Post by mrpalmetto on Jul 31, 2014 18:51:48 GMT -5
I'm reading all these mods and if you free flow the air box and free flow the exhaust, I've read that you should up jet the carb and raise the needle. I saw a video that cut One winding off the carb spring and drilled one of the holes in the slide slightly larger. What does raising the needle do? And what does cutting one winding off the carb spring do.? What does increasing the size of the hole do?
Another question.......how does an exhaust leak, like a poor donut seal, or a hole in the pipe, lend itself to poor running? Should it not run ok with no exhaust on the block at all? Or does a certain amount of back pressure come into play.
The slide of a CV type carb is raised by vacuum which translates in to demand from the motor. Raising the needle in relation to the slide allows more fuel for the same amount of demand. Chopping the spring so that is weaker allows the slide (and the needle) to raise higher with the same amount of demand. Which accomplishes the same thing as raising the needle. Mostly this has an effect on the midrange as the needle is fully closed at idle, (all the fuel comes through the idle jet), and fully open at high speed, (all fuel is metered by the main jet).
Big displacement engines from Detroit don't care about minor nuances like a leaky exhaust. The smaller the motor gets, the more sensitive it is to everything, including ambient temperature and humidity. (Model airplane engines down to 0.049cc need to have the mixture screw adjusted a few degrees every time they run for optimum performance.) Changing the flow of air in or out of a scooter motor will effect the mixture. Yes, opening it up will allow it to breath better, affording potential gains in power, but you will need to compensate for the increased air flow by installing larger jets.
The actual size and length of the intake and exhaust runs are where you get in to voodoo science of tuning to resonant frequencies for induction boost and exhaust scavenging effieicency. Lots of trial and error there.
Post by JerryScript on Jul 31, 2014 23:18:18 GMT -5
Just a little extra FYI- Needles come in many shapes and sizes. There are different widths, lengths, and tapers. The smaller the width, the greater the flow at all times, but especially at 1/4 throttle. The shorter the length, the faster extra fuel is fed. The quicker the taper, the faster the fuel flows at mid-range. Here is a good writeup on Keihin carbs and how jets and needles affect engine performance at various throttle ranges: www.duncanracing.com/TechCenter/KeihinCarbJetting.pdf
The engine has a pulse that is evident in the intake and creates a resonance at various speeds. The intake and filter box plumbing are tuned to this pulse both in length and in placement of a venturie to maximize low and high end torque. The CV slide compensates fuel mixture based on differential in intake vacuum so the engine does not get chocked with fuel however when you modify any part of the system be it the head, valves, intake, exhaust or airbox you change the levels of intake vacuum and the pulse of the engine with the most common results being lower intake vacuum so that the engine can't draw/suck the required amount of fuel up from the fuel bowl and though the carburetors jets anymore making it so you have to resize and adjust to compensate. The slides needle then has to be raised to prevent a lean condition at high speeds so you don't burn holes in the top of the piston or destroy the exhaust valve. Fuel injection does this automatically for you if you don't go to wild with your modifications otherwise then you may need to install a larger fuel injector and may have to raise the pressure being regulated to the injector along with download a new fuel map to the ECM.
Last Edit: Jul 31, 2014 23:48:06 GMT -5 by rockynv
A few more years until fuel injection technology trickles down into the lower price points and becomes the norm instead of a luxury. All the collected wisdom of a hundred years of carburetor subtleties will be abandoned. We'll all become tuner boyz.
Better performance. Better fuel economy. No more chokes. You'll just open the app on your smartphone to connect to the engine management computer via bluetooth. Click an icon to change the fuel mapping from "economy" to "sport" or choose edit and adjust the fuel mapping curve just like you used to do with the stereo equalizer. Boop, done. Didn't even get my fingernails dirty.
Despite the advancements afforded by fuel injection there are still places I would rather be with a non-computerized carburetor equipped bike such as off-road and wilderness trekking. On a basic magneto and carbureted bike most times you can scavenge enough wire or leather bits to get it going well enough to get you home even if the battery gives up the ghost. On a fuel injected bike once the battery quits that's all she wrote since you won't be able to get the fuel injector pump going without it.