Missing & Backfiring?

[tywebb3]

I picked up an '05 400ex a couple weeks ago and have been cleaning it up and going over it since I got it. I pulled the carb and it looked very clean, changed the oil, oil filter, etc. I noticed today while driving it that it misfires, backfires (through the intake I think), and generally carries on at low throttle and when let out of it it even shuts off every once in a while. If I just let it sit in neutral it idles perfectly? I noticed this when I first rode the bike but thought the carb might need to be cleaned....but it looked good?

I pulled the air box lid just to see what it would do and it seemed to make the backfire worse which leads me to believe it is a lean condition? Could it be that I need a fatter pilot jet, or could it be something else all together? Stock jets are in the carb right now, 148 main and 38 pilot.

[Ryan'07400ex]

I've always wondered same thing but it may help to know what mods have been done to it. Like an aftermarket exhuast would probably make it lean

[tywebb3]

Sorry...I thought I put that in there. The bike is stock except for one of those billet exhaust tips and I put some 450r front shocks on it this weekend. Other than that it is bone stock.

It seems like the problem is the worst when I am just lightly on the throttle. Like when I was giving my daughter a ride and we were in 2nd or 3rd gear and I was just lightly on the throttle it was bucking, surging, and missing on me pretty bad. Also I had it shut off on me a couple times when I just pulled the clutch in, let off the throttle and was drifting to a stop...say to cross a road. But open it up and it seems fine or let it sit and idle on it's own and it seems fine? I guess I could try a 40 or 42 pilot and see if it helps. I was just curious if this is a common problem with these bikes and if so what the fix is.

[Ryan'07400ex]

Originally posted by tywebb3
Sorry...I thought I put that in there. The bike is stock except for one of those billet exhaust tips and I put some 450r front shocks on it this weekend. Other than that it is bone stock.

It seems like the problem is the worst when I am just lightly on the throttle. Like when I was giving my daughter a ride and we were in 2nd or 3rd gear and I was just lightly on the throttle it was bucking, surging, and missing on me pretty bad. Also I had it shut off on me a couple times when I just pulled the clutch in, let off the throttle and was drifting to a stop...say to cross a road. But open it up and it seems fine or let it sit and idle on it's own and it seems fine? I guess I could try a 40 or 42 pilot and see if it helps. I was just curious if this is a common problem with these bikes and if so what the fix is.

I doubt this would cause this but is your choke on

It shouldn't need rejetting with just that tip cause that wount change the airflow very much.

Your elevation could be making it lean but idk whats your spark plug look like? What color?

[bmid450]

do you know were the fuel screw is set at? if your bike is stock it should be out 1 1/2 from lightly seated

[tywebb3]

I'll have to check the fuel screw tomorrow to see where it is set. The choke is off. I tried to pull the spark plug this weekend but don't have a socket that works so that is on my list of things to get. I definately would like to pull the plug and see what it looks like.

[tywebb3]

When you say fuel screw are you talking about the pilot screw on the underside of the carb? I just went out and took a look at it and where it looks like there should be a pilot screw there appears to be a brass blank up in there? I scraped around with the screwdriver thinking I would knock some dirt out and find the slot of the screw head but couldn't? I am looking at my Clymer manual and was in the right spot.

[78ta]

yes that is the pilot/fuel screw. pilot screws on 05+ I believe are the "D" shaped head, take a special screw driver bit to turn them. I made a tool to take them out, once out I cut a slot in the "D" so I can insert a small flat blde screw driver. I would suggest taking the carb off and taking it completely apart and cleaning everything including the pilot circut and take out all jets ect and clean and blow everything out. be careful, under the pilot/fuels screw there is a spring o-ring and washer...don't loose these!

also check spark plug and make sure it is tight, I had one do some funky things like what you are saying yours does and all it was was a loose plug!

[tywebb3]

Nothing like making it easy to adjust....but then again I guess that's the point. I removed the pilot screw tonight and cut a slot in it. From what I could tell it was at about 1 3/4 turn out. I guess once I get the carb back on the bike I will mess with it to see where it likes it best.

Once I get my new 18mm Craftsman spark plug socket ground down I will check the plug...maybe later tonight after the kids go to bed.


