Carb Jetting Guide, ATV Options

[GoFastCat]

~~Carb Jetting Guide~~

This is a step-by-step procedure for re-jetting a carburetor, in this case it is the stock Mikuni BSR36 SS carb on the DVX 400.


- The first step before you even begin to tear into the carb is to give your ATV a good cleaning, you do not the risk of dirt contaminating the carburetor or engine while you are working on it.

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- Next you must be able to get at the carburetor to work on it. This may involve removing fenders/side panels, fuel tank and air box depending on application.
For the DVX the whole fender assembly needed to be removed along with the gas tank to get full access to the carb.



- Now with everything out of the way its time to remove the carb to be worked on. If the tank wasn’t removed shut off the fuel and drain the carb bowl. Once the fuel has been drained unhook the fuel line and vent line from the carb, unhook the choke cable and throttle cable. Then loosen the carb boot clamps and remove the carburetor.


CHANGING MAIN JET


- The main jet controls the fuel flow from ¾ throttle to full throttle. A larger numerical main jet increases the fuel flow from ¾ and beyond.

- To change the main jet turn the carb upside down and remove the idle mount screw on the carb bowl and then remove the 4 screws on the bottom of the carb (see picture below). These screws are very tight so make sure you have a good screw driver or impact driver to remove them, or they will strip.
If you round out the head of the screw, dont worry just take a dremel tool and cut a slot in the screw to remremove it with a flat head screw driver.



- With the carb bowl off you now have access to the main jet (along with the starter jet and pilot jet). With a wrench hold the Main jet holder and unscrew the main jet with a flat head screw driver and replace with correct jet. Thats all there is to it so now you can fasten the carb bowl back on.

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ADJUSTING NEEDLE JET CLIP


- The needle jet controls the amount of fuel from ¼ to ¾ throttle positions. By moving the e-clip up you lean out the mixture and lower the e-clip richens the mixture.
The stock needle on the DVX is non-adjustable so an aftermarket needle will be needed.

- To get to the needle assembly remove the two screws holding the diaphragm cover on and beware of the large spring underneath it.

- With the cover off gently pull up on the rubber diaphragm and remove the slide. With the slide out of the carb take a pair of pliers and pull on the needle holder to take out the needle assembly. There is a tiny spring under the needle holder that can pop out so keep an eye open for that.



- Now that the needle is out you can make the adjustments needed. Just take note of the placement and order of the washers, spring and spacer are in for when its time to re install it.

- When putting the needle back in make sure it clicks into place in the diaphragm. Then put the diaphragm and slide back into the carb and put the diaphragm cover back on, be sure that the rubber seal is positioned right.




ADJUSTING THE PILOT SCREW


- The Pilot screw tunes the low end circuit of the carb from idle to ¼ throttle. Turning the screw out richens the mixture and turning it in cuts back the fuel flow, making it leaner.

- On the DVX carb the Pilot screw is located under a brass plug that needs to be drilled out and then take a self tapping screw and tread into the hole. With a pair of pliers pull the screw out which will bring the brass plug out with it. Now make the adjustments.


TESTING


- Now comes the fun part and seeing if your new jetting setup works properly. Install a new spark plug to do a plug chop. With the ATV up to operating temperature do a run at full throttle and then hit the kill switch and cost to a stop. Remove the plug and take a look at it, the porcelain on the plug should be a nice tan color, if not then some more tuning will be needed. A white plug indicates it being lean and black is rich, remember that this was done at WOT so is verifying your main jet. If that looks good then check your midrange (needle jet clip) and work your way down to idle making proper adjustments.


Thats all there is to it, and its pretty easy to do, infact the hardest job is just getting the carburetor out so you can work on it easily and getting it tunned properly. Feel free to add more info or suggestions for rejetting a carb.

[GoFastCat]

For you 650 V-twin owners. One of the sites sponsers, Velocity Devices Inc (VDI) have some nice intsructions for installing their snorkle mod and rejetting on their website.

