Carburetor Jetting throughout the RPM range - Throttle openings explained!

in the last video we looked at the two major types of carburetors and their differences in this video we're going to look at the throttle openings and what adjustments are in play at each one of those openings this is what a carburetor looks like when the engine is idling in this case the butterfly valve and the slide are closed and the mixture is only going through the pilot circuit to keep the engine running let's take a look at what adjustments are in play here the pilot jet the pilot air jet which is usually not adjustable in most carburetors the pilot air jet is just a term for the passage that the small amount of air to mix with the fuel enters and the mixture screw alright with that out of the way let's take a look at some of the problems that happen with this because that's what you're really here for okay the most common problem in the idle circuit is a blockage of either of these two passages I know I sound like a broken record but clean fuel entering these passages specifically the pilot jet or the pilot jet passage is one of the number one problems with this adjustment or poor idle quality the air jet or air passage can get blocked if you have a dirty air filter but it's by far usually a fuel problem the correction to this problem is very simple clean the passages make sure they're free of dirt and debris and use clean fresh fuel but now you're gonna say I've done all that and I still have idle problems well I have a whole video about that you can click the card up top or the link in the description that goes into more detail but it can be as simple as twisting the mixture screw now identifying it is important and I've already gone over that so either you have a fuel screw or an air screw each controls one side of the mixture and I think it's pretty obvious but see my video all about it now let's look at slow speed operation air entering the carburetor also enters the air jet which sucks fuel up through the pilot jet and as the slide is opening a little bit of fuel begins to come out the main jet but it's metered by the jet needle so what adjustments can we make at slow speed well we can adjust the size of the pilot jet by changing it out and now the mixture screw has no effect on the actual mixture through the pilot circuit because the pilot jet is flowing full fuel through its orifice we can also change the clip position on the jet needle this controls how far in or out of the main jet the needle is relative to where the slide is traveling up and down in the bore lower the needle the leaner it will be at this setting higher the needle and the richer the mixture will be at this throttle opening now at medium speed or about half throttle or half to 3/4 throttle the same adjustments are in play it's just that now the slide is higher up in the bore so is the needle and it's opening that main jet more and more and that has a larger effect on the mixture the main jet size while the pilot jet is flowing at full fuel it is contributing less and less as the throttle opens now at full throttle it's much the same except for now the slide is clear up all the way into the bore and the main jet is fully open now the jet needle is not doing anything to control the fuel through the main jet so the main jet is flowing at maximum capacity at full throttle so the only real adjustments you have at full throttle or high speed is that main jet now that we've gone over all the basics of all the different throttle openings and you got to remember this is progressive as it opens and closes now we can go over some of the symptoms associated with jetting problems and how to correct them starting with the idle if the idle is erratic and you have to keep twisting the throttle to keep the bike running or it runs better with the choke or enrichment circuit engaged which there will be a video separate all about that that means that you're too lean for some reason so try reaching up the mixture if the idle kind of is Logie or there's kind of black smoke or exhaust at idle that means you're too rich so try and lean out the mixture at idle the same thing goes with all the other throttle openings if the bike seems sluggish or jerky and stuttering that is usually a sign of the mixture being too lean also if the engine tends to want to overheat or lose power as it warms up can be a symptom of a overly lean mixture try larger jets or changing that jet needle clip position and find that nice happy medium on the flipside if the engine runs pretty good but there's excessive black smoke or it kind of bogs or is very low-key but you're not experiencing overheating or it isn't coughing or stuttering that is sometimes a indication of an overly rich mixture also sometimes the spark plug will be very in these conditions remember at the end of the day the whole goal is to try to get the mixture to that nice 14 or 15 to 1 mix ratio which is what is about optimal for most engines and if you want to learn more about that I have a video detailing all about how stoichiometry works and how to try and achieve it with carburetors click the link in the description or the card up top well I thank you for watching please subscribe like this video and I'll catch you in the next video and go back and watch my other carburetor videos I think you'll learn a lot

Channel: thejunkman

Views: 133,544

Keywords: Carburetor, carb, jetting, air fuel mixture, setting the mixture, idle mix, pit bike carb, chinese carb, carb jet, main jet, pilot jet, motorcycle carb, bs carb, mikuni tuning, kehin, gas oil mix, 2 stroke tuning, fuel setting

Id: SkoMCP9QreQ

Length: 7min 55sec (475 seconds)

Published: Thu Nov 14 2019