....the plug definately was not tight in the head as I was able to tighten it up at least a full turn. I took it out and it was gapped good at .026" and the color looked good as well. The ceramic has a nice light brown color to it and it is clean and dry without any deposits. The is some carbon build-up covering the portion just below where the threads stop.

I think at this point I will just put the plug back in good and tight and run through my carb one more time real good. Then when I get it back together I will mess with the pilot screw to get the best idle. Maybe that will take care of the problem?

[78ta]

the loose plug was prob your whole problem, but clean that carb, make sure you clean all passages and blow them all out with compressed air. I would start with the pilot/fuel screw 2 1/2 turns out. and go from there.

[tywebb3]

I got it back together tonight and it seems to like the pilot screw 3 turns out. At 2.5, 2, and 1.5 it acts up, misses, terrible throttle response, etc.

So does turning the pilot screw out introduce more air or more fuel into the circuit? Did I fatten it up or lean it out?

[bmid450]

you are allowing more fuel for the pilot circuit the further out you go.does your EX still have the 38 pilot in it and is your air box lid on or off? sounds like you could use a bigger pilot,I put a 42 in mine and was able to completely remove the chock and it starts better than ever.

[bmid450]

I also should of ask if it has any other mods,slip on or after market filter?

[tywebb3]

Everything is stock except it came with one of those billet exhaust tips in the stock pipe, and it has a stock air filter with the box lid on. The pilot is a 38 and the main is a 148. I was going to wait to rejet after I get a K&N and make an outerwears airbox lid.

[78ta]

turning out is more fuel, in is less. 3 turns out is fine, but seems a bit excessive for a stock bike, at 2 or 2 1/2 turns out it shouldn't miss and run horrible bad, it might be a bit hard to start and not want to idle when cold if the fuel screw is a bit lean.

did you take the pilot/fuel scew out and clean that passage?

[tywebb3]

Yes, I cleaned everything thoroughly including the pilot screw area. Is running a stock bike 3 turns out unheard of? Mine sure seemed to like it. Is that a sign that I need a larger pilot jet?

[bmid450]

3 turns out seems excessive for being bone stock,make sure your not pulling air from somewhere else,like around the intake boot or oring,you can haize a little starting fluid around connections while running,if it picks up rpm then you have a leak.If you confirm there is no leak then adjust fuel screw out to the highest smooth idle.In regards to a bigger pilot it will definately help with starting on the 400ex as they tend to be cold blooded and also will keep you from having to back fuel screw out so far,If I have to go past 2 1/2 turns I will step up to the next bigger pilot,as you back it so far out you lose pressure on the spring, I have had one vibrate out on my 450 while out in the dunes.

[tywebb3]

I was back out messing with this again tonight. I noticed that at 3 turns out the bike will sit and idle perfectly...but if I push the throttle to 1/8 - 1/4 throttle and hold it, it will sit there and pretty much consistantly backfire out the exhaust. It just pops, cackles, and carries on. I held the throttle and let it backfire and at the same time gave it some choke and it rev'd up, smoothed out, and the backfiring went away which leads me to believe it is a lean condition?

I have sprayed starting fluid on the intake boot and all around the carb and the engine never picked up one tick so no leaks were found. Is is possible that with a gutted stock muffler and the billet tip it would improve the flow that much and lean it out? The plug looked good so the main jet seems right. I have a 40 and 42 pilot on the way to try out. I was just wondering if I adjusting the needle any would help? I was also thinking of trying the pilot screw 3.5 and 4 turns out to see if that does anything. I wouldn't leave it there but am just curious if it helps any.

[78ta]

was the needle in the middle clip? airbox lid on?

going out on the pilot more won't hurt anything. you could try it, just keep an eye on it so it doesn't fall out.

stock motor really shouldn't need a bigger pilot to run right, it will help it start much better, but it shouldn't backfire and run that lean. I think you have something else going on here.

I've seen 400ex's with full exhaust and lid on run fine with stock pilot and main, I'm not saying they wouldn't run better after they were jetted, but they didn't run so lean they backfire like you are describing.

[tywebb3]

Could a leak were the head pipe bolts up to the engine cause something like this? I don't remember seeing any signs of this but I can check the header bolts later to make sure they are tight.

How about on the ignition side. Would a defective ignition contol module or ignition pulse generator cause something like this? Or when they go bad would the bike simply be dead?

