View Full Version : JETTING PROBLEMS?

08-10-2001, 03:10 PM
I understand the whole jetting thing for the most part, but could something else be wrong? I have a K&N air filter, Greedy er... White Brothers exhaust, and 3 1" airbox holes, and my elevation is about 800 ft above sea level.
I have been trying to get the jetting right on my 400EX for about two months now, with little success. Today I got two new spark plugs, and decided to rip into it again. I rode for about an hour with the new plugs to get some color to them, and then started doing wide open plug runs. I first tried the 160 I have been running (the whole time thinking I am ruining my motor from running lean), and the plug was as white as Marilyn Manson's skin. I went right to a 170, and it was just white. I then tried a 175, and I got a gray color. Now I am at a 180, and it is a darker shade of gray. I know myself that anything over a 165 - 170 main jet on a stock 400EX is crazy, and I am at a 180, right where I was when I last asked about jetting. I have raised the needle one notch and it is the dynojet needle that comes with the jet kit (which I found worthless).
I have checked every intake point on the quad, and everything is looking good. I cleaned my filter yesterday, and the petcock flows just fine. The carb is all clean, and as far as I know, everything is tip top on the whole machine.
My question is, could something else be wrong to create this condition? I don't know of anything, but maybe I am missing something...

The quad ran fine with the 160, but the mid range feels choppy, so does anyone have any suggestions for a needle position (Dynojet needle and I don't have the stock needle) with my setup? The 3/4 to WOT is choppy now with the 180, and got worse as I went up from the 160 to the 180.

In a perfect world, someone with a WB exhaust, K&N filter, and airbox vents or no lid could tell me their setup, so I could give it a try and work from there. Thank you so much for any help you can give me.

08-10-2001, 03:27 PM
i live around 800ft above sea level, and run the white brothers slip on with the last disk out, making me have no spark arrestor and its now pracitcaly a free flowing exhuast, K&n filter,and outerwear airbox lid. so i practicaly have the highest flowing intake and exhuast and i am running a 152 main and a 40 pilot. 185 seems very very high for just a filter and pipe.

08-10-2001, 03:54 PM
I know, it seems very high. I went out again and did it 4 more times, run it check the plug, run it check the plug..... I took a picture of the best (darkest) color that I got. This is with a 180 and it runs like crap.

08-10-2001, 06:12 PM
Well I have pretty much the same set up as you only i have the Pro Circut T-4. Only thing i did different from the directions was i raised the needle one notch from the recommended directions. I am running 3 turns out on the air/fuel screw. 170 main on the dynojet kit. Ummmmmm I cant really think of anything else. I am running right on too. Nice brown color. You might want to try restiricting your air flow intake to richen it up and see what happens. Try to cover up your air lid holes. or wrap something around your air cleaner then do another WOT run and then test your plug. I dont know.................sorry but i am not much help!!


08-10-2001, 06:31 PM
make sure your fuel is flowing correctly. Use an older plug with some color already on it, gets you a reading easier. Either keep going up with the jets or take it to a professional. I have heard of a few fluke 400exs that seem to need larger jets, but yup getting around 180+ seems very high. But I'd believe it.

08-10-2001, 08:01 PM
Thanks for the replies, but keep them coming.
I have kept it up, with no success. I put the stock filter back on, and it was still white, so I then covered the vents in the lid, still white. I then wrapped duct tape around the stock filter and left about half of it uncovered, and got a little color, but it was pretty much white. I think I will go back to the old spark plug and see if I can get anything out of that. This is starting to get on my nerves, 6 hours of doing this and no success! I don't want to take it anywhere, but I might have to. You see, I was using an older plug, but I was getting a very light gray color using a 180 main jet. It is so weird, it feels strong with a 160 to 170, but the plug is crispy white. When I go up from a 170, it runs horribly, but the plug is the same exact color. :huh
I will try the older plug, and take it slowly, and try not to get frustrated about it. I just want to stop worrying about damaging the motor whenever I ride it.

It may seem like I am a moron that doesn't know what he is doing, but I am fairly mechanically inclined, and usually know what is going on, but I think in this situation something else is wrong. I have done the same thing over and over, and really had no success.

Thanks guys, any more help would be great!

08-10-2001, 08:15 PM
I am running a 170 main with no airbox lid and my quad flys. But I haven't done a plug test yet. I hope I don't have this problem too. I will do a test tomorrow.

08-10-2001, 08:57 PM
I am running an E-Series and Modified stock filter with a hole in my airbox lid about the size of the lid itself. I am running a 160 main (thinking about a 162 or 165) and the clip on the needle lowered one notch (stock needle).

