HOW-TO JET YOUR CARB
This information was taken from December '96 issue of
DIRT WHEELS.
WHAT MEAN JET? :
Carb jetting can be easily understood if we understand the basic principles of
carb and engine operation. A carb mixes fuel with air before it goes into the
engine. When the mixture is correct the engine runs well. The bottom line is a
carb must be adjusted to deliver fuel and air to the engine at a precise ratio.
This precise ratio can be affected by a number of outside and inside infuences.
If you are aware of these influences you can re-jet your carb to compensate for
the changes. I'm going to show you some examples of how you can change your
jetting for better performance and in some cases increased engine life. As with
any engine work be sure you have good tools the correct parts and a good manual
before you get your hands dirty!
ALTITUDE COMPENSATION:
For our first example let's say we find a new riding area WAAY up in the
mountains. Our jetting is dialed in for our usual riding area which ranges from
sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from
there. Going to a higher elevation will require will require a jetting change
but which way? Like our fuel density, air density can also change. Higher
elevations have less air density then lower ones. At high elevations our engines
are getting less air, so they need less fuel to maintain the proper air/fuel
ratio. Generally you would go down 1 main jet size for every 1750 to 2000 feet
of elevation you go up (info for Mikuni carbs). If you normally run a 260
mainjet at sea level you would drop down to a 240 at 4000 feet. Something else
goes down as you go up in elevation is horsepower.You can figure on losing about
3% or your power for every 1000 feet you go up. At 4000 feet your power will be
down about 12%-even though you rejetted! For our second example let's say we are
still at our new 4000-feet elevation riding area and a storm comes in. We head
back to camp and ride it out overnight. The next day there's a foot of snow on
the ground the skies are clear and it's COLD!. Aside from getting the campfire
going and making some coffee you should be thinking about jetting again! Cold
air is dense air and dense air requires bigger jets. If the 240 jet ran good the
day before you will need a bigger jet to run properly today. If the temperature
is 50 degrees colder than it was the day before you can actually go back to your
sea level jetting, a 260 mainjet! If you don't rejet you can kiss your assets
goodbye when you rebuild the seized engine. Air temperature makes that much
difference!
Our final example will deal with something often overlooked. We are still up in
the hills enjoying our NEW riding area when we notice the old fuel supply
getting shorter. No biggie;there's alittle store/gas station just down the road.
A short trip a few bucks change hands and we are ready to go again. Out on the
trail the bikes are running funny, sometimes "pinging" and running HOT. What
happened?!?! When we changed jets to compensate for altitude and temperature we
were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a
different blend of additives to compensate for the altitude. Generally high
elevation gasoline is less dense to compensate for less available air going into
the engine and to aid starting. The lighter specific gravity of the high
elevation fuel actually "leaned out" our mixture! A 1 to 2 sizes bigger mainjet
will get us back into the hunt. If you ride in vastly different areas try to
bring enough or your normal fuel along to last the entire ride. It will save you
hassles and gray hair in the long run!
PILOTS,NEEDLES,MAINS:
So far we have only talked about main jet changes to compensate for altitude,
temperature and fuel density. As most of you know there is a pile of jets in a
carb. While mainjets are the most critical for ensuring full power operation and
engine longevity, the other jets are equally as important for a good running
engine. Let's run through them quickly. Pilot jets control the low-speed and
idle mixtures. Many times an adjustable jet is used in conjunction with the
pilot jet. The adjustable jet allows a precise setting of the idle mixture. If
the adjustable jet is located to the rear of the carb and usually on one side it
is a AIR adjustment. It controls the amount of air that mixes with the fuel
coming from the pilot jet. If the adjustable jet is to the front of the carb,on
the side or bottom, it controls the amount of air/fuel mixture going into the
engine. In either case if adjusting the mixture screw won't improve the low-end
running speed it's time for a different pilot jet. Throttle valves(the slide)
control the off idle, to 1-quarter open ,mixture. Some aftermarket carbs have
replacement slides available with different "cutaways". Changing the cutaway
changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do
not have different slides available, so you have to compensate by changing the
mixture on the idle circuit or needle circuit. Partial throttle hesitation or
rough running can be caused by the slide cutaway. Needle jets control the amount
of fuel going by the needly and into the engine at low to mid throttle. There
are 2 types of needle jets used in a carb. One is a primary type that has a very
precise hole hole drilled through the middle of it, along it's length. The size
of the hole relative to the size of the needle determines how much fuel goes
into the engine. The other type of needle jet is constructed essentially the
same except for a bunch of holes drilled into the side of the jet. These holes
allow air to mix with the fuel before it's metered into the engine. Either type
of needle jet works well in most cases but there is power to be gained on HIGH
PERFORMANCE 4-STROKES by going to the needle with the holes in the side. These
are called "bleed" type needle jets and produce more midrange power in a
4stroke. In any engine going to a leaner(smaller) needle jet is the easiest way
to rejet the midrange running when going to higher elevations. Changing the
needle jet leans out the mixture evenly at all the midrange throttle settings
moving the needle clip doesn't. Jet needles more commonly know as the "needle"
control the fuel mixture throughout the midrange. The shape or taper of the
needle dictates how much fuel goes into the engine at a given throttle opening.
