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Thread: Theory Discussion: Can a mainjet be an accurate "rule of thumb" for engine power?

  1. #31
    Join Date
    Mar 2016
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    NorthwestGermany
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    If you have 2 times totally the same engine and only change the cc size on one,then yes.
    But in reality i would say no.

    From my experience its really the intake charge speed across the powerband that dictates the jet size,much more than changes in CC's,or cubic inches for you stateside folks :-)
    So a overly agressive,race type engine with a large carb but small cubic inches will need one hell of a large jet to run OK.While that big cube stocker gets away with a smallish one and burn very nice.
    I like how Honda invested a lot of time and money and came up with that 4 valve engine with a extremly mild cam.That way they created a very wide powerband and still got decent peak horsepower.One can say it just sucks way better then the old 2 valve,large valved,big cam engines.

    If one is familiar with the old V8s,just take a look at the 1960/70s Ford Cleveland 351cui (5,8L) engine,with its sewer intake ports/rough cam,large carb and compare that to a modern 4.6 32valve OHC V8 engine.We all know what runs harder,once you ride a modern stick shift mustang,lol..
    Olli

  2. #32
    Join Date
    Mar 2016
    Location
    NorthwestGermany
    --
    35
    If you have 2 times totally the same engine and only change the cc size on one,then yes.
    But in reality i would say no.

    From my experience its really the intake charge speed across the powerband that dictates the jet size,much more than changes in CC's,or cubic inches for you stateside folks :-)
    So a overly agressive,race type engine with a large carb but small cubic inches will need one hell of a large jet to run OK.While that big cube stocker gets away with a smallish one and burn very nice.
    I like how Honda invested a lot of time and money and came up with that 4 valve engine with a extremly mild cam.That way they created a very wide powerband and still got decent peak horsepower.One can say it just sucks way better then the old 2 valve,large valved,big cam engines.

    If one is familiar with the old V8s,just take a look at the 1960/70s Ford Cleveland 351cui (5,8L) engine,with its sewer intake ports/rough cam,large carb and compare that to a modern 4.6 32valve OHC V8 engine.We all know what runs harder,once you ride a modern stick shift mustang,lol..
    Olli

  3. #33
    RamsesRibb's Avatar
    RamsesRibb is offline At The Back Of The Pack Arm chair racerAt the back of the pack
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    From what I've read this phenomenon is caused by intake velocity. It's the reason some people use CV carbs.
    Previously known as HighFlying101 since 2003
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  4. #34
    Join Date
    May 2010
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    Wichita, Kansas
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    Looking at how carburetors work: These work off vacuum. The fuel isn't necessarily pulled from the carburetor, it's boiled out of the carburetor. The more vacuum there is, the faster the boiling occurs. The more surface area exposed from the jet, the more fuel can be boiled at once.

    Lets go back to the scenario of the different sized engines with the exact same carb configuration.

    We'll say the set up is a mikuni vm 34.

    YZ125 with 34mm. This engine would use something around a 300-330 main jet
    Tecate 250 with a 34mm. would require around a 250-280 main jet
    Polaris 400l with a 34mm - comes stock with a 200-210 main. <-

    The YZ is going to be the smallest. With the large carb, the amount of vacuum it is pulling is going to be low. It's going to need more surface area on the jet in order to get enough fuel out of the carburetor. The high end RPM needs to breath, so the big carb is an advantage.

    The Polaris, a big 400 2 stroke, has a big bore and a carb that is a little bit small for it. It's going to pull a massive vacuum on the carb, boiling much more fuel out of the carb. In turn, a smaller jet is needed.

    The Tecate, in the middle, would require a jet between the two.

    Restrictive intakes can cause a larger vacuum. It an idle, you actually have the most vacuum, so the main jet is plugged with the needle and the machine operates off the pilot jet. At WOT, which the carb all the way open, your vacuum is going to drop, Tons of air can move it due to the carb slide no longer restricting the air flow.
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