Bad missfire at less than 1/8 throttle and 3000-6000RPM

So I recently did a lot of work to my bike, had the head off and did a lot things while it was apart.

Put it all back together and now it missfires really bad when trying to hold 3000-6000 rpm in neutral, or 1st gear on flat ground. ( and less in 2nd or more poweretc.)

It still idles well, and seems to run great under heavy throttle. it does have a little bog if I snap the throttle from idle, but it has always done that.

The only thing i changed during the overhaul that I think would affect it is the removal of the AIS.... I used the plugs and blankoff plate from the GYTR kit.

Does the AIS removal really affect the jetting that much that I should have a major difference with and without?

I have done a lot of jetting searches, but I am a jetting nooby, so I am trying to learn, but I want to know if this is a jetting problem, or if I should look for a mistake made during assembly. Like some kind of leak, or a wire disconnected etc.

To me the logical thing is a clip position might fix it... idle mixture seems good, and so does wide open, so it would not be the main jet or the idle mixture screw. But i am not sure.

i have an 08 WR450 with AIS removed, YZ head pipe and Muffler, Air box snorkles and cover plate removed. grey wire pulled, throttle stop screw from GYTR kit. Zipty fuel screw at 2 turns out. and JD jetting with red needle at 4th clip, 168 main jet, and the big oring on the accelerator pump.

The day it was doing this bad missfire was REALLY hot out, probably 95-105 degrees... and I did not notice the same problem when riding it around the block a couple times when it was colder right after assembly. I dont think it smelled too rich when it was missing, but its hard to say.

Any thoughts would be much appreciated... thanks,

Have you checked the TPS? Course if it was fine before you did all the work, then that may not be the case, or either removing the AIS and installing all the upgrades just made you find the TPS stutter that these WR's seem to be plagued with. Is it a really bad missfire, or just a stutter when holding a steady throttle with not much load?

You're jetting sounds about right, thats what I'm running at 5000+ feet in Utah for all my mods in my sig, except I'm running a 165 main.

Course you could rule out the TPS real quick, just unplug it, and take it for a spin. Its the grey 3 wire plug up under the tank, make sure you unplug it while it's off.

It is a pretty bad missfire.... The kind that makes you think you ought to ride straight back to the truck and do some troubleshooting. But the bike is still rideable though.

Yea, I have been reading posts about jetting for hours now and i still have no idea what to do.

It is a pretty bad missfire.... The kind that makes you think you ought to ride straight back to the truck and do some troubleshooting. But the bike is still rideable though.

Yea, I have been reading posts about jetting for hours now and i still have no idea what to do.


Unplug the TPS (grey plug at tank/seat junction). If it gets better, that is the issue: TPS is out of adjustment, and you have one of the many 'bad' CDI boxes that do not track RPM properly. Most of us have one ('03-'05), and still ride with it.

No change when un plugged?

- Check the battery connections

- inspect wire harness for loose connections

- inspect kill button for wire shorts

- inspect spark plug and lead at both ends (change plug)


+1, unplug TPS, and report back.

I will try the TPS. Thanks for the Idea...

gotta wait till tomorrow though, 96db is too much for the neighborhood at 10:00pm, and I want to warm it up and completely reproduce the problem before I unplug it.

I warmed it up for about 10 minutes, and was able to repeat the missfire, but not quite as bad as sunday when it was really hot out. But still missing more than should be considered acceptable.

When I pulled it back into the garage(near dusk) the yz header was glowing slightly but closer to the head, not in the elbow. This makes me think it is way too lean.

I pulled the seat and tank, unplugged the TPS. threw the tank back on and no change to the missfire performance.

I also sprayed carb cleaner around every spot I could think of that might have a leak and got nothing.

Think it could actually be a jetting problem?

I will pull the plug in a little bit to inspect for rich/lean evidence.

Any thoughts would be much appriciated!!!!!!


I used the jetting database format below, and copied in the stock numbers for reference.

