Jon Stoodley of JSE Trials, Muskogee, Oklahoma talks us through….
Let’s talk about Carburetors. They can be fairly simple, like the pictured flat slide Keihin PWK or the Dellorto PHBL, both common on Trials bikes today. Or, they can be fairly complicated, Like the electronically controlled Mikuni on a YZ250 I modified. For this post, I’ll try to keep it simple and helpful for new riders.
Here’s one of my old articles about basic carburetors and jetting that might be helpful to newer riders. Trials engines are probably a little more difficult to properly jet and adjust because they must perform over a wider range of throttle settings, engine loads and ambient conditions than just about other form of motorcycle competition in my experience.
Here’s a couple of tips to get started. There seems to be some controversy surrounding adjusting the air/fuel screw for some reason. I’ve had riders argue with me that the factory setting (number of turns out) is what it should be set at, period. Other riders tell me that they read on the Internet that so many turns out is recommended by an “Expert” and that’s what it is supposed to be.
Here’s how I do it before each ride. The air/fuel screw “fine tunes” the low rpm circuits to handle the low-speed throttle response by compensating for changing ambient weather conditions (temperature, barometric pressure, humidity etc.). I warm up the engine to operating temperature, place it in Neutral and quickly “blip” the throttle (quickly open the throttle and let go of the grip) and adjust the air/fuel screw in or out to get the best engine response. For ever how many turns out I end up with (no matter how many turns out), that is the best setting, for this specific day, and under these specific weather conditions. This is why the screw is adjustable. I add knurled knobs to the air/fuel and idle screws so I can adjust them easily with gloved hands. When you bottom out an air/fuel screw, do it very lightly as they, like suspension adjusters, can be delicate and easily damaged.
Engine idle speed is generally a matter of personal preference, but I would suggest that you set it this way. Put the warmed up engine in gear, with the clutch lever pulled in normally the way you would ride which is usually back to a knuckle and not fully back to the grip. Then adjust the idle speed and in this way, you compensate for any “clutch drag” (GasGas riders take particular note). If you set the idle speed in Neutral, and you have too much clutch drag, when you are in a section stopped, or close to stopping, clutch drag will pull down the rpm below where you set it and you will have a much greater chance of stalling the engine.
A slight amount of clutch drag can be a positive thing as it keeps the engine’s drivetrain “loaded” (mechanical slack taken up) so that clutch modulation is much smoother than if the clutch released totally. This is particularly important under less-than-ideal traction conditions and slow going. Say you were in a tough section almost completely stopped, full-lock turn, off camber, muddy with roots, the clutch modulation (if it totally released) would result in a “jerky” take-up (and loss of traction) as the drivetrain loaded and unloaded.
One of the most frequent questions that riders ask me about is how they can properly set up their jetting. It seems like a lot of riders put up with poor performance from their bikes without realizing that, with a little time, effort and most important, a basic understanding of how that nasty little piece of aluminum and brass that some engineer stuck on their engine works, things could be a lot better. I’ll try to give a crash course (excuse the term) on what the carburetor is required to do and how you can help it do its job better.
Why would we need to change jetting anyway? Jetting doesn’t change by itself unless there is a mechanical problem in the carburetor. Conditions outside of the carburetor change and they have an effect on the engine’s air/fuel requirements.
For example, most bikes are jetted rich from the factory when they are assembled because they are shipped to various countries with a wide variety of conditions and fuel. The engineers can’t possibly anticipate and jet for all the conditions and areas they ship to, so they put in “safe” (ie rich) jetting with the idea that the individual rider or dealer will make the final adjustment to the engine requirements.
Riders who travel to different Trials find that their engines don’t operate as efficiently as they desire unless they adjust the jetting to suit the area. You may have bought a used bike that is jetted incorrectly (jetted for high altitude and brought down to sea level) and want to set it up for the area where you ride. Temperature, humidity and altitude have a direct effect on the amount of oxygen available to the engine. Ask any Ute Cup rider how much of an effect altitude has on engine performance. Fine tuning an engine to maximize performance is so important in racing that years ago when I was campaigning fuel dragsters, we would build special engines just for high altitude competition and we would refer to them as our “Denver motors”.
What is a carburetor and what does it really do? A carburetor is a device that atomizes fuel with air and meters that mixture to the engine over a wide variety of throttle openings. The stoichiometric (or, chemically correct) ratio under perfect conditions of the air and fuel is 14.7 pounds of air to one pound of non-oxygenated gasoline, although AF (air/fuel) ratios in the range of 12/1 to 14/1 seem to produce the best power in motorcycle engines.
