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

[Billy Golightly]

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"?

[200x Basket]

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.

[SWIGIN]

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

[OldSchoolin86]

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.

[SWIGIN]

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

[Billy Golightly]

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?

[tecat-z]

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.

[200x Basket]

what does needle size have to do with WOT settings? nothing, so needles are not even part of this equation.

[SCEADU]

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.

[tecat-z]

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.

[200x Basket]

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.

[tecat-z]

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!

[Billy Golightly]

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 ?

[tecat-z]

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.

[Daddio]

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.

[Billy Golightly]

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?

[300rman]

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.

[lndy650]

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

[Billy Golightly]

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.

[TRITecate350]

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.

[lndy650]

very well said TRITecate350

[madmanwithmouse]

dont forget to ask how well is the fuel being atomized.

[Dammit!]

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.

[Billy Golightly]

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.

[Daddio]

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.

[Daddio]

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 ...

[300rman]

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?

[Daddio]

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.

[tullier855R]

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.

[300rman]

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"

[lincolnolli]

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

[lincolnolli]

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

[RamsesRibb]

From what I've read this phenomenon is caused by intake velocity. It's the reason some people use CV carbs.

[RubberSalt]

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.