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Thread: Fireblade engine won't rev.

  1. #1
    Junior Member
    Join Date
    Aug 2008

    Fireblade engine won't rev.

    I have installed a 1996 carb spec Fireblade engine into a 4-wheeled version of the Bond Bug, but cannot get the engine to run properly.

    The engine was heard running in the donor bike (CBR900RRT) before purchase, and I have no reason to believe that there were any problems then.
    I removed the cylinder head, and the motor is in good condition. I reground the valves and reset valve clearances. Valve timing set correctly and subsequently re-checked.

    The engine starts and runs perfectly well, but when it reaches around 6000 rpm, it starts to run badly, misfires seemingly on all cylinders, and will not exceed 6,500 rpm. Back off the throttle and rpm, and normal running is resumed.

    I have run the engine with the original airbox, but had to modify it (chop off about 30 per cent) and delete the air cleaner element, so it is just a cold air box now supplying a ram air supply. I have tried it with this airbox fitted, and without, on open trumpets, but with similar results.
    I found that the voltage regulator had melted, and replaced it with a genuine new unit of improved design.
    The crankshaft sensor checks out on its resistance value, as does the throttle position sensor, which rises/falls as specified.
    I have substituted a known good ignition control module to no effect.
    I have fitted another complete set of carbs (with TPS) to no effect.
    The ignition coils check out, and provide a spark across 2 cm test gap.
    The fuel delivery (via Facet electronic pump) is 50 per cent greater than the specified minimum, and I have run it from 1 psi to 5 psi via a pressure regulator.
    When it starts misfiring, applying some choke makes a small improvement, but only about 500 rpm. Fuel is fresh.
    I have tried running with the fuel cap removed to no effect.
    The exhaust uses a home made 4-2-1 manifold made without scientific calculation, but just to fit into the available space, but is not disimilar to the original, and uses the original quiet o.e. silencer. I have also tried a straight through sports silencer.
    I am using a digital tacho, but have tried disconnecting this to no effect.

    I am running out of ideas now. I guess it may need bigger main jets running without any air cleaners, but would this stop it revving at all? Currently running 115/118 mains.

    Any help or advice would be sincerely appreciated.

  2. #2
    Senior Member
    Join Date
    May 2007

    Re: Fireblade engine won't rev.

    I'm not a bike mech, but have been aircraft and car mech for more than 20 years. If it backfires basically it's a timing issue with the spark. But cutting out at high RPM it could be a misfunctioning coil pack or even plug wires that have worn out. It doesn't sound like it's the lack of air, or choking the air making the air/fuel richer would not increase RPM but flood it instead. Cutting out? It sounds like you have good exhaust flow, but how does it run with stock manifold at RPM? Remember the old potato in the tail pipe trick? Anything above idle created serious problems due to back pressure. I would not re-jet until you are sure. Remember it ran good with the jets the way they were before. What changed between then and now in the configuration? That's where I would look first. Your exhaust.

  3. #3
    Senior Member
    Join Date
    Jun 2008

    Re: Fireblade engine won't rev.

    i had an o2 gsxr 750 that hit what sounded like a rev limiter at 6500 also and found out later it was a pinched fuel line .

  4. #4
    Moderator FunnyWheels's Avatar
    Join Date
    Nov 2002
    Tampa, FL

    Re: Fireblade engine won't rev.

    I found on my Ducati that it would backfire if there were air getting in between the intake manifold and cylinder head. I had a Harley once that would backfire due to a loose exhaust pipe from the head. Make sure the intake is sealed properly and the exhaust is tight.

    Mu .02 worth for you.

    If you're not confused, you're not paying attention.

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