Tuning motorcycle carburetors for the use of pod filters can be a long pain staking task to anyone who doesn’t have a grasp on identifying what the carburetor is telling them with the symptoms given. How do you know which jets need to change? How much bigger or smaller do I need to go? Wouldn’t it be easier to toss in a staged jet-kit for my application?
In this post, I want to dive head first into some practical knowledge you can use instantly when it comes to the art of carburetor tuning. Here you will gain a full understanding of the techniques I use to diagnose and make adjustments to carburetor problems such as hesitations off idle when quick throttle response is given, flat spots at wide open throttle, drop outs or surging at mid-range, and deceleration backfire when it comes to your pod filter set up.
First, let’s set some ground rules and drop some knowledge bombs
Why Do I have To Re-Tune My Carburetor For A Pod Filter?
What many people don’t understand is the purpose of an enclosed factory air box with one or two air inlets. I can assure you is that they are not just there to take up space. Believe it or not, they are designed specifically to restrict the amount of air that is readily available for the carburetor and cylinder. The size of both air box and air inlet is set in place for optimal “regulated fuel efficiency” and what the motor works best with out of the box around the world. The second you drill huge holes, remove the air box lid or add the ever popular pod filter. All stock settings are thrown out the window and the carburetor must be brought up to speed with the increased/unrestricted volume of air flow.
Example, if you shape your mouth as if you where about to whistle and suck in as much air as possible you created your own restriction point for the surrounding air to feed your lungs. Now open your mouth as wide as possible and breath in.. The amount of air you were able to receive is much different as well as the intake rate. This is very similar to what happens when air boxes are removed and the pod filter is slapped on.
The Ugly Truth About Jetting Pod filters
The truth is, in some cases (depending on the carburetor style) you may NEVER get all of the bugs out of your carburetors response at certain transitional phases throughout the throttle. Most of the time people battle between poor idle responses such as hesitation when the throttle is flipped or choppy breakups throughout mid-range throttle. This is more common on much older carburetor systems that may not use systems like accelerator pumps or air-cut valves.
“Finding the sweet spot between poor throttle response and poor mid-range to wide open throttle is key”
Going Against The Grain
There are hundreds of different posts on carburetor jetting that involve scientific theory’s, algorithms, charts and math that can be very helpful yet confusing when setting up the fundamentals or “starting point” for what needs to change inside the carburetor. The way I dial in carburetors is not the ONLY way… It’s just what works best for my brain.
For example I found this pretty nifty write up from the guys over at CafeRacersUnited.com
They have a cool way of calculating jet sizes that I can agree with!
What I don’t advise you to do is to go out and spend 100$ or more on a fancy staged jet kit and expect it to be a sure-fire bolt on fix for all things carburetor. Often times it can end up leading you into deeper water with overly drastic jet and needle changes that simply are just not needed. Don’t get me wrong, I have heard plenty of people swear by certain set ups from aftermarket companies as the “wham bam thank you mam” fix for their bike’s application. So To that I say congratulations on finding the perfect fix….but for most people, it’s just not that simple. In some cases I do believe kits like these can serve it’s purpose as a general platform to start on.
“The one thing that isn’t factored in with jet kits is your physical location. Meaning that If I live in Colorado and buy a high dollar stage 2 jet kit. Then Tommy boy buys that same stage 2 kit but he is located on the east coast beaches of Virginia. There will be very different results.”
That being said.. LET’S BEGIN!
Diagnosing Carburetor Symptoms of Pod Filters
One of the quickest most useful tools you can use to diagnose any carburetor problem is the choke system designed with the bike. Once you have installed the pod filter(s) and have evaluated how the bike is responding to them, you can use the choke to determine what helps the problem spot.
Example 1: New pods are on and the bike seems to idle “okay” but as soon as any throttle is given the bike shudders and shuts off or it has a bad hesitation and shudders/backfires until you increase the throttle more. If you apply the choke during the throttle operation what happens? Does the bike’s RPMs suddenly pick up and respond better to the throttle? Or does it get worse and shut off instantly because of the richer mixer you added. If better is your answer, than start here. An increase in either the idle jet or main jet needle would be the first to increase.
There are three different styles of choke systems that Honda uses on their bikes, ATV’s and scooters. Although they are different they are all serving the exact same purpose, which is to RICHEN UP THE MIXTURE entering the combustion chamber.
Those three systems are
Butterfly choke – A simple plate that when actuated, closes off the mouth of the carburetor (air box side) causing a drastic pressure change in the throat of the carburetor forcing more gas out of the jets and into the cylinder. Thus producing a RICHER MIXTURE for cold starting.
*Mainly found on many older pre 90’s bikes but still used on many smaller ATV’s and Dirtbikes
Enrichner valve – This system uses a plunger and needle to open an additional fuel passage to pass more fuel into the mix. Keyword being enRICHNER, it serves its purpose beautifully by RICHENING UP THE MIXTURE.
*This is the most popular system used on almost all full size bikes from the 1990’s to current date.
Automatic choke system – This system operates very similar to the enrichner valve because it uses the same plunger and needle parts to operate the additional fuel passage but it takes the manual control away from the rider and allows electricity to do it’s thing. This system uses temperature changes to move the plunger up and down to best accommodate the mixture needed. Naturally the plunger is in the up position allowing the mixture to be rich right off the bat to make starting easier. Inside the unit is a coiled wire that when heated, slowly pushes the plunger down closing off the extra fuel passage. Genius right?
* This system is most commonly found on Honda scooters like the Ruckus and Metropolitan.