Old vet rider, new to site, have a carb issue on for my 07 yz450f.


i just installed a two brothers m-7 slip-on, a dyno jet kit- installed just per the instructions, a uni filter and a fuel/air screw. it starts cold 1st or second kick, starts on 1st kick when warmed. it starts great just like the day i brought it home, stock.

my small issue now is; from idle if i snap the gas to rev it, it'll stall sometimes now, didn't do it stock at all. i can tap the gas and make it short rev over and over, but if i turn the throttle deep/ fast, it'll stall.

also motoring around the trails or yard in 1st gear, i notice the rpm's falling off a tad even with my hand steady on the throttle, it won't stall while rolling down low, it just doesn't seem to like motoring at a set rpm low below 1/8th throttle without me working it. it idle's fine and constant, runs perfectly eveywhere in any gear other then this stumble down low.

any help would be appreciated,


huntsville, alabama

Mixture screw adjustment would be my first guess.

What did the jet kit consist of? That bike shouldn't have needed much (if any) in the way of jetting changes for a slip on.

I wouldn't have thought so, either, and I usually have a reflexively negative view of "jet kits" in general. I much prefer to work with OEM jetting one step at a time, correcting whatever might actually be necessary.

I'd go back to stock and start over on that track.

Welcome to the site!

Tweak the mixture screw some.

The kit instructed me to do a main jet, a needle change but to the same groove 4th slot, lastly turn the fuel screw to 1.5 out. So, two parts really......

  1. The upper straight section of the needle has a great deal of influence on the transition from the pilot circuit to the main circuit, which is where your bog is happening.
  2. The bike didn't do it before the kit

Although we have no way of knowing whether you have the pilot screw set correctly (read: http://www.thumperfaq.com/jetting.htm#PJ and: http://www.thumpertalk.com/forum/showthread.php?t=327405 ), my first step would be to return to the OEM needle. In almost all cases, it works fine.

As to the main jet, $5 says they wanted you to go bigger, right?


took your recommendation, put the stock needle back in, left it in the 4th slot as it was when i removed it. followed the link you supplied, rotating the fuel screw in while idling the rpm drops off a tad, i catch it before it stalls and rotate it back to between 1 1/2 and 2 turns out, seems to be happiest around there. while i had the tank off, i wanted to check the spark plug, it was/is a med tan maybe a bit on the dark side out where the threads begin and the center point as well. the idle's now clear and i can snap the throttle and get an instant rev out of it without stallling near as much as before, i adjusted the idle as well as i thought it was really low, i'll play with the fuel screw this weekend when the ground drys out from the massive rains here.

oh, the kitted main jet (installed) was alot larger then the stocker.


Edited by JDV500
adjusted text,

Along with the air/fuel screw, your pilot jet also plays a role in engine starting, idle and "off idle" throttle response behavior. After reading your mods, from what your are describing, I think your bike would respond nicely to a one size richer pilot jet (from stock). I run a one size richer pilot on my bone stock '07 and like it in all but the warmest summer months of riding.

I too frown on pre-determined "jet kits" . Jet kits are only as good as the ambient air temps at which they were tested within. If temps go 15 degrees warmer or colder, now your "jet kit" is not providing peak performance. You can also browse the users jetting database stickie for some other's jetting experiences as a cross reference. It's funny to me how some people seem to loose sleep over "is my bike jetted right ?" Me personally ain't that good of a rider to notice a difference between a 170 and a 172 main jet change. In reality for the "average" rider if your bike is jetting a bit rich or lean, it's no big deal. I simply jet for a good "compramise" of riding all year. I ride my YZ year round from summer MX to winter ice fun. I have a summer jet spec and a winter jet spec. Is my bike outputting peak hp year round ? Nope. Does my bike run good year round ? Yes.

It's definately a good skill to learn the basics of jetting and how your bike reacts to jet changes. Keep in mind, these comments are strictly my opinions, and not law...

Edited by Polar_Bus
...After reading your mods, from what your are describing, I think your bike would respond nicely to a one size richer pilot jet (from stock)...

After reading his post following the return to stock jetting, I would disagree. His pilot now sets up correctly, according to what he has said, and going richer to improve throttle response is a mistake made far too often. A too rich pilot will make a YZ450 hard to start hot, and will cause the engine to stall when returning to idle from speed, especially if the throttle is opened and then closed suddenly. People will complain of such things and yet persist in the changes that caused them.

The pilot has some effect on the transition from idle to main circuit, but it's not really primarily responsible for it. The cut out of the slide together with the straight top section of the needle does most of this, and the accelerator pump fills in the rest. In realistic terms, you should not be able to "snap" the throttle open from idle on a YZ450 in any case, but it should respond immediately to a very slightly progressive "rolling" opening.

I do agree regarding the obsession with perfect jetting. I change mine twice a year, from a 168 main for the winter to a 165 for the summer, and ride at anywhere from 4000 feet to 80 below sea level without more than a fuel screw tweak for the weather.

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