I'm just thinking maybe it isn't a carb problem and I need to start looking at other things.

[tywebb3]

The pipes are tight at the head with no leaks. But I did notice tonight that when warming it up once I turned the choke off and gave it about 1/8-1/4 throttle to get it pop, the head pipe (especially the left side) was cherry red for about the first 4-6". I have had this happen on a snowblower in the past and it ended up being the exhaust valve was not sealing due to carbon build up.

Could this all be because I adjusted the valves incorrectly or do you think I have a more substantial problem in the engine? I followed the specs in the Clymer manual for the valve adjustment but maybe I goofed? Should I be checking the valve clearance with the engine cold or hot? When I did it the other week the engine was cold and it was in the 40's deg F outside.

[78ta]

could very well be a valve problem. You need to check them cold, the clearance is there because as the engine heats up everything expands, If you had them too tight as the engine heated up the valves could be held open slightly and not allowing them valve to seat and seal up.

pay close attention to exhaust valve clearance, it will make exhaust header glow if they are too tight and leaking, just like your snowblower.

If the clearance checks out ok, you need to do a leak down test and see if they are sealing up.

[jasonwayne222]

when u cleaned ur carb did u pull out the main jet and poke out all the holes, just to make sure ur carb is cleaned properly get some carb bath, comes in a gallon can, take carb apart including all jets, everything and let it soak overnight, if u had an exhaust leak then u would have it pop on decel, it will throw fire out the exhaust if it is a bad leak, check valve clearance, and if all that checks out then do a comp. check, if it fails comp. check then u may have a screwed up valve.

[tywebb3]

I readjusted the valve and they are now .005" exhaust and .004" intake. The bike still backfires when you hold the throttle at about 1/8th - 1/4 throttle....and I noticed that is when the header pipe gets red. This makes sense considering it must be lean at this point. As soon as you let off the trottle and let the bike idle the header pipe cools right down and no more red.

I did a compression check and it was a consistent 105psi each of the three times I tried. My Clymer manual says spec is 100-124psi so I guess I am ok although I don't really like being on the low end of the range.

I also did a leak down test and it was 4% which is pretty good...right?

I guess at this point I am just going to wait for my 40 and 42 slow jets to show up and I will see if they make a difference. If not, I guess I'll take the carb apart again.

[jasonwayne222]

clean the carb how i suggested, if ur bike ran fine and all of a sudden its doing this then you shouldnt need a bigger pilot, you didnt change anything to need a bigger one, the old one may be plugged, and if you dont clean ur carb how i suggested alot of the time there will be old pieces that were gummed up in there and come loose and clog it again so do it right. if its not ur carb then possibly u have jumped timing, mine was one tooth off awhile back before i knew how to time it and it did something simialar to that.

[tywebb3]

I bought the bike used a few weeks ago and it has been acting strange like this since day one. I have just been chasing the problem. The muffler is gutted and it has one of those billet tips with a screen in it. Could that be effecting it?

I am going to try installing the slow jet to see if it improves things. It is simple enough and will only take a few minutes.

If that doesn't fix it I will disassemble and dip the carb. My brother just picked up a used '06 400ex that is stock so I may also try swaping carbs with him to see if that fixes things. If not then I will start looking elsewhere.

By the way, if the timing was off wouldn't my compression numbers be way off?

[jasonwayne222]

with it being off a tooth the valves are gonna all be closed at some point just not when they should be the piston will be in the wrong degree into relation to the valves proper open and closing sequence that may be why u are on the low side for compression as your piston is compressing if the timing is off a tooth then the valves would be closing just a little late, so u would still get a comp. reading. and ya swappin carbs is always a great way to illiminate the problem. Is your muffler gutted with no packing at all? maybe it would affect it but i wouldnt think in that way. but if u get access to another one then try it too.

[tywebb3]

Could the timing chain "jump" a tooth. The guy I bought it from said the bike is bone stock except for the tip he put in the stock muffler. He didn't strike me like the kind of guy that would be digging in the engine and messing around with the timing chain.

With a 4% leak rate should I expect a higher compression reading from a stock bike?

One more thing, when I have the engine at TDC all (4) rockers are loose. Wouldn't that mean the timing is correct?

