for most of the time iv owned my bike, probably for the last few years, my bike has always cut out over the whoops. im in Florida, and EVERYTHING is whooped out when im trail riding so it can become annoying. what happens is im cruising along 4th or 5th gear with he throttle pinned, and it runs great, hit the whoops and lean back and as the bike is jolted around over the whoops it looses power, it cuts in and out(no backfires or anything to crazy tho) its as if it starves for fuel, or possibly a loose wire jiggling around. iv been meaning to make some room in the garage to get a chance to look at the bike but it would be hard to trouble shoot it since i cant really test if i fixed the problem with the small test track behind the garage, no whoops, so i was hoping that you guys may have some experience with this, maybe its common?

right now what im thinking is i need to set the float level on the carb, possibly the carbs bowl just doesn't hold enough juice, iv never heard anything good about this series of mikunis. or possibly the wire where i spliced in my tether kill switch is loose. possibly the ecm is just going bad, when i turn the key off sometimes the bike may run for several seconds before it actually dies, this occurs mainly when the bike has been ridden really hard and its running hot. just what i can think of right now, maybe you guys can narrow it down and save me some time.

the bike is all stock other then a yoshi trs system, k$n, and i jetted it to yoshis specs... i think a 150 or 155 main jet in there, i wouldn't think that would be enough to drain the bowl out under harsh conditions.

do a search. i remember a thread about this being on here just a couple months ago. i cant remember what ended up being the problem though. bad memory. im sure though if you do a search it will turn up the thread im thinking of.

Look at the electrical very first. Ignition system checks. Negative battery cable, and ground that's hooked to the battery at the top of that checklist. There are two wires going into the terminal that hooks to the Neg on the battery. The small one breaks often, where it was soldered. Common. It's not gonna be fuel, I'll bet on it. It takes awhile to run the bowl out of fuel, and a quick smack on a whoop is not gonna do that. I'm just sayin'...

alright, ill check that out. that should save me time not having to pull the carb.

thanks for the input.

i just had a similar issue and believe it or not it ended up being my choke. might wanna give it a look.

that's interesting, how so? the choke cabel was hung up on somethings? i dont have a choke cable or anything like that, iv got a small choke plunger on the side of the carb, i added it long ago to clean up the handle bars.

i had a pro design tether kill switch, that switch has always been a pos, the bike will run with the tether removed. i have a really good feeling that could have been it so i got rid of it. i took the key apart, it looked to damn simple to have anything wrong with it. although, i have had to remove my headlights from the key and put them on a switch because there was so much corrosion that the headlight wires pretty much just pulled free from the key, i couldn't get solder to stick so i gave up and put them on a toggle switch.. i might just go ahead and replace the key with a marine toggle switch for the time being just to be sure.

the tether kill switch was not only a pos, but it had a really bad ground too.


normalz, the battery cables are all good.

Sounds like you're on the right track with looking at that key switch. I had a Pingel switch soldered into my harness, and had a problem with it. I used too much solder, and it made that section brittle. All but 3 of the strands of wire were broken because of it, and it made contact most of the time. The damn thing would cut out on me only when I turned left... It drove me nuts trying to figure it out, but when I found it, the fix was easy - cut out the bad section of wire, and re-soldered it, using less solder the next time. Never had a problem with it again, and I fixed it in 2005. Electrical issues can be a biotch sometimes...

yea they sure can, i remember after a put a new motor in my truck the wipers didn't work, methanol injection didnt work, radio, gauges, you name it. sometimes they worked, others they didn't. turned out i just left a ground off of one of the transmission bolts. i hate dealing with electrical but it almost always is something simple.

yrs the plunger was sticking open a little bit bike still ran fine and had power would just cut out over bumps and die when i let off the throttle while riding.. went berserk looking for the problem went over grounds cleaned carb (never checked choke)etc. finally checked choke cleaned it all out etc and its perfect now

so basically, your bike did this because of a rich condition, my choke may not be an issue, but maybe the bike is jetted to rich.


i took it out for a test run yesterday and it did the same thing. it seemed like it got worse as the day got on, as the motor got hotter. towards the end of the day i poped the fuse under the seat, this happened soon after i turned my headlights on which are wired to make the highs and lows come on at the same time, iv got the high beams wired like this on my truck for the extra lighting offroad and it works well, maybe its to much for the z400s electrical system to be powering both the highs and the lows, or maybe this problem is related to my other problem, the bike running bad over the whoops. who knows.


so next line of business is to get rid of the key, if that doesn't do it, drop the main jet a size and see if i possibly just have a rich condition. if that doesn't do it then maybe the ecm/revbox, or whatever the hell its called on an atv is going bad.

i was also thinking that maybe this is ok, i mean, it runs good otherwise, maybe i just ride it to hard. i wouldn't really expect a carbureted motor to do well beings bounced around so hard. another thought, maybe i rev the w00ter out of it too much, maybe im just hitting the rev limiter, maybe im so close to the limiter that once my back tires leave the ground for a moment it abruptly hits the limiter. i can handle the choppiest of whoops, and i nail them with the bike winding out at the very top of 5th gear, i really let them motor scream... i just don't know what to think of it tho, i could swear i used to be able to do this with this bike, its been so long tho. like i said, iv had this issue for years.

and for my last though, the petcock, its one of those stupid vacuum controlled ones. although i switch it to the free flow position, maybe that not enough, what if there's more to it then that. iv never really though about it before, iv seen them go bad a million times but its normally just as simple as they puke fuel out of the vacuum line, i replace the whole petcock, problem solved.

Check the diaphragm on the carb. This happened to a friends quad. It turns out the Slide was bouncing around due to the diaphragm issue.

i could see it, if the diaphragm was ripped it wouldn't have a strong enough vacuum to hold it open in the choppy stuff. iv seen ripped diaphragms before tho, normally it results in the bike just being slow and the slide never really opening in the first place.

took apart the carb today, diaphragms not ripped, choke is fine. i did have to turn the fuel screw about 4 turns out to get it to run and idle right tho. i thought the fuel screw was 3.5 turns out(may have miscounted) so put it there and the idle surged up and down, i thought that was a rich condition so i put it up to 2 turns, got worse, backfired on the down rev, lean. so i droped it down to 4 turns and now it runs right again, it just seems odd tho because i could swear it wasn't that far out before... i guess its not surprising, iv been seeing that its common to need to fatten up the idle circuit to compensate for this new fuel that burns richer then the old stuff.

the bottom of the carb was all clean, the top half was pretty gritty, nothing totally plugged up or nasty, just gritty. when i pushed the slide up with my finger sometimes it wouldn't return till i gave it a jiggle. so maybe that was contributing to the problem. the choke was fine, i took it out and it was all clean, a dab of never seize and it went right back in. float level is good as well.

so the carb is out of the way. i put a vacuum plug on the nipple on the carb that goes to the petcock to eliminate the whole vacuum operated deal, switched it to free flow. so ill eliminate the petcock as a potential problem. and then ill eliminate the key in favor of a switch and then its off to the riding spot for test run #2

i think its time to sell this turd. the valve lash is tight so the valves must be cupping already, cam chain is way loose too. these 4 strokes are pathetic, 3 years of use and it already needs a valve job and has an unknown electrical problem. well i tried it, back to the 2 strokes hopefully... i just dont know what the hell i want, the z is nice, but i dont like the motor. 450s are good but dont want a 4 stroke. 250rs are hard to find, banshees dont handle good...