'86 VF500F lack of top-end

Discussion in 'Mechanics Garage' started by Cherryriver, Jul 11, 2021.

  1. Cherryriver

    Cherryriver New Member

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    I've just gotten my "barn find" '86 VF500F going. It had been sitting in a fellow's garage for what I guess would be four to ten years. The bodywork and cosmetics are surprisingly good but the carbs were, of course, gummed up.
    He listed it for sale on Craigslist when he got frustrated with the carbs. He'd pulled them and started with the carb cleaner, but without removing the diaphragms and pistons. He convinced himself he'd ruined the diaphragms and simply sold the bike.
    Miraculously, every single part was still there in the box when I came to buy the bike, every nut and hose clip.
    After a fairly complete cleaning session which included new slow jets (couldn't get those clean) but without a complete dismantling from the plenum, it started right up and began to run halfway decently.
    I don't get too bold with new-to-me non-runners but this morning, with the Missus alongside for backup, we did a 20-mile loop that concluded with a fast-ish run down an industrial road, and here's where I ran into a problem, I think.
    We were batting along at 75-ish; I was in fifth gear and unwilling to go too much faster as it still is not fully proven and is wearing 15-year-old tires.
    The engine seemed to be pulling decently in fifth (within the bounds just mentioned) but when I went to sixth, it pretty much acted as if it was tapped out. Full throttle produced almost no detectable accleration.
    The engine is running very nicely apart from that. Even the idle is settling down.
    The weather conditions were cloudy, 68 degrees, and a modest 10+mph headwind.
    I confess to putting ordinary 87 octane unleaded when I fueled it with two initial gallons. It appears from what I see in the service manual (don't have an owner's manual) that I should have put in 91.
    I did not detect any knocking, though. In fact, I thought the acceleration in the first four gears was probably satisfactory. Since I have no baseline I can't be sure, but the Missus was nearly full-throttle on her new Ninja 400 and I was able to keep up okay. Not done scientifically or with any plan, so not a perfect data point.
    I have not checked the valves yet; the clock shows 16K miles and there is abundant evidence of good maintenance all over the bike.
    Nor have I checked the float levels, my bad.
    I did check the #1 spark plug (only) and it looked excellent, so I left the others alone for the time being.
    Basically, apart from the gummed carbs, this bike came in really good condition, surprisingly so.
    This photo is from the ride home before laying a finger on it.
    Suggestions on the lack of sixth-gear acceleration welcomed.
    Bill VF500F day one.jpg
     
  2. Diving Pete

    Diving Pete Member

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  3. squirrelman

    squirrelman Member

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    check compression first thing ! low psi=low power. bikes that sat for years may have stuck/rusted rings.

    good luck with it.
     
    Last edited: Jul 11, 2021
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  4. Cherryriver

    Cherryriver New Member

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    Another test ride convinces me it's not making full power. Struggling to get past 80mph just can't be right.
    Next time on the rack, a look at (all) the plugs and yes, compression.
    It's a fooler, it runs so smoothly at more modest speeds and power settings, not like it's missing something.
    Like one cylinders' worth of power.
     
  5. squirrelman

    squirrelman Member

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    do you know how to inspect valve stem tops to spot the common problem, "mushrooming" ?
     
  6. Cherryriver

    Cherryriver New Member

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    I will give that a look as soon as the cam covers are off. Given the bike's low mileage and apparent lack of abuse, plus being an '86, I wasn't really expecting that kind of trouble.
     
  7. jethro

    jethro New Member

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    Running rich will kill the top end, but so will slides that don't open fully due to diaphragm issues. Make sure they are not kinked and sealing fully or have holes in them which will preclude them opening fully.
     
  8. Cherryriver

    Cherryriver New Member

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    Yes, the carbs are next on the suspect list.
    Checked valve clearances, couldn't be more perfect. (16K on the clock)
    Ran a compression check: 140-153 on this particular gauge, so no trouble there.
    #2 plug came out a trifle damp. Might be a clue.
    New plugs, the off come the dang carbs for a look.
     
  9. Captain 80s

    Captain 80s Member

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    Don't half-ass on the carbs. Read up if necessary and do them right. Worth the time and effort.
     
