VF500F Tech Question - what causes valves to drop

Discussion in '1st & 2nd Generation 1983-1989' started by invisible cities, Dec 14, 2010.

  1. invisible cities

    invisible cities New Member

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    I was giving some thought to the often maligned valve issues on the VF500F and thought I would post an RFI here.

    I have read in previous posts that the number one cause of dropped valves on the VF500F is valve spring float due to the limits of manufacturing back in the day (as noted, this can be corrected with modern springs and retainers from KPMI or R/D Springs).

    My tech question is whether an incorrect valve lash setting can also contribute to premature wear and subsequently a dropped valve as well.

    For example:
    • too small (lash < .13mm) and the oil does not have enough room to properly lubricate between cam and the follower leading to premature wear of the surfaces of each
    • too large (lash > .13mm) and the rocker arm tappet screw creates a hammering effect on the top of the valve stem which can lead to the spring retainer coming loose - never a good thing

    Any insight would be most appreciated.

    Thx!
     
  2. creaky

    creaky New Member

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    I think that both of your points merit consideration. I'll weigh in for what it's worth. As you know, I have very limited experience with the VF500, but I do have quite a bit of experience with 4 stroke dirt bike engines, mostly XRs, have worked on several engines of various sizes that had dropped a valve or two. It is difficult to pinpoint the exact cause of failure due to the extensive damage done when a valve is dropped. My observations include:
    1. Heat damaged valve due to lean mixture/many hours of usage, valve crystalizes, valve head breaks off.
    2. Valves float and the keepers fly out, valve drops, probably a result of heat sacked springs not providing proper tension/harmonics.
    3. A piece of piston breaks off, piece gets between remainder of piston and valve, valve head breaks off.
    4. Valve becomes very worn at the seat area, valve "tulips" and since the edge of the valve at the seat is now very thin, heat damage can cause the valve head to break off.

    Something else on the VF500. The valve stems have a very small diameter, maybe this might contribute if high heat is an issue.

    P.S. I forgot to mention another cause.......broken retainer.
     
    Last edited: Dec 14, 2010
  3. invisible cities

    invisible cities New Member

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    Thanks creaky. All good intel.

    I was thinking of only going for an inner and outer valve spring upgrade but I am giving some more thought to ordering up on the valves and retainers as well.
     
  4. GreyVF750F

    GreyVF750F Member

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    "* too large (lash > .13mm) and the rocker arm tappet screw creates a hammering effect on the top of the valve stem which can lead to the spring retainer coming loose - never a good thing"

    The hammer effect will only damage the rocker and valve tip. There are various reasons why valve springs wear out. Starting with design, tensile strength, stack height and rpm.

    Here's what a good set of dual springs look like running at 10k rpms. Look at how they dance.[video=youtube;yfmb-tCo2yA]http://www.youtube.com/watch?v=yfmb-tCo2yA&feature=related[/video]

    Here's what valve float looks like at 10k rpms. That latter half of the vid shows the actual valve bounce
    from the chamber side. [video=youtube;_REQ1PUM0rY]http://www.youtube.com/watch?v=_REQ1PUM0rY&feature=related[/video]

    Now imagine what the 500 looks like at 10k with weak springs. Then you will have an idea on how the valve locks and retainers come off and how a valve will beat itself in to the head. Then break the valve head off.

    Love those spintron vids.
     
  5. creaky

    creaky New Member

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    I have installed Kibblewhite valves & valve spring kits in a good number of XR engines, no problems. I have also used R & D spring kits with OEM valves when specified by the owner, again, no problems.
     
  6. Michael E

    Michael E New Member

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    Awesome post Grey! Thanks.
     
  7. cbfazer

    cbfazer New Member

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    Looking at the R & D website, I see a price of $200 for valve spring kit for honda 500 VF. That can't possibly be a whole set of valves for the bike can it?
     
  8. invisible cities

    invisible cities New Member

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  9. GreyVF750F

    GreyVF750F Member

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    Sounds like springs are hard to come by. Thinking if you can still get stock valves you may only need a proper set of springs. If you can find out the specs for valve seat pressure , nose pressure (fully opened), installed (stack) height, coil bind and diameter you should be able to find already made springs that could be used. You can use the same spring in different applications depending on stack height and coil bind. Which will change the open/closed pressure it will have depending how you install them.

    The big problem I see is finding those specs from Honda for various springs they use, unless you find a good head and take measurements. Even that may not do. I don't know why the 500 has the valve problems it does. Is it the spring material doesn't hold up, not the best engineered to begin with, or something else. I would venture to say with a good set of properly setup springs the problem can be solved. It may only need a stronger inner damping spring or the stack height reduced to fix the problem.

    Anybody know a Honda engineer ex or otherwise. I know one, but haven't talked to him in years. We were involved in 24hr racing at the time. His name is Keith Marshall and he was with Bob Hesh in Warrendale PA at Bob's mc shop/store (multi time overall 24hr winners). He might be able to answer some questions on the engineering of the valve train. I might be able to track him down if someone really wants me to.
     
  10. invisible cities

    invisible cities New Member

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    Yes, please! I would very much appreciate this.
     
  11. JamieDaugherty

    JamieDaugherty New Member

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    All VF500F dropped valves are caused by the keepers "slide hammer"ing the keeper groove off the end of the valve stem. Valve lash has nothing to do with this at all. It's the valve spring design being marginal and the excessive stresses causing the material to fatigue which results in the failure. No other causes have ever been identified.
     
  12. invisible cities

    invisible cities New Member

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    Thank you for the post.

    Given this, would it be best to replace the valve springs and valves/retainers with KPMI upgrades at the same time - or - is this 'simply' a spring replacement issue?

    On a side note, do you agree with setting the intake and exhaust lash to .13mm for both (vs .12mm and .13mm respectively)?
     
  13. JamieDaugherty

    JamieDaugherty New Member

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    While they were still available, the solution was to replace the springs with new Honda parts every race season. Street mileage intervals varied, but somewhere in the 10-20k was the generally accepted range. Now that the springs are NLA it's not an option. I don't have a lot of confidence in Kibblewhite having something off-the-shelf at this time. Plus it would take some testing to validate their performance. That is the right patjh as "new" valve springs is the solution. The keepers don't really matter (in this discussion). Exotic designs like Titanium retainers would be a great idea. The problem there is you would invest more in those parts than the bike is worth (probably).

    I always have set my valve clearances to the Honda manual spec.
     
  14. invisible cities

    invisible cities New Member

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    Thanks for the post.

    The kicker as noted is that KPMI and R/D Springs, while originally offering kits for the VTR250, have not made kits specifically for the VF500F.

    I'm happy to send either KPMI or R/D a set of heads (in this case from an '85) but without a proven track record it is an expensive ($600) speculation.

    The 500 wants to rev well above the 9k mark but for now I'll plan on keeping the revs in check while dialing in a source for the valve springs.
     
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