'86 VF500 Engine suddenly lacks power (or any real urgency) Rpms roll on slowly..

Discussion in '1st & 2nd Generation 1983-1989' started by HeavyNova, May 26, 2014.

  1. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Hi,

    I am looking for some troubleshooting assistance. This bike was running in top-form on my way to work. Truly amazing power, acceleration and performance! On my way home, I noticed it was initially having a hard time starting and idling. Being relatively low on fuel, I filled it with 92 octane at the Costco gas station before leaving the parking lot. I was able to ride it home but I noticed a huge loss on power, and an unresponsive throttle. Usually, this bike will just bury the tachometer without even trying. Now, it doesn't want to go past 8k and seems to accelerate sluggishly.. boggy. The engine still sounds pretty much the same (smooth), it doesn't run rough or sound like it is having trouble on any particular cylinders. But, it now has a lower pitch at WOT. Instead of a higher pitched "RIIIINNNN" sound when goosing the throttle, now it sounds like "RUUUUNNNN". However, overall it still sounds and runs relatively similar as it used to except that it seems "muted" now. The exhaust still looks clear, no smoke. I checked choke cable operation to make sure it wasn't stuck open and that it didn't break loose of its fasteners -looked operational, normal.

    I've only had the bike two months and I got it running better than ever right before it started misbehaving. This bike was a real performance champ! right before it started having problems. To help troubleshoot and rule some things out, here is maintenance I've recently performed on the bike in the past two months.

    Valves checked/adjusted, new tires, new K&N Air Filter, new NGK sparkplugs, fresh oil change, new fork oil seals, new battery (electrolyte level perfect) Coolant flush/fill.

    It has been running really well on 91 octane non-oxygenated fuel this whole time but started exhibiting problems right before I refueled a near-empty tank. Just an hour ago, I drained the gas tank and verified there is fuel reaching the two banks of carbs via the sucking pressure from the intake vacuum fuel line port (which appears functional and not clogged) I have verified there is no fuel pump on the '86 VF500, so there is no potential for a bad fuel pump. What could be the problem?
     
    Last edited: May 26, 2014
  2. jeremyr62

    jeremyr62 New Member

    Country:
    Ireland
    Joined:
    May 14, 2011
    Messages:
    609
    Likes Received:
    40
    Trophy Points:
    28
    Location:
    Limerick, Ireland
    Map
    Is it running on all four cylinders? Check (carefully) the temp of the header pipes.
     
  3. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Yes, it is running on all four cylinders.
     
  4. jeremyr62

    jeremyr62 New Member

    Country:
    Ireland
    Joined:
    May 14, 2011
    Messages:
    609
    Likes Received:
    40
    Trophy Points:
    28
    Location:
    Limerick, Ireland
    Map
    If it was mine I would tear off the valve covers and make sure all the valves/springs are present and correct. If they are OK then it's looking like carb or coil related.
     
    Last edited: May 27, 2014
  5. commrad

    commrad New Member

    Joined:
    Oct 29, 2010
    Messages:
    229
    Likes Received:
    1
    Trophy Points:
    0
    Location:
    Red Hill, GA
    the deep moaning sound is generally a sign of the carbs opening but not getting enough fuel. First thing I'd check would be the petcock diaphragm, drain the float bowls and check for water (it won't pass through the jets, too heavy). Also that K&N filter will delay the slides opening a little bit and actually hurt performance if you don't modify the carb slides to work with the filter.
     
  6. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Hmm. Ok. I checked the petcock diaphragm and there is fuel flow when vacuum is applied, although I stopped short of taking it apart when I tested it and found it to be working. Is it still necessary to take it apart? The engine was more responsive with the K&N. The old air filter wasn't even really dirty but I replaced it anyway. I didn't think a K&N was supposed to give you so much additional air (power) that you had to fiddle with carb slides? I was just trying to eek out the one and only (very minor) performance upgrade on the machine. What is it, maybe an additional 1-2 hp? Would you raise the slides up or down to accommodate the K&N? Looks like I need to check the float bowls for water. But, how would water get in there if the bike ran perfectly 4 hours earlier? The bike hasn't even gotten wet in the two months I've owned it. Maybe a bad coil, clogged carb? The valves have just been checked and adjusted and the machine was really running crisp before this happened..

    UPDATE: A previous owner has gerrymandered the fuel lines. I drained the fuel cup on the petcock. No fuel strainer (filter) present. Also, no in-line fuel filter either, meaning probably no fuel filtering anywhere before the fuel goes directly into the carbs. The main fuel line has been simply attached to a Y-splitter. Each branch of the splitter feeds directly into a bank of carbs.
     
  7. jeremyr62

    jeremyr62 New Member

    Country:
    Ireland
    Joined:
    May 14, 2011
    Messages:
    609
    Likes Received:
    40
    Trophy Points:
    28
    Location:
    Limerick, Ireland
    Map
    Do you mean there was no tall fuel strainer sat on top on the petcock that fits inside the tank? This is the only filter before the carbs on 86 version. The fuel line does split as you descriibe so that sounds OK. Each carb has a very small fuel filter inside too I seem to recall.
     
  8. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Yes, there was no tall fuel strainer sitting inside the petcock -it was just empty space.. It sounds like I need to find out where/how to access and clean these tiny, carburetor fuel filters..It is good to know the stock fuel lines weren't completely gerrymandered. Does anyone have a detailed schematic of an exploded diagram of an '86 VF500 carburetor? The carbs on the '86 were a bit different than those on the '84 and '85 models..
     