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  10. Cherryriver

    Cherryriver New Member

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    Carbs.... why does it always have to be carbs?

    Indeed, the float on #2 was stuck upwards and the cylinder was starving. Now, for some reason, I'm completely unable to set a float height correctly anymore.
    That was one good thing about the Amal Monobloc on my BSA- setting the float was a no-fail cinch.
    Back to work.
     
  11. Captain 80s

    Captain 80s Member

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    Because it's always the carbs. And everybody tries to convince themselves it's not. It's always the fucking carbs. Usually. ; - )

    Good luck!!
     
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  12. Terry Smith

    Terry Smith Member

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    Except when you had the carbs apart 6 or 7 times then you find out all the backfiring was due to low voltage reaching the ECU from a dodgy connector...DAMHIK. (not a VF500F).
     
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  13. TOE CUTTER

    TOE CUTTER Mullet Man

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    Like you read my mind or my diary. I just did not realize you were capable of dropping the F bombs but it was appropriate.
     
  14. Captain 80s

    Captain 80s Member

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    "Usually. ; - )"
     
  15. squirrelman

    squirrelman Member

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    it's important not to switch the jets and emulsion tubes from front to back.
     
  16. Diving Pete

    Diving Pete Member

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    The reason its always carbs is that no-one ever empties them of fuel. They are a precision instrument that works in a vast range of conditions & yet still they are pretty much ignored - until they no longer function.

    Don't forget to balance them after. Sounds like this is going to be a quick and easy fix. Great job.

    For setting float height I use an old credit card cut to the correct settings (yes, you can buy the correct tools) as it makes it:
    1. Repeatable on all,
    2. It sits in the tool box marked for this bike ready to reuse,
    3. I then don't have to think about it.
    4. Permanent marker the bike model on the card.
     
    Last edited: Jul 13, 2021
  17. Cherryriver

    Cherryriver New Member

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    Looks like I got it. Finally.
    Had to go back over a couple of assembly errors, then it fired up right nicely.
    Sure is a different motorcycle when all four cylinders are working, needless to say.
    Now, leaks.
    The petcock's leaking pretty badly. I have an O-ring kit on order that is still a couple days out, hope that takes care of it. It seems to vary quite a bit in intensity. I haven't taken it apart yet, but since I've seen no trash at all in any of the carb strainers I wasn't worried about rust flakes jamming up the petcock seals.
    And the alternator cover is leaking oil. The original gasket went to pieces when I pulled the cover to do the valve adjustment. I tried a light coat of Hondabond 4 but no dice. I've never been a big fan of liquid gaskets, especially when paper ones work so well (on modern motorcycles anyway. Never mind the Brit bikes.) but I thought it was worth a try to at least be able to run the thing to test out my work.
    A rainy morning here, hard to restrain the right hand, but 15-year-old tires are not a thing to be taking liberties with. The replacement Pirelli Sport Demons are due tomorrow, so no shortage of tasks to pass the time.
     
  18. Captain 80s

    Captain 80s Member

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    You might consider eliminating the vacuum operation of the petcock when you rebuild it. It's just another thing that can / will fail.

    Oh, and please post which petcock kit you ordered and update when it arrives and you dive in please.
     
  19. Diving Pete

    Diving Pete Member

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    As a suggestion, get used to turning the tap on & off if the engines not running.
     
  20. Cherryriver

    Cherryriver New Member

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    I may do that, although I don't have the habit, never having had troubles with those systems in the past, including several bikes run past 100K: a Connie to 121K, a GPZ1100E to 106K, and a GL18 to 116K. Petcock troubles seem to befall bikes not in use, mostly.
    Plus, the Missus, who may well be the main rider, doesn't have the habit atall, having only started riding four years ago. Those modern bikes, you know.
    No icky carburetors for her.
    The petcock's leak is from that vent opening in the front of the unit, the little extension pointing towards the front of the bike that looks for all the world to be needing a hose attached to it. It's tempting to connect one and run it up over the fuel level in the tank just to be able to ride it a bit.
    I should be too mature for that, though.
     
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