My brother in law has a 400ex that is sputtering awfully bad. It seems to be getting worse. Haven't pulled the spark plug yet as I don't have a wrench to get to it. It's much worse today, and got worse the more we rode, than it was the last time.

At first, it had a bobble at 1/8 throttle, still kinda does, that is intermittent but now it's doing it at all rpms. It's most noticeable when it's under load, such as in soft dirt and/or acceleration.

He just bought this quad and I'm wondering if maybe it is jetted for the higher altitudes the fellow said they sometimes rode at, although I was under the impression it's been a while since then. We're at 4,000' altitude.

Any help would be appreciated.

kyle

 

i would first clean the carb out and check to see what it is jetted at.

 

Got an idea what size(s) I should have? I was thinking about taking it apart before I posted but decided to go ahead and see what I came up with first.

 

Stock jetting is 148 and 38 for the pilot. The needle is in the second spot. If it is piped a 160 to 170 and 40 pilot should keep it running good.

Get that spark plug out and change it. They go bad and it will still run, just not very well.

 

stock jetting should be a keihin 148, 38 pilot jet, and needle clip in the 3rd position, NOT the second.

your jetting may change if you have mods done to the bike.

 

Thanks for the replies. Here's what we've found so far. Getting the spark plug out is a #$!@#$!@#$ without the spark plug wrench made for the 400ex. So far, we're still working on that. A 16mm spark plug wrench is too small but a 17mm seems too big. Anyone got any insight on that?

The main jet is a 160 The bike is all stock. My BIL just bought it and that's all we know about the bike.

I think the engine is running rich. It smokes more than it should and doesn't accelerate like it should, or at least I think so. That leads me to thinking it's rich. And my 240lb frame runs side by side on my 350x vs 160lb on the 400ex. I think he should outrun me, don't you think?

Is there anyway to get the spark plug out without the factory spark plug wrench. The wrenches didn't come with the quad.

Forgot to add, it stumbles whenever you "blip" the throttle. It sputters for a second and then revs.

 

look at the mainjet again and see if there is a K by the 160. this K almost looks like a star. it that is present, you have keihin jets in. if not, you are probably using dynojets. also, look at the clip on the needle to see where it is located at. when you have the needle off, see if there is a washer under the needle. also, look and see what size pilot jet is in the bike. either way, your too rich on the mainjet whether it has keihin or dynojets in. but, i think your spuddering problem is either a incorrect pilot jet or wrong needle clip location unless you have an electricalproblem.

 

UPDATE:

Cal, the main has the K on it. I then pulled what I think is the pilot jet. Does it have a number somewhere on it? I couldn't find one anywhere. This would be on the bottom right of the carb if you're looking at it from the left side. I put it back in. I unscrewed the air screw and it looked fine, put it back in at 1.5 turns out. It was at 3.0 turns out. Put the gas tank back on, and took it for a spin. It was a different machine. Didn't sputter at all and didn't seem to smoke although it was hard to tell in the barn with not much light. It felt much stronger when I rode it.

I didn't disassemble the carb so I don't know about the needle position. I wonder if there was a speck of dirt somewhere restricting or what else I don't know. I was pessimistic because it wouldn't sputter and then took it for a spin and you know the results.

 

the pilot jet is right next to the mainjet but doesn't come out as far as the mainjet. you will also have to remove the pilot jet with a flat blade skrewdriver. yes, there will be a # on it. you may just have to look close. it will probally be somewhere between a 38 and 45.

 

also, 1.5 turns out on the fuel skrew is probably not enough if your running a stock pilot jet (38). are you sure your went out 1.5 full turns? just making sure because that is a full 360 + another 180 degrees. some people tend to go out 180 degrees and think that is one turn but in reality that is only 1/2 a turn.