[78ta]

did you hold throttle wide open when cranking while doing compression test?

stock cam in the 400ex have an auto compression release, so cranking compression will be on the low side, generally anything over 100psi with a stock cam and piston is acceptable.

try swapping carbs and go from there, 4% leakdown sounds good.


It could have jumped a tooth on timing, but for that to happen the timing chain has to be extremely loose. I would think you would be able to hear it slapping around in the engine while it is idleing

[tywebb3]

I did hold the throttle open while doing the compression test. I'll report back after I mess around with the carb(s).

[tywebb3]

I tried the 40 pilot tonight with the pilot screw out 2.5 turns and it didn't make a difference.

I started disassembling the carb so that I could dip it and have everything off except for the throttle plate, shaft, and assembly. The screws that hold the throttle plate on are split and bent over on the back side I guess to prevent the screws from backing out and getting sucked into the engine. Is there an easy way to remove these or should I just leave them in and forget dipping the carb body? I have the carb dip ready to go but I don't want any plastic parts going in it. I could always just spary it down with carb cleaner and blower it out real good. I know the carb dip would do a better job though.

I checked my needle and it is not adjustable. There is only one groove in it? I thought I read guys were moving the clip to various setting?

[78ta]

05+ 400ex's have non-adjustable needles in them. Its a common mod to install an older adjustable needle in them for tuning purposes.

Don't worry about soaking the carb body, but completey dis-asemble everything off of the carb except for stuff that isn't easily removed then just soak all metal parts and jets in the carb dip and blow them out with compressed air once clean. I normally leave stuff in carb dip for a couple days to get good results, but my dip is old so you may have better results in shorter time.

Then before assembly spray carb body and all ports and holes with carb cleaner or brake cleaner and blow them out with compressed air also.

be very carful with carb dip, it is really nasty stuff!


on a side note, I have been cleaning carbs with purple power degreaser recently ( soak in it if really nasty), then rinse with hot water and blow off, then I give it a once over with brake cleaner and a little wd-40 to keep from corroding. Purple power cleans carb varnish very well, plus its not as harful on plastic and rubber like carb dip and brake cleaner is.

[tywebb3]

I picked up some of the Purple Power today but it cautions against use on aluminum. I also read someone online saying not to use it on bare aluminum. 78ta, did using 100% strength Purple Power on your carbs damage the aluminum or the finish in any way? I was going to dip the entire carb body in it because there are a few plastic parts that I can't get off of it in the throttle assembly/linkage, but I have concerns since the carb is aluminum.

[78ta]

purple power is kinda acidic, it will discolor polished aluminum, won't harm raw cast aluminum, but if you have any polished aluminum it will stain it white from oxcidation and you will be re polishing it! It will clean off baked on mud off engine cases heads and cylinders well too. just make sure you get it all rinsed off and dried completely and spray a little wd-40 on it after your done to keep it from oxidising

your carb body will be fine, I use it full strength, but if your worried about it use it 50%. You won't need to soak in it very long...like 10 mins tops. I use a spray bottle filled with purple power and spray carb body down very well and let it sit on it for a bit and rinse with hot water. repeat if nes. I only submerge jets and other completely metal parts in a small pan filled with purple power, I have never had to submerge a hole carb body in a very long time.


I use purple power to clean a lot of stuff, the only thing you have to be careful with is like I said polished aluminum and annodized alum...like stock wheels...it will stain them too if left on very long. and once annodized gets stained it will never polish out!!! found that out the hard way!! a lot of stuff that looks like plain alum on bikes and quads actually is clear annodized so becareful. It will also discolor painted parts as well.

oh yea, wear eye protection and rubber gloves!!! I got it in my eye once, it hurts very mutch badly!!!

[tywebb3]

I did a super cleaning of the carb tonight. It was already disassembled so I sprayed the body, bowl, and accel pump housing down with purple power and let them sit. I washed the parts off with plenty of warm water and then blew it all off with compressed air. Then I sprayed carb cleaner through all of the various ports and passages in the carb followed with another round of compressed air through and over everything. The thing looks like new! I also decided to leave the choke off, blocking the hole for the shaft, and installed a 42 pilot.

