Noob Question: Explain Rejetting

[annguyen1981]

I understand it modifies the carbs, but how?  I'm not used to have a carburator in ANYTHING.

[makenzie71]

google would help you out a lot here...

Basically there's a series of tubes and passages inside your carburetor.  When the piston goes down every other time, it creates a vacuum, and draws air through the carbs, thus sucking fuel from te various passages and tubes.

Fuel is delivered primarilly through two passages:

The Pilot Jet and Main Jet.

The pilot is operates basically as a hole in a screw.  It stays open at all times.  When vacuum is created, fuel is pulled through it.  This is your primary source of fuel up intil 1/8~1/4 throttle.

The main is primary source of fuel after 1/8~1/4 throttle.  This jet works basically the same as the pilot, but there is a needle that closes and meters the fuel delivery up until 3/4 throttle.  This needle is tapered and slides into the tube that the main jet feeds.  After the the engine has built enough vacuum it raises the needle out of the tube via a rubber diaphram.  In the higher RPM band, the needle will be complete raised out of the tube, allowing the jet to flow unhindered.

Rejetting is the resizing of the pilot and main jets and adjusting the needle to provide an adequate amount of fuel to the engine through out the rpm band.

[annguyen1981]

So rejetting is just replacing some parts and that's it?  I've read all over on this board that the right combination is needed depending on what application you're using.

I originally understood it as physical changing the current parts, like boaring a cylinder in a car.

[makenzie71]

So rejetting is just replacing some parts and that's it?  I've read all over on this board that the right combination is needed depending on what application you're using.

I originally understood it as physical changing the current parts, like boaring a cylinder in a car.

Yup, just swapping parts, for the most part.  Full jet kits include a new, notched needle to eliminate the need for shims, but you can simply buy the jets by themselves and use generic shims or washers on the stock needle.

However, you DO need the right combination of parts and adjustments to make the whole thing work propperly.  How you've altered the bie's breathing abillity (intake/exhaust) and the bike's reaction to those alterations is what determines the parts you need.

[pantablo]

you ARE replacing some parts (jets) , and you are physically changing current parts (drilling out and readjusting the air mixture screws). I ahve a couple good writeups I can post later (also check FAQ's for the links).

[annguyen1981]

Is there any reason I would rejet without modifying the can or intake?  I'm assuming that the stock jets are EXACTLY what I need with stock parts.

[makenzie71]

Is there any reason I would rejet without modifying the can or intake?  I'm assuming that the stock jets are EXACTLY what I need with stock parts.

That depends.  Does the bike run well now?

[annguyen1981]

Is there any reason I would rejet without modifying the can or intake?  I'm assuming that the stock jets are EXACTLY what I need with stock parts.

That depends.  Does the bike run well now?

Not since I parked her on Thanksgiving Day.   :lol:  :lol:  :lol:

Seriously, yes she does run well.  Just has a little problem starting on COLD days, but I think that's common for ALL bikes.  Actually, ALL motors (Cars, bikes, snowblowers,...)

[makenzie71]

Seriously, yes she does run well.  Just has a little problem starting on COLD days, but I think that's common for ALL bikes.  Actually, ALL motors (Cars, bikes, snowblowers,...)

Then don't worry about it.

...oh...and this is a plus to own a propperly working EFI apparatus.

TLS = runs smooth all the time it's not broken.

[pantablo]

the only other possible reason to rejet is that the bikes come lean from the factory and when warm the idle speed can sometimes shoot up to or past 3k rpm. This is one of the signs of lean condition. still, doesnt mean you have to do it. I never did.