yz 400 problems


8 replies to this topic
  • jasonbase

Posted January 27, 2010 - 05:57 PM

#1

Just bought a 1998 yz 400 from a guy that was the original owner, he ended up getting hurt and parked it for 4 years, anyway I took the carb off and had it cleaned and checked out at a local repair shop. Brought it home and couldn't get it to start, towed it up and down the street and got it to fire but won't idle, and it doesn't run well enough to put it in gear and take off. any ideas? valve problem?

  • brentn

Posted January 27, 2010 - 06:01 PM

#2

What kind of compression are you getting?

Abnormal clicking?

  • 426 NOOB

Posted January 27, 2010 - 06:05 PM

#3

Just bought a 1998 yz 400 from a guy that was the original owner, he ended up getting hurt and parked it for 4 years, anyway I took the carb off and had it cleaned and checked out at a local repair shop. Brought it home and couldn't get it to start, towed it up and down the street and got it to fire but won't idle, and it doesn't run well enough to put it in gear and take off. any ideas? valve problem?


I would start by installing a new pilot jet before doing anything else. They are cheap, and even when they look clean they may still be plugged. If it doesnt change anything after that, move on to check valve timing and clearences.
There are yamaha shop manuals located in the common threads sticky to download also.

  • myoung

Posted January 27, 2010 - 06:23 PM

#4

Got all three, fuel, fire and air?

4 years is a long time, there could still be some clogged passages in the carb or even the petcock.

Are you getting fuel from the tank?

Are you getting fuel from the A/P?

Are you getting spark?

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  • jasonbase

Posted January 28, 2010 - 12:30 AM

#5

Yeah I had the carb looked at by a professional mechanic, and he replaced the needle valve, and said everything else looked good with some good cleaning. I had also put a new spark plug in, but I haven't checked for a spark, and I haven't checked the compression, so I will do that and reply back. Thanks so much for the feedback!

  • DeeCee

Posted January 28, 2010 - 04:33 AM

#6

What else did the mechanic do to the carburettor ?

The diagphram in the A/P needs checking to see if it is persihed.

The fact that it is running (after some effort) suggests the spark is OK. As it has been left standing for a long time, the carburettor passageways will have been clogged with gunk/varnish. It is possible they will have have to be cleaned numerous times before you get it right.

I speak from experience because I too brought a bike that had been left standing for a number of years and the fule had not been drained.

Also check that the hot start valve is closing off properly as this can be the source of an air leak.

You should also check the condition of the carburettor rubbers (intake and air cleaner side) as they may have perished/cracked.

Finally, have you checked the sticky's for the starting procedure.

Good luck.

  • jasonbase

Posted January 29, 2010 - 03:05 AM

#7

He said he had cleaned the whole carb, all the jets including the air jets, which were all blocked up. He said everything else looked good. Intake rubber looks good with no cracks, start valve seems to be working (hoses look good and fit securely), yeah I've got the general idea on the starting procedure, but kicking is still not producing any results, I can only get it to fire when pulling it down the street, it sputters and won't idle. Could the carb have been over looked still? Any ideas?
Thank you so much!

  • 426 NOOB

Posted January 29, 2010 - 11:54 AM

#8

He said he had cleaned the whole carb, all the jets including the air jets, which were all blocked up. He said everything else looked good. Intake rubber looks good with no cracks, start valve seems to be working (hoses look good and fit securely), yeah I've got the general idea on the starting procedure, but kicking is still not producing any results, I can only get it to fire when pulling it down the street, it sputters and won't idle. Could the carb have been over looked still? Any ideas?
Thank you so much!


Once again, I would start by installing a new pilot jet before doing anything else. They are cheap, and even when they look clean they may still be plugged.

  • myoung

Posted January 29, 2010 - 12:33 PM

#9

If the guy completely disassembled the carb, did he put the slide plate back in correctly? It should be square end down. There's also a seal that could be missing or damaged, you'd have to remove the slide to check that.

There's the possibility that the fuel screw has a bad or missing o-ring or a broken tip lodged in it's orifice. Also a possibility the jets still aren't clean enough, maybe just buy new ones to rule them out.

If that's all good, then I'd be looking at the passages for the pilot circuit. These can be very clogged and very hard to clean. It's even possible that gasket material can be blocking the passage between the pilot jet and the orifice, this is what happened to mine.

Is the A/P working? If this is still the original carb, there is a passage with a check ball that in the worst cases of clogged up carbs, this check ball will get stuck and you won't have any A/P function. If the air jets were clogged on yours, then it was clogged pretty bad and you might have a problem here as well. It's non serviceable other than soaking with carb cleaner.

If it's still the original carb, you should still have the remote hot start. Try blocking it off all together in case there's a leak you cant see.

You should check your valve lash just to rule out any tight clearances on the intake valves and do a compression check. It may not be carb related if there's a lot of hours on this bike.





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