You can also use it for pilots, but pilots sometimes go the other way (larger at high altitude). To submit your questions or ideas, or to simply learn more about It Still Works, These jet sizing tips are designed to help you understand the basics of carburetor jet sizing and what engine/exterior factors may require a jet sizing change. The float bowl cover is held in place by 4 screws - on most carburetors. Carb 3: same as Carb 2. To submit your questions or ideas, or to simply learn more about It Still Works, Each jet is stamped with its size, so adjustments can be made without measuring. (I would like feedback if you find it different). Remember, its a good place to start but is not the same as jetting by someone who is good at it. 42.5 (pilot jet). Carburetor Jet Size Calculator. The Higher the Jet Number the Bigger the hole init so more fuel goes into engine to make it richer, the smaller = vice versa less fuel means lean running. Threaded with the standard 1/4-32 in. If the vehicle will be operated at sea level, but the temperature is 35 degrees, you'll need to reduce the size of the jets by 1 size, as indicated by the stamp on the head of the jet.This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. A Stage 1 Carb Kit will normally offer you a proper range of jets for your particular model. Holley manufactures each carburetor with a set of jets that will allow the carburetor to perform well at sea level and around 70 degree air temperature. However by adjusting these settings will help you achieve a more accurate calculation. Once I fully close it, the bike shuts off even when the engine is warm.
The smaller the jet size, the less fuel will flow through it at a given pressure difference across the orifice boundary. Replace the Pilot Jet with one size larger. Rich conditions allow the engine to operate at a cooler temp.Unbolt the fuel bowl bolts from the metering block of the carburetor. In most cases screw adjustments won't correct the lean back firing, installing a one size larger pilot jet will do the trick for higher elevations (Colorado).Carburetor diameter(12mm) x 5 =  60(or number it calculates) x 0.9 = sea level jet sizeTake 0.948(Denver atmospheric pressure at 70 degrees) x jet size(60) = new jet size(56.88) round up 57 would be the proper jet size to start at for a stock scooter using a 12mm carburetor.the CF is correction factor. The jets are under the float bowl cover. Remove the jets and insert a smaller/larger set based on the vehicles operating altitude. An incorrect ratio will show in decreased engine performance.Unscrew the fuel bowl bolts with a wrench and pull the fuel bowl off the carburetor. For Keihin Carbs the jet sizes refer to the metering orifice diameter, expressed in millimeters.

Holley jets range in size from 40 to 99, with the stock jet size differing based on the carburetor model and engine application. However by adjusting these settings will help you achieve a more accurate calculation. Test the engine to ensure the jet sizing change has compensated for the temp change, indicated by the engine running smooth during idle and crisp under acceleration.Change the jets whenever you increase the performance characteristics of the engine. 2-2.5 turns out on a fuel screw is proper for Colorado. Keihin Jets are sized in steps of 2 and 3. This would improve power and mileage. You can calculate that the jets should be increased by exactly one size. First identify the carb manufacturer, mikuni, keihin, hitachi, holley. The could be anything from a larger intake manifold, long duration cam shaft or milling the cylinders for increased displacement. You take your main jet that is for a CF=1, say a 200. In those cases use drill bits to measure the hole, then convert the drill bit to decimal. .989. The bike only stays on if the choke is on. Second the jets need to be located. The bike wouldn’t stay on everytime I give it a little throttle. As mentioned in step 1, you'll need to increase the jet size for lower altitudes, while a smaller jet size will be needed for higher altitudes. Eg: 100,102,105,108,110. Holley jets range in size from 40 to 99, with the stock jet size differing based on the carburetor model and engine application. Browse our library of automotive, truck, marine, industrial, and agricultural carburetor parts to find what you need for less. This is probably to avoid engraving “.5” on the jets PART NUMBER PICTURE DIMENSIONS SIZES 99-101-357-SIZE 6mm Hex head M5x0.8 Thread 16.5mm Length 80-190 (Δ 2/3) 195-220 (Δ 5) 220-230 (Δ 10) 99-101-393-SIZE 6mm Slot Head M5x0.8 Thread 8mm Length 60-200 (Δ 2/3)… Read More » Carb 2: 110 (main). The first three numbers are the Day of the Year the carburetor was built; the last number is the Year.
Lets say the main jet that runs best is a 180. If the vehicle will be operated at sea level, but the temperature is 35 degrees, you'll need to reduce the size of the jets by 1 size, as indicated by the stamp on the head of the jet.This article was written by the It Still Works team, copy edited and fact checked through a multi-point auditing system, in efforts to ensure our readers only receive the best information. Holley jets range in size from 40 to 99, with the stock jet size differing based on the carburetor model and engine application. Holley manufactures each carburetor with a set of jets …