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Jetting Question.

7 posts in this topic

I have a 2000 426 that did the same thing. I set the fuel screw at 2.5 turns (stock was 1.75, I think). This cured the popping.

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Id put the pipe on before you do to much jetting because it may change w/ the e-series pipe.

Nick

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To add a little to what the previous two guys posted, you might want to make sure that you don't have an air leak between the air box and cylinder head. My 2000 426 had an air leak at the little adaptor where it bolts to the inlet of the carb. There are several different places in the intake where an air leak can develop. No amount of jetting will cure this, plus, if there is a leak, dirty air gets sucked into the engine.

Also, the accelerator pump diaphram bowl seems to accumlate dirt at an alarming rate. My float bowl also had a lot debris in it when I took the carb apart for cleaning. A good cleaning of the jets with carb cleaner would be a good place to start when you do your jetting. If dirt is caught in any of your jets, you'll get a lean condition. A dirty pilot jet will cause leanness at idle and right off idle. A dirty main jet will cause leanness at 3/4 throttle to wide open. If the accelerator pump nozzle is dirty, you would get poor response when you whack the throttle open.

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I don't want to bother anyone but I have a jetting question. I couldn't find any info about the 99 in the midwest.

I have a '99 YZ400F and it runs real lean. Backfiring and popping when rolling off the throttle to slight throttle. I can run with the choke on it is not a problem. I checked the plug and it is reading lean.

I am in Missouri, Stock engine and pipe configuration, pumped gas(92 octane), opened up airbox(but does have the flap behind the tank installed for drainage off the tank, not sure this is relavent or not), Aftermarket Air Filter. The carb hasn't been touch. So I am wondering how much on the jetting should I go. I also have a WB E Series Pipe. But not installed yet. I want to make sure that the bike is jetted properly for the stocker before I go after the WB set-up.

Thanks in Advance.

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Scott #431

Some Fear Racing "Cause if you don't have any you ain't going fast enough"

'99 YZ400F(Coming to a Theater near you soon)

'92 ZX-7R and '97 ZX-7RR

"Doesn't hurt till the bone is exposed"

"When cut do you bleed green? I do!"

Pics

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Originally posted by scottzx7rr:

I have a '99 YZ400F and it runs real lean. Backfiring and popping when rolling off the throttle to slight throttle. I can run with the choke on it is not a problem.

It sounds like you are lean on pilot and idle. Try richening your idle mix ¼ turn at a time (that is a significant amount). You may want to verify stock position by screwing it all the way in first and counting the turns, it should be 1.75 turns out. Make sure your bike is fully warmed up before deciding on the effectiveness of any jetting changes.

The idle mix or “pilot screw” is accessed via a recessed hole in the float bowl and requires a small flat-blade screwdriver. The problem is finding one short enough to fit between the bowl and cases, I just detach the metal bit-driver portion of one of those cheap screwdrivers and mark it with something so I can count the turns, out is richer, in is leaner.

If you get past 2.5 turns out then you should probably try a richer pilot (#48 is one step richer for your ’99) and start over at 1.75 turns on the idle mix. You need to remove the float bowl to change the pilot jet, it is next to the main jet and is slotted for a screwdriver (the main is hex-shaped).

There are many ways to skin this cat, you may want to go the minimalist approach and not remove anything, loosening only the two clamps on either side of the carb and rotating the carb clockwise to enable the fuel bowl to be removed from the left side of the bike. I think this is too difficult, especially without removing the throttle cables.

The opposite extreme is remove coil, throttle cables, clamps, seat, subframe and then carb. This may take longer but is actually easier.

Middle of the road method is leave the subframe on, this is easiest but reinstalling carb between the air boot and manifold and routing the carb vent lines are made harder this way. Removing the coil makes disconnecting the throttle cables much easier (you don’t have to remove the spark plug boot).

Hope this is what you were looking for. For specific jetting suggestions you may try another post to the WR side with your symptoms, elevation, climate etc. A lot of the WR guys have YZ timing and jetting on their bikes.

I’m betting a richer pilot will really help your low rpm response, especially in colder weather. FWIW my less restrictive PC pipe tended to lean things up a bit down low, so the WB may exacerbate your lean spot.

In addition to what Boit said a leak at your header will also cause the popping you describe.

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this method works well, too. take a dental mirror w/ a very small flathesd screwdriver and some good lighting. it takes a minute for your eyes to adjust to this, but works well.

TMX AND MY DOGS BITE!!!!!!!!!!!!!!

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Thanks for the advice. I can rule out the air leak theory. I had checked that seeing how the motor was recently rebuilt and wanted to rule it out before any jetting changes.

I would normally agree with you there NPUZA about putting the WB on then make the jetting changes. But being a former roadracer I tend to handle my dirtbikes the same. I want to have the stock baseline settings to compare to when I make changes. I want to make sure that it is running properly stock then I have notes to compare back to.

I will check the pilot screw and the accelerator pump diaphram also tonight.

------------------

Scott #431

Some Fear Racing "Cause if you don't have any you ain't going fast enough"

'99 YZ400F(Coming to a Theater near you soon)

'92 ZX-7R and '97 ZX-7RR

"Doesn't hurt till the bone is exposed"

"When cut do you bleed green? I do!"

Pics

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