Year: 2008

Model (400 - 426 - 450): 450

Cam Timing (WR/YZ): WR

Main jet: 168 (JD)

Pilot Jet: 45 (stock)

PAJ: 80 (stock)

Leak Jet: 50 (stock)

Starter Jet: 65 (stock)

Fuel screw (turns out): 2.0

Needle Model/Clip position: JD red 4th clip

Grey Wire Mod (Y/N): Yes

TPS Connected (Y/N): Yes

Airbox Snorkel/Lid (on or off): Off

AIS Removed (Y/N): Yes

Airfilter Brand: Stock

Exhaust Brand: 08 yz large head pipe and shorty muffler with fresh packing

Average Altitude: 1500ft

Average Temperature: 90 in the summer

Average Humidity: dry(california)

Degree of Satisfaction (0-10): Bad missfire.

Additional Mods/Comments: JD kit with large O-ring on Accelerator Pump circuit. followed JDs instructions for setup.


Main: 165, Pilot: 45, Starter: 65, Leak: 50, Pilot Air Jet: 80, Main Air Jet: 200(Non-adjustable)

Warm up is done when the tops of the radiators are too hot to touch. Anything past that is a waste of gas and time. 10 minutes is WAY too long.

I believe you probably have either an electrical problem; it could be a jetting problem. I would try electrical first, as a jetting problem is usually limited to specific throttle positions, not rpms.

Plug check doesn't always work good with all the additives in pump gas, but if you had a really black plug, then yeah, something is up. Should be an almost grey color with the pump gas.

Since your at 1500 feet and you have a less restrictive exhaust and airbox, I would try maybe 5th from the top on the needle, and a 48pj 1 1/2 turns out to 2 turns out. 168 main sounds right, though you could try a 170 anyhow.

I'm running your jetting except for a 165 main for 5000 and above.

I'm with Krannie on the electrical problem though, check all your connections first and simple things. Heck even check your vent in your gas cap line. Mine was junk from the showroom floor and had to be replaced.

Also if all else fails, you said you had the head off, so I'm assuming you removed the cam chain and all that, I would double check your timing, being off one tooth could allow the bike to run alright but cause all sorts of quirky problems, believe me I've done it.:doh: Check the valve clearance too while you're in there.

It is definitely a throttle position sensitive problem. I should have described it as that. It is most reproducable and most obvious in the 3000-6000 rpm range.

I spent a couple hours reading about jetting last night.... I think I will try a JD red needle at position 5 or the JD blue needle at 3. Then I will see what pilot I might need. It took a while to read all the way through the jetting database, but that seems like a popular setup. A lot of people are running the JD red at clip4 needle though.

I am going to check all the connectors too. Also I will be rechecking valve clearance soon, so I can look at the timing then.

The thing that doesnt make sense though, is that I had more problems when the air temp was hotter, but I am going to try a richer setup.


I find myself having to adjust the idle speed and pilot screw almost every ride or every other ride. Something keeps changing. I definitly have a significant performance shift from early in the day to mid day. Could be the air heating up, but a great example is that On sunday, mid day high heat,after riding for an hour, I had to adjust the Idle from about 1300 up to 1800.... and then last night at my house(25 deg cooler), I had to turn the Idle down from 2600 to 1800. That cant be normal... could it? Maybe I should also take the carb out and look for a loose jet or sticky choke or something?????

I have an 07 and am in CA as well. I went with the 48 pilot as I believe is what was called for in the JDJetting instructions. I also installed all of the other jets that came with the GYTR AIS removal/JDJetting. Red needle on the 4th clip. I ride from the desert at 2,500' to 9,500' in Kennedy Meadows and have not experienced what you seem to be.

Yeah you have some weird problems going on there for sure, haha. I would think your 45 pilot would be a little lean so the idle going way up at night in the cooler temps would say its rich however, I would say this isn't the case however. Make sure you have your fuel screw set up right. I actually had the oring come off the fuel screw upon installation and get shoved up in the port which caused some weird idle behaviors.

