Inside A Motorcycle Carburetor - Main Jet Tuning | MC Garage

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I'm on the main jet have you ever heard

that expression while bench racing with

your buddies what the heck does that

even mean well after this episode of MC

garage you'll know today we talk about

the main jet for the past few weeks

we've been going through a carburetor

piece by piece if you haven't watched

the first videos in the series jump back

and check them out we'll have the links

in the description down below cover the

basic lay of the carburetor we've

covered the float Bowl and we've covered

the pilot jet the next piece in a

properly running carbureted motorcycle

equation is the main jet the main jet is

responsible for supplying the fuel that

mixes with the air as it makes its way

into the engine after the pilot jet is

done doing its job at idle at about 20%

throttle all the fuel goes to this bad

boy right here there is another part

that meters of fuel until about 80%

throttle but we'll cover that next week

when you're wide open you're on the main

jet the main jet is located at the

bottom of the carburetor inside the flow

pole it's the larger of the two Jets and

it's installed into the needle jet

that's the part we're gonna be covering

next week

if this guy's plugged up you're in for a

full carb cleaning rituals everything

will usually be clogged up if this guy

is plugged check out Ares video on how

to clean a carburetor links it down

below so we are going to assume you have

a clean carburetor so we can get right

into knowing if your main jet is too

rich or tuning first let's start with

the manufacturers recommendation in the

service manual just like we discussed in

last week's video there'll be a


according to temp and altitude this is

where you want to begin a lean condition

means the ratio of air to fuel is too

high there's just not enough fuel this

can cause issues from not making full

power at wide open throttle and rpm to

detonation and pinging that could damage

your engine over time lean jetting is

the kiss of death for two strokes with a

possibility of a quick meltdown and an

eventual seizure in 4 strokes the damage

could take a little bit longer but it

can still happen it's fairly easy to

hear pinging on a two-stroke think of a

towel tick that sounds like a rattling

ball bearing in a tin can that's the

sound of the fuel charge detonating

before it should smashing it on top of

the piston before it reaches top dead

center that same sound can be heard on a

four-stroke but the sound is masked by

the sound of the valve train you hear

this you're gonna need to be jet before

you do some serious damage also a lean

condition will have a crystal response

the top-end is gonna feel weak and the

bike will run way hotter than it should

there's many times I've heard two-stroke

racer savers running so crisp and so

snappy and then it just stopped that's a

classic case of seizure from being too

lean to fix that lean position go up a

jet size and run the bike again repeat

until you find the right size now these

numbers on the sides of the Jets they

might not increase or decrease in

increments of one usually it's a number

like five or ten when your bike's rich

it means there's more than the optimal

amount of fuel in the air charge this

leads to stumbling and poor overall

running it doesn't lead to premature

death like a link addition can so if you

have to err on the side of being rich if

the bike moves through the mid range and

onto the top end the power is gonna

stumble and miss it's an easier

condition to feel here for most people

and it's very clear that it's running

bad just drop a jet size and retest it

on a two-stroke you can also do a plug

chop to check the proper jetty well

that's a pretty involved process and

we're gonna save that for a separate

video for now that's it for checking the

main jet and that's it for this episode

a messy garage the main jet is one of

the simpler jets to get dialed in but

it's not right it's gonna cause issues

with next week's topic the needle jet

and needle if you like what we're doing

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below thanks for watching