JETS-R-US: REJETTING BASICS

These jetting instructions are for engines that have high performance parts added. This procedure is not for vehicles being taken to higher elevations or to lean out engines. These instructions are a general guideline to use. If you are not qualified or cannot perform this take your engine to a qualified service technician.

What symptoms indicate the engine is too lean?
a. Acceleration poor, surging.
b. Performance improves slightly as engine warms.
c. Better performance when choke/starter is used.
d. No color, overheated spark plug electrode (see reading plugs FAQ).
e. Removal of air cleaner makes condition worse.
f. Popping back thru carburetor.
g. Engine pings or knocks, excessive engine heat.

What jets do I buy?
First determine what jets are in your carb. There are two easy ways. (1) Open the carb and visually check the carburetor jets to confirm the number on the jets (note: if the engine was rejetted by a previous owner the jets may be generic and the numbers will be meaningless) . (2) I if this a new vehicle new then look in the service manual. If the jets are generic aftermarket the numbers usually do not match the factory jet numbering system. Our jet numbers are the correct numbers. Do not mix our jets with aftermarket jets from a used parts bin or other manufacturers, doing so will waste time, result in poor jetting results and lead to frustration. Remember all engines are unique and using a jet size just because someone said it works will not always work. Buy a few sizes in addition to what other sources stated to make sure you can get it dialed in to account for different setups in engines. Don't forget pilot or slow jets and dont forget carb bowl screws, the factory screws strip out and ours will not strip.

THE REJETTING PROCEDURE:

Do not deviate from the order listed below. The following instructions are for jetting an engine that has had performance parts added and is running lean. A lean engine needs larger numbered main and pilot/slow jets. If you are rejetting for a trip to a high altitude then usually smaller number main and pilot/slow jets will be needed.

Do not change more than 1 thing at a time, for example don't change the pilot jet AND the main jet. If more than one item is changed no determination can be made with regard to which change caused the result.

Always make sure to clean the air filter before adjusting the jets. If the carb is dirty when the jetting is performed the engine will be "lean" later when the air filter has been cleaned.

In what order do I rejet? The order which provides the best performance is correct the: 1) idle, 2) slow or pilot jet, 3) re-adjust the idle with the new slow or pilot jet, and finally 4) the main jet. As the temperature fluctuates thru the year rejetting will be needed to compensate for the change in the A/F. Since the A/F changes it is good to have a few extra sizes of slow or pilot jets and several extra main jets.

Idle screw: Let engine warm up and adjust for smooth idle.

Pilot screw: (sometimes called a mixture screw) Turn screw in GENTLY until it bottoms. Turn out 1/4 turn at a time until smooth idle is obtained. Normally do not turn the pilot screw out more than 2.5 turns. If you must turn it out more than 2.5 turns go up to the next larger pilot screw.

If the above adjustments did not get the engine running properly, the jets will need to be changed.

How do I get to the jets? Shown below is a typical carburetor. The jets are located in the bottom of the carb. Remove the 4 screws which attach the fuel bowl to the carburetor.

Once the 4 screws are removed the main jet and slow (or pilot) jet can be removed.

Pilot jet (Slow jet): If unable to obtain a smooth idle with pilot screw, increase the pilot/slow jet until a smooth idle can be obtained. The pilot/slow jet will eliminate or reduce popping that occurs when letting off wide open throttle. The pilot/slow jet will also improve cold weather starts. Go up 1 pilot/slow jet at a time, don't skip sizes. After installing a larger pilot/slow jet, re-adjust the pilot screw.

Main jet: Start with the biggest numbered main jet and run at wide open throttle, the engine should stumble at wide open throttle. Install the next smaller size until the stumble is gone. Once the main jet is correct re-adjust the pilot screw first and the pilot(or slow) jet if needed until proper idle is obtained.

Needle: It is common for some to recommend changing the needle. We suggest not spending money on a needle (which may or may not improve performance) until every clip position available on the needle has been tried. If needle is not adjustable try shimming it with a small washer from the hardware store.

Reading plugs: At this point it is best to do a plug check, go to FAQ-READING PLUGS via the following link.

What to do when the weather changes: The temperature, humidity, elevation etc all affect the jetting. Extra jets are always needed so the engine can be adjusted to meet changes in the climate. Hot weather will normally require smaller numbered jets and cooler weather will need larger numbered jets.

To link to this page copy and paste the following:
http://www.jetsrus.com/FAQ_rejetting.htm