Seeking guidance - possible valve issue

Yamaha YZ400 Yamaha YZ400F 1998

21 replies to this topic
  • ShsFunny

Posted June 25, 2013 - 06:22 PM

#1

I am an old school dirt rider who has been out of the game for a decade or so. I decided to begin riding again so I found a 98 YZF 400 locally that I purchased for 500.00. The guy was up front about issues with the bike which include, no idle, will not kickstart cold - must bump start, bogs out when throttle is dumped (accelerates fine when throttle is applied slowly). He is telling me that the center intake valve is bent and is causing these issues. Although I have been out of the game for awhile, its been my experience that when a valve is bent, the engine wont run at all. This bike runs well, VERY well actually minus the idle and starting issues (lots of power, awesome acceleration). How would any of this be possible with a bent valve? I havent had the time to break the engine down yet but I wanted to get a few opinions on here before I even begin down that road.

  • Yz400f Jeff

Posted June 26, 2013 - 05:54 AM

#2

I had some of the same problems with my 400f when i first got it. I started by taking the carb off and learning how everything worked, the idleing issue will probably be the pilot jet it controls the idle they get plugged up all the time especially if the bike has been sitting around, or it could be an air leak in the carb boots making it lean, fire it up and spray carb cleaner around them to see if your rpms go up. Your bog issue when you wack the throttle is probably the accelerator pump, it gives the bike a small shot of fuel at low rpms when the throttle is opened fast because there is not enough vaccum to draw fuel at low rpms. Search those 2 issues on here their is almost too much info on them hope this helped abit as im still learning the ropes myself.

  • grayracer513

Posted June 26, 2013 - 06:56 AM

#3

Valve clearance is actually tighter cold than hot, and you can create a condition where they have clearance when hot, but not when cold. Check them.

Could be the carb, too. The old FCR's can develop internal leaks that make them almost impossible to tune. Check the starter jet (near the pilot, marked #65, or close to that). Check the vacuum release plate on the engine side of the slide for cracks or missing pieces.

http://www.thumperta...726#post8021726

  • ShsFunny

Posted June 26, 2013 - 07:40 AM

#4

Am I correct on if the valve was bent it wouldn't run? Thats the part that makes NO sense to me at all. Thanks for the input!! I will definitely pull the carb and clean it this weekend.

Edited by ShsFunny, June 26, 2013 - 07:40 AM.


  • grayracer513

Posted June 26, 2013 - 10:44 AM

#5

A bent valve would prevent it from running, yes, but what I suggest you do first is check the valve clearance for reasons already given.

  • ShsFunny

Posted June 26, 2013 - 11:22 AM

#6

Thanks guys. I will definitely give it a look!!

  • ShsFunny

Posted July 27, 2013 - 02:28 PM

#7

Well Im a bit further in to this now as I have pulled the carb and cleaned it, checked the jets, cleaned the accelerator pump (the gasket in the pump was stuck to the carb with gunk.). My symptoms are the same. I havent pulled the head to check clearances yet as I just got my feeler gauges yesterday but that will be very soon. bike is still running good and actually has more power now than it did before the carb cleaning. I just cant get an idle or get it to kick start. I always have to bump it. I love that bike though man.......Talk about power. Second gear is unreal on that thing. I guess where I was used to riding 250cc. Anyway, the head pull is up next and Im afraid of what Im going to see!!! :)

I do want to add one thing to this that I noticed today that the idle adjustment on my carb is completely stuck. I cant turn it one way or the other. Have you ever seen that before?

  • WouldsAssassin

Posted July 27, 2013 - 04:07 PM

#8

Good you pulled the carb and cleaned it up some. I'd recommend pulling it again and completely dismantling it down to the very last nut, bolt and adjustment screw. Then clean all the small orifices and passages within the carb body and jets. The last time I cleaned a carb the pilot jet got plugged with fuel resin at least a dozen times before it actually started running right.

  • Crguy713

Posted July 27, 2013 - 04:26 PM

#9

Dep on how bent a valve is I've had many xr's bend valves....then I pull em get them somewhat true slap em back in ..runs great ...the valve guides aren't damaged due to the bend at the neck of the face not the shaft...and sounds like you need a valve job /adjustment...how was the diaphragm on the carb any holes?