  9. jeremyr62

    jeremyr62 New Member

    Country:
    Ireland
    Joined:
    May 14, 2011
    Messages:
    609
    Likes Received:
    40
    Trophy Points:
    28
    Location:
    Limerick, Ireland
    Map
    For info, the small fuel filters are located above the float needle seats inside the carb bodies.
     
  10. NormK

    NormK New Member

    Joined:
    Jul 12, 2012
    Messages:
    1,821
    Likes Received:
    3
    Trophy Points:
    0
    Don't want to be picky but you have had the bike for 2 months, it is an 86 model so it is nearly 30 years old and it seems to bog down at 8000 rpm, keeping that up will only end in tears.
     
  11. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Haha. We'll see Norm. Everyone loves to hate on the reliability of these little bikes. However, at the end of the life cycle of the VF500 ('86) designers had worked out most of the kinks of the '84 and '85 VF500s. Of course, it goes without saying that the VF500 was also unfairly disliked out of association with the problems of the older, larger V-Fours. Every bike needs work now and then, but I'll make argument that the '86 VF500 is still one of the more reliable 1st or 2nd Gen Interceptors. For what it's worth, after adjusting the valves recently the engine ran and sounded WAY smoother than my '97 Suzuki GSXR ever did. Two weeks ago and 30 years after it was made, it was showing some amazing performance characteristics..It is only a machine. I will bring it back. Fortunately I live only 40 minutes away from the largest motorcycle junkyard in the country which has several of these VF500s lying around.
     
  12. Dr. Jay

    Dr. Jay New Member

    Joined:
    Nov 17, 2010
    Messages:
    80
    Likes Received:
    0
    Trophy Points:
    0
    Location:
    Deland Florida
    Map
    Given that the problem started after you were very low on gas and are missing the petcock filter, I would suspect that some crud from the bottom of the tank got through and is partially restricting one or more of the main jets. The fact that the problem occurs when you are on the main jet circuit as opposed to the pilot or needle circuit is also suggestive. I sure would try to eliminate all other possibilities first though, pulling the carbs is a PITA.
    Let us know when you do correct it. Learning from members trouble shooting experiences is one of the great aspects of VFRworld.
     
  13. squirrelman

    squirrelman Member

    Country:
    United States
    Joined:
    Jul 27, 2006
    Messages:
    9,838
    Likes Received:
    743
    Trophy Points:
    128
    Location:
    Buffalo, NY
    Map
    IMG_1145.jpg

    First thing to check is your plugs for signs of too much or not enuff fuel (or spark). You need a nice light tan color on the center electrode if things are all good.
     
  14. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    UPDATE: I assumed I was running on all 4-cylinders because the engine seemed to be running smooth but with a general lack of power. However, I have discovered the front bank of cylinders is losing spark as the engine heats up..What would be the faulty part that causes me to lose spark on just one bank of cylinders? Any ideas? Thanks for any help.
     
  15. MPH Racing

    MPH Racing New Member

    Country:
    Canada
    Joined:
    Dec 13, 2013
    Messages:
    171
    Likes Received:
    1
    Trophy Points:
    18
    Location:
    Calgary Alberta Canada
    Map
    My back 2 were doing that and it ended up being the coil was worn out. Can also be the CDI box but not as likely. Remember the coils are Batch fire so 1 coil fires the front 2 and 1 fires the back 2.
     
  16. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Thanks. Since my front cylinders aren't firing, would these be on the secondary or primary ignition coil? The front cylinders are numbered 2 & 4 while cylinders 1 & 3 are in the back row..If I can figure out which sub-system it is I will be in business. Also are these "primary" and "secondary" coils interchangeable and identical to one another, or are they different?

    btw, can anyone recommend any decent aftermarket coils for this bike, or are used OEM replacements the way to go? I am leaning towards used OEM for the good price and original quality but was wondering whether buying one really old, used coil is going to be the way to go on this one (for reliability/longevity/performance sake) I am fine taking my chances with another used original coil but was just curious. Is it recommended to replace both of the coils at once with new ones, or a matched-worn set from another old machine, or just replace the bad coil with any other used coil?
     
  17. MPH Racing

    MPH Racing New Member

    Country:
    Canada
    Joined:
    Dec 13, 2013
    Messages:
    171
    Likes Received:
    1
    Trophy Points:
    18
    Location:
    Calgary Alberta Canada
    Map
    pop your tank off and you will see the 2 coils, the lower one on the mount is for the front and the upper one is for the back. They are the same, you can get them off a VF700/750 as well. Accel make's aftermarket ones but in the 30 years I have been building stuff I have found Accel is CRAP and won't use any of their stuff. You can check the coils with a multimeter, put the positive on where yellow wire connects to the coil and negative on the other, you should see around 3- 3.5 K ohms or below (no lower the 2.6) if it's above that or below that the coil is bad.

    Remember also they might read ok till they heat up then the metal inside expands and you lose continuity causing the coil not to fire.
     
  18. HeavyNova

    HeavyNova New Member

    Joined:
    Feb 17, 2014
    Messages:
    23
    Likes Received:
    0
    Trophy Points:
    0
    Yes, indeed-absolutely right. One of the A,B spark units isn't working right. There's something really satisfying about buying another OEM tested unit on eBay for only $20. Fortunately, their location on the bike (right under the seat) easily makes this one of the most accessible and simple 'plug and play' repairs on the machine.
     
  19. fastenough

    fastenough New Member

    Joined:
    May 10, 2014
    Messages:
    26
    Likes Received:
    0
    Trophy Points:
    0
    can you elaborate on this? ive searched and searched and found nothing about this... i run a k&n and noticed a little 'dead spot' at about 5500 - 6000 after installing it, maybe this is my problem?
     
Related Topics

Share This Page