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

  1. #16
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    OK, so we've deduced that engines with bigger strokes don't need as much of a main jet diameter because they have more draw time and intake pulse. So lets say now we're comparing the same engines. I like your example Daddio, the stock 250R needed a 108 main, and the highly worked over 300 now uses at least a 160.

    Lets ask again then, if we have 2 engines of the same type (250R, Tecate, Tri-Z, whatever and what have ya), both are at equal or nearly equal altitudes and same types of fuel so as to not skew the #s any (or, not nearly as much). Is a main jet size an accurate rule of thumb for the engines power output now?

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    ok, heres a real doozie. my 300 kit is running a cr500 carb, jetted for a cr500.
    i bolted it onto my 300, and didnt touch the jetting. it was spot on perfect.
    now, i have ported the jug and added Vforce reeds, and the once perfect carb is now running a tad richer than before. the blug is a darker, almost black color, and im running a better oil now too.

    slightly off topic, but 2 stroke related:
    i have noticed that small 2 stroke motors have insane power differences compared to equal cc 4 strokes. but, from what i have HEARD,as the cc's of each motor climb, the differences become less apparent, up to the point where, say, a 650 dirtbike 4 poke will actually be faster than a cr500. again, this is just from reading others experiences, never ridden either bikes.

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    i only have two things to say that havent yet been mentioned.... squish and compression.... those 2 things will change how much fuel is acually used and turned into power and how much is just blown out the exhaust

  4. #19
    Billy Golightly's Avatar
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    I don't disagree with you at all Indy650, and theres undoubtedly a lot of times when an engine is over jetted. For the hypothetical, I'm thinking that they are spot on. There are plenty of engines that need a lot bigger jet sizes added just to keep from holing the piston from being leaned out.

    I guess what I'm asking and getting at is, on engines of the same type, and most things being equal (fuel types, altitudes, and the CORRECT a/f mixture) can an engine with a larger required main jet size be used "as a rule of thumb" thought of to make more power then another one with the same attributes above I mentioned, that has a smaller jet size. I think it is.

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    The main jet size cant be used as a rule of thumb to judge power output. The carburetor and intake duration is only one process of many that work together to create power.
    Here is a quick list of other variables;
    Ignition timing, is the explosion working at the right time to rotate the engine
    How much a/f is lost out the exhaust
    How long is the power stroke?
    Is some of the explosion blowing past the rings
    How heavy is the piston/crank/ rotating mass.
    How much a/f is the pipe pushing back into the cylinder
    How much a/f is being burnt
    ........lot of variables
    Most people don't know how a carburetor even works, when the engine sucks air though the venturi of the carb, the result is a pressure drop, how much of a pressure drop depends on the speed of the air. When the atmospheric pressure is greater than that of the venturi the outside pressure pushes the gas though the jet into the lower pressure of the venturi. So with that said, if you have a pwk 39 carb on both engines, I think we all agree that the bigger engine will suck more and could have a longer duration of intake stroke. So if the bigger engine sucks harder, and air speed is greater thought the venturi(lower pressure than the smaller engine), more fuel will be pushed though the main jet at higher pressure. So even with same jet sizes, the fuel is being fed at different pressures, smaller engine lower pressure, big engine higher pressure.

  6. #21
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    very well said TRITecate350

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    dont forget to ask how well is the fuel being atomized.
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    I think everybody is over thinking this. Billy, if you're asking what I think you are, I believe you're right.

    If two 250Rs that are more or less the same mod wise and are running the same carb but one has a bigger main jet (and assuming they're both jetted right... one just needed the bigger jet to get the right mixture) the one with the bigger main is going to be making more power most of the time. It won't be accurate 100% of the time but I think it would usually be a safe bet. That's been my experience with 250R's anyway.

    Now if we're talking about two radically different motors with different strokes and displacements, obviously there's too many factors involved and you can throw it all out the window.