Trials engines seem to work better on the lean end of the A/F ratios due to the conditions that they operate under. You can see that if your carb had only one metering orifice (jet) it wouldn’t be able to maintain that ratio from closed to full throttle opening. That’s why they have all those other fuel and air metering gizmos, in order to adjust the amount of fuel to the amount of air allowed into the engine by the slide opening from almost shut, to wide open.
Most modern carbs have six (and sometimes seven, a “power jet”) different air and fuel metering jets that affect the fuel mixture over the range of throttle openings. Those are the air screw (sometimes this a fuel metering needle), throttle slide, air jet, pilot jet, needle jet and main jet. Most riders can just replace the pilot and/or main jet to get the desired performance and fine-tune the engine with the air/fuel screw and needle clip adjustments.
Let’s look at where those little thingies do most of their work. The throttle settings and the air/fuel jets that affect the air/fuel ratio at those openings are:
-Closed to 1/8 throttle opening – air (or fuel) screw, pilot jet
-1/4 to ½ to full throttle opening – throttle slide, jet needle
-1/2 to full throttle – jet needle, needle jet, main jet, air jet
As you can see, most of the air/fuel circuits overlap in their functions so it’s not a black/white decision as to what to change and how far to go rich or lean. A savvy tuner spends a lot of time looking for the signs that the engine exhibits in order to make a well-educated guess, and its not unusual to see such tools as exhaust gas analyzers, air/fuel electronic sensors and relative air density meters used at the professional level of racing.
What do all those numbers and letters stamped on the parts mean? In the pilot and main jets the larger the number, the richer the jet. On the slide, the larger the number, the leaner it is. Needles have various codes, but as a general rule the smaller the number (if it doesn’t have letters), the richer the needle. To richen the needle setting when it is installed in the carburetor, lower the circlip one groove at a time. And to lean the needle, raise the circlip in the grooves (lower clip = raise the needle and vice versa).
Air/fuel screws are a little trickier. Although they may look the same, an air screw is turned clockwise to richen the mixture but a fuel screw is turned counter-clockwise to richen the mixture. To tell if its an air or fuel screw, look at the carb from the side. If the screw is located on the carburetor towards the front of the slide (manifold/reedcage area), it’s probably a fuel screw. If it’s located towards the back on the carburetor (air box side), it’s probably an air screw adjuster. Air screws usually have a blunt end and fuel screws have a sharp needle-shaped end.
The air jet, and sometimes the needle jet, are usually not replaceable on some carbs. For example, the needle jet is replaceable on most Keihin carbs and on most Mikuni carbs.
The numbers on jets, throttle slides and needles will allow you to tell how rich or lean they are from stock settings. The jets are stamped with their numbers but don’t be confused by the radical difference in the numbers between models of carbs as some jets are rated according to flow rate and some are rated according to the metric size of the orifice. For example, a 172 Keihin jet is roughly equivalent to a 350 Mikuni jet in Motocross bikes.
Needles have the number or letters stamped at the top by the clip slots. Those numbers or letters relate to the thickness and taper of the needle which will dictate how much fuel it will allow to flow around it as it is retracted out of the needle jet by the slide as the throttle is opened.
As the needle is tapered, the more it is pulled out of the needle jet by the slide, the more fuel it will allow to pass into the throat of the carb, relatively speaking. A thinner needle will pass more fuel around it than a thicker one, and it therefore a “richer” needle. Some needles have compound degrees of taper that allow individual adjustments to various throttle settings. Slides are usually stamped on the bottom front of the slide. That beveled cut on the front of the slide, near where the needle comes out (called the cutaway), has numbers that usually relate to the height of that bevel from the bottom of the slide. A #4 slide will have the top of that bevel 4mm from the bottom of the slide, a #5 slide will have a bevel 5mm high, etc. The higher the number, the leaner the slide as higher bevels allow more air to be funneled over the needle jet tower (that little protrusion that the needle retracts from in the center-lower part of the carb throat).
Before we set out jetting, we must eliminate the possibility that other problems exist that could have an effect on the jetting requirements of our engine. The engine must be in good shape with no leaking crank seals, broken reeds, air leaks in the intake system or crankcase, weak ignition system, cylinder head coolant leaks or blown-out packing in the muffler. The carburetor float level must be a factory specification, the fuel inlet float level needle must not lead and the vent hoses should be replaced if there is any possibility of clogging. Also, check to make sure that the air cleaner is clean and the engine has fresh pre-mix.
