Boyesen esque port..? Please explain, I don't have a clue what that term means...
39mm carb, too big?
- Thread starter SwedeMix
- Start date
Boyesen esque port..? Please explain, I don't have a clue what that term means...
i know it seems like common logic that a 39mm carb works on a 125 then it really should have no problem being utilized on a 200 engine ,but instead of size alone consider thinking of an engine as a pump* the 2 motors couldnt be more different :a modern 125 dirtbike engine can attain rpm over 11,000 ..while the archaic yfsblaster motor with a pipe would be fortunate to hit half that.huge difference in CFM needs and pressure.its how much air and fuel the engine can efficiently pump through it~ not the size stamped on the jug casting
that cylinder intake looks strange as hell, ive never seen one worked like that. i would really like to hear what our resident builders think of that? :-/
It does look weird, doesn´t it? I´m hoping it´s not screwed up, would suck to have bought the piston if it won´t work properly!
Seems to me as the previous owner done quite a bit (of damage..) to it, there was an xtra gasket in the cyl.bottom too, made of thick steel - thanx for cutting off all high-rev-abilities dude...
So that´s gotta go, I don´t know if I wanna do the exhaust either in case it´s been milled too - chance is I´ll completely f*ck up the port-timing! Would be nice to have another cyl to compare it to, cuz this one seems to be anything but stock..
My 250 motor-swap might just come earlier than I expected..! X(
I'm pretty sure the gasket is related to the portingjob somehow, will try without the xtra gasket first - carefully checking the piston dont bang in to the head ofcourse. But in the short time I had it, I noticed that it never seemed to rev properly, when I picked it apart I obviously thought that was because of the cracked piston but I'm getting increasingly worried it's not that easy to fix... I'm not going to invest any big amounts of money or alot of time in this motor though, but if it runs at least semi-ok I'll keep it on during the winter. But we'll see what happens, keepin my fingers crossed!
Actually I disagree. I dont think he will run lean, I think he will run RICH as hell.
I was running a 36mm on mine (thats what came on it - not sure why the dumbass who did that one did it but whatever), and no matter how low I jetted it, I couldnt get the stupid thing to run correctly.
Larger the carb, you have to jet DOWN. And you get to a point where you just cant jet down anymore.
Or at least this is my experience with a mildly modded blaster. FMF Fatty and an outerwears pod filter.
I just couldnt jet down enough. Finally fixed my issue by trading that 36mm PWK for a 28mm PWK and it runs great now.
I was running a 125 main and a 30 pilot on my 36mm and it was rich as hell.. I have a 138 main and a 35 pilot on my 28mm and it is slightly rich, but its running good and not fouling plugs.
I was running a 36mm on mine (thats what came on it - not sure why the dumbass who did that one did it but whatever), and no matter how low I jetted it, I couldnt get the stupid thing to run correctly.
Larger the carb, you have to jet DOWN. And you get to a point where you just cant jet down anymore.
Or at least this is my experience with a mildly modded blaster. FMF Fatty and an outerwears pod filter.
I just couldnt jet down enough. Finally fixed my issue by trading that 36mm PWK for a 28mm PWK and it runs great now.
I was running a 125 main and a 30 pilot on my 36mm and it was rich as hell.. I have a 138 main and a 35 pilot on my 28mm and it is slightly rich, but its running good and not fouling plugs.
it may have been your carb because my stuff would run lean pretty much no matter what i did for the pilot. it was on a similar setup to yours luni.
the science behind it works against how yours was acting, and makes me think it may have been a problem inside the carb
the science behind it works against how yours was acting, and makes me think it may have been a problem inside the carb
You can run that carb all day. Two words. Modulator Ring. Use it to tune velocity.
Brandonz28 is right as well as flotek. There is a lack of low end velocity to draw air threw the jet wells to pull the fuel out at lower rpms. Might pick up around 6 grand when the blasty is dyeing off. But if you run modulator rings to inherently choke the flow at the front and cause a high pressure wave at the front of the carb you will be amazed how well it actually works if not better than any other suggested carb.
