Loss of mid/high rpm power.

Discussion in 'Mechanics Garage' started by RotaryRocketeer, Apr 23, 2014.

  1. RotaryRocketeer

    RotaryRocketeer New Member

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    Here's the deal. My bike ('86 VFR700 12,400 miles) has experienced a serious loss in power from 5k to redline in every gear at every speed. This started about 3 days after I ran the bike out of fuel waiting to turn across traffic into the gas station. It doesn't sputter and die, just not nearly the power it was making before. It will accelerate smoothly its just anemic. No backfires but you can feel it surge and buck a little occasionally when holding steady throttle at RPM greater than 5k. On one test ride I lugged it pretty hard up a mild short hill in 4th at 35mph to see what would happen and it was fine until it died at the upcoming light (started right back up though.) Just had carbs professionally cleaned and rebuilt, plugs are new and look good (I'd call them light chocolatey tan), fuel pump flow tested (spot on), tank cleaned and sealed while carbs were being doctored, fuel pump relay good, fuel lines and filter replaced, and tank vent clear. Also, the in-line fuel filter I have is always less than 1/4 full after riding. I checked it the other night in a parking lot while troubleshooting. The fuel pump was pumping its little metal heart out with the bike idling at about 1k but the fuel was barely flowing through the filter.

    Any input is greatly appreciated.
     
  2. TNRabbit

    TNRabbit New Member

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    If you ran it dry, I'm guessing some crud that was floating in the tank is clogging the line. I'd pull the pump & double check all lines going in & out.
     
  3. RotaryRocketeer

    RotaryRocketeer New Member

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    Good call, I actually just replaced the lines today and haven't gotten a test ride in yet. Waiting on my low fuel sensor to be rust free, it's soaking right now. Keep 'em coming.
     
  4. squirrelman

    squirrelman Member

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    Your bike started out running fine, right ? And strong ??

    Seems like sufficient fuel isn't flowing to carbs. Since water (if any) collects at the bottom of the tank, best practice is to always drain float bowls after running out to eliminate any water, rust flakes.

    On a '86 700 (# 343), i've had a recurring fuel problem during my 24-year ownership where, for reasons unknown, randomly, the petcock will slow its flow so that pump clicks wildly while engine still runs but not strong. Never figgered it out, but pulling fuel line at petcock always shows a trickling flow, not full, when issues occur. If pump clicks continuously it can be a sign that no fuel is flowing into it. I've cleared the line by blowing compressed air backwards into the fuel line with tank filler cap open. Try it !
     
  5. RotaryRocketeer

    RotaryRocketeer New Member

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    Forgot to mention, I did drain the bowls today but haven't run her yet. Interesting, that last part about the fuel flow. Have you heard the same thing from other owners?
     
  6. squirrelman

    squirrelman Member

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    I really should have posted that years ago but never got around to it !! I'd love to hear if other '86/'87 700/750 owners have had similar fuel flow problems. Can't say how many times i've tried and tried to find out what's causing it, no fix after 24 years !
     
  7. RotaryRocketeer

    RotaryRocketeer New Member

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    I searched it and came up with nada. Guess it's just us.
     
  8. squirrelman

    squirrelman Member

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    Gotta be more !
     
  9. RotaryRocketeer

    RotaryRocketeer New Member

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    Bike is back to running like a top. Changed all the lines, blew compressed air into the tank plumbing, then soaked the bottom 1/2 of the tank in Evaporust to clean the pick-up screens, primed the system and drained the float bowls then primed it again. By the way, if you've never used Evaporust for anything, it is awesome! Safe for skin and won't blind you if it gets in your eyes either. Also, you can re-use it over and over until it loses it's potency. Thanks for the suggestions amigos!
     
  10. JasonWW

    JasonWW New Member

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    I'm glad you got it fixed.

    I just wanted to mention that when the fuel tank level is low, below petcock level, the pump has to pull a vacuum to get fuel flowing so it's important to replace the fuel lines from the pump to the tank. Even a tiny crack or leak can cause a loss of vacuum and prevent fuel from flowing.

    Squirrelman, do you think your issue was because the fuel level in the tank was below petcock level and your pump was clicking away wildly because it was dry and needed to be primed (like priming a well water pump to get it flowing)?

    These bikes have high mounted petcocks which is odd. Most bikes have them below the tank and gravity can do all the work.
     
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