Jet Size!

[chrisscott]

How do i know what the biggest size jet i can use is ?

i have a puegeot speedfight 2 air cooled.
i have a "https://www.adrenalin-pedstop.co.uk/p/TECNIGAS-NEXT-R-SPORTS-EXHAUST-93.aspx" that exhaust !

Help Please

[Kickstart]

Hi

You don't want the biggest jet, you want the right jet. Jets are fairly cheap, so get a few going up in size by (say) 10, start at the largest and work down (rich is safe, lean is dangerous, but you will lose power either side of the perfect jetting).

You will also likely need to have a fiddle with the needle jet position.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Ant132]

Have you done a plug chop? If so, whats colour plug are you getting?

Personally, if the plug chop suggests you should upjet, then i'd buy a range of jets going up in small sizes. Then go up a size, do another plug chop and repeat until you find the optimum jet size
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1980 Suzuki TS50 ~ 1984 Honda CB400N ~ 1992 Triumph Trident ~ 1998 Suzuki Bandit 1200

[chrisscott]

[ms51ves3]

You will need either a 72 or 74 (can't remember which).

You can leave the needle where it is.

[Ant132]

Make sure you have a clean plug, warm the bike up and run it at full throttle on a long stretch of road for a good few seconds.
Kill the ignition (dont release the throttle or idle prior to killing the ignition) and come to a stop. Take out the plug and read it.

Black and wet: jetting is too rich

Chocolate brown: jetting is spot on

White and burnt smelling: jetting is too lean! upjet!
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1980 Suzuki TS50 ~ 1984 Honda CB400N ~ 1992 Triumph Trident ~ 1998 Suzuki Bandit 1200

[ms51ves3]

[Kickstart]

Hi

Jet is a small tube through which the fuel passes . Larger jet (to an extent) flows more fuel. The carb will have different types of jet which will each work over a different rev range. Some jets can be changed for different ones which flow more or less fuel.

Main jet is the normal one that is changed. This affects the fueling on large throttle openings (which only do you any good at high revs). These are generally small brass "bolts" with a small and very accuratly drilled hole in the middle.

The needle jet is basically the tapered needle that is connected to the carb slide. Often these have several different grooves that the clip that locates them can be put in to change the mixture. The needle jet affects the fueling in most of the throttle range.

There is also the idle mixture screw, which affects the fueling at idle and on very small openings. These can generally be adjusted. However there are different types, depending on it they control the fuel at idle or the airflow through the fuel system at idle. First one will generally richen the mixture when turned anti clockwise, the second one will generally richen the mixture when turned clockwise.

A plug chop is done by using the bike at the throttle / rev range you are interested in, then pulling up and checking the plug colour. However to do this you generally need a steep hill (so you can put the engine under enough load for large throttle openings without accelerating away rapidly), and you should pull the clutch and turn off the engine as just slowing down will change the plug colour.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing