Air fuel mixture role?

hi im confused on air fuel mixture role particularly on the designated jets on the carb and the the after effect on the engine, correct me if im wrong, my analysis is the the air fuel mixture thus only give effect on the lower rpm which is from 1/4 throttle to 1/2 throttle and it only supplies the idle rigth??? and the role of the main jet is only for the topspeed rigth? please enligthen me on this please. thanks thanks

The air-fuel mixture in any gasoline 2-stroke or 4-stroke engine is not for low to mid rpm’s only. Actually, air-fuel mixture OR air-fuel ratio is the mass ratio of air to fuel present in the combustion chamber during compression / combustion stage. We all know that there is a LEAN or RICH mixture. But did you know that there is a STOICHIOMETRIC mixture? A stoich mixture is the ideal combustion process, meaning, this is where fuel is burned completely giving the engine its maximum combustion making it efficient. And as a result, an efficient engine makes more power compared to a same engine that has lean / rich of a mixture. Hope this helps.

hi thanks so much on the info much appreciated, but does it also applies to delorto si carbs of px’s? you mean when you adjust the air fuel mix the effect does crossed over up to the main jet? all the while i thougth it only affects the idle jet mix…please need your advise, thanks

Yup, plug reading is done with 3rd gear, max it out and shut off the engine. This is the only thing you should do to read your mixture. Don’t worry about the idle jet, your Main Jet works with it always. Once you set your mixture right at high rpm, you don’t need to worry about the mid range rpm. Just make sure your mixture is slightly on the RICH side at high rpm.

To get a perfect tune you’ll probably gonna need to change carb to the larger ones (28 mm etc). As these bigger carbs allow you to adjust the fuel/air mixture in low mid and high. Definitely recommended to run the engine a touch richer to extend life and reduce the possibly of a seize.

i don’t really know about si carbs as i’ve only ever replaced them :slight_smile: with a carb like a phbh you can plug chop the different stages of throtle appature. basically to plug chop at idle, you’d warm yout scoot up for a few minutes, then leave it to idle for maybe a minute, then switch it off, take the plug out… thats it, its not complicated, if its really black or oily then your either using too much oil or you idle circuit is rich, you can lean this with the idle jets, and or the slide, and or the fuel/air screw. plug chopping won’t give you readings at different rev ranges, but only at different throttle openings. th colour of the plugs is an indicator of the air/oil ratio regardless of the revs. black= rich, nice dry chocolate= good (or ‚stoichiometric‘ if you want to sound clever) and light brown-white is lean. different throttle openings rely on different jets, fully open is all about the main jet, mid’ish is about the needle/atomiser. things differ depending on what carb you use. fiddle around and you’ll work it out, it can be frustrating learning about jetting, but once you’ve sussed out what your doing, you be able to tell rich from lean from 500meters. i used to have a bunch of jets, a spanner and screwdriver in my pocket all the time :slight_smile:

got it thanks thanks, so how do you plug read the mixture of the idle jet? plug reading the main jet is on 3rd gera wide open throttle and switch off ignition key rigth? im worried about the mid range reading if i have the correct mixture, i heard that seizure always happen on mid range especially when you’re using geniune expansion chamber. so how do i check my plug read on midrange? by the way im using the stock idle jet 160/55…should i used a leaner or reacher jet??? thanks.

got it many thnks mate.

Air-fuel mixture is applied to all induction process, whether its a carburetor or a fuel injection set-up. Remember that an engine is basically  an "air-pump", its sucks air, the volume and the burn rate of which is dependent on the compression raio of the engine. But "air" alone wont ignite inside the combustion chamber, it needs gasoline, by means of a carburetor, for it to ignite. Now, when adjusting air-fuel mixture, it affects the whole carburetor. In the case of a PX carb, when you loosen the Pilot Screw, you are "leaning" your mixture, this means you are letting more air to pass through the carb/jets before it is ingested by the engine, and vise versa. Getting a good air-fuel mixture right is a trial & error process. Plug reading is one way of checking if your motor has a good enough mixture.