  9. #24
    Billy Golightly's Avatar
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    Quote Originally Posted by Dammit! View Post
    I think everybody is over thinking this. Billy, if you're asking what I think you are, I believe you're right.

    If two 250Rs that are more or less the same mod wise and are running the same carb but one has a bigger main jet (and assuming they're both jetted right... one just needed the bigger jet to get the right mixture) the one with the bigger main is going to be making more power most of the time. It won't be accurate 100% of the time but I think it would usually be a safe bet. That's been my experience with 250R's anyway.

    Now if we're talking about two radically different motors with different strokes and displacements, obviously there's too many factors involved and you can throw it all out the window.


    Yeah, that wasn't my initial question, but thats what I was working into. I've seen evidence to support the theory too, but wasn't sure on it which was why I posted it here.

  10. #25
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    Well after some racing and getting the 300 completely dialed in we have decided that a 152 main jet is what the machine likes. At first it seemed to run better with the 160 but after the clutch failure we backed up and looked at all of the variables. With the current porting it didn't have nearly as much grunt on the bottom end as it did last year. The one thing that we hadn't changed from last year was the gearing. Last year we geared it higher to take advantage of the extra low end power. It just didn't work with the current engine. It put too much of a strain on the clutch. So we went back to stock gearing after putting the new clutch in. That made a huge difference but it totally threw the jetting off. That is why we were looking for smaller jets at Trikefest. We ended up running the 155 but it still wasn't as crisp on the top as it could be so the next time he races it he will be running a 152 to start with and smaller if need be. I think the 152 will do it. With the engine set up like it is it makes the most power at a much higher rpm but it still needed to be leaned out to run as well as it did earlier in the year. Another thing is that the temperature is considerably warmer than it was in April. At least 20 degrees warmer. One thing that I have learned over the past few years is that if you make any changes on a 2stroke machine you will invariably have to change the jetting too.
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  11. #26
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    Something else that I forgot to mention is fuel/oil mixture. He runs and has been running 110 octane leaded gas and amsoil dominator racing oil at a 20 to1 ratio. This is another big factor in your jetting. If you change fuel types oils or ratios you will need to change your jetting and if you start messing with your timing ...
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  12. #27
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    Quote Originally Posted by Daddio View Post
    Something else that I forgot to mention is fuel/oil mixture. He runs and has been running 110 octane leaded gas and amsoil dominator racing oil at a 20 to1 ratio. This is another big factor in your jetting. If you change fuel types oils or ratios you will need to change your jetting and if you start messing with your timing ...
    wow, i am running a 188 currently. i think i may need to back down a jet size, but i had a lock-up with a 175 main due to it overheating......and we have very similar machines. why do you think there is such a huge difference?

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    Quote Originally Posted by 300rman View Post
    wow, i am running a 188 currently. i think i may need to back down a jet size, but i had a lock-up with a 175 main due to it overheating......and we have very similar machines. why do you think there is such a huge difference?
    That is a very hard question to answer. For starters what type of fuel do you use? What type of oil do you use? What is your mix ratio? Do you run it WOT like in dragracing or do you just ride it?
    When we tune the 300 we tune it as a complete machine, rider included, taking into consideration that it is going to be run WOT most of the time.
    If you are trail riding the pilot jet is probably more important than the main jet however it is still important to get everything dialed in as close as possible to get the most out of your machine.
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  14. #29
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    Hey im new to this forum and here is what i think. I think everyone is looking over the fact that of corse the jets in a 2 stroke carb put the gas to the motor, but the gas has to go through the reeds to pick air so if you rejet to a bigger size, change the reeds also.

  15. #30
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    Quote Originally Posted by SWIGIN View Post
    i have a better one for you .....

    im running a 200x carb on my built 110 and instead of jetting it down i had to go up to a 120 main

    different motors need different fule supplys
    intake velocity. the larger carb combined with the lesser CC's of the 110= lesser intake velocity, or less "suction" on the jets, thus requiring a bigger jet to compensate for the lack of "suction"

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