>>TAKE A LOOK HERE<<


Here is a list of links that have some very good info about jetting:

• http://www.thumperfaq.com/jetting.htm
• http://www.motorcyclecarbs.com/jetting.htm
• http://www.mikuni.com/fs-tuning_guide.html
• http://www.4strokes.com/tech/4sjetting.pdf
• http://www.4strokes.com/tech/howtojet.asp
• http://www.4strokes.com/tech/carbover.asp
• http://www.yellowdogracing.com/techstuff.htm
• http://www.cdracingonline.com/id57.htm
• http://www.powroll.com/tech_specs_jetting.htm
• http://jimrace.com/faq/CVK40.html

Here are a couple links about spark plug readings

• http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
• http://www.off-road.com/dirtbike/tech/2002sparkplug/
• http://www.4strokes.com/tech/sparkplg.asp

[shanekee-h1]

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i found it here is a dyno jet troubleshooting guide it helped me greatly click on the page you want to view

[400ex]

also remember to take your time cause if you dont and you rush you will screw something up or strip a screw so remember take your time it saves a lot of time in the long run!!!!!

[B_Brown]

thanks for the post, that helps alot man

[DemonCat400]

Great information!! Really helped. I might add that during reassembly of the needle and diaphram that you make absolutly certain that the order is exact and that the tiny little spring hasn't dislodged your assembly. If it does, your bike will not start. She runs like a champ now! Horsfalls here I come...

[ArcticMonster]

I'm going to be rejetting, so that is nice to know...

Thanks all.

[DVXDUDE]

i just finished jetting my 06dvx for the very first time and this guide helped a lot, changing the jet and needle isnt the most difficult thing after you figure out what everything is and how to get it off. the hardest part is tuning it after. i have a big gun full exhaust wit kn air filter and using a 155 dj main and 3rd clip on the needle with 2 turns out on the mixture screw....this set up is differnet from what dynojet recommended, i talked to them on the phone and they told me to use a 130main which was WAY lean so i bumped it up to a 155 and it runs like a beast...o ya and buy 2 new plugs before you start so you can plug check after you ride it to make sure its not too lean or rich and watch the 4 little brass screws on the bottom of the bowl on the carb, they are tight as a nuns c*nt and EASILY strip


my bike ran really well at one point, didnt bog or sputter at all through all rpm levels, but when i checked the plug it was still on the white side, turned the mixure screw out a half turn and its looking better so make sure you do a plug check even after you think its running at full potential

[HTRX400EXRider]

I just got a 2001 400ex...everything in it is stock...i had a couple friends ride it who all own 400ex's and they all said that something is messed up. It doesn't pull near as hard as what it should be. I can barely do a wheelie on it!!! At idle, it always stalls out unless I am giving it a little bit of gas. So it was suggested that i should try and rebuild the carburetor. I have knowledge of motors but I've never even touched a carburetor on a four wheeler. do you have any Carburetor Jetting Guides for a stock carburetor for a 400ex?

[cataway]

can some one please explain why its necessary to take all the fenders off ? MG all your tiring t do is get to the carb.

[PBPoison]

Keihin "Jet List."

Lists the jet part numbers, sizes, and the carbs they fit.

http://www.keihin-us.com/am/_media/pdf/jetlist.pdf

[remotruker]

Would be nice if the pictures loaded.... a few broken links here....

[GoFastCat]

Would be nice if the pictures loaded.... a few broken links here....

Yeah some pictures were lost in one of the later forum updates. I'm not sure if they can be recovered or not but I will bring it to the admins attention. Unfortunatly I sitll dont have those pictures on my computer

[rango726]

i have a quick question. i just installed my sw pipe on my 400 and rejetted but now im runnning real rich. do i just turn the air to fuel mixture screw in a quarter of a turn at a time until perfection or is it out im assuming to cut fuel down(leaner) to the carb i will tighten the screw and to (richen) add more fuel i loosen .right now im at 3 turns out from seated

[GoFastCat]

i have a quick question. i just installed my sw pipe on my 400 and rejetted but now im runnning real rich. do i just turn the air to fuel mixture screw in a quarter of a turn at a time until perfection or is it out im assuming to cut fuel down(leaner) to the carb i will tighten the screw and to (richen) add more fuel i loosen .right now im at 3 turns out from seated

You are correct rango and turning the mixture screw IN will lean out the your idle to 1/4 throttle.