Deceleration back-fire, rough idle, and throttle lag can be quickly adjusted by tuning your carburetor. In my how-to video I will walk you through the basic principles and fundamentals of what you are adjusting and how it will impact your bike’s response. With this knowledge you should be able to make adjustments on almost any style of carburetor.

Deceleration back-fire or “after-burn”
on carbureted motorcycles will typically
result from one of these three things:

You can skip straight to my deceleration back-fire HOW-TO-VIDEO by scrolling down to How do I make it stop?

1. The burning of excess fuel in the exhaust ports or exhaust piping from a drastic change in pressure, causing a very LEAN air/fuel mixture from the carburetor.

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Don’t be mistaken…

After-burn is similar to back-fire, which is the burning of fuel in the intake port of the carburetor. Both conditions occur when a very lean air/fuel mixture burns so slowly in the combustion chamber that it is still burning when the exhaust and/or intake valves open. Now back to the list…

2. An extreme or “constantly rich” mixture where unburned fuel ignites in the exhaust ports or exhaust piping (same as number #1 with a different cause).

3. Low compression can cause the motor to become unable to burn the mixture at the rate at which the air/fuel mixture is being introduced. This results in a “slow burn” causing the same effect through out the exhaust/intake ports and piping.

 

 

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What the heck does that all mean?!

When you’re riding at a constant rate of speed, the velocity of the air/fuel mixture moving through the carburetor is much greaterbackfire than when you’re sitting at idle. So when Sally is on her phone and cuts you off or the green light you tried to make turns yellow, you let off the throttle while in gear to let the transmission begin your slow down (typically).

The carburetor has now been manually told to try to idle. With Newton’s law in full effect, the motion or RPMs of the motor are trying to slow down accordingly. The drastic change in velocity (back to idle/from the high pressure to low pressure) slows down the air/fuel mix abruptly. This makes the mixture linger and hang around the combustion chamber longer. When this abnormal condition takes place, it results in most commonly an audible “POP” or series of sputter noises out of the exhaust pipes.

Now, there is always some degree of this after-burn or back-fire happening. When the case becomes extreme, this reaction is louder and more noticeable.

 

NEED HELP_

 

How do I make it stop?

In this video tutorial I will be tuning the slow-speed mix in the carburetor by means of the pilot (fuel) screw to decrease that after-burn or back-fire result. Doing this will either lean or richen this mixture according to what adjustments you make (IF everything in the carburetor is clean and working properly).

The goal is to richen this slow-speed mixture slightly. The technique I am using to make my adjustments is called the “Idle Drop Procedure”.

OH AND BY THE WAY

LEAN MIXTURE = Less fuel or more air in the air/fuel mix

RICH MIXTURE = Less air or more fuel in the air/fuel mix

 

Does the thought of Welch plug removal make you queazy?

I understand! Removal of these covers does not always go according to plan. Remember to be patient and precise. Rushing the task can empty your pockets quickly if things don’t go according to plan.

Fortunately for you, a fellow DIY friend of mine, Matthew Bocknak put out an awesome video detailing this exact procedure step by step. CHECK IT OUT!

SEE ALSO:
– Explaining Proper Pilot Jet Sizing

– Finding Vacuum Leaks

 

“What if my carburetor adjustment screw is in a different location?”


EASY, let’s break this down.

There are TWO different styles of adjustment screws on your carburetor for your slow-speed circuits:

1. AIR SCREW: This adjustment screw is located in the air-bleed passage.  It controls the flow rate of the air being delivered to the perforated tube located just above (or attached to) the low-speed fuel jet or “Idle Jet”. The final air-fuel mixture is combined with the air in the carburetor bore. This adjustment screw is turned CLOCKWISE to ENRICHEN the mixture by REDUCING aeration.

air screwAir adjustment screw

 

2. PILOT (FUEL) SCREW: This adjustment screw is located in the passage between the low-speed jet or “Idle Jet” and the “Idle Fuel Discharge Orifice”. It controls the flow rate of aerated FUEL delivered to the carburetor bore. Like the air screw, the final air/fuel mixture ratio is achieved when the aerated pre-mix is combined with the air in the carburetor bore. The pilot screw is turned COUNTERCLOCKWISE to ENRICHEN the mixture by INCREASING fuel flow.

pilotpilot fuel adjustment screw

 

Typically Honda brand motorcycles utilize two different screw head patterns to make the adjustments. Most commonly you will see a simple flat-head screw adjuster. On many other bikes and off-road units you will see the infamous D-shaped head. This “D” style will require a special tool to make your adjustments, but can be obtained at any specialty tool website.

NPS50_pilot_mixture_screw_special_tool_d-shaped_wrench

 

 

What if my adjustments did not make a difference?

Depending on the type of mixture screw you are using (air screw vs. fuel screw), the results of making these changes can tell you a lot about the health of your idle circuit system inside the carburetor.

For example, let’s say you are adjusting a pilot (fuel) screw. You turn it all the way out from its initial setting to find out that nothing changes or that it only changes when the mixture screw is turned all the way until it bottoms out. Knowing how the fuel and air mixture screws impact the air/fuel mixture will help you diagnose what’s going on. Ideally, you want your mixture adjustments to be close to the spec setting (2.5 – 3 turns out). Whether you have just swapped in a new jet kit to get more power or you are simply adding some bolt-on upgrades to increase performance, this rule does not change.

 

SO, let’s go over some possible outcomes and common problems.

No Change or Fluctuation with Adjustments

– Clogged slow-jet circuits

– Clogged transfer ports

– Clogged choke circuits

– Clogged fuel screw or air jet passages

Time to clean that carburetor.

RPMs Increase Only When Screw is Turned Completely In

– Too rich of a mixture at idle (IF adjusting pilot fuel screw type)

– Too lean of mixture at idle ( IF adjusting air screw type)

– Too large/small of slow-speed or idle jets

– Possible slight clogging

 

Check out “How to Diagnose a Carburetor Problem” and

How to Tune a Carburetor for New Exhaust” for more answers!

 

 

 

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