I just purchased a used 2003 z400. WOW, what an awesome bike! The problems I have is that it will not idle properly without the choke engaged slightly.

Also, when I give it a little throttle and hold it steady in neutral, it seems to backfire through the exhaust.

Otherwise, it has a lot of power and runs great under a load.

Any suggestions are appreciated.

 

For the idling part, just turn the idle up on it.

And as for the backfiring, sounds like a jetting issue to me, probably need to jet it up some.

 

Well the fact that it's not running properly unless choked and it's backfiring through exhaust indicates that it is lean. It could be lean for a number of reasons. Aftermarket exhaust, intake, or even an airleak. Could just be carb plugged up too, which is a pretty easy fix and should probably be first step (Just take it apart and clean out all the passages and jets.).

 

Thanks Sparky. I will try to clean the carb. This bike has been sitting for a little while so maybe that is the problem. I am a little mechanically inclined. Is this an easy job that I can do myself, or should I take it in for a carb service? The local shop wants $100 for a standard carb service.

 

I've never had it apat on my z. On my lt, and every other carb I've ever worked on including the holley I had on my bronco they were pretty easy. I got to put some new jets in my z once I install my pipe. I'm gonna put those in myself. $100 doesn't sound bad if they're gonna clean AND make sure it's jetted properly.

 

I was just dealing with this very topic this past weekend. Went up in altitude (Denver into the mountains) and wanted to keep all my power. When I hit the throttle, power would be fine until higher RPMs where the power dies, then I would have the same symptoms (small backfires, had to have choke up slightly to keep the engine running). The spark plug was also black, meaning the fuel/air mixture was too rich (too much gas getting into the carb/engine). Also of note, I was running with the air box lid off (otherwise my '03 LTZ400 is stock).

Had a 120 jet installed when I started and tried going down in sizes. Got down to a 112.5, and the symptoms got worse. After asking other riders around, they said the fuel/air mixture was not right (knew that). A couple suggestions they had was to tape the air box shut (left the top at home; tip to everyone, take your parts with you when you ride) to see if that helps things. After trying that, the backfiring cleared up and got back the high end power. We also tried a bigger jet; tried a 124. This helped things some more. If I would have had more jets at my disposal I would have tried about a 130.

So I'm not really sure what the answer is considering I'm not done tuning. I think the info to come away with is, it is likely that your fuel/air mixture isn't right. Perhaps cleaning the carb is a good thing, but if they are going to be taking the carb apart, you might as well trying rejetting!

 

When you go up in altitude you need to go richer. If your plug was black it was probably because it was rich at your normal elevation, but not rich enough for the elevation you went to. That's why when you put in the bigger jet it helped. Mine has a 130 stock, but my carb is also 2mm bigger.

 

Maybe it is running rich. I changed the plug and the element looked ok, it was a light brown color. However, the first quarter inch of thread on the plug was black. Do the THREADS being black indicate a rich carb?

 

Frank...check your mixture screw settings...and your pilot jet...they will both mess up your idle...main jet is more evident higher up the RPM , hope that helps

 

A spark plug isn't meant to be in and out over and over. They have, what is meant to be, a one time crush washer. They seal when tourqued down. If the plugs been out a few times, you might notice some black $hit on the threads from the combustion gases blowing by. If it was running rich it would die, or just about die, when you choked it. Does it ever backfire when you get up to speed, then let off throttle real quick? If it does then you do have issues with main jet. But check screw like rolsmojave3 said.


Warm it up to operating temp, then turn the screw all the way in until it stops. Don't torque it down, just until it lightly seats. Now start turning out slowly and count the turns out. Make sure you count 1 turn as 360 degrees (Duh I know, but I've watched somebody counting before. Each time it went around 180 degrees they counted 1,2,3...) At some point it should be idling at it's highest level, and as you go beyond this point it will start coming down again. Turn it back to that point where it's idling the highest. If you have to go up to 3-3.5 turns range then you either need next bigger pilot jet, or you got some junk partially blocking that fuel circuit.