[400ex carb jetting with slight twist] - ATV Riders Forums - Honda, Yamaha, Suzuki, Kawasaki, Polaris, CanAm, Arctic Cat, Kymco ATVs

Pages:1

400ex carb jetting with slight twist
(Click here to view the original thread with full colors/images)

Posted by: MY FYN 79

My story starts with a clean 400ex carb I bought from a reputable ATV salvage business on Ebay. Its a nice carb, but the problem was that it had a Dynojet kit in it when I pulled it apart to jet it. I sourced jets and should have everything I need.

Last night I installed a DJ 138 main jet and set the needle to the 3rd notch down. (As per Dynojet's instructions on the 400EX kit) I still havent touched the idle or mixture screw, and never checked which pilot jet it has in it.

It actually runs pretty decent from the little bit of putting around the yard I did. With 20" tires and 14/38 gearing, once its warmed up, I cannot even get it to bog or hiccup when pinning the throttle from a rolling idle in 1st gear. It starts right up and idles pretty decent (albeit a little chop in the idle), but the only problems I'm noticing are these:

If I have it in first gear and clutch a little wheelie from a dead stop, riding the wheelie with smooth throttle it'll blubber and come back down. Also, when cruising with steady throttle the motor feels to be laboring (lean?)when I lightly tip into the throttle.

The top end is fresh with a +.040 11 to 1 Wiseco, Stage 1 Hotcam, lightly cleaned up (by me) head, good valve job, aftermarket header with a stock 450R muffler, 400ex carb and no airbox mods. I'm running 93 octane fuel in it.

I'm planning to take my machine out to the inlaws and make a wide open pass and shut it down to check the plug. Any insight on the couple running issues it has? These carbs are pretty new to me, Holleys I can understand.

Just for reference, here are the instructions showing what Dynojet is asking for when the carb is used on a 400. http://www.dynojet.com/pdf/Q107.pdf

Any input would be greatly appreciated!!

Posted by: bwamos

You could easily swap back to the Keihin main jets so you can get them from the dealership.

You're probably at a good starting point as the 400ex carb w/ stock jetting works well on a stock 300ex motor w/ aftermarket exhaust and no airbox lid. You're a bit richer and you have more flow, so you're probably fairly close.

I run a 165 Keihin main, 42 pilot.
But I have a 330cc 11:1 w/ portwork, oversized valves, cam, etc...

I'd estimate you'd be running about a 150 keihin w/ a 40 pilot. So the 138 DJ is probably fairly close. This is using the WAG method though. (WAG = Wild *** Guess.)

Posted by: maddmatt02

man I hate it when you type something and get an error and when you go back its gone!!!!!

anyways, sorry to steal this in a sense, but maybe this will be useful for the poster too.

I got a 400 carb, and it came with an unlabeled 114 main, and it is in this threaded spacer/adapter thing that threads into the bottom of the needle jet(think thats what its referred to as) and the main goes in this. well i went and got a 148 keihin main, and it would not thread into this, but it will thread into the threads where the adapter threads into. is that the dynojet adapter? the threads are very close. the keihin jet will start where the other jet goes but gets VERY tight and you cant go more than 2 threads and it take ALOT of force, the original jet will however thread into and tighten where the keihin will thread in. but with this adapter the jet is almost to the bottom of the large hex hed access bolt on the bottom of the float bowl, without it, its about even with the bottom surface of the float bowl? is this deep enough or is there a spacefr/adapter that the keihin jet goes into the make the jet deeper in the bowl.

Posted by: maddmatt02

see what I mean.

Posted by: markvette

the new jet you put in is the wrong jet. the need the hex shaped main jet not the round button style. with the right jet you dont need the dyno jet adapter.

Mark

Posted by: markvette

this is the type jet you need.

http://www.motorcyclecarbs.com/Keih...101-357_C77.cfm

Posted by: maddmatt02

ok, when i first opened it up and saw it i just figured i needed round mains. didnt know that this carb used the same hex jets like the fcr's

Posted by: bwamos

Yea, you need the "long hex" style of jets.

Posted by: maddmatt02

alright, I got 150-160 hex mains for my brothers crf, Ill just use those and see if one is what I need and then replace it before I go to mexico to jet his bike. so ill only need to pay for the jet I need.

anyways Ill let the original guy have this thread back now.

Posted by: MY FYN 79

Dont worry about it Matt, I have nothing against learning more about these carbs!

Any other input on tweaking the carb? Which circuit of the carb meters low throttle angle and slight tip in? I still havent had the quad out to go wide open and check the plug, but any input on adjusting what I have now would be great.

Posted by: maddmatt02

Im pretty sure it goes like this.

the pilot circuit controls fuel flow from idle to around 1/4 throttle, then the needle meters the fuel between 1/4 - 3/4 and then the main is basically almost WOT to WOT.

also a general way of jetting the pilot circuit ive read is if you have to turn your fuel screw in more than 1.5 turns out go to the next smaller, and if you need to turn it past 2.5 turns out go the next larger pilot. also if you are riding and you let off the throttle and it pops on deceleration the pilot is most likely lean.

Posted by: MY FYN 79

Thanks for the info.

Just to keep this up to date, last night I dropped the needle clip one notch (4th down), and surprisingly it got blubbery during steady low throttle. Turns out I went the wrong way with it, and when I raised it back up to the 2nd notch down, it seemed to run good.

I took the machine out last night and ran it to WOT then shut it down. With the 138 DJ main jet, the spark plug is showing I'm quite lean yet. I'll throw more jet at it and update tonight.

Posted by: bwamos

Yea the needle is pretty easy to set. Either it's good, or it's really bad.. lol. Getting into tapers/diameters, etc.. for maximum power is another issue.

Dont forget when you shut it down, you need to kill the spark while maintaining the throttle to avoid a false lean reading. You also need to read the plug down inside the collar not up on the tip.

But, it sounds like your getting close. up a size or 2 should do ya. May require to readjsut your needle as well, since you're increaseing the fuel flow through the same port.

I hate Jetting. I've gotten to the point that I get it "close" then take it to a Dyno, lol. It's worth the money to me, lol.

Posted by: 440exmadness

did you get it figured out.

Posted by: MY FYN 79

It seems to be pretty decent now. I ended up with a 142 Dynojet main and the clip on the top needle notch. It can be a little bucky until it warms up, but that could also be the 38 pilot jet thats still in it.

This coming weekend we're going upnorth riding, so I can report back how the EX does after that.