I just bought the k/n power kit and fmf megamax II.
I put the 155 main jet in as per instructions for under 5000 feet.
I am running all 11 discs and no quiet core in the pipe.
It idles fine, low end torque seems to be great, but when i'm on it...like really widing it out there's a miss.
It happens i'd say in 3rd-5th gears.
I looked at the plug reading and it seemed fine.
I know that at full throttle the main jet is responsible but i cant see this being the problem because that's what it said in the instructions.

By the way it's a 2001 wolverine.
Thanks[img]i/expressions/face-icon-small-confused.gif[/img]

 

Jetting has no specifics. I could have the exact same setup as you at the same location and still be off. Just because it says 155 main, doesn't mean that is the right jet, it should be close though. If the plug reads fine then it may be electrical.

 

I sure sounds like the main jet is too big.

Side note: I've read countless "expert" answers (mags) stating "it doesn't matter what gear you're in, it just matters what position your throttle is at." i.e., if it isn't running good at full throttle, then it's your main jet. My experience, as well as many others on this forum is that it does somewhat matter, because when you're in higher gears, the engine has to work harder. Although the above is technically correct, the experts fail to realize the latter.

When I'm testing my jetting (especially the main jet), 1st gear is of little use to me. It's too easy for the quad to rev out without giving me any indication of jetting abnormalities. I've found the best way to check the main jetting is to put the quad in 2nd gear (400ex) and, while lugging it, wick the throttle wide open and hold it full throttle. It works especially well to do this under conditions that make the engine work as hard as possible, i.e., climbing a hill or coming out of a corner. Since you have the throttle wide open throughout the entire rpm range, any engine hesitation or stumbling is a direct result of the main jet - that is, with a mildly modified quad - throw different piston, cam and its a bit more complex (testing other carb curcuits requires that you hold the throttle at a different position.).

Anyway, a main jet that is too big will give the symptoms you described - stumbling, bubbering (kinda acts like your running out of gas). A main jet that is too lean will give you more of a "power falling off" feeling.

If I were in your position, I would drop the main jet down one size and go test it. If it still bubbers, drop the main jet down one more size. If, however, the stumble is gone, then that is the correct size (some drop the main jet size down one more - but that's it, or else you're at risk by running the machine too lean).

By the way, I assume that when you say the engine "misses," you mean it stumbles, bubbers (then maybe pulls out of it). I certainly don't want to lead you down the wrong path by saying for certain that the main jet is too rich - because your definition of "miss" may be actually too lean of a main jet.


P.S. - I've heard that plug reading is becoming increasingly difficult, given all the new additives in fuel today and longer life plugs.

ogre

 

I just rode the bike today for a good while...a better way to describe it is more of a hesitation. It happens at full throttle more noticible in higher gear or when the engine is working harder. It's like it cuts out.
It's hard to explain.
But i will try the 150 jet and hopefully that will cure the problem.
Thanks for the info.

 

"Cutting out" is a good description of the main jet being too big.

I've kinda noticed, after doing extensive jetting, that it's pretty linear. That is, if it just cuts out a little as you go up the rpm range, the main jet is pretty close to "right." On the flip side, if it refuses to accelerate at all (like your running out of gas), then you'll usually have to drop quite a few sizes. (Sorry if this just makes light of the obvious).

Anyway, I'm not sure if the next size down from a 155 is a 150, maybe there's a 152. I would guess, however, that the 150 will eliminate the problem (if it's just a slight hesitation) without the machine getting leaned out too much (fourstrokes are pretty forgiving) - if indeed there is a main jet between 155 and 150.

Let us know how it goes.

ogre

 

Well,
I stopped at the yamaha dealer and picked up a 152.5 main jet and installed that today also while i was at it i back the air screw out a quarter or turn and took it for a drive.
It runs great now with out the hesitations or cut-outs.
I wound it clear out in fifth and it didnt stutter once.
I probably could have gotten away with the 150 but for $2.13 i bought the 152.5
Thanks for all the input.
Also, I got my ram mount for my etrex Gps and installed that....it's really a great piece along with my 'water-resistant' 12volt outlet i'm ready to hit the trails hardcore and toast my buddy's blaster.

[img]i/expressions/face-icon-small-cool.gif[/img]

 

Sounds good.

If you have the K&N kit that includes the dynojet jetting kit, many have said that dynojet main jets actually have a different sizing system. Therefore, the jet from dynojet might not translate to the same size as the one from the dealership (reason 107 why I don't do business with dynojet).


If that 155 main was from dynojet, you might want to go to the dealership and pick up a standard 155 main - throw it in, and see if it stumbles like before. It might be cheap insurance. Do not call dynojet to see if they're actually comparable, they will only try to confuse you with smoke and mirrors then try and sell you "their" 152.5 main jet (reason 108 I don't do business with dynojet).

Again, this has kinda been an on-going discussion on this board, with most claiming that the sizing system is different. Take it for what its worth.