As far as special tools are concerned, you should have a good metric scale short ruler (for float level), a small magnifying glass to read jet/needle stampings accurately (Mikuni jets are notorious for being hard to read because of shallow stampings), a long, narrow screwdriver for pilot jets, a long 6mm socket for hex jets and good sharp screwdrivers, as the screws and brass jets are soft and you’ll have a hard time extracting them after you’ve rounded them off with poor tools.
As far as jets are concerned, I usually buy one size larger and three sizes smaller than stock on the main jet, and one size larger and two sizes smaller than stock on the pilot jet. I’ve usually never needed to go out of this range when jetting bikes from below sea level to about 8500 feet. A good way to store jets is in a film canister between layers of foam so they don’t rattle around.
Now for the actual jetting ritual. First, try to figure what throttle setting is not responding well by riding the bike in a practice section. Atmospheric changes have a dramatic effect on the amount of oxygen available to the engine, as mentioned before.
Higher temperature, altitude and humidity will sometimes require going leaner on the jetting. Lower temperature, altitude and humidity sometimes call for richer settings. Decide what jet to change according to what throttle setting needs adjustment. Only change one jet at a time. If jetting is a new experience for you, always start by going rich at first. This is safer than initially going too lean and it will give you a direct experience of how an engine acts when it’s running too rich.
Diagnosing jetting by listening to the exhaust note takes experience as even practiced ears can sometimes have a problem discerning the difference between a “surging” (lean) and a “bogging” (rich) sound coming from the exhaust pipe. If the engine runs worse with your change, then go two steps leaner, which will actually be one step leaner than where you started. The engine will tell you if you’re on the right track. Test the bike again to see if that solves the problem but keep an eye on spark color (it should resemble a milk chocolate color with most grades of gasoline) with any changes of the needle or main jet.
Let’s look at some common conditions and possible adjustments, starting with what to try first.
-ENGINE RUNS “FLAT” AT MID-THROTTLE – Adjust the needle, change main jet
-ENGINE STUMBLES WHEN THROTTLE IS OPENED FROM IDLE TO 1/8th – Adjust air/fuel screw, change pilot jet
-THROTTLE OPENED QUICK, ENGINE BOGS THEN CATCHES – Adjust air/fuel screw, change pilot jet, change needle clip position
-ON LONG UPHILL, POWER STARTS OUT OK, THEN FALLS OFF – Check for blocked vent tubes
-ENGINE HAS INTERMITTENT MISS FROM ABOUT 1/8TH THROTTLE ON – Check for water in float bowl.
Here are some hints and tips that might prove helpful:
-Gray, thick wall Tygon fuel line is the best I’ve found. Some small engine repair shops can get it for you. Be sure that it’s Tygon type, as that’s the tubing they make for use with fuel. The clear is not as good. For dusty, dry conditions, keep an extra, clean/oiled air cleaner in a sealed plastic bag to use in the Trial in case practice has gunked up the one on your bike. This will keep the engine from running rich. Run the vent lines of the carb relatively short (about 4-inches), as fuel can collect in longer lines and a hard hit, like splattering rock step, can cause the fuel globules to quickly drain out of the lines (creating a partial vacuum), and lower the float bowl pressure, making the engine bog. If you like long lines, run a “T” fitting at the vent hole and run the top line up under the fuel tank. This will allow the fuel to drain out without lowering the float bowl pressure.
-Check the spark plug color often to monitor mid and upper range jetting. Run a fuel filter. I like the small cone-shaped, sintered brass, clear ones. You’d be surprised how much gunk can collect in your fuel can and make its way into the fuel tank and then clog the float needle and possibly the jets. Don’t forget to clean the filter on the sides of the Dellorto carbs once in a while.
-Clean the throttle often and make sure there are no kinks. It’s a good idea to safety-wire the ends to the housings. Drain your floatbowl after wet weather Trials. Most standard bike service manuals have altitude/air temperature jetting correction charts with instructions for use. Ask a buddy who has one if you can copy it and keep it in your toolbox. They can come in handy. Keep a notebook in your toolbox of any jetting changes (along with ambient weather conditions) when you travel to a different Trials area so when you return, you have a good place to start when setting up the bike again. This goes for any other changes like tire pressure and suspension settings, too.
This should give you a good place to start. Take your time. Properly jetting an engine isn’t as esoteric as a lot of riders seem to believe it is.
That’s it for now… or have a look at my other Gearhead Alerts
Jon Stoodley, Muskogee, Oklahoma