The needle must work in conjunction with the fueling requirements of the engine
relative to slide position. If you have an engine with a strong hit in the
mirange the needle will probable have a noticable reduction in size the the
slide is half open. Remember it takes fuel to make power and when the engine
makes power it needs fuel NOW! If it doesn't get the right amount of fuel it
pings or misses. You many have cleared up alittle midrange pinging by moving the
needle up a notch but at the same time you may have overrichened some other
areas. If the problem isn't oo bad you won't even notice the rich condition. If
the machine stutters before it comes on the power that part of the needle's
taper is too small and the only way to cure it is to get a needle with a
different taper. Finding the right needle can be difficult so hopefully moving
the clip will do the job. Finally the good old mainjet comes into play at
3-quarters open to full throttle conditions. Most of you already know a bigger
mainjet has a bigger hole so it lets more gas into the engine! Pretty simple!!
As simple as it is the mainjet is absolutely CRITICAL to high-speed engine
operation. Not onlt does it meter the gas into the engine, it can aid in cooling
the engine as well. A properly sized mainjet will let the engine make good power
for a long time. A 1 size smaller mainjet may make greater power for awhile. A
slightly rich mixture burns cooler than a lean one so be sure the mainjet is big
enough! One final note on jets. All of them and the carburetion functions then
perform tend to overlap into some other jet's territory. If you mess with one
jet, you may have to mess with a few of the others.My best advice is to not
change more than one jet at a time. Slowly work out the correct jetting and keep
notes on what you are doing. If you get totally fouled up at least you can go
back to where you started.
SIGNS, SYMPTOMS AND CAUSES: How would
you know if there was something wrong with your jetting? If you listen, your
engine will tell you! All you need is an interpreter. Since i speak and
understand several different engine dialects, i will give you a hand. Let's
start with lean conditions because they can cause the most damage. In a lean
condition the engine will surge and sometimes ping under acceleration. The
engine will also be "cold-blooded" (hard to start and keep running) but will run
better when hot. The spark plug will look bone white or burned in extreme cases.
The engine may spit back or sneeze throught the carb once in awhile too.. If the
engine is running rich the throttle response will be fuzzy and not too quick.
The engine will burble, miss and blow black smoke. It will start easy but will
run funny when fully warmed up. The plug will be dark, wet or fouled (possible
all 3!). Ok so what do you do first to cure the problem? The very first thingis
to check and adjust the float level. If it's off one way or another it can throw
the jetting off too. Set the float to the specs and retest the running. The next
iten is to determine a rich or lean condition. Let's say the engine gets hot and
doesn't pull well. This is a lean condition so the engine wants more fuel. Stick
in at least a 2size bigger main jet and try it again. If it's better but still
not right go even bigger on the jet. and try it again. Bear in mind that drastic
or sudden changes in jetting usually mean an air leak has developed somewhere in
the engine. Find it and FIX IT!! When the engine burbles on the topend come down
1 jet size at a time until it winds all the way down. Don't drop and more sizes!
If the engine seems sluggish and lumpy or want to load up on the bottom end the
mixture is TOO RICH. Adjusting the low speed mixture screw helps alittle but
doesn't cure the problem completely. What you need now is a new pilot jet. Go 1
size smaller and try the adjustment again. When the engine runs smooth with the
adjustment screw about 1 and a half turns out from the seat you have it!!
IS THAT ALL THERE IS TO JETTING??:
There's alot more to jetting than just stuffing jets in holes and hoping the
problem goes away. If you can understand what your engine is trying to tell you
when it runs funny you will have a better chance or correcting the problem than
someone who doesn't have a clue. When you rejet, go slowly and carefully until
the problem is solved. As a final thought let me remind you that jetting is alot
like life, if you have a choice it's always better to be alittle rich!!