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

  1. #1
    Billy Golightly's Avatar
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    Theory Discussion: Can a mainjet be an accurate "rule of thumb" for engine power?

    Lets, hypothetically think that we have several engines. All burning at the right a/f mixture level. None of them are rich, and none of them are lean. Lets also assume they also all use the same type and model of carb, but they all require different jetting to get to their optimal spot where they are not rich or lean. These engines being 2 strokes, I will for example say we have a tecate engine, a tri-z engine, a 250r engine. Lets say the 250R needs a 165 main jet to be in its spot. The Tri-Z needs a 152, and the Tecate needs a 178. Everything else being equal, the Tecate should be burning and using the most amount of fuel because it has the largest main jet, correct? So can we use main jet size as a rule of thumb for power ouput (again, supposing the mixture level is right, and its not over or under jetted)?





    Heres another hypothetical, How about a Tri-Z motor with a 185 main jet and a KX500 motor with a 168. Is the smaller Tri-Z motor in theory actually making use and power from the additional fuel then the KX500? I've seen some discussion here in the past about the jetting differences between small bore 250s and the big bores on how the big bore engines do not seem to need as much main jet as the smaller motors, I tend to agree with this from my experience. But, its also my experience that the big bore motors out perform the smaller ones even with the smaller jetting. So what is the cause of this? Are the big bore motors more efficiently using the less amount of fuel? Or Does a larger bore and stroke produce a stronger intake resonance and pulse that does not require as large of jetting because its pulling in the mixture "quicker"?

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    i have noticed the same trend. I have a 165 in my 500r and i recently put a 180 in a stockish 250r.

    i had thought it was rpm related but the intake pulse could be a factor. There is no doubt that a cr500 is moving more air then a 250r.

    all engines are air pumps. the more air pumped per given period of time means more work/power.

    the power will require more fuel.


    so i would "guess" that the intake signal is stronger on a big bore.




    on another note. there is a theoritical limit on how much nitrous a drag car can take based on the size of the engine. anymore then that and it sprays out the top of the carb!!! on my last car i actually had to drop the size of the carb when i got a larger N2O kit. i built it to run a 600HP shot. on both kits the carb signal was to small and it the car ran funky. i swapped to a custom HP950 and the problem went away.
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    what you seem to be forgeting is that the neddle has everything to do with main jet size. and comparing totaly different motors is crazy.

    my 350x motor is built the same as my buddys down to carb and pipe.....so you would think they would have the same main jets right?.......nope

    he is running a thicker needle and a 220 main to my thinner needle and 160 main

    and mine runns stronger
    80s......185 atc, Yamaha tri-moto 200, 85 200x with tons of work
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    2012.....Replaced the 350x motor for a 444cc YZ426
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    Explain this, I have a 300ex carb on a 230 quadsport. The quadsport is now 236cc and the 300ex was 282cc. The 300ex needed a 128 main, stock needle middle clip, and a 38 pilot 2 turns out for a filter and pipe. The 230 needs a 148 main, stock needle one clip down, and the same 38 pilot 3 turns out to run rite with a pipe and filter. The 230 quadsport without a doubt makes less power then the 282 yet it needs more fuel. I would say no, a main jet is not a rule of thumb for engine power.
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    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
    80s......185 atc, Yamaha tri-moto 200, 85 200x with tons of work
    90s......89 Suzuki quad racer 250 (raced 250 A class for 6 years, late 90s.. custom framed 250x with long travel shocks and a built 350x motor.... built Honda 110
    00s...... chomeoly framed 350 RX with all the goodies (thanks to my bro)
    2012.....Replaced the 350x motor for a 444cc YZ426
    WWW.HREATV.COM

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    Billy Golightly's Avatar
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    But why do the smaller bore engines (that make less power) typically need more jet then the bigger, more powerful engines? How can it burn more fuel and not make more power? Obviously its got to have more air too, or it'd be over fueling/rich, but they arent.

    I can't explain your experience either Frank, just like I can't explain how my 270cc flat tracker motor was running a 185 mainjet and the 500 is running a 158 with the exact same needle and pilot (the #55 pilot by the way, is insanely to rich for the 500, it needs like a 50 it feels like). Engine internals and specs aside, both engines are being fed with the exact same carb, the exact same air filter. Yet the engine using less fuel, and less air, is making more power. How? And what is happening to the motor that is using more air, and more fuel, and making less hp?

