I know this is a silly question for you engine techies out there, but I have been told to have the jetting checked on my SP 500. I feel dumb, what the heck is jetting on a carb and what does it do? I should have stayed in high school, instead of leaving to join the circus..hehehe

 

Jetting refers to the adjusting of the carburator. Basically it allows you to adjust the fuel to air ratios and a few other carb. controlled items by changing the needles (or other items) inside of the carb. If you live in a higher altitude, or colder regions, you may want to research the different options further. Certian combinations of adjustments can make a big difference in a machine..

 

Think of a jet like this:

A jet is a tiny hole. If you took a standard sewing needle and pushed it through the hole it would be a very close fit. This gives you an idea of how small the jet hole is. Now, fuel passes through this hole and mixes with air that is passing through your card at a very high speed. The amount of fuel passing through this hole is generally controlled by you pushing the throttle open farther. Think of the sewing needle again sitting in the jet. As it is a close fit no fuel can pass through the jet. This is when your ATV is idling. As you open the throttle, the sewing needle
which is tapered, fat on top and skinnier at the bottom, creates enough room for fuel to get through the jet and up into the carb. Here it mixes with air and becomes highly explosive. (Combustion.

Depending on the ambient air temp. and what elevation you live at the size of the needle and jet need to be changed. Jets can vary widely from the size of a human hair to the diameter of good sized electrical solder. The higher you live above sea level the smaller the jets. The folks living at sea level have fatter jets. This is because air becomes less dense at high elevations and the engines perfomance will decrease significantly. As air becomes colder in the winter it also becomes less dense. When it gets really hot outside your 2 stroke motor will tend to act up, as hotter air is denser.

Two stroke motors are way more finiky and require alot more attention to jetting. 4 strokes need attention too but don't seem to whine for so much attention.

Hope this helps.

Keith

 

kiethb..Have you re-jetted for your riding in P.George. I don't mean for racing, just general riding. Have you found that a stock bike (4 stroke) generally runs to rich with factory settings in B.C.
I just bought a 00 Suz. LT F500, rode it for a few 100 klics, checked my plug , and it was black. I'm really cautious about re-jetting cause of all the different opinions I've heard.

 

Thanks keithb. you have simplified the carburetion and jetting for me. I am sure now I will have the jetting checked in service. I am at the same elevation (or close to) of the Polaris factory. Maybe the service guys can find an opertional glitch in my engine. Bolt-ons have not provided any solutions. Time to take it to the experts....I hope they are!

 

R34:

I regards to my XR350R Dirt Bike I have not had to rejet it. It runs fine and was set up just perfect when I bought it.

However you point out that your bike is producing black plugs. This is indeed an a/f mixture problem. It is true that most always the factory jetting is rich. The manufacturers always jet rich as they need to save their butts. They really don't know where the bike is going to be ridden so they play it safe and go rich. In my experience even the dealers won't set up your carbs to the sweet spot unless to specifically tell them so. This is so true with 2 stroke motors. A dealer that sets your carbs up perfectly in the shop could see potential problems when you go riding in the mountains and seize your engine. Wouldn't you be a little upset? Demand a free warranty engine?
This is why they usually run them rich.

However 4 strokes are not as bad. But the black plug in your bike is a sign that it is not running to it's full potential. You did say that your 00 LT F500 was brand new right? You need to allow time for proper break in. You could still be burning oil as the rings may not be completely seated. You could also have a dirty air filter.
Do you notice a slight flat spot while accelerating? If so at what RPM? IS it hard to start when cold or hot?

In general rejetting a 4 stroke will not really harm the engine's major components. Unlike a 2 stroke, improper jetting is sure to cause a complete engine seizure.

Keith

 

"Jetting" is checked by "reading" the spark plug; you can perform this check yourself. If the engine runs o.k. and the plug's the "right" color, you can assume jetting is o.k.

Otherwise, a black plug (generally) indicates too fuel-rich a mixture; a whitish or chalky color indicates too lean. After a brief wide-open run, hit the kill switch and close the throttle, When you come to a stop, remove the plug, looking for that cafe' au lait, coffee with cream color. Solid!

If you vary engine component configuration, we're talking trial-and-error for optimum jetting, if you change altitudes, re-jetting's more straightforward.

Tree Farmer

 

SWAC,all these guys are dead wrong.Jetting is something you aboard on the ground and fly in the sky that travels at high speeds.[lol]Actually,Keith and Tree Farmer did excellant jobs explaining jetting to you,so I therefore will shut up.Good job fellows.====BILL

 

kiethb.... The machine is new, 500 km now. Its hard to give an answer to acceleration flatspots, because of the damn snow, if I had some dry area to ride, I could probably answer that question. When the machine is warm it starts fine, but anything below -10 celc. its hard to start. I must say, that this is'nt a performance issue, 'cause the machine pulls really hard, and thats a fact, but I am just interested in "tuning" it properly for the elevation that I'm at. If I put a high flow exhaust on it, that would probably lean it out enough to give me the amber colored plug, but I am not willing to spend $300.00, when a set of $20.00 jets could do the same thing. I asked the Dealer about it, and got the response that you mentioned, although he said that in the winter, it's better to have it running a little "fatter", which in fact, could be true.

 

R34:

I have a really great article about jetting and how to set up your carbs. It was published in an old dirt bike magazine that I have in my basement.
I keep all my old mags as they make a great reference. If you are interested I can scan it for you and email it to you, or anyone else who is interested. Let me know and include you email address.

Keith