Here's how to find and adjust the mixture screws on your carburetor.
Note that metering-block style carbs such as Holley and Demon more often have one or 2 mixture screws in each metering block depending on the number of venturi. Also those may not be located at the bottom of the carb as in factory carburetors.
Auto Repair & Maintenance : How to Adjust Carburetors
Idle speed and idle mixture are the two most common adjustments made to
carburetors, and these adjustments start by warming up the vehicle to full
engine-operating temperature. Find out how to perform a lean-drop
adjustment with help from an ASE-certified master automobile technician in
this free video on carburetors.
Expert: Clark Mason
Bio: Clark Mason is an ASE-certified master automobile technician.
Filmmaker: Christian Munoz-Donoso
How a carburetor works, schemes and explanations
This video explains, not in too much detail, how a 28mm Walbro diaphragm
carburetor works (using mainly schemes to point out what happens inside).
It explains what occurs at the venturi, what the butterfly valve is for,
how the needle doses the amount of fuel, and how does the diaphragm slide
DIY Carburetor Synchronize / Balance Tool
This is a homemade bottle style carburetor synchronization tool ( homemade
manometer ) and balancing tool. If you are going to attempt to, or want to
know how to sync your carburetors, there are a few things to know first.
1. Carburetor Synchronization = Adjusting your carburetor throttle plates
or throttle slides so they move from your carburetors idle setting to wide
open throttle simultaneously, or in sync with each other.
2. Balancing Carburetors = Adjusting your carburetors so they allow each
cylinder to flow or pull the same amount of air and fuel at your desired
idle speed. Carburetor sync and carburetor balancing go hand in hand.
3. Idle speed screw adjustments must be made at normal operation
4. Air fuel mixture screw adjustments must be made at normal operating
5. Air fuel mixture adjustments only affect your carburetors pilot circuit,
which means from idle to part throttle. Air fuel mixture screw adjustments
have zero effect on wide open throttle.
6. Carburetor running rich = Too much gas not enough air.
Carburetor running lean = Too much air not enough gas.
7. Wide open throttle rich or lean conditions are adjusted two ways:
A. Carburetors with throttle slides have a clip on the carburetor main
jet needle. Raising the main jet needle clip drops the needle deeper into
the main jet and will lean the carburetor out. Lowering the main jet needle
clip will raise the needle and enrich your carburetors mix.
B. Carburetors with throttle plates (butterfly style) need to have the
carburetors main jet changed to a larger diameter jet to richen the mix or
a smaller diameter jet to lean your mix.
8. If your engine will not idle correctly or your air fuel mixture screws
seem to have no affect on the engine at idle, then most likely your pilot
jet circuit is clogged and you should clean it.
The snowmobile in the video is a 1986 Arctic cat cougar 500 fan cooled, it
has Mikuni vm34 roundslide carburetors which have a primer nipple on the
engine side of the throttle slide. Perfect place to attach a homemade
manometer to balance everything. These mikuni vm34 roundslide carburetors
were common on many motorcycles, snowmobiles and atv's. Suzuki, yamaha,
polaris, arctic cat, all used them at one time or another, they are good
carburetors when they are dialed in.
Setting the timing and adjusting the carb
Set the timing by ear, and got the idle slowed down so I could do a little
adjusting on the carb. I'll do more when it's cold, so I can set the
choke and all. Love the way it's sounding..
How To Adjust The Idle Mixture Screws On Holley Carburetors
Tips on adjusting the idle mixture on Holley carburetors, using a simple
vacuum gauge. Proper idle mixture adjustments can prevent bogging,
hesitation, rough idle, backfire, stalling and hard starting. Additional
information can be found on Holley's Blog:
How To: Setting the Timing
How to set the timing on a 3 cyl Geo Metro engine, 1995 - 2001 models.
Visit GeoMetroForum.com for more information on the repair of Geo Metros
How To Adjust The Accelerator Pump On Holley Carburetors
How-to adjust the accelerator pump on Holley carburetors to avoid stumbling
and hesitation upon acceleration. Tips include: How to check for the
clearance between the accelerator pump diaphragm arm & operating lever;
how-to select the correct accelerator pump nozzle size, and accelerator
pump cam selection and installation. Additional information can be found on
Holley carb adjustment basics
-just a quick overview of how to adjust a holley and what to know. happy to
answer any tuning issues. i know far more than id like to about these old
holleys. not a very good public speaker clearly.
HOLLEY CARB FLOODING & HARD STARTING SOLUTIONS
How Motorcycle Carburetors work and how to tune and clean them
Here's a link to the kind of carb cleaner I like best (it IS the best)
hey... awesome video..nicely explained !
just one question. Please do tel me wts AIR NEEDLE and LOW JET used for
again. ? i mean how and what do they work?
The air needle is like a transitional restriction to increase vacuum
pressure and tapered to prime the main jet flow gradually to ramp up fuel
flow from the idle/low circuit to the main circuit. Think of the circuits
like gears or stairs. The needle covers transition between 3rd gear to
about 5th gear on a six speed. It overlaps basically. Fuel is getting by
it from the idle/low- on through to where it's completely out of the way of
the main jet and the main jet fuel flow is roaring like the
Without the needle the carb would work, but not as smoothly through middle
throttle (where most people ride and have the most fine input or throttle
roll back and forth.
The low jet is like second gear on a transmission. You can shift from 1st
to 3rd, but again you risk stalling and it won't be as smooth. Also there
isn't really the mechanical inertia to carry the process on like you have
with a rolling vehicle with a flywheel on the engine and the wheels
turning. The pistons still create vacuum every other time they go down,
but air just doesn't have the punch that steel does when in motion.
The circuits in a carburetor have to be there or you would have a limit on
RPM range. An idle jet can't deliver enough fuel to really scream, and the
main jet couldn't go slow and would be VERY hard to start because the
engine would have to be spun up really fast. So you have usually four or
five circuits (I want to say jets, but their physically different) that get
air flow like stairs as the throttle opens. They are typically 1. Starting
circuit (choke on) 2. Idle circuit 3. low jet 4. Jet Needle 5. Main jet.
How Motorcycle Carburetors work and how to tune and clean them Check out
my other channel here http://www.youtube.com/watch?v=1KKvwMaIotA