Jetting experts please look

I have been riding my 04 XR 650 L for about 6 months since I put a Mikuni Flatslide 38 MM carb on it. Also all the other usual mods have been done.

The bike starts great hot or cold. When I shut it off though it backfires through the exhaust with authority. It's a very loud backfire. I checked the plug and it's a medium brown color. I installed a new plug, fired it up, ran it through the gears wide open a few times and then hauled the plug out. It looks fine.

Why would the bike backfire? Also there is a small flatspot around 1/4 throttle, then it's a beast.

Main Jet: 260

Pilot Jet: 27.5

Needle in middle position

Please help :thumbsup:


I run a TM40 pumper on my 650R. Those Mikuni's come setup goofy sometimes. Your carb takes the same jets as mine, and that 260 main jet CAN'T be right. Even if you mistyped and it's a 160, that is still WAY too rich. Also, you need to get in and check your needle jet (long brass tube that the main jet screws into, and that the needle goes into), and the needle # itself. Also, there is a small brass air jet in the mouth of the carb intake, see what this is also.

My setup is;

22.5 pilot

1.2 air jet

Y-4 needle jet

9DJY03 needle, 4th clip, or 9DJY04 needle, 3rd clip

140-145 main jet (depending on exhaust pipe)

Other than your hotcam, your 10.25:1 piston and my 10:1 piston, our motors should be jetted within the same ballpark. Of course your smaller bore carb will make a difference...

I am confused. The jets that I have are stamped with 230 through 270 on the main's and 17.5 through 30 on the pilot jet.

Today i tried moving the needle up one notch. It's now 2nd from the bottom. I put in a 20 pilot and kept the 260 main.

I have a 2.0 air bleed in.

Still backfires.

Anyone have any jetting info???


What needle jet and needle are you using? I'm still confused about those main jets, my genuine Mikuni jets for my TM40 (same carb, bigger bore) are in the low to mid 100 range. For example, I'm running a 140 main on my uncorked 650R, guys with big motor mods run 150 main or less.

Regardless, it sounds like your popping is at less than 1/2 throttle. Your pilot jet needs to be about 22.5 or 25. I'm using a Y-4 needle jet, but was using the richer y-6 needle jet previously. Then your needle, which may be different from mine, who knows. You said you raised your needle, so your needle jet is the last thing to check. It simply unscrews from the bottom of the carb body (within the bowl). The main jet (my main jets are round headed by the way, maybe yours are hex head?) just screws into the end of the needle jet.

Just to go a different direction and say that you have a leak in the exhaust system.

If the system is sealed then the only gas in the pipe is your burn fuel/air charge, the oxygen is consummed but there still is gas leftover, there is still (and always will be) unburnt fuel in the exhaust gases, if fresh air (oxygen) enters the pipe with the unspent fuel charge then the backfire can occur. Where as i never had this happen on a bike, it definately has in a car.


Well I checked for exhaust leaks and couldn't find any. Im sure it's not leaking.

As far as the jets go yes they have a hexed head..... Do any other companies make jets that will fit the Mikuni TM style carb????

Check out SUDCO info My main jets look like 4/042 an the pilot jets look like VM 22/210 .

My jets have a small weird stamping on them also.


I have installed a 210 main jet and a 25 pilot. The needle is in the middle position. 2.0 air bleed in also.

The bike is running a little better. The plug is a light brown after 4 long Wide open hauls. I really don't know what else to do. It's still not right.

Any help....should I really be down 150-160 range with my main?

Backfiring after shutoff is the result of having the engine turning over after the spark is shut off, and having that explodable cylinder mixture pumped into the hot exhaust where it ignites.

My guess is your idle is a bit high (extra mixture pumped into the exhaust), or maybe your idle mixture is off and you are setting the idle throttle setting high to compensate.

I'm guessing a too-rich idle mixture will tend to backfire more than a too lean one- but that's a guess.


Yes it gets better with the idle down. That still does not fix my flatspot. The backfire is actually almost gone.

Thanks for the reply Dave. I am almost dead set on a FCR or Edelbrock.

Needle jet Q-4

Needle 6fj41

pilot 25

main 210

The bike still works like crap but plug is ok.

For what it's worth the jetting for my TM38-85 from XRS only call for:

Main Jet 250

Piolot Jet 20

Needle Jet 6FJ41-4

Nozzle 389-Q4

Slide 4.0

Thats for low elev.

250 main 20pilot clip 3rd from top low elev.

240 main 17.5 pilot 3rd from the top std. elev.

put mine on today.

If you are tired and go with another carb, send it my way and I will try to figure it out!!

For what it's worth the jetting for my TM38-85 from XRS only call for:

Main Jet 250

Piolot Jet 20

Needle Jet 6FJ41-4

Nozzle 389-Q4

Slide 4.0

Thats for low elev.

Hmmmm. That's almost identical to the way it was setup when I bought the carb. I wonder if it was close and i have gone farther from being right????

Maybe this just aint the right carb for me???

Do you guys that posted earlier have piston and cam mods???

With a cam, piston , bored carb, and pipe, it probobly will have a flat spot. Your first post said it pulled like a beast. Start on the main jet, i say 260, run it full throttle in 3rd hit the kill switch. see how the plug reads . then go from there. The more moded. the engine , the more quirks.


Have a 06 YZ450F and its awesome. But at 0 to 1/4 throttle, not so good. If you just twist the throttle hard in nuetral it will die. Is this something i have to put up with? Or can i fix it with jetting.


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