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FCR Main Jet too small?

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Hi Folks, 2006 DRZ400SM with tt 39fcr.

Was running stock header with pro circuit slip on 165 main pulled hard all the way to redline. Installed full Leo Vince system with no baffles or spark arresters--straight thru. Now when I go full throttle from 3/4 to full it starts breaking up on top as though it is too lean. JD says use 170 main with cams and free flowing exhaust. Should I go 170 or 180? Has 3x3 and KN filter. Engine is stock internally.

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Go for a ride with the airbox door off. If it is better, try a SMALLER main. If it is worse (and I doubt it) try a larger main.

 

Even a 165 should be too rich. A 160 or 158 should be more appropriate.

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Go for a ride with the airbox door off. If it is better, try a SMALLER main. If it is worse (and I doubt it) try a larger main.

 

Even a 165 should be too rich. A 160 or 158 should be more appropriate.

Thanks William, Wouldn't a more free flowing exhaust lean the mixture out at higher rpm? It was running great with a 165 main but now with no other changes than a full exhaust it is going into what feels like a lean misfire at max rpm. I am running the red JD needle on the third slot down from the top also, any effect at WFO? Lastly, and I appreciate your help, might I be banging it in to the rev limiter? It starts stuttering and acting like it is dropping a cylinder, so to speak at the very top of each gear but it didn't do that before the exhaust swap. I'll pop off the airbox door as you suggest next. Thanks for the help.

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Stuttering is usually too rich, not too lean.

Flat on your face bog, is too lean.

If it was fine with the stock header and a slip on muffler and now with a full ex system it is stuttering and misfiring at full throttle it is now too rich? I am sorry but I don't understand how with  a more free flowing exhaust the air/fuel mixture could get richer and not leaner. I am thinking along the lines of my Cuda, when I installed headers I had to go from 68 to 72 on the main jets in the double pumper. I'm talking running fine until the last 7/8s to full throttle that the misfire is occurring, feels like it is running out of fuel/ fuel starvation. I am going to try taping off part of the 3x3 opening, if it runs better wfo then I'll know its too lean, if it gets crappier then I think I am too rich, thanks for your input, let everyone know what happens after my Frankenstein mods. :banghead:  

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Going past 3x3 is not a good idea.

It has been proven by the smartest and brightest to be the best size.

Your jetting is what it is. You best thinking got you here, so why would that thinking solve the problem too?

 

SMALLER MAIN JET

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Sounds like its lean to me since its a 4 stroke . Id go up 10 and try . Clean plug and run wfo and pull clutch shut bike off and look at plug again .

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With the 3 by 3 youll need more fuel to . Not sure if jd jettin is figuring that in ? It prob tells you though .

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This is for the "E" model @ sea level...the higher you go the less Oxygen you have......165 is way too rich unless you have a BB kit.

Also that exhaust is more restrictive so even less airflow for that motor. I would try 155mj and go from there.

 

160MJ Open MRD/SSW or Yosh Ti 
Eddie Mod (Slant only)
Remove CE (Slant only)
45pj
Stock OBDXP 5th clip / EMN needle 3rd clip
Fuel screw 2 to 2.25 turns out
all with the 3X3 hole

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Hey everyone, finally got back to working on my jetting problem after some med issues (diabetes sucks) and followed William1's advice. Pulled the airbox cover off and it ran 100% better. Swapped the 165 main for a 160, reinstalled the airbox cover and voila, it's runs the way it did with the other exhaust, it rips. I still can't figure out how it got too rich with a full system as opposed to a slip on but wtf, I'll take it. Thanks everyone for the advise and help, much appreciated. :ride:

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