03 Jan 2013
January 3, 2013

How to change Pilot jets

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Pilot jets which are sometimes called slow jets are the fuel metering system in modern carburettors that determine how your engine will run at lower throttle openings.

Basically what we know as a normally aspirated petrol engine uses a carburettor to feed fuel to you engine operating by suction from the engine. The amount of petrol that is mixed with air is important and has to be monitored accurately. There are several elements to any carburettor important in achieving this and these are the throttle slide and its cutaway angle, the throttle needle that is held into the throttle slide by a circlip, the slow or Pilot jets and then the main jet. The mainjet is screwed into a brass component located in the float bowl of the carburettor and is simple to remove by draining the float bowl carefully and using a screwdriver or a hex wrench to remove the old mainjet and exchanging for the new chosen size.

Mainjets control top end or larger throttle opening performance, Pilot jets and needle position control the slow and mid range performance.

The pilot or slow jet is also accessed from inside the float bowl area but is a small diameter brass jet with a screwdriver slot just in front of the larger brass mainjet. Pilot jets can be removed and changed for a chosen size after determining what is wrong with the engines performance beforehand.

Warm the engine, set your engine idle to 1500 rpm.

If there is spitting as you pull away or popping in the exhaust the pilot jet is normally too small and a larger size should be chosen. A similar effect can be obtained by unscrewing the spring loaded screw on the side of your carburettor but if you have wound this out approx three turns and there is no effect the pilot jet size needs increasing. When changing exhausts and filters to a different to stock design or when you ride in a significant temperature change , altitude or humidity difference a pilot jet change may be needed.

If the Pilot jets is too large the engine will run rich, plugs may soot up and excess fuel consumption and dull performance will be experienced. In such cases you need smaller Pilot jets. To test this screw the idle screw right in and undo one quarter turn. If the engine still performs OK it is running too rich, normally an engine should run best at 2-3 turns OUT on the pilot screw.