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    My experience on this topic is long stroke engines require smaller mains and leaner needle settings than big bore short stroke combos. Reason being----- well the intake pulse is longer, and it's inhaling fuel for a longer period of time through the same size jet. Which means more fuel and a richer condition. And larger engines produce more signal. Short stroke engines get through their cycle in theory faster so more fuel is required sooner through a larger jet. The 330R we just built is a perfect example of this. So far all of the jetting is smaller that my 85 tecate, '256cc' and the 330 is still way fat! This is on all 3 phases of fuel delivery. And my Tecate is tuned very crisp. Traditional thinking would lead you to believe bigger engine = bigger jets. Engine design 'bore/stroke' has lots to do with it.

    My Tecate 71mm / 64.9mm

    330r 76mm / 76mm "4mm stroke added"

    The tecate was designed way oversquare where a liquid R is way under. We added both bore and stroke to the R, so the stroke is now way long and will now inhale fuel for an even longer period of time, which makes it richer. Where the Tecate has to fulfill it's thirst in a much shorter period of time, thus requiring a richer needle, and larger main. If you think about this it will make perfect sence, and it's true.

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    what does needle size have to do with WOT settings? nothing, so needles are not even part of this equation.
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    I did not read all the posts so someone may have already said this. But here is a thougth about how the fuel comes through the jets. Air passes over them createing a syphon effect. A 500 would have more of a syphon than that of a 25o. A smaller jet doesnot necessarily mean less fuel at this point because more air travels over the jet pulling more fuel through it.

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    Quote Originally Posted by 200x Basket View Post
    what does needle size have to do with WOT settings? nothing, so needles are not even part of this equation.
    It directly affects WHEN you begin to pull fuel through the main. Besides, i am answering the original question as to why, not basic jetting problems.

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    Quote Originally Posted by tecat-z View Post
    It directly affects WHEN you begin to pull fuel through the main. Besides, i am answering the original question as to why, not basic jetting problems.

    exacty it has NOTHING TO DO WITH THE MAIN. the main is 3/4 to WOT.
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    You need to re-read my first post. More fuel is being pulled through every step of the process because the time that the piston its traveling is longer. Therefore more fuel is being pulled. Not just the needle, but everywhere. And needle length, taper and diameter have a huge effect on the main jet and the engines ability to handle the rush of fuel from being fully on it, before hand. " loading up" This is basic carb tuning. Billys question wasn't on this. We already know how fuel is pulled from the carb. This is on the duration it is being pulled in all 3 phases. And long stroke engines are way different than short. Go re-read than think. If you still don't understand i'll create an analogy for you. It's really very simple!

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    Billy Golightly's Avatar
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    So the answer is the larger engines have more draw, and are in effect having a higher velocity of the fuel flow requiring smaller jet sizes then a smaller, less powerful engine that requires more jet (diameter) to make up for the lack of a powerful vacuum/intake pulse/syphon effect ?

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    The carburetor has no idea how it is supposed to be jetted. Nor does it know what engine it is being used on. It only provides fuel on an, as needed basis. Just as on a car, max rpm has a greater effect on cfms needed. Not displacement. And on a valve engine, when stroke is added, say a 383 chevy, a different cam profile is needed to fully realize the added displacement. So a higher lift and longer duration cam " the time the valve is off the seat" is needed to fill the cylinder. The stock grind 350 cam will work, but not optimized. In 2 srtrokes, port timing and stroke determine how long or short the intake pulse is. Long intake pulse, with a lower revving engine doesnt require as much fuel as you think. Small example. Two people have garden hoses. On the count of 3 they both squeeze the nozzle. One person lets go after 5 seconds, and the other after 6. Same size hose, but obviously the guy will have more water in his bucket after 6 seconds than 5. Why, because it was held open longer. Valves can be machanically controlled, where 2 stroke, reed valve bikes and the like are controlled by length of pulse. Longer stroke bikes have longer pulse times, thus more fuel through same size jet as a short stroke higher revving engine. No two engines are the same. But this a general rule that applies to engines. And Billy seems to understand this. Again, if you think about it, it makes perfect sense.

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    I am definitely no expert on this subject but this is what I found out helping 1upfront dial in his 250r over the last 3 years.
    The original 250 cc engine with stock carb and boysen pro series reeds liked a 108 main jet.
    With the 300r kit and the same reeds with a 36 pwk it liked a 125 main jet.
    This year with the ported 300, V force 3 reeds and a 39 pwk with the 160 main that came with the carb it is running a little lean. He will try a 165 main this weekend. That should be just right.
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