Not to question your mechanical abilities, are you running it long enough to get a good color reading?? A 180 seems really high.
Try to pull off your silencer to see what the color is inside of the header pipe. Also the plug cap might be shorted out, which will cause more resistance resulting in more voltage and hotter fire. A long shot, but a few more ideas. If it runs best within the 160 range, my guess is that is probably the right jet. Good Luck


What do you mean by choppy??

08-10-2001, 09:19 PM
Do you run with your bike with the petcock at "On" or "Reserve"? I may be way off base here because I don't know the 400 that well but many bikes use a small filter in the gas tank. When the gas is "on" it flows through a 1"-2" stem that extends up into the tank with a small filter on the end of it and when you go to "reserve" it bypasses this stem and goes straight into the petcock. This is how most setups allow for a "reserve" without running 2 tanks. It's possible that you got some crap down in your gas tank and it's impeding the flow ever-so-slightly. Not enough to hurt performance all that much but enough to throw off plug readings.

Another long shot is some people have talked of their local gas stations in which the fuel has detergents and other additives in it which will always burn white on the plug. This could explain why the bike runs so strong around the 160 range but craps out at WOT with the 180. You may be jetted correctly at 160 and the plug comes out white. The only way to tell 100% if you're jetted correctly is to have an O2 test done. Call around to local car repair facilities and ask them if they do "sniffer" tests. They could stick the wand in your quad and tell you your burnt air/fuel mixture. Good luck, keep us posted!

08-10-2001, 10:04 PM
I might be mistaken, but I don't think the Dynojet #'s correspond at all to the Keihn numbers..

I think there is a huge difference between a 180 keihn and a 180 dynojet main...

(you said you had a dynojet needle, so I'm assuming you have the whole kit with the dynojet mains also....)


08-10-2001, 11:11 PM
No, I am only using the Dynojet needle, I lost all the Dynojet jets and my stock needle. You know how it goes when mom cleans the garage? She doesn't mean any harm, but stuff always ends up missing. It was okay because she bought me more main jets, and I am going to buy a new stock needle as well.

I am running 100 octane Av Gas, my step dad hauls it. I might try some pump 93 octane and see how that goes. I will also try putting it on reserve. This is weird, because when I added just the K&N filter and modified my airbox, I had it jetted perfectly at a 160 dynojet main. When I added the pipe, I began to have problems. I can feel that something isn't right, but the visual evidence (plug) isn't there.

EDIT: I will also look at my gas tank, sometimes we get dead bugs in our gas cans, so I usually try to strain it through something, or use the can with a screen in the spout.

Thanks for the suggestions!! :D

08-11-2001, 12:52 AM
Jetting can be very difficult given the way you're approaching it. Plug reading has become unreliable given newer gas additives and longer life plugs.

Another (and safer) way to jet your machine is to "over-jet" it. The point is to make your machine run too rich - it'll give you a reference point.

First, you have to test if your machine is already running rich (with the 180 in it).

Put the machine in 2nd gear, get rolling enough so the machine isn't quite lugging, then roll on the throttle. Hold it wide open til it either (A) skips, blubbers, cuts out (can't miss it) OR (B) you've reached the top of the revs without the above symtoms occuring.

If you get A, then you're too rich on the main.

Else, if you get B, then run to your dealership and pick up a 182, 185, 188, 190. Retest with the 190 in. If you get A, drop to a 188 and retest.

Continue to do this until you achieve B. Then, drop back one more - you should be perfect then.

If 190 isn't high enough, go get more!

Although 180 does appear a bit high, only YOUR machine knows what it wants.

This is basically a fail-safe method, you will see results.

What to do with all the extra jets? Just return them, most dealers are happy to take back the unneeded sizes. Or if you're like me, you'll keep em, never know when the next engine mod will come along!

I've performed this jetting technique countless times - it is much easier and more effective. Plus, you won't have to burn your fingers trying to get the darn plug out!

Hope this helps,

08-11-2001, 01:36 AM
Here's my opinion... Guy660R and Ogre mentioned something that I ran into when jetting my 400EX, and it seems as though you've encountered the same problem--the detergent additives in the fuel you're using are keeping the plug clean no matter how rich you jet. Although I have no experience in running AV gas, I would venture to say it has quite a bit of additives to keep aircraft engines running clean, as the higher in altitude you go the leaner your mixture needs to be. Follow my logic? Anyway, I ran into the same problem trying to do plug reads...always read white...as the gas here in California is loaded with additives due to state EPA regulations. It wasn't until after my 416cc mods that I started running a 50/50 mix of 111 leaded race gas and 92 octane premium, and now when I read the plug it shows a nice light brown color. Must be the lead in the race fuel...