I just don't want your machine running too lean with the standard 152.5 jet. Maybe the dynojet 155 main actually translates to a standard 160 main. See my point?

Hope this helps,
ogre

 

well, i'm going to run it for a bit and get a plug reading(well try to atleast). I dont really feel like pulling the carb again.
But i'll go from there.
Thanks for your help, ogre.

 

Fella's-

Good info on the jet stuff. I'm finding that 1-3 hears my 400ex pulls hard and 4&5th gears, it stops powering... Maybe I'm running too rich.

I'm using the dynojets- and I called them company when I first installed everything. What they told me is that you can't do plug readings because of the additives they use in fuel will give you false readings. Also they mentioned a jetting test:

Tape your filter 1/3 of the way up and run the machine-.. if you get an increase in power- your jet size is too small.

Now run the machine WITH-OUT the filter- if you get a power increase- you are running too BIG on the jet size.

If you get NO power increase with either of the tests- You've hit the sweet spot.


-also- (Mr. Wolverine)

You can change the jets WITH-OUT taking the carb off. Just loosen your clamp to the airbox and TWIST your carb so the jet faces you. You should be able to change your jet in less than a minute.

 

RotGutBottom -

(Reason 109 I don't do business with dynojet, bad jetting advice!)

Actually, that will kinda work, in the sense that it'll get you close. But its way too much work!

Sorry to hear you purchased from dynojet (not your fault), I too have been down that path.

The only thing you really get from them is a different needle. The main jets are the same as you would get from a dealer (with possible size discrepancies - addressed above). Some say the dynojet needle provides you with more power, I never saw any (except maybe imaginary power - it's gotta have more power, I just spent $60!). I've heard numerous people say that the needle dimensions only really come into play with extremely modified engines - but I have no experience to back those claims.

As for your jetting, You can do it one of two ways: first, if you have enough dynojet mains - use them. Else, if you don't have the proper range of dynojet mains (after testing) - go buy a bunch of mains from the dealership.

Don't get into the infinite loop of running to dynojet for mains. Since their sizing is (possibly) different, they know you'll come back to them in order for you to keep the jetting linear in your head.

Trust me, if you're anything like me, the next upgrade to your machine may be sooner than you think. It's MUCH easier to run to the dealer for jets then to call dynojet.

Back on subject: First, if the dynojet needle has 5 clip positions (middle position being stock), I would lower the clip one notch (raises the needle a little to richen midrange). Then, start with the biggest dynojet main you got with the kit. Ride it the way I described above (2nd gear lug/full throttle).

***By the way, ride it a little to warm engine first before testing (cold engines like a richer mixture, so it may run fine the first 5 minutes).***

Basically, you want to make it run rich (by using largest dynojet main). If, with the largest dynojet main in, it still doesn't cough and sputter, run to the dealership and get like a 155, 157, 160, 162, 165, and 167. Then try the 167. If that doesn't cause it to cough and sputter, then go get a 170, 172... You get the idea. Once you get it to cough and sputter(whether a dynojet main or reg. main), you know you're too rich.

Continue to jet by installing one size smaller main jet, and test. Do this until the coughing and sputtering ceases (some drop one more size down after it stops sputtering). You now have the correct main jet.

As for the needle adjustment, lots of people just raise needle one position and leave it. You can attempt to plug read (hold the throttle 1/2 open for 1/2 mile or so, then cut engine and read the plug). I've had very little success getting a good reading by doing this (dynojet was right about that). Anyway, most people with just a pipe/filter raise needle one clip position, and have good results. If however, you find that it coughs and sputters at mid throttle with the needle raised one notch, lower it one notch (it's pretty easy to tell when the needle curcuit is too rich).

On the pilot jet, you usually don't need to get a bigger size with your mods. You might want to turn the air/fuel mixture screw (fine adjustment for pilot jet) out 1/2 turn though (easier starting, even with stock machines). Its located directly under the carb, just in front of the float bowl.

Take this guide for what it is, a guide. I gave you jet sizes that I would buy, given your mods and MY elevation (1100-1400 ft.).

By the way, when you say "it stops powering," I can't be sure if that's a overly rich or overly lean condition. Check my descriptions above for a sense of what you're dealing with. If in doubt, go richer, that way you won't run the risk of being overly lean. If it acts worse, then you know you were too rich in the first place.

Oh, by the way, its pretty easy to leave the carb in place on the 400ex while jetting. I just shut off fuel, use screwdriver to open float bowl drain (working on the left side of the quad), remove breather hose connected to engine case, use driver with 17mm? socket to remove main jet access on bottom of carb float bowl, use 1/4"? socket to remove main jet - then do the reverse for installation.

Edit: Never mind, if you're using dynojet mains, I think they have a slotted head, not a hex head. You would need to turn your carb over to gain access using a screwdriver (reason 110 I don't do business with dynojet, slotted heads on their main jets makes jetting harder!).

By the way, how do you like the 250ex? I also bought one for my wife, she really likes it. I enjoy riding it too (probably way too much!).

ogre