Make sure your Hot-start lever isn't sticking as well. Sometimes the cable can get binded up under the tank, or hung up on the regulator on the side of the steering stem and kind of hang the hot start open.

The needle is the easiest thing to get to on the carb, just pull the tank, and remove the top motor mounts and play with the needle, see if you can fix that mid-range missfire. If needle has no affect on the problem, then I think your problem is else where.

What was the reasoning for taking the head off the bike?

If your mixture AND IDLE is changing during the day I would check:

1. Hot start : it may not be seated into the top of the carb correctly

2. Hot start : the plunger can get corroded

3. Slide plate: there is a removable plate in the front of your slide. It can come loose and leak. Plays hell with idle and partial throttle. Glue it in with 3m yellow.

4. Pilot jet : it only takes one little grain of sand to ruin your day

5. Needle jet: same as above

6. Grounds, grounds, grounds: check them all, especially the harness plugs

7. Plug wire: cut off the end and re-attach it

8. Plug: if it wasn't bad before, it is now with all that bad running.

9. Coil ground: remove and re-attach.

10. CDI : you could have an overheated one: they act strange.

It's going to be in the harness, the plug/plug wire, the grounds, or the CDI.

The ignition timing has too much advance in that range, and the spark duration across the plug gap is too short. This creates the misfire. You can try different coils from other bikes, but its much simpler to just unplug the TPS and let the CDI resort to less advance at that range.


Yamaha WR250/WR450F Stutter Condition-

This is an ignition problem that Yamaha has designed into the bike to make the motor run cooler at light throttle and medium speeds. The stutter is VERY COMMON, and can sometimes be reduced by disconnecting the TPS electrical lead from your carburetor. The condition is most common on flat roads at light throttle positions, when producing under 10hp at the rear wheel. The stutter is harmless and will not affect the power under acceleration, if it is not too annoying, leave the bike as-is and just ride it. The problem is mostly noticed when riders want to do road riding with their bikes.

Try disconnecting the TPS lead and see if it helps your condition. The other alternative is to try running either clip position #2 or #4 on the Red marked needle. This is a work-around that forces you to use more throttle than normal and lightens the stutter, but the bike will make less power when accelerating.

See- and and with replacement CDI and no jetting changes-

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Didn't you get the head ported and polished?

so either you need to rejet based on your head work, the port and polish job. You can email JD on that one. you bought his kit he will help you.

or You have a re assembly issue. you probably got vacuum lines connected wrong durning reasembly or an air boot or carb vacuum leak. maybe the cam is not degreed right to the crank.

I think Krannie had the right call:

"4. Pilot jet : it only takes one little grain of sand to ruin your day

5. Needle jet: same as above"

I had the head off due to a siezed cam. and I pulled the AIS when i put it back together, so there were a lot of variables. No leaks... I coated it in carb cleaner during idle.

After a lot of research and some soul searching, I decided to go to the 48 pilot jet and the JD blue needle on clip 3. I started the pilot screw at 1 turn out.

When I had it apart I found some mud in the bottom of the float bowl. i think that was root cause. It was probably clogging the pilot jet, and maybe the main intermittently. I must be a bad person, and let some dust get into the fuel line when the tank was off.

How often would you guys pull that float bowl cap to check for dust?

Just started it up... started super easy, and ran great with the choke off just 2 seconds after start up. reved it up and it sounds fantastic. no off idle bog either. Only revved it in the garage though... need to get it out on the trail. hopefully early tomorrow morning.

Thanks for all the input!

I had those same problems. A friend told me to do somthing and it worked perfectly. Pull your tank off. there is a wire that plugs into the side of the carb. follow the wire to the plug. It;s located right under the tank and above the head. unplug that and tape it up with electrical tape. Doing that fixed that bog in the throtle and all hesitations. After rejeting the bike 3 times this is all I had to do. I used the JD jetting kit. It actually fixed mine a 2 other of my buddy's bike. It's crazy, but thats all it takes

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