How To Determine Carburetor Jet Size?

How do I know if I need a bigger main jet?

Other than performance, the way to test your main jet is by doing WOT Plug Chops. The color of the plug will tell you if you need to do any fine tuning. DO NOT assume it is correct just because the engine appears to be running good. Remember, white and light gray is LEAN which means you need a larger main jet.

What does the number on a carb jet mean?

the number is the jet bore diameter in 1/100 mm. A #65 main jet has 0,65mm diameter. The larger the number, the larger the hole -> more fuel.

What happens if main jet is too small?

Be careful: most engine jams occur when the main jet is too small. A lean fuel/air mixture will cause the engine to overheat fast.

What happens if main jet size is smaller?

Installing a smaller size jet will reduce fuel flow, which is ideal for high-altitude conditions where air is limited, where a larger jet size increases fuel flow for oxygen-rich, low altitudes.

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What Is a Stage 2 jet kit?

Dynojet Stage 2 kits are intended for motorcycles with a stock or mildly tuned engine using a well designed aftermarket pipe with a modified airbox. Designed for applications where airbox modification improves the engine’s performance and in most cases power increases of approximately 8% can be achieved.

How do you adjust the main jet on a carburetor?

Carb Quick Guide

  1. Verify carburetor is set to stock settings:
  2. Start bike, bring to operating temperature.
  3. Set idle speed adjusting screw, clockwise to increase rpm, counter-clockwise to decrease rpm.
  4. Adjust idle mixture by turning idle mixture screw slowly clockwise until the engine runs poorly.

Does jet size affect idle?

As far as jetting goes, the jet size will not affect idle, as the jets don’t come into play until the carb transitions to the primary circuit, unless the throttle blades are open far enough to expose the transfer slot.

How do I know if my jet is too small?

If the bike surges or hunts, the pilot jet is probably too small (lean). Other indications that the pilot circuit is lean are popping or spitting through the carburetor when the throttle is opened and popping or backfiring through the exhaust when the throttle is shut.

Is the pilot jet the slow jet?

Slow (Pilot) Jet On most carburetors, the slow jet — also called the pilot jet by many — is recessed farther from the float bowl plug and smaller in size than the main jet.

What will a bigger main jet do?

A larger jet number means a bigger hole and more fuel. A smaller jet number has smaller holes and should be used if your engine needs to run more lean. Whether your machine is running too lean or too rich is a good starting point for choosing your jets, but using a jet calculator minimizes trial and error later.

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What is the pilot jet in a carburetor?

The pilot jet handles the mixture from idle to 15 to 20 percent throttle. Air comes in through the front of the carburetor drawing fuel up through the pilot with a vacuum that is created as the air flows through the pilot circuit.

What size pilot jet should I use?

The pilot jet is sized correctly if the fuel screw setting falls between 2.5-3 turns out. The pilot jet is sized correctly if the air screw setting falls between 1-2 turns out, with 1.5 being the sweet spot.

Can you drill out carb jets?

By drilling out your stock main jet to larger sizes you will be able to narrow down what jet size your carburetor needs. You will need a numbered drill bit set. If it’s lean and you need to increase the size of your main jet, drill the jet out with the next size larger drill bit, which in this case is the #60 bit.

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