Brandonz28 is right as well as flotek. There is a lack of low end velocity to draw air threw the jet wells to pull the fuel out at lower rpms. Might pick up around 6 grand when the blasty is dyeing off. But if you run modulator rings to inherently choke the flow at the front and cause a high pressure wave at the front of the carb you will be amazed how well it actually works if not better than any other suggested carb.
Modulator ring is just what it sounds like a ring that modulates flow into the carb. Simple way is take a measurement of outside diameter. Cut it out . Start buy cutting that out at 28mm in the middle. Making a ring. This will change the venturi flow pattern and casue more of a suction effect then a 28mm at a given velocity. Why there is a bigger Draw from the inside causing a turbine/sucking effect threw the ring. We used to use these on Vw's that where running Blowthrough turbo system to get some on boost enrichment before we had fancy adjustable baffles.
this also cam from the factory on the Lotus turbo espirit that ran blowthrough turbo as well.
Is this more work maybe. But the bigger draw at least for someone like me who tunes indefinitely is the ability to adjust the inner hole to tune a specific rpm range.
this also cam from the factory on the Lotus turbo espirit that ran blowthrough turbo as well.
Is this more work maybe. But the bigger draw at least for someone like me who tunes indefinitely is the ability to adjust the inner hole to tune a specific rpm range.
I´ll second that, this sounds interesting!
EDIT: You were to fast, the reply came before I´ve posted mine...
Swede, save that spacer. My DR ported cylinder has the port timing adjusted so that I must run a 1mm spacer with two stock gaskets (thicker than most aftermarket ones) under my jug. Although your jug looks odd where they put the boyesen ports (two round holes in intake port) it still looks like a professional porting job of some kind. Has your head that came on it been seriously milled to compensate?
Think of the velocity a motor creates at a given vacuum level. Its happily sucking air threw a 28mm mouth. It pulls fuel out so on so forth. Slap a 39 or whatever on there. Open the throttle. BOOOOOOOOOOOOOOOG. Youre just opening a hole into the intake tract and its so big it cant build a velocity wave to pull fuel. Now throw a modulator ring on there. You opened a big gapping hole that causes a huge pressure differential right in front of the carb. this effect causes a stronger deeper pull into the motor and promotes better atomization and burn rate. Because there is Such a huge vacuum right in front of the carb it causes a almost turbo effect. Not gonna see any positive pressure but youre gonna at least double the intake flow rate.
Thats odd. Well I hope it wasnt the carb, because then I hosed royalt whom I did the trade with.
Cause I agree with you guys on the physics for sure. I just know what I personally encountered. And I did bog bad when I hit WOT, but my spark plug and fuel consumption disagrees with the lean thing.
So Dude, what else could be wrong inside the car to have caused mine to behave the way it did?
I know there was nothing physically wrong with it, the carb is in tip top shape. Everything functions, seals, all the jets were always clean, the float works properly, no plugged air passages, etc.
Cause I agree with you guys on the physics for sure. I just know what I personally encountered. And I did bog bad when I hit WOT, but my spark plug and fuel consumption disagrees with the lean thing.
So Dude, what else could be wrong inside the car to have caused mine to behave the way it did?
I know there was nothing physically wrong with it, the carb is in tip top shape. Everything functions, seals, all the jets were always clean, the float works properly, no plugged air passages, etc.
The edges on the inside of the ring, are they supposed to be left sharp..? Isn´t that causing alot of turbulence? This sounds interesting as h*ll actually, but why exactly cut it 28mm?
Thanks // SwedeMix
So, sure, adding a restriction will induce a low pressure zone behind it, ventruri effect. But why would this possibly be better than a properly sized carb? Just the tunablity? You wouldn't normally see someone drop a 1050 Dominator Holley on a mild 350 just to have to choke it with a restriction.
Yeah you would if they knew how tune things the right way. tuneability is everything. How many things can you change to achieve a effect that is desired. To me it priceless. I guess if you like to keep things simple its fine but I like to screw with sh*t and be able to change all kinds of things.
Demon Race Demon RS Carburetors - Oval Track - JEGS
The edges must be smoothed yes. I just used 28mm as a reference you can go from the minimum to maximum bore allowed buy carb.
Hence the tune-ability. AS you make it bigger or smaller youre going to change this effect in a diffrent rpm range. Smaller hole larger low end effect......