With the pilot screw out 2.5 turns the bike still seemed to act the same. Maybe not quite as bad but I still detected some backfiring out of the exhaust when under light throttle. I reved it up to 1/2 throttle...maybe more and it sounded good. It just seems to be lean at around 1/8th - 1/4 throttle. I also noticed while it was cold that it was backfiring (hiccuping) through the carb. I felt air blowing back out the air box snorkel when it did this. Once it warmed up this stopped. Is this normal for these bikes?

It may be a couple weeks before I can get over to my bros house to swap carbs so I was wondering what else I could check. I was thinking I could pull the intake and inspect it? Maybe pull the header and install new gaskets? And as a last resort I could check the timing. I didn't really want to dig into the engine that much though.

[78ta]

its normal for them to cough a bit when first fired up if your trying to rev a completely cold engine, but should clear up within a few minutes of run time.

did you try to adjust fuel/pilot screw out a bit farther to see if that would clear it up?


the thing your going to run into is if you get the pilot rich enough to start and run perfect when cold, it will be too rich and foul plugs once it warms up, so you have to find an acceptable middle ground.


You can do what you want, but IMO removing the choke is going to cause you more problems than its woth power wise. It is an acceptable mod for a complete race motor that your trying to get every fraction of horsepower out of it, but thats it. I don't know where you live or what your normal outside temps are, but for me anything below 50`f and my bikes need to be choked to start cold if you don't wanna have to sit on them and play with the throttle for 10 mins while it warms up

[78ta]

forgot to ask , so once it warms up it runs perfect?

[tywebb3]

The coughing did clear up once it warmed up. I still noticed a little backfiring at 1/8 to 1/4 throttle. It seemed a little better but was still there. I guess I will put the choke back on it and see if adjusting the pilot screw out 2 3/4 and maybe 3 turns out clears it up the rest of the way?

[78ta]

try that and get back with us. is the muffler completely gutted? or just the tip removed and a "performance tip" in place?

[texasman20]

mine does this exact same thing. runs like a champ when i get on it but when i just put around with very little throttle i can feel the engine pulsing like it is missing or something. I am gonna throw a new plug in it and clean the carb again too. I will post if i have any success with fixing it.

[tywebb3]

texasman20....sorry to hear you are suffering from the same problem. I hope you have better luck solving it then I have. Are you running a stock exhaust and stock jetting?

78ta....I pulled the carb and intake boot tonight. The intake looked like it was sealing good so it confirmed the spray test that there is no leak there. I reinstalled the choke assembly and put a 150 jet in it. I know this is just for 3/4 to full throttle but I figured what the heck. I was wondering, since the needle controls 1/8th to 3/4 throttle should I try shimming it up a little with a couple washers since mine only has one groove?

I pulled off the "performance" exhaust tip tonight to see what was going on behind it and there was nothing but a screen. So basically I am running with a gutted muffler and a "fancy" piece of billet aluminum on the end of it with a screen to knock down any sparks I guess? Would running a 42 pilot and 150 main be too terribly crazy for a stock bike with a gutted muffler? I also just got a UNI air filter that I am going to install but haven't yet.

I hope to get it back together and have time to run it on Saturday.

[78ta]

inside the stock muffler behind the stock tip/spark arres. there will be a a wall a bout 6" in or so with 1 or 2 small pipe ends you can see ( i don't remember exactly what the inside looks like, its been a while since i've looked at one)

do you see what i'm talking about? or can you see hollow pipe all the way to where the head pipe clamps on to muffler?

the reason i ask, is because it is possible to grind out the all the inside baffles, but it is a lot of work. I doubt somone completely gutted the stock muffler, but if they did remove all baffles (i'm not just talking about your performance tip) it could be making your bike run funky. If its just the tip thats different, I don't think that is gonna make enough of a difference to srew up factory jetting so bad that it doesn't run right.

now on to the main jet, the main jet will have an effect on the entire throttle range above idle..I know they say 3/4 throttle and up....but that is not 100% the way it works. Think about it, the needle rides inside the main jet passage. So anytime the needle isn't completely closed and sealed the main jet is feeding fuel.

so, yes changing the main jet might change a/f ratio enough to notice a difference under 3/4 throttle. but only going up 1 main jet size won't have a drastic effect on under 3/4 throttle. Adjusting the needle will have a greater effect 1/8-3/4 throttle.