My advice is to re-jet using Ogre's advice. Start with the largest main you have and ensure it's rich by running at WOT and noticing a blubbering condition, and then work down just until that condition goes away. At this point you still may be a tad rich, but your motor will run a little cooler and will last longer.

08-11-2001, 02:20 AM
NVR2L8 -

I agree with you, as soon as the blubbering goes away - it isn't a bad idea to stop there. The engine will run cooler.

When I installed my cam, however, I noticed quite a bit of carbon on the piston dome (rejetted previously for pipe, piston).

At that time, I was dropping mains til it stopped blubbering.

Now I drop it one more size down after the blubbering ceases. That's why I mentioned in the previous post to go down one size more.

Either way though, everything should turn out great.

Oh, and by the way, even after jetting with this method, my plug still reads grey - yet I know it's borderline rich. Goes to show that plug reading really doesn't work very well.

PS - I usually run 91-93 octane - NVR2L8 may be onto something with the leaded gas giving color to the plug. Using 91-93 octane, all I EVER see is white, grey, or black!


08-11-2001, 03:42 AM
You guys are great! I am definetely going to get some pump 93 octane, I asked my boss at the gas station I work at if there are a lot of additives and my step dad who hauls gas and they both said the normal pump gas shouldn't have too many additives.

After that, if I still get no color, I will try ogre's method. Sounds like a good way to do it.

Oh yeah, I discovered something that made me wonder if I had found the problem, I took off my seat and noticed the carb was tilted probably 20 degrees to the right. I think when I last had the gas tank off, I put it back on and twisted the throttle arm and made the carb crooked. I fixed that and looked very closely at the carb and intake boots, and I also discovered that the bottom of the intake manifold on the motor was tucked under a bit into the carb. I am sure both these things would affect something.

I think it is a lot of things all mixed together, and I need to get some good gas, try the machine after I corrected those problems, try ogre's method (if need be) and report back within a few days when I don't work.

Thanks you guys so much for all the help you have given me. :D

08-11-2001, 08:54 AM
Last year before I did any serious mods I just had a K&N filter/outerwear (w/lid),and FMF MegaMax.My jetting was 160 which was probably a shade rich because I had the stock lid and my stock needle was raised one notch.I would try the 162 and raise the needle up one notch from stock.Also I would try the 93 octane and that will probably help with the plug reading.I also live in Wisconsin so elevation should be pretty close.Good luck

08-11-2001, 09:54 AM
When I ran 104 and 110 octane in my almost stock bike the plug was always white,:huh because it burned so hot. Now I hear running higher octane will actualy lower your HP. Now i just run 93 or 94. So if your running 100 octane, than thats a good start changing the gas like ya said.

08-11-2001, 11:56 AM
The AV gas is the reason your plug is white, it's not burning lean, it's burning clean!! Contrary to most beliefs the higher octane fuel will give you MORE power and COOLER running. I've run pump gas, AV gas, and Race fuel in my quad numerous times and pump gas sucks once you burn the good stuff a few times.

08-13-2001, 05:37 PM
I recently experienced the same problem.

I am running a stock filter with a combo of different airbox mods, and a modified stock exhaust...........

stock jetting is a 148........I went to a 150 and a 155 and I am still burning white........I was going to go to a 160 & higher, but after reading this I might not.

The Washington DC area has been said to have alot of blended fuels which may be why I am running such a clean looking plug.

My old plug looked carbon fouled when I removed it for my plug tests. The old plug had been in the bike since before I modified the airbox & it might have been running rich to begin with.

I will wait awhile & check it again to see if I get any color after a few rides.

08-14-2001, 08:30 AM
Leo - this should probably be moved to the "best of" forum at some point; a lot of good information for people who have jetting questions.

08-14-2001, 03:24 PM
After reading when you run race fuel or av gas it burns clean and white i went and checked my plug. Well I do run some race gas that i have left over from the weekends from my race car. I run about 40/60. I see that my electrode is nice and brown but on the edge of the plug right above the threads to screw in the plug it is white on that space. So it must be burning clean then???? Least i am not the only one getting the white................

08-14-2001, 03:46 PM
Well guys, I got some pump 92, drained the Av. gas out, and ran it for an hour, then went wide open, checked the plug, and it was still white. This seems weird, because before I added the pipe, my plug was always nice and brown after dialing it in for the air filter and airbox lid modification.

Oh well. I have a 172 in there now (I think :rolleyes: )

I even tried an older plug, and it turned from dark brown to light gray. I think it is fine now, it runs good, and anything above the 172 was too rich.

