What causes "tight" valves?


14 replies to this topic
  • Slow and Jerky

Posted August 09, 2008 - 04:47 AM

#1

Hello all,

Can someone explain to me how (if) valve clearances get tighter as they wear with shims? I have a 98 WR400 with a starting issue and several people have suggested valve clearance.

Also, how tight is too tight? The minimum spec in manual (for intake) is .15 mm and I have one that is .13 mm...?

Thanks,
Rob

  • PBDBLUE

Posted August 09, 2008 - 05:26 AM

#2

Valves become tight due to seat/face wear. Although you should shim the tight valve I don't think it has anything to do with your starting problem as it's not that far out. Yamaha's in general and particularly the early ones (ss valves) have very good valve longevity. I'd take a serious look at the carb. Partially blocked pilot jet, float height, float valve leaking can all contribute to difficult starting.

  • WR_Dave

Posted August 09, 2008 - 05:32 AM

#3

As a mechanics instructor I have to say that if the spec is .15mm and you are at.13mm, then you are out of specs and need an adjustment. Having said that I agree with PBDBlue, it is more likely a carb issue. Give the carb a good cleaning and remove the float valve seat and clean that part really well and put a new o-ring on it. The o-rings tend to leak after a while and allow fuel to bypass the float, causing a flooding condition. If it is the original float needle it may also be worn to the point of not sealing properly and should be replaced. Good Luck -- WR Dave

  • Slow and Jerky

Posted August 09, 2008 - 05:47 AM

#4

I did clean the carb, but I did not replace any of the O-Rings. In my frustration I tried starting fluid, both sprayed directly into the carb and into a rag with no change. Am I correct in assuming that the starting fluid should bypass the normal carb functions and pull the fuel in directly..?

Thanks,
Rob

  • WR_Dave

Posted August 09, 2008 - 05:51 AM

#5

I wouldn't use starting fluid at all, but it will be drawn in by the air passing through the carb so you don't need to spray it in after the carb. I would take the time to go through the carb again and check the parts mentioned above and also pay close attention to setting the float height as well. Also make sure the inlet boot between the carb and the engine is sealing properly. Is there a good spark, have you tried a new plug? Check that the cam chain has not stretched and gotten the cams out of time. WR Dave.

  • Slow and Jerky

Posted August 09, 2008 - 07:10 AM

#6

How do you check the cam chain?

  • WR_Dave

Posted August 09, 2008 - 08:47 AM

#7

When you have the valve cover off for valve checking ,I believe there is a measurement for the chain in the manual. If you don't have a manual, download one from the WR Performance sticky thread at the top of the forum. WR Dave

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  • Slow and Jerky

Posted August 09, 2008 - 09:17 AM

#8

I just finished double checking the valves and only one is slightly out of spec (.2 mm), as I was doing this I realized that I can turn the engine over using the crank nut fairly easy (probably not a good sign), but when I use the kick starter it will support my weight for a few seconds before bleeding off.

Is this possibly normal?

  • chickenstrips

Posted August 09, 2008 - 09:57 AM

#9

I just finished double checking the valves and only one is slightly out of spec (.2 mm), as I was doing this I realized that I can turn the engine over using the crank nut fairly easy (probably not a good sign), but when I use the kick starter it will support my weight for a few seconds before bleeding off.

Is this possibly normal?



First off, if one of the valves is out of spec, adjust it. Quit telling us how much it is out of spec, remove the cams, and change the shim.

  • WR_Dave

Posted August 09, 2008 - 11:12 AM

#10

When you are turning the engine over with the crank nut, you are turning it at a 1-1 ratio and when you use the kicker it is around a 200 to 1 ratio ( a number I heard somewhere a long time ago). So you are turning the engine over a lot faster and the compression doesn't have time to bleed off when using the kicker. Also since your in there do like Chikenstrips and a few others have suggested, get the valve back into spec, it won't get any better by waiting for someone to say it's OK the way it is. WR Dave.

  • Slow and Jerky

Posted August 09, 2008 - 12:09 PM

#11

Sorry if I am starting to sound like a whiner. My issue is geography, I am 40 miles one way from the nearest Yamaha shop and a couple of days for anything from the Internet, so I am just trying to rule out as much as possible before chasing parts.

Thanks,
Rob

  • WR_Dave

Posted August 09, 2008 - 02:39 PM

#12

No worries, when I did the valves on my old 426 i was able to adjust the valves by mixing the existing shims to different valves and it all went into spec. I usually adjust for the loose end of the spec so it has some room before it gets tight again, but I'm not a revver and run more on the middle of the RPM range. Higher RPM's require more diligent valve service. WR Dave

  • grayracer513

Posted August 09, 2008 - 05:19 PM

#13

When you are turning the engine over with the crank nut, you are turning it at a 1-1 ratio and when you use the kicker it is around a 200 to 1 ratio ( a number I heard somewhere a long time ago). So you are turning the engine over a lot faster and the compression doesn't have time to bleed off when using the kicker. Also since your in there do like Chikenstrips and a few others have suggested, get the valve back into spec, it won't get any better by waiting for someone to say it's OK the way it is. WR Dave.

A one third (approximately) rotationj of the kick crank shaft will take the engine from the start of one compression stroke to the next. Since that's two revolutions of the engine, the ratio is 0.3:2, or 1:6. While that is nowhere near 200:1, the same principal applies. It's like trying to turn the crank nut with one sixth of the leverage of the wrench used.

Everything else said here is right:
Valves tighten because they wear at the seat and face, so the stem moves up into the head.

When the spec is .15 and the valve is .13, the valve is out of spec and needs shimming.

The intake valve being .02mm tighter than called for IS NOT the cause of your starting problems, even though it needs to be corrected.

The early FCR is a very fussy carb, sensitive even to changes in weather, and minor dirt invasions or just being slightly our of adjustment are 80% of all starting troubles on a 400. Weak ignition that demands a spark plug in very good condition is another 15%.

  • Slow and Jerky

Posted August 10, 2008 - 04:43 AM

#14

Thanks for the responses, this is the first time I have dealt with valve shims.

  • Slow and Jerky

Posted August 10, 2008 - 10:38 AM

#15

Thanks for the tips guys, I was able to switch a shim from the exhaust that was on the upper end of the spec (but still in) with the intake that was out and now they are both in. Carb is soaking, so hopefully tomorrow I will have good news!


Rob




 
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