Consider the jets that will be used
The first thing to consider before rejetting is to get all jets from 1 place that is strict about the quality of their jets. Our goal is consistent jetting results. All Jetsrus jets have been manufactured in the same plants since we opened. That means if you need a jet 1 size up or down that is what you will get from us. If you buy from a different store today, and need more jets in the future they may not be correctly sized. Many manufacturers have their own numbering system. Many dealerships and stores buy the jets from the cheapest manufacturer and those numbers won't match each other. Many counterfeit jets are now on the market that won't match anyone else.

What is Rejetting? Rejetting is a repetitive process in which jets are swapped in the carb until the desired A/F (air to fuel) ratio is obtained.

What is the Air to Fuel Ratio? The air to fuel ratio (A/F) is the amount of air in proportion to the amount of fuel. The stock engine from the factory has been jetted to provide the correct A/F ratio.

Why do I need to rejet? To maintain the correct A/F ratio to avoid damaging the engine.

What kind of things would cause the need to rejet? 1)If performance parts are added to the engine, 2) if the temperature changes, 3) if the elevation you normally ride at changes, 4) even the barometric pressure effects the A/F. For example an aftermarket exhaust increases the flow of air thru the engine and that leans the A/F ratio out. Rejetting is the process of correcting the A/F ratio for any performance modifications. When the A/F is leaned out larger fuel jets are required to correct the A/F ratio.

What jets do I need? Due to variations in each engine from the manufacturer, different intake modifications, different exhaust modifications, elevation, temperature, humidity, barometric pressure and other differences, it is nearly impossible to to determine with any accuracy the exact jets needed.

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 READING PLUGS
e. Removal of air cleaner makes condition worse.
f. Popping back thru carburetor.
g. Engine pings or knocks, excessive engine heat.

Does temperature effect the jetting? Yes it does.

Does elevation effect jetting? Yes it does.

Does barometric pressure effect jetting? Yes it does.

Will a pilot or slow jet be needed? Pilots or slow jets usually help if the engine backfires when you let off the throttle. Even if no backfiring is present the pilot (slow) jet may be too lean. The pilot (slow) jet is one of the most overlooked jets in the carburetor. If the engine needs the choke for a long time in cold weather a larger pilot or slow jet will help. If unable to obtain a smooth idle with the mixture screw going up a size or two in pilot (slow) jet will help. We recommend purchasing pilot or slow jets with orders to avoid having to re-order them later, that will save shipping costs.

I put on an air filter and exhaust and my engine seems to run fine with the stock jets, do I need to rejet? The engine may appear to run fine but must be checked make sure the engine is not running lean. Most intake and exhaust modifications require rejetting.

Will the engine be damaged if I run it lean and don't rejet? Yes it can be damaged. The fuel in the air and fuel mixture helps cool the cylinder. When the air flow thru the engine is increased with performance parts the cooling effect of the fuel decreases. The engine now runs hotter than it was designed to and may be damaged with prolonged high temperature use.

Do the instructions provided with Jetsrus kits tell me how to keep the engine from being jetted lean? Yes they do.

If I buy individual jets do I get instructions on jetting with them? No

Am I going to have to change jets in the engine in a repetitive manner to find the right jet setting? Yes.

Is rejetting hard to do? Rejetting is not hard to do. The process is redundant but it is not difficult

I don't want to spend the time rejetting my engine. I have a performance exhaust, a K&N air filter and I drilled three holes in the airbox lid. Can't you just tell me what jet to use? Each brand and model of exhaust and air filters flow a different amount of air Your elevation is different than someone else. These variables are some of the differences from one engine to another. Due to the differences there is no way to 100% accurately predict the air flow rate of two engines and therefore there is no way to tell what jet to use. We still have to compensate for different elevations and temperatures. The odds of two quads being equipped the same, run at similar ambient temperatures and elevations is very low.

The jet kit from company ‘X’ says to use jet 142 for aftermarket exhaust and air filter at elevations of 3000 feet. Why can't you just tell me what jet to use like that kit? This is similar to the above question. Any kit that gives instructions like that is assuming all exhaust and air filters affect the A/F ratio the same, and of course all exhaust and air filters flow different. If a kit is bought with that type of instruction there is a risk of damaging the engine and poor performance

I bought a bunch of jets really cheap, did I get a bargain? Usually there is a reason the parts are cheap. Many cheap parts are manufactured to low standards, they have burrs in the holes, the holes are egg shaped, the threads don't match up with the carburetors. Most aftermarket jet numbers do not match the stock jets, nor do they match anyone else. A lot of money is spent on the vehicle and more money is spent on performance parts, it makes good sense to buy they best jets for a little more.