IMO I wouldn't "shim" the stock needle, just because its going to be a guessing game on thickness and trying to find a tiny washer that is perfectly the right size isn't going to be easy. You would be better off getting a stock adjustable needle for an 99-04 carb.

sorry for the book, hope that helps.


where do you live?

[katch26]

Originally posted by tywebb3
I was wondering, since the needle controls 1/8th to 3/4 throttle should I try shimming it up a little with a couple washers since mine only has one groove?


from HMF website


"****** NOTE: you MUST use the N424-26 series Pilot jet. (It is the smaller 23.5 milimeter version) NOT the more common 28 milimeter version.*********
Since the needle is not adjustable, use a .032 shim under the needle stop to richen the 1/4 to 3/4 if needed.

[tywebb3]

Ok....I think/hope I got it fixed tonight? Here is my carb jetting and adjustments:

Main: 150
Pilot: 42
Needle: .024" shim washer under clip
Pilot Screw: 2 1/4 turns out

I have not had a chance to ride the bike but was able to run it in my garage and it seems to be much better. It idles great, and when I give it 1/8 - 1/4 throttle the popping and backfiring seems to be gone. If I hold it there the head pipes don't get cherry red any more so it looks like I got rid of the lean condition. 2 1/2 turns out on the pilot screw gave me a large stumble when I hit the gas. 2 1/4 turns out seemed to be nice and crisp with no stumble, not lean popping/backfiring, so I left it there.

I will take it out tomorrow and see if it acts better while riding and will let you know.

I also installed my new UNI air filter and cleaned out the air box but I doubt that had much of an effect on it?

One more question....the 40 and 42 pilot jets that I got from Rocky Mtn ATV measure 28mm in overall length where as the original 38 pilot jet measured 23.5mm. The narrow part with the holes that inserts first into the carb seems to measure the same it is just the head seems to be longer. Are these ok to use or will they cause a problem?

[78ta]

hope you got it fixed, I still don't see how a "performance tip" would require such drastic jetting changes, but at the very least you are ready for a aftermarket slip on!!

was your pipe completely gutted or just the tip different?

[tywebb3]

The muffler was not gutted. Just the tip has been changed out.

[tywebb3]

I had a chance to do a little trail riding today and the bike is running MUCH better now. No more popping/backfiring. No more shutting off when I pull in the clutch and coast. It might be a touch on the fat side but with winter coming I think I will leave it the way it is. Today it was 72F here. If anything I might try taking the shim out of the needle or going back to a 148 main jet. One change at a time to see if either or makes a difference. But for now I am just going to ride it as is through the winter.

I never did find a leak anywhere. The intake boot looks good and I took the header pipe off today to grind the welds and it looked like it was sealing nice and tight. I installed new copper header gaskets before reinstalling the header pipe so it is nice and tight once again. I have no idea why my bike needed the carb tweaking that it did but oh well....it seems to like it now.

Thanks everyone for all of your help.

[texasman20]

Got mine fixed today! Took the carb out and soaked it in cleaner over night. Guess there was something clogged somewhere. Runs like a champ now!

[f4iracer]

What would be the best shim washer to use on these non adjustable needles?

I've got a uni air filter and lexx slip-on and i'm looking to go up to a 42 pilot and a 155 main.

I have no idea what size shim washer i might need or how to put it in exactly.

.032" or .024"?

[tywebb3]

I just found a small washer I had in my washer bin. It measured .024". I placed it under the clip in the needle to raise the needle.

I took the shim out today to see if it made a difference and I really didn't notice a hole lot. I thought I caught a slight pop but could have been hearing things. It seemed to run well without it. So as of right not I am at a 150 main, 42 pilot, and 2 1/4 turns out on the pilot screw. I think I am going to stick with that until I get a slip on or mod the air box lid.

[texasman20]

So I was wrong on having it fixed the other day.?I actually got it fixed today. I bumped up the pilot jet to a 42 and all of my problems went away. I guess it was to lean and causing it to starve on the low rpms. The reason I thought I had it fixed the other day is it was very warm outside making it run a little richer. Hope this is the last I see of it. It all seemed to make sense so I think it will.

Glad to hear you fixed yours as well. It is a very annoying problem.

[tywebb3]

I think that 42 pilot is the key!