yz400f running problems

Hello after having many two strokes i decided to purchased a 1999 yz 400f after riding my brothers 426.... before i bought the bike i was told it needed a rebuild but still runs, i started it up and it idles fine and starts pretty much first kick, it has good compression and all, but when i give it throttle it backfires and splatters, i changed the oil and filter, and when i took it up the street it runs fine but just doesnt have the power or balls and is slow as and when i back of it backfires i cleaned the carby and airfilter checked exhaust no leaks & good packing. could it be the timing or valves?? or am i just in need of a Rebuild! any help is much appreciated Thanks Adam.:excuseme: Here is a youtube video of the bike running.

Hello after having many two strokes i decided to purchased a 1999 yz 400f after riding my brothers 426.... before i bought the bike i was told it needed a rebuild but still runs, i started it up and it idles fine and starts pretty much first kick, it has good compression and all, but when i give it throttle it backfires and splatters, i changed the oil and filter, and when i took it up the street it runs fine but just doesnt have the power or balls and is slow as and when i back of it backfires i cleaned the carby and airfilter checked exhaust no leaks & good packing. could it be the timing or valves?? or am i just in need of a Rebuild! any help is much appreciated Thanks Adam.:excuseme:

I would check the valves. I just recently sold my yz400f and upgraded to a '07 yz450f, and the yz400f was never low on power. It had the meanest low end power of any bike I have ridden. Good luck figuring out the problem.

sounds like u are running a little lean. adjust your slide needle to the #4 groove and start from there. pilot screw should be about 1 1/2 turns out

Check the valves by all means but if it starts OK these should be OK. I would however check the valve timing. After that it is most likely the carburettor. If it has good compression and is not burning oil (blue smoke out the exhaust) a rebuild may not be required, just a good tune up.

Another common mistake made is to fit the throttle slide around the wrong way which obviously adversely affects the running of the bike.

Check the common/maintenance threads out on this forum for tips on the common faults.

yeah ive checked the carby adjusted the slide needle to the 4th groove and adjusted the pilot screw, put it back together and it started first kick but still played up it idles fine and all but just backfires after i give it throttle, and has no power it doesnt rev right out like they should and sometimes cuts out. it doesnt blow any smoke and the compression is still good. im going to check the valves and timing chain tomorrow. i suppose ill just eliminate one thing at a time. thanks for the help:thumbsup:

checked the timing chain it was tight and valves seem ok so im moving on back to the carburettor, its got me what the problem is. if anyone gets a moment to look at this youtube video to try and diagnose this problem it would be a great help thanks.

Does anyone else have an idea what this problem could be???:doh:

hello i have just rebuilt my y400f 99 model and i still have the same problem as described :bonk: it still starts first kick and all no problems but if you give it gas it runs fine up untill high rpm it doesnt rev out like hit the rev limiter if i take it up the street it just has no power no snap i thought the clutch might be slipping but it does the same in neutral doesnt snap like these bikes should and has a huge backfire withblue flames, ive done the following

- Rebuilt top end

-raised needle

- new airfilter cleaned airbox

-cleaned carby, put in new diaprahm for accelerator pump

-new fmf muffler changed - main jet to 185 instead of stock 175

- checked all couplers and wiring and cleaned coil where it earths frame.

i dont now what else to do except take it back to the shop and ive already spent lots of dollars. does anyone think it could be the flywheel? or pickup coil/stator or maybe the cdi?? anyhelp would be much appreciated i just want to:ride: thankyou Adam:thumbsup:

You say you checked the timing chain but did you check the valve timing ?

If you can take an ignition off another 400 and try it on yours you will eliminate or verify the ignition system is OK.

- checked all couplers and wiring and cleaned coil where it earths frame.

i dont now what else to do except take it back to the shop and ive already spent lots of dollars. does anyone think it could be the flywheel? or pickup coil/stator or maybe the cdi?? anyhelp would be much appreciated i just want to:ride: thankyou Adam:thumbsup:

It could be the CDI, but when you checked the wiring, did you happen to check the light blue lead coming from the neutral switch? If the switch or the wire is shorted at any point when the bike is in a gear other than neutral, the rev limit will be reduced, and the timing altered, on the assumption that the bike is still in neutral.

TPS adjustment?

Unplug the tps and see if that helps.

yes i finnaly found the problem it was the CDI:thumbsup: i took a guess assuming i checked everything else i got one of ebay for 50 bucks from the states good price. it would of saved me a rebuild and i would of had a cheap bike if i new it was the cdi:foul: oh well im stoked now time 2 ride. id just like 2 thank everyone for thier help and time:thumbsup:

glad to hear you worked it out and thanks for letting us know for future reference

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