There aren't any differences in the jets I buy at the dealers is there? Yes jets can be different. Many jets are produced which do not conform to factory original. Jets may look correct but the orifice (hole) sizes are incorrect. Many places that sell jets are unaware of the differences hence the trouble encountered with buying jets from different dealers and the 'used-jets' bin at the dealer.

Don't stores sell GENUINE MIKUNI or KEIHIN jet? Many dealerships and part stores sell keihin and mikuni, they are not GENUINE KEIHIN or GENUINE MIKUNI. Most stores don't know there is a difference.

How do I know if a jet is GENUINE MIKUNI, GENUINE KEIHIN or generic and what does that mean? The OEM equips a Mikuni carb with GENUINE MIKUNI jets and a Keihin carb with GENUINE KEIHIN jets. Many providers make Generic jets that may not match the GENUINE jet.

GENUINE MIKUNI jets will have a stamping on them which is a "square within a square" on them:

GENUINE KEIHIN jets will have a "K star" stamping on them:

My engine is still lean, I need bigger jets than what was in the kit, can you sell me more jets? Yes, the higher the jet number the more fuel that flows.

I am planning a trip to the mountains and will be at a higher elevation, I need lean jets for that don't I? Yes, the lower the jet number means less fuel flows. See elevation jetting.

Do you carry lean jets for mountain riding? Yes

I was looking at other kits provided by other companies for the 300EX, 400EX, TRX450R, YFZ450,etc., I noticed other kits require you to install main jet adapters, do I have to use adapters with Jets R Us kits? Jets R Us jets do not require adapters, we sell the correct jets. Some other manufactures require the use of an additional adapter to band-aid their unique jet to fit. Always ask other kit manufacturers if they require the use of jet adapters. Jetsrus jets DO NOT use adapters which interfere with precise fuel flow.

Some jet kits promise big power gains of 5% to 25% and better fuel economy on STOCK engines, can every jet kit do that? No, Jets cannot by their self increase horsepower 5% to 25% on stock engines. Jets alter the amount of fuel into the carb. Placing bigger jets in the carb without performance parts will richen A/F ratio, that will not improve horsepower or economy, just foul plugs. After studying the large horsepower claims of other kits it is usually determined that some or all of the following had been done: 1) airboxes where cut or modified and 2) engines had been modified with cams etc and were not stock and 3) exhaust added, etc.

Why do other kits sell jets that are smaller than the stock jets? Smaller numbered jets are "lean" jets made for high elevation mountain riding. Many kits are mass marketed to appeal to everyone. The Jetsrus kits sold here are made specifically for high performance aftermarket parts. If smaller jets for high altitude riding are needed we can provide them.

Some kits don't list the jet sizes in their kit, how do I know if those kits will work for my engine? If the kit doesn't list the size of the jets it contains there is no way to determine if those jets will work for the application. Always determine the size of the jets in the kit you buy.

Other kits indicate they are for different stages, what stage are these kits? The designation of stages can be misleading as the stages overlap each other. How is a "stage" defined? It is marketing hype. The overlap of the stages makes defining whether a kit meets a certain stage ambiguous. There is a description on each Jetsrus jet kit page that indicates what each kit is for.

Do you offer needles? We offer some needles, however it is unknown if the needle needs adjustment until after the main jet, pilot/slow jet and pilot screw have been correctly jetted (the instructions that come with Jetsrus jet kits and FAQ's show how to correctly set these). If you want to adjust your needle and it is the non adjustable type the most economical approach is to place small washers under the c-clip to raise the needle.

How can I effectively rejet if I dont know where to start? Lets use an example: A slip on pipe and an aftermarket filter are put on. The stock main jet is 150 and the stock slow or pilot is a 40. The engine runs poorly at wide open throttle and backfires when backing off throttle. The backfiring can be fixed by going to a larger slow or pilot jet, in this example the next size up is 42.5 and then 45. Both sizes could be purchased to get the low end of the carb adjusted. If the low end is still performing poorly the next bigger sizes could be purchased later. The main jet affects wide open throttle but it is hard to know what sizes to guesstimate, a 155, 160, 165, 170, 175 would be good sizes to try. If the 160 is too small and the 165 is too big an in between size may be purchased at a later date to get it dialed in perfectly.

Remember the extra jets that have been purchased are not a waste of money, when the temperature changes or if there is a trip where elevation changes those jets will be used.

To read more re-jetting basics go to: