohm meter once the bike is hot. You shouldn't have any more resistance than what the manual recommends ( 4K?)Do you have the 450 cam mod? If so, verify the timing is correct. If you have the original cam, also check the cam timing and make sure the decomp cable is not too tight by turning the bars lock-to-lock and checking for proper cable slack. That's a lot to check but these bikes are all about the details...

[crf450f]

Sounds like when the throddle is cracked open, the fuel is becomming very lean.

Make sure the accelerator pump is working, located in the carberator. If you do not have a manual, get one!!! It will help a lot, it contains diagnostic procedures that will aid in getting that monster running again.

[YZ426FPilot]

Thanks for the suggestions. I think the first thing I'm going to do give the carb a good cleaning. Valves did cross my mind, but from what Ive seen with tight valves on these motors is that they will not even idle with out the choke on. And it does not take long for the condition to reach that point after you notice the first signs. This had been steady and I'm pretty due its a too much or lack of fuel issue. Or dirty carb. The AC pump squirt is about .4 sec, that might be too lean. Stock is about 1.3 on the '02.

I'll check all that other stuff as I go through it.

Thanks

[Satch0922]

I gave mine a good cleaning and the intermittent cough and bog went away. I knew that was causing it because it just started doing it a few weeks ago. Sure enough after the cleaning it fired up first kick and barks like no other when you stab the throttle.

[Fastest1]

.4 of a second should be enough of a squirt. I suspect a dirty carb. When cleaning the carb DO NOT REMOVE THE SCREWS WITH THE YELLOW PAINT ON THEM! The special O ring is not available and you wont be able to make one either.

[fatmancantfly]

I'm surprized no one has mentioned the accelerator pump timing screw. It's the one at the top/forward side of the accel pump lever. Make sure you do everything else suggested first. That screw times when your fuel pump starts spraying. Make 1/8 turns or less when you adjust it. A small adjustment makes a big difference on the "start" of the fuel spray. If you have too adjust it very much. Check your fuel spray duration. It can change it depending on which way you had to move the screw. Your manual should give you a good starting point but you'll have to have the bike running to get it just right.

[YZ426FPilot]

I have adjusted this. The timing of the AC pump was actually my first guess as to what was happening. The squirt would clear the slide if you just snapped open the throttle. One thing I noticed is that of you opened and closed the trottle rather rapidly not exceeding 1/8 throttle the squirt would miss the front of the slide but hit the far side & gas would puddle up. It took almost 2 turns in before I could get a change & 3 before it missed all together. It will still just miss the slide if you stab the throttle. After I did all this, the condtion did not change.

I think im going to undo all of the changes. Clean the carb, and start over.

[Indy_WR450]

#40 leak jet will fix your problem! ? That is why it was worse after you put in the power now. Too lean a squirt. Not duration will cause the bog. #40 leak jet will give you a larger volume of gas and eliminate the dreaded bog.

[skthom2320]

#40 leak jet will fix your problem! ? That is why it was worse after you put in the power now. Too lean a squirt. Not duration will cause the bog. #40 leak jet will give you a larger volume of gas and eliminate the dreaded bog.

I don't think the '02 YZF had a leak jet.

[skthom2320]

I have adjusted this. The timing of the AC pump was actually my first guess as to what was happening. The squirt would clear the slide if you just snapped open the throttle. One thing I noticed is that of you opened and closed the trottle rather rapidly not exceeding 1/8 throttle the squirt would miss the front of the slide but hit the far side & gas would puddle up. It took almost 2 turns in before I could get a change & 3 before it missed all together. It will still just miss the slide if you stab the throttle. After I did all this, the condtion did not change.

I think im going to undo all of the changes. Clean the carb, and start over.

Here's some additional comments:

1. As mentioned by others, you may need to clean the carb. But I would try the things I mention below first to save yourself some time.

2. Don't discount the AP timing screw. From your comments above it sounds as if you set it on the bench. I did the same thing but found that I had to reset it according to the seat of my pants at the track. Take the cover off and have the bike running. Go do a lap. Turn it an 1/8 turn and do another lap. Continue until it gets better or worse. If it gets better. Good. If it gets worse, try the other direction.

3. Your pilot may be clogged with a speck of dust. I've had that happen and had it screw up the jetting. It's simple enough to pull this, clean it, then put it back. Maybe a 2 minute job.

4. Your pilot jet may be too rich. I've adjusted the fuel screw as you mentioned and "thought" I had the best spot. But still had an off-idle bog. Guess what? A 40 pilot jet cured it. Once again, a 2 minute job.

If none of those things work (about 15-20 minutes worth of work), THEN pull your carb and clean it.

[TonyDeCoster]

What he said :jawdrop:The '02 YZ426 doesn't use a leak jet...

[fatmancantfly]

Do as Skthom2320 says. The only way to adjust the AP correctly is to have the bike running. I don't do it at the track though. I do it on the stand with a fan blowing through the radiators. The 525 I had was the only bike that came from the factory properly adjusted. The 450sx was the worst one. It still only took a very few minutes to adjust the accel pump to where it never bogged. That is as long as you have the right pilot installed. Usually the stock one works best on the newer models.