I will try ogre's method too see how that works out. I feel better about the plug being white now. You guys have been a great help, thanks!

08-14-2001, 04:25 PM
skemp, are your intake boots tight around the carb and at the head? There shouldn't be any reason why you're still burning white. If I were you I'd have the bike sniffed that way you know for sure what your air/fuel mix is. Something just isn't right...

08-14-2001, 07:23 PM
I am up to a 165 and mine is still burning white & it still runs fine........no real power increase or decrease.........plug is white with some carbon on the threads.

08-14-2001, 08:19 PM
Guy, I think maybe my boss was pulling my leg when I asked him about the gas. Right on the pump it says "With Super XHP Additive to keep engines running longer and increase spark plug life" I think that explains it right there. Actually, the plug is getting some brown color to it, so I think it was just the gas, and I needed to run all the Av. gas that was left in the tank out.
I will definetely keep you posted on how everything is going however, and I called around to see which auto shops do a sniffer test, and found two nearby, so I can always go there if all else fails. Again, thanks for the help you have given me thus far. I will be back when I need more, or just to tell you how this is working out. Like I said, it feels strong, like I can tell a difference when I ride it after getting the jetting correct. I think I might have it right this time.

08-16-2001, 11:38 AM
i ran 110 octane from a store near silver lake and i noticed the end of my stock pipe had a light grey and with carbon powder on it ...i think it was from thelead in the fuel

08-16-2001, 08:15 PM
I think some of your problem is with the av gas. It has special driers in for the the air planes and will naturally give you a whiter looking plug. Try some regular 93 and see what you get. Also check your carb intake boot where it bolts onto the head. I have replaced several of these because the get a stress crack on them and your bike will suck air and run lean. The always crack at the bottom and you should be able to see some fuel leakage there. I have replaced about 60 of these in the last few months, including the one on my 400 two times.


08-19-2001, 01:58 AM
All is well with my problem. I am happy with the setup I have right now. I have the Dynojet tapered needle on the 3rs groove, a Keihin 172 main jet, and soon to have a 40 pilot to remedy the idle-1/4 throttle hesitation I get. It runs great in all other aspects; no popping on deceleration, skipping on acceleration, or the like. The plug finally has color, and it is the most beautiful tan color I have seen. :D
You guys were helpful, informative, and never underestimated me. I appreciate that. Now that Leo put this in the "Best Of" forum, others can use it if they experience unusual jetting problems like I have. I feel so much more confident in myself after the suggestions you gave me. I'll see ya on the board!

09-03-2001, 11:09 PM
great thread, good info!!

09-18-2001, 03:31 PM
you might want to check tihs out!!!pull the vent hose that goes from between the intake and the carb and over the carb to the heat shield. pull it out start it up let it idle and put a finger over it.if you feel a vaccum, if you do you've got a problem..don't know the fix just yet but hopefully soon enough.any suggestions on this problem would be greatly appreciated!maybe something to do with the air cutoff valve.i don't know?

09-18-2001, 03:39 PM
you might try this also.pull the vent hose that goes over the carb and into the heat shield start the motor let idle and stick a finger over the end.if you feel a vaccum you have a problem.don't know the fix just yet but i have the same problem,this suction is probably going straight into the motor causing a lean condition.i think every one should check this but its just my opinion.......any ideas of what is causing this would be greatly appreciated....

09-20-2001, 08:27 PM
Get rid of the AV-Gas. It will give false plug readings. Av-Gas is designed to run in high altitudes and has different additives and properties than pump gas or race fuel. If you have NOT bumped the compression, then use 93 pump gas. I use Unocal 110 on my hot rod stuff. If your carb was sucking air from the folded boot then it won't matter how big of a main you put in, it will still run lean. Good luck...

09-29-2001, 12:45 PM
Lose the av-gas.Another old wives tale that needs put to rest. No professional race team anywhere runs av-gas!NONE!NADA! This is a big clue! Airplanes aren't the same as land based vehicles in their fuel requirements.Check out the pro's fuel. VP,Elf,Trick,and others are the common choices.Your carb being tilted will give you false readings big time.Don't get sucked in to buying "race" gas just because you think you need it. Excess octane will not help your quad run better unless you count a lighter wallet a perfomance mod. Only buy what your bike needs. Even a heavily modified engine will usually run on 50/50 mix of pump gas and and race fuel.Hi-compression motors need the octane,not stock or close to stock.If,after all your jetting trys,you still can't get it right,please check your ignition timing. If it moved when your mom cleaned the garage,it could also give a false jet reading.(only kidding about the Mom part,not the timing) Check out the VP web site to see what they recommend in fuel.:p