stalling FZR400

[zed]

I've searched the board - didn't find a clear answer to the problem we're having:
My boy's stock FZR400 has an issue - it will start fine and take off, but after a few miles will suddenly lose power completely and die. Less than a minute after dying, it will start again, ride off no problem, only to die again after a few miles. It behaves exactly as my pick-up does when its petrol filter is dirty, or the capacitor or plug wires need replacing. We changed the plugs, and the fuel filter - no improvement. I read that the FZR is very sensitive to voltage - but I assume that if the starter will crank the motor, the battery must be ok? I know very little about bike motors... any advice would be much appreciated
thankyou

[DonTZ125]

Your inlet vent in the cap is fouled, preventing air from being drawn in as fuel flows out. A vacuum builds, until fuel can't flow out anymore.

[zed]

thankyou - I'll tell him to quickly take the cap off next time it stalls to see if there's a vacuum built up in the tank. Hope it's this easy...

[zed]

well, if anyone's interested, we're finally getting to the end of this saga. The problem isn't a fuel vent blockage - the mechanic showed me if you shake the bike and listen at the cap you can hear the gas hissing out at the cap vent.
Bike's been standing over a month now, kid's driving us crazy... mechanic says he's replaced O-rings under the carbs, plus the fuel pump - this has fixed the stalling problem, but now it won't idle! So he's ordered smaller jets - says those in the bike now are bigger than standard. Which doesn't make a lot of sense to me, because it ran fine for months on those same jets... I dunno, at this point buying my son an FZR was not a smart move!

[pefrey]

Have you replaced the fuel filter? Sometimes when they have a lot of crap in them, the crap settles to the bottom of the filter when the bike is turned off. When the bike is running, it stirs the stuff that's settled to the bottom of the filter which then clogs the filter entirely. The bike will restart when the stuff settles out again.

Also check your gas tank for crap. Replacing the filter can be useless if there's crap in the tank to re-clog the filter.

When those two steps are complety, drain the carb float bowls. There could be crap in there as well which probably won't cause your bike to run then stall out, but cleaning them out now will confirm that there's no more crap in the tank or filter when you check the float bowls after running the bike for a while.

I hope this makes sense and I hope it is what is wrong with the bike. Otherwise, somebody else is going to have to take a guess at it.

[pefrey]

Oh, and the vent cap thing Don mentioned is pretty common, I'm surprised it was not that.

[Evilchicken0]

I think maybe you had an air leak getting past the carbs (hence the new o rings). Too much air makes the engine rev higher and it could be the last owner tried to sort it out with bigger jets ... well maybe
When these bikes run right they're nice and actually quite reliable

[zed]

thanks for the advice. I did replace the fuel filter - but the filter I removed was clean. The mechanic now says there are square 'o' rings under the carbs, and he reckons the bike flooding at idle is caused by fuel being drawn past them... so we're waiting for new 'o' rings from Yamaha Japan! another 3 weeks... this mechanic has a good reputation here, and used to ride an FZR1000 himself... when I suggested he use Loctite Grey to 'help' the 'o'rings seal, he didn't agree

[Evilchicken0]

If you use anything other than a propped o ring it can break down and foul the jets ... After that it can go into the combustion chamber and you don't want any of that.
Trust your mechanic on this one.

[zed]

well, we have the bike back at last, and going really nicely so far. Cost a packet but worth it to see my boy smiling on his FZR again.. Seems like those O-rings are important parts. Thanks for all the replies here