Main jet size question

Looking at the jetting tables I can't help but notice that most people have changed the standard main jet (#165) for a #170 or #172 on their '99/'00 WR's even though they haven't changed their exhaust or removed the airbox lid.

My '00 WR (without airbox snorkle, otherwise totally standard) has a problem where it will sometimes cough and sputter on hard accelleration when the weather is cold (below 10 C) and humid. If I don't ease off the throttle it will die with a lot of backfires and black puffs of smoke.

Could changing my #165 main jet for a #170 have any effect on this? Or could it be something else entirely? :)

I'm totally in the blue about carbs and jetting so any suggestions are welcome

.david

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David Reksten, Oslo, Norway

2000 Yamaha WR400F

1998 Aprilia RS250

Originally posted by David Reksten:

Looking at the jetting tables I can't help but notice that most people have changed the standard main jet (#165) for a #170 or #172 on their '99/'00 WR's even though they haven't changed their exhaust or removed the airbox lid.

My '00 WR (without airbox snorkle, otherwise totally standard) has a problem where it will sometimes cough and sputter on hard accelleration when the weather is cold (below 10 C) and humid. If I don't ease off the throttle it will die with a lot of backfires and black puffs of smoke.

Could changing my #165 main jet for a #170 have any effect on this? Or could it be something else entirely? :)

I'm totally in the blue about carbs and jetting so any suggestions are welcome

.david

Don't bother what the 99's are using since they use a different carb. My Y2K WR400 is c

jetted with a 168 main and a 45 pilot jet. The service rep for Yamaha Canada told me that this jetting would solve most of the problems I was having and so far he has been right.

Willy

Originally posted by Willy:

Don't bother what the 99's are using since they use a different carb. My Y2K WR400 is c

jetted with a 168 main and a 45 pilot jet. The service rep for Yamaha Canada told me that this jetting would solve most of the problems I was having and so far he has been right.

Willy

Thanks for the info about the '99 carb.

Could I just ask what kind of problems you were experiencing before rejetting?

My dealer here in Oslo isn't very helpful, giving me a lot of mumbo-jumbo when asked about jetting. Things like "it's supposed to be like that 'cause it's a race engine, you know." :)

.david

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David Reksten, Oslo, Norway

2000 Yamaha WR400F

1998 Aprilia RS250

Yep, ignore the 99 specs. Let's face reality, the bikes come lean from the factory. Jetting richer is almost mandatory. Cold , dense airr has more oxygen than the same volume of warn air, thus more fuel is needed to burn at the optimum appx 14-14.5:1 mixture. Higher elevation offsets this as the air is "thinner" (less oxygen). Your popping confirms a lean condition, at least at low thottle when you ease off the gas, but the black smoke is almost like the gas isn't being burned properly (tto much gas). I had the same problem except the black smoke, but it was my clip position that solved it. (along with 45PJ/172MJ). I wonder if your TPS isn't adjusted correctly?

Try some basic stuff first-make sure the carb is sealed tight, all carb hoses clean, fresh plug, etc. But I think you will find like so many others that jetting richer not only fixes some bugs, but increases performance as well.

Also you have to roll the throtle on with this bike, are you just going from 0-full throttle too fast?

Since different jets control varying throttle postions, you have to feel at what throttle position is doing what. Bogging at low throttle positon (not necessarily RPM's) during hard acceleration has a different cure than bogging at 3/4 or full throttle.

I rejetted the bike almost immediately after getting the bike. The plug reading was very lean after having it out the first time. Since the changes I have not noticed any of the symptoms you are discussing.

Willy

Originally posted by mcarp:

I had the same problem except the black smoke, but it was my clip position that solved it. (along with 45PJ/172MJ).

(...)

Since different jets control varying throttle postions, you have to feel at what throttle position is doing what. Bogging at low throttle positon (not necessarily RPM's) during hard acceleration has a different cure than bogging at 3/4 or full throttle.

Which clip position are you currently using?

My bike will cough and sputter if I accellerate hard up through the gears. The problems seem appear from about 3/4 throttle and up - which I suppose could be a combination of main jet and clip position?

.david

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David Reksten, Oslo, Norway

2000 Yamaha WR400F

1998 Aprilia RS250

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