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Thread: V twin 'lazy' cylinder.

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    V twin 'lazy' cylinder.

    Hi all,
    can anyone please explain how a 'lazy' cylinder comes about in a V twin and how to correct it?

    Thanks.

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    Injected or carbs?

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    apriliaforum expert pete roper's Avatar
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    What do you mean by a 'Lazy' cylinder Chris?
    Professional Goat Burster.

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    Honest always, feared often Micah / AF1 Racing's Avatar
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    Confusing terminology to me here. All odd fire motors do breathe slightly differently cylinder to cylinder due to acceleration and deceleration of the crankshaft but even in a v-twin it is not a crazy big difference in volumetric efficiency.
    Diminished expectations is the key to happiness in life.

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    Quote Originally Posted by pete roper View Post
    What do you mean by a 'Lazy' cylinder Chris?
    Well when researching this I came across the term in a post of yours years ago in the V11 forum.
    (You mentioned small blocks having to use smaller jets on the 'lazy' cylinder in order to pass pollution laws.)

    It refers to the following cylinder running rich due to the fact that the compression stroke is aided by the closely preceeding other cylinder - at least thats my guess, hence this posting.

    Just had a talk with a Ducati super bike mechanic and he said its not unknown for the rear cylinder to run 50% richer than the front!
    Last edited by Chris Wilson; 07-07-2017 at 10:05 PM.

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    apriliaforum expert pete roper's Avatar
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    If it's running 50% richer there's something very wrong with it!

    Yup, I thought you might be referencing something like that.

    The firing intervals on a shared crankpin 90* twin are 270/450 degrees. The longer interval allows longer for the crank to slow due to frictional losses so even if the difference is miniscule at lower crank speeds especially it will result in differences in volumetric efficiency between the two cylinders and consequent fueling differences. It's why modern (ish.) twins all use a delta map to compensate for differences between the two cylinders. This is particularly true on a bike like the Griso with its odd, asymmetric exhaust system.

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    Honest always, feared often Micah / AF1 Racing's Avatar
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    If you have a 50% fuel table Delta on any twin cylinder motor that motor or the base mapping is FUCKED. The only times I intentionally heavy bias fueling deltas in tuning is towards the rear cylinder on narrow angle v twins like HD motors, this is not a power strategy but rather a thermal control strategy. One HD motors the rear cylinder gets almost zero effective air cooling at low speeds typical of the breed while the front gets fantastic cooling. You can run the front cylinder at peak power mixture and the rear biased as much as 2 full afr points richer with say -2 to -4 degrees ignition lead biased rear to get a somewhat more thermally stable engine at street and highway cruise speeds. Water cooled motors are in no way biased for thermal control only to correct the very small VE difference between cylinders. When talking carburetor tuning yes sometimes you have a 4 main jet size or greater difference front to rear on a dual carb twin, carburetors are infinitely more fickle about small pressure and time differences of intake open event than EFI equipped machines.
    Diminished expectations is the key to happiness in life.

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    Quote Originally Posted by sburns2421 View Post
    Injected or carbs?
    Sorry, missed this for a while but I suppose both systems are an issue.

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    Thanks for the replies guys.

    The 50% richer on the rear seems hard to believe I must admit so I will chase it up to see if it was just bravado or not.

  10. #10
    Honest always, feared often Micah / AF1 Racing's Avatar
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    That would mean on a superbike engine the front cylinder was making 125bhp or so and rear 75bhp or so...such a thing would shake itself to bits in little time.
    Diminished expectations is the key to happiness in life.

    Micah Shoemaker
    AF1 Racing
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