  • ShsFunny

Posted July 27, 2013 - 07:54 PM

#10

Dep on how bent a valve is I've had many xr's bend valves....then I pull em get them somewhat true slap em back in ..runs great ...the valve guides aren't damaged due to the bend at the neck of the face not the shaft...and sounds like you need a valve job /adjustment...how was the diaphragm on the carb any holes?


No holes but it was pretty much glued to the seat. Completely filthy. I soaked it in carb cleaner and cleaned all the dirt from it. Heres an image of it AFTER i cleaned it.

Posted Image

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  • Crguy713

Posted July 27, 2013 - 08:52 PM

#11

No holes but it was pretty much glued to the seat. Completely filthy. I soaked it in carb cleaner and cleaned all the dirt from it. Heres an image of it AFTER i cleaned it.

Posted Image

Rubber should be supple and rubbery flexible...also a tiny pin hole will make it run like shit! Alignment of diaphragm is mucho importante !

  • NitrousR1

Posted July 28, 2013 - 09:31 AM

#12

I'd do a leak down test on it. See what percentage the tester tells u. If u can hear leakage coming through the intake port or carb then u know.
You might end up backwards in this bike depending what it needs. By the time u replace the valve and or valves and get the seats cut u could have more into it then what it's worth. Depending if its been taken care of it might need a piston. I don't know what your markets like but around here a perfectly maintained yz400 or 426 with zero issues is only worth $800
As long as ur having fun and enjoying it then its alright, that's what it's all about

  • ShsFunny

Posted July 28, 2013 - 09:49 AM

#13

I'd do a leak down test on it. See what percentage the tester tells u. If u can hear leakage coming through the intake port or carb then u know.
You might end up backwards in this bike depending what it needs. By the time u replace the valve and or valves and get the seats cut u could have more into it then what it's worth. Depending if its been taken care of it might need a piston. I don't know what your markets like but around here a perfectly maintained yz400 or 426 with zero issues is only worth $800
As long as ur having fun and enjoying it then its alright, that's what it's all about


Wish I lived where you do. Good running bike here is about 1600-2000. I wanted a fixer though. I enjoy the hobby and learning new things.

Edited by ShsFunny, July 28, 2013 - 09:50 AM.


  • ShsFunny

Posted July 28, 2013 - 02:31 PM

#14

So I measured the clearances today. See the photo. Sorry for the shotty artwork but I think you get the picture so to speak. Im not terribly out of spec as I expected. Manual tolerances are:

[color=#282828][font=helvetica, arial, sans-serif][background=rgb(252, 252, 252)]Exhaust- .010 -.012"[/background][/font][/color]
[color=#282828][font=helvetica, arial, sans-serif][background=rgb(252, 252, 252)]Intake - .006 -.008"[/background][/font][/color]

ImageUploadedByThumper Talk1375050676.379490.jpg


Cams are remarkably clean. Timing chain is brand new and clean as a whistle.

Posted Image

As bad as I hate to drag multiple issues into the same thread, while I was removing the head today something caught my eye on the header. Should there be this much distance between the header and the engine? I do get some popping on decel on this thing. See the below image.

Posted Image

Edited by ShsFunny, July 28, 2013 - 07:28 PM.


  • ShsFunny

Posted July 29, 2013 - 12:13 PM

#15

Something I also realized today after reading is that this bike doesnt have great compression. I am not "able to stand" on the kicker as I should be able to with this bike. I never have to touch the decomp lever to get it to turn over. Naturally this is related to the valves but Im not sure how. They arent that far out of spec. What else can cause compression issues?

  • grayracer513

Posted July 29, 2013 - 12:26 PM

#16

The fact that they have clearance doesn't mean they will seal when closed. Wear at the faces and/or seat, carbon, etc. can interfere with that. Frankly, your problem is more likely the rings.

Have a leak down test done, This will tell you where the leakage is occurring.

  • ShsFunny

Posted July 29, 2013 - 05:45 PM

#17

I pulled the plug today and it was as black as night. Does this strengthen the cause for bad rings?



Posted Image

  • grayracer513

Posted July 29, 2013 - 06:02 PM

#18

No. That's typical of an engine that hasn't been firing much.

  • ShsFunny

Posted July 29, 2013 - 06:17 PM

#19

Thanks GreyRacer. Ill get my hands on a compression tester soon and see what I come up with. You have a link to a decent one that wont cost me an arm and a leg?

  • grayracer513

Posted July 30, 2013 - 07:13 AM

#20

Not a compression tester, a leak down tester.





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