Loss of power at higher RPM's

Discussion in '5th Generation 1998-2001' started by fonebone77, Oct 10, 2019.

  1. fonebone77

    fonebone77 New Member

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    I have a 99 VFR that I have worked on quite a bit. I am not sure exactly when this issue began because I have only been riding the bike every once in a while lately and not pushing it very hard. What is happening, as I get over 7k rpm the bike just starts losing power and the delivery gets very choppy. The wider I open the throttle at that point the worse it bogs. I can coax it as high as maybe 9500 rpm and that is all she will do. I thought perhaps I had a vaccum leak because the last major repair i did on the bike was a thermostat change which obviously required removing the throttle body. I completely redid all the vacuum hoses, and do believe I had a leak because previously a quick squirt of brake cleaner under the air box would cause it to bog a bit , now that is fixed and the bike runs fantastic below about 6k rpm and does not bog with a sprtiz of brake cleaner anymore. It still is having the higher rpm issue. Now, the tank is definitely a little rusty and is something i should have addressed some time ago. Does this maybe sound like a failing fuel pump/clogged fuel filter? It is so consistent that i was skeptical of that at first, but now I am not sure what else to look at.

    Adding to my post, this condition is only under load. I can rev the bike to redline in neutral with no issues.
     
  2. ottablue

    ottablue New Member

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    I had very dirty air filter which caused the bike not to rev passed 9k rpm and was feeling sluggish. Replacing air filter solved it. But your case might be little different since you can rev all the way to red line. So, check that if not I would check fuel filter then possible fuel pump. Good luck.

    Sent from my SM-N950F using Tapatalk
     
  3. GreginDenver

    GreginDenver New Member

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    The single statement you made in this post that really got my attention is: "... and the delivery gets very choppy"

    It sounds like you're describing a situation that progresses (as load is demanded) to the point where individual cylinders are "missing" (failing to fire) due to some sort of problem. Given that your description shows the bike running well (enough) in the lower RPM and low load-demand parts of its operating envelope, I would be inclined to believe (as you already seem to suspect) that you may have a fuel delivery problem. The feeling of "choppy" power delivery you describe could be cylinders reaching a too-lean-to-burn condition as the RPMs and demand ramp up. You didn't mention any FI codes so I'm guessing there are none, which means the ECU is going about its job in a business-as-usual way, commanding normal fuel injector pulse-widths that would normally deliver the proper amount of fuel for the situation at hand.

    Your spark plugs might give you a clue as to whether this too-lean theory has any truth to it. You could pull one or two and see what they look like, compare them to some internet-search results for spark plug condition indications of problems.

    Good luck with this.
     
  4. jfrahm

    jfrahm New Member

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    You don't have the resources to run the engine under load at high RPM, right when you need the most fuel and air (but not too much air).
    And the tank is rusty. So probably the fuel filter is partially clogged with rust and/or the fuel pump is clogged or damaged from pumping rust.
     
  5. fonebone77

    fonebone77 New Member

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    I acutally just changed the plugs and they looked mostly normal for 20 year old plugs. I am going to take the tank apart, have the tank cleaned and sealed and put in a new fuel filter and see where that gets me. Ill let you guys know how it works out though it may be a while before it all comes together.
     
  6. Rio800

    Rio800 New Member

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    It sounds like fuel delivery to me. Exactly where will be your problem. Before going as far as removing and refurbing the tank, change your filter, check all connections for leakage flush through after filter change and try it out . If no good change the pump.
     
  7. fonebone77

    fonebone77 New Member

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    So for anyone who is interested. Pretty sure I have found the problem. I took the tank off, and removed the pump assembly and there is some rust particulates in the little mesh prefilter but i dont think that is the real issue. Someone else has has this tank apart at some point and broke one of the studs that holds the nuts that holds the pump assembly in. Their solution to that was to go kinda nuts with some red rtv around the lips where the assembly seats. It actually was fairly effective for who knows how long, but that rtv has broken down and now there is a pretty thick slime of the rtv all over the mesh filter. I am going to go ahead and have the tank cleaned professionally (local place does its for $150 which seems pretty fair) and then put it all back together but this time i am going to try to find a gas friendly sealent to cover that stud and use a whole lot less of it than they did. The stud would be pretty diffidult to repair imo. The studs are sandwiched between a metal plate and the tank itself and the plate is spot welded to the tank. Ill maybe take some pics and post another update when I start reassembling everything.
     
  8. bk94si

    bk94si Member

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    JB weld
     
  9. GreginDenver

    GreginDenver New Member

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    The "mesh filter" isn't a filter, it's function is to perform as an anti-splash for the returned excess fuel flow from the fuel rail (the returned fuel outlet sprays directly into this wad of stainless steel mesh) . This prevents fuel pump cavitation (due to bubbles/frothing) as the fuel quantity within the tank reaches low levels.
     
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