99 yamaha yz400f need help ! :(

Basically i traded my "91 suzuki dr 650 a couple weeks ago for this "99 yamaha yz400f. It was running very well and was a ton of fun to ride, but while test driving it was backfiring a lot. After having it for about a week the idle went worse, so i decided to go ahead and clean out the carburetor and give it a nice soak. The jets were in great shape and screws (I've cleaned out a couple carburetors in my past as well as rebuild ed some). After this i put it all back together and tried to start it, but the idle was even worse as well as didn't even hold a second of non-throttle. I then adjusted the pilot screw, but still just the same thing would happen (I did check the pilot screw if there was any junk in it while cleaning out the carburetor and it was all fine). A day later me and my uncle wanted to go trial riding and so i decided to say yes, and now the bike lost a lot of compression and won't even start anymore!!! (The kick start goes down all the way just by pushing it down with my hand!).


Short Form:

- Barely compression.

- Idle bad.

- Backfiring.

- Spark plug is brand new and tested!


EDIT: I think it might be a bend valve, but im not sure. I really would appreciate any help or advice!!! :)

Edited by fishtime9

The problem with buying an old YZ is that they really do last a long time......but by then, EVERYTHING is worn out !

Never a great idea to buy a used dirtbike with more than 250 hours on it (that's only about 7,000 miles or much less...)


No way to know why you have no compression other than to pull the motor apart.


The motor originally probably had a  skipped cam chain due to a worn out cam chain and tensioner....hence all the back firing and poor idling.....and then it skipped again, causing you to bend a valve.

Did you do any maintenance on it when you got it,like give it a oil change, change the filter, clean air filter. And the back firing happens on most early yzf's it is possible to stop or decrease with a lot of fine tuning but I say not to worry about it.

Something that can happen with an older 4-stroke is for carbon to build at the point on the valve stem where it meets the valve guide when closed.  This can hold the valve slightly off its seat.    Oft times this can be temporarily cleared up by lightly tapping down on each lifter at TDC/compression to "bounce them shut" against their seats.  Check the cam timing and valve clearance as you do this.  


As to the idle, you may think you cleaned the pilot jet, but did you?  http://www.thumpertalk.com/topic/681801-bike-wont-start-dont-know-what-to-do/#entry6879695


Also, you said you "soaked" the carb.  Did you remove the throttle position switch first?  If not, you may have damaged it.  If you did, it may be out of adjustment.


As to backfire, be aware that leaky exhaust systems will cause that, too.

I've also seen the carb slide taken apart and reassembled with the plate upside down. This also makes the bike pop. As for compression loss I've seen the decomp lever stick and a cable that was routed wrong.

Gray beat me to it, but my 99 400 backfired like an angry grizzly when i first got it, and exhaust gaskets fixed that right up.

Okay, im planning on taking the dirt-bike apart on Saturday. I thank you all for advice as well as i hope that its just a bit of that carbon stuff making the valve stick out. I do plan on putting new gaskets in, and to check the valve clearances as i go along on the process.


I'm going to update this thread constantly on my bike process, till its fixed :).



Also i was thinking on installing a hot cam if i have to switch the valves out, do i have to buy the shim size kit at the same time? (I was hopping only to purchase the hot cam exhaust thingy for 140$).

I've pulled everything apart and did a liquid test on my valves, and one of them is slightly bend (This might cause the compression to be a bit lower then usual). I'm thinking on just replacing all of the valves (Brand new ones).


Also i'm replacing all of the gaskets, as well as going to have to check out the valve specifications.

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