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matthewdcross

When to re-jet?

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When is it nessary to re-jet your carburator? What affects your jetting, and what exactly does re-jetting do? All stupid questions but none I know the answer too.

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Hi,

Pilot jet and fuel screw effect up to 1/2 throttle opened.

Needle from 1/8 to 3/4 and main jet from 1/2 to full.

You need to have device to measure exhaust gases and rolling road or dyno to get some load to the engine. Then you'll be able to set-up fuel air ratio as it should be. When this is done once, you can use JDs excellent excel table to figure out which air temperature and altitude etc effect what, when and how.

So in rejetting you have following components:

Fuel screw

Pilot jet

Needle

Main jet

all these effect each others also.

If you change exhaust or type of airfilter etc. it will also change jetting.

Hopefully this helped you atleast a bit

Timo Mc

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When is it nessary to re-jet your carburator? What affects your jetting, and what exactly does re-jetting do? All stupid questions but none I know the answer too.

1. Any change to intake or exhaust flow requires rejetting (from an optimal set-up). Such changes could inlcude (for example) airbox mod, different exhaust. Also note changes in ALT/TEMP/Rel Humidity etc will affect airmass flow so for best performance a rejet is required. (less air [O2] mass requires less fuel)

Also, take into account new bikes generally come jetted lean to comply with emission standards - some need rejetting out of the crate, others dont...

2. Rejetting (different jets come into play at different throttle settings) alters the fuel:air ratio at different throttle settings as stated above. If you have a manual, a brief description of the changes are included. Otherwise, check out the jetting FORUM stickies and general tips/info.

3. No such thing as a stupid Question, just one you think needs answered...

:thumbsup:

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As OneToGo said in order to meet emissions they are lean out of the crate. I rejetted when mine was new because it is much better to break in with a richer condition than a lean one. The sticky at the top of the page will explain just about everything you need to know.

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Also, take into account new bikes generally come jetted lean to comply with emission standards - some need rejetting out of the crate, others dont...

Not for emission standards, that is what the long throttle stop is there for. It takes the main jet out of the equation.

The stock jetting was nearly spot on for the stock bike for me, it is hot and humid where I live in Australia, just like in Japan. :thumbsup:

Just about all bikes need rejetting anyway, the OEM settings couldn't be right for everyone, unless it was EFI!!!! :thumbsup:

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All new motors these days will come out way too lean of settings. This is for Emissions tests, especially California.

Buy a new bike, need to re-jet richer.

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