Carburetor jetting and adjusting guidelines for your Trials Motorcycle.

Before starting any carburetor adjustments;

Clean and properly oil the air filter.
Inspect the rubber boot between the carb and engine, confirm it is in good condition with no cracks or leaks and it is secure on the carb as well as the engine.
Have your bike at operating temperature. A warm bike will run much different than a cool or warming bike; get the radiator fan to kick on at least once before making any adjustments to your bikes' carburetor.
Also an engine excessively worn and rattling will be difficult if not or impossible to make adjustments on; nothing will make the engine run right if it is time for a rebuild.

Step 1: Pilot Jet;
Your modern Trials bike with a Dellorto or Keihin Carburetor will have an air screw located on the left side of the carburetor towards the rear or air boot end of the carb.

With the air screw turned all the way in, clockwise, it should stall the bike.

If you have a turn, in, to go, but the bike is stalling the pilot jet is to rich.

With the air screw closed all the way, and the bike is still running then you have a lean pilot jet.

Once you have the right pilot jet, step two...

Step 2: Air screw adjustment;
With engine at operating temps turn air screw in until the engine starts dying then turn it out, counterclockwise, slowly until the idle increases. Once you have found the high idle point by turning the air screw in and out, turn the air screw in a quarter turn.

Step 3: Needle adjustment, 1/4 to 3/4 throttle;

Roll the throttle on from about 1/4 to 3/4 throttle. If it builds rpm fast but doesn't necessarily build power, the needle is lean, move the needle clip towards the bottom of the needle. This moves the needle up, richer.

If the acceleration is blubbery and not smooth, move the needle clip towards top, dropping the needle, leaner.

Step 4: main jet;
The main jet primarily meters fuel in conjunction with the needle, in the wide open scenarios of riding.
If the engine makes rpm but no power, get a size bigger main jet.
If it blubbers and fall on its face, go smaller on main jet.

At 4200 feet elevation; I have had good results using the following jetting.
Main, 116 +/- a size or 2 Pilot, 36
Needle clip, 1 notch from the top,
Air screw 2 to 3 turns out

Main, 120 Pilot, 50
Needle clip, 1 notch from the top,
Air screw 2 to 3 turns out

This jetting and needle clip position has worked well at events held at 8500+ feet all the way down to 3000 feet in elevation. You may need to adjust the air screw a 1/2 turn in or out at elevation.
Below 3000 feet, stock jetting works well.

Additional side notes;
Check and adjust as needed the carburetor floats; at the closed position the float body should rest parallel to the sealing surface between the top of the carb and the float bowl. While the carburetor is apart doing jetting, tip the carb top upside down with the fuel flow needle and floats attached as they would be in the carb and check that the float(s) rest on the needle as close to level or parallel to the sealing surface of the top half of the carb. All carburetors have 2 floats in them; make sure they are set level with each other as well.

Don't use fuel that has Ethanol in it if possible. Ethanol in fuel will pull water out of the fuel and air and concentrate it in the carburetor. Ethanol is hard on all small engine carburetors; along with leaving water deposits in all parts of the carb and slowly plugging jets and small ports, and leaving a flaky film that floats around the carb until it finds a jet to lodge in, Ethanol also hardens fuel lines and can collect water in the fuel tank.

As I noted at the first of this write up, have a clean air filter! Trials bike have small air filters and most are located near the rear wheel where dust, dirt, sticks, seeds, etc., can get pulled into the air filter. Clean your air filter and properly re-oil after each ride.

Contact Thumbs Up Trials Supply if you have any questions about your carburetor, jetting or adjustment.