I have tired it both ways. The 1.5" was perfect for my 98SP500 (Which I bought used so there is no telling what jet was in it) The 2000SP500 with K&N will not run properly with a 1.5%. I tried it and had to up to a 2". I am still using the stock 162.5 jet. The plug is a medium to light brown as it should ne in 80 degree weather. I may need to up it to a 165 or 167.5 for the cold weather. The last time I checked it was in 25 degree weather and the plug was a little lighter than what I wanted to see, but still in the acceptable range.

Greg

 

One thing to watch for in all this, is that mfrs are beginning to used "tuned" air boxes like more commonly found in HP snowmobiles. Sometimes altering may go the wrong direction...

 

I noticed in one of your posts you said that your sp500 only went 45. Check with your dealer. There is a service bulletin for the main jets. If you purchased it in NY then it most likely has a 157.5 main jet. The service bulletin suggests changing to a 162.5 main. I did this on my Magnum 500 and it came alive. Also check your belt for excessive wear. This will also affect top speed.

 

Airbox restriction is a fact of life on all stock atv's. It you experience that the engine will not run with the lid off or the filter changed to a K&N, be thankful. That means that your carb was properly jetted to begin with for the temp. and alt. that you are running. Anyone who messed with the air intake at all must realized that rejetting is necessary, when you change the flow into the engine. Also, with more flow in you need to get more flow out or you will not appreciate a gain. That's where the exhaust comes in. The more you do to the motor, the more it is necessary to put a non-restrictive exhaust system on. When we switched to the IDS2 from IDS1 is was very noticeable, we had to make a jettin change. That tells me that there are definitely gains to be made with exhaust systems. Just adddding a pipe/muffler alone doesn't do much at all to a stock 4-stroke machine. But with intake, carb, cam, and piston, etc. changes, the exhaust becomes quite important. Unfortunately most seem to want to go that route first, as with a 2-stroke. With a 4-stroke getting more air and fuel into the motor is just as important as getting the exhaust out.

www.godigital-design.com/schultzmotorsports

 

Jack,
You are obviously more knowledgable about this than I, and I am not saying that adding an axhaust pipe at some point won't help. I am just saying that the airbox is the most restrictive part of the stock system.

In response to your comment that anyone who alters the stock intake will need to rejet, I disagree. I have a late model 2000 SP500 manufactured May 22,2000. The first time I looked at my plug, it was very black and even had a little biuld up on it. The motor was running very rich. After removing the airbox lid, and noticeing the bike lean out, I realized that the motor and exhaust would flow considerbly more air. I altered the airbox to allow for more air intake, then kept a close eye on the plug. I have not changed jet as of yet, and I am not sure I need to.

I did not make this modification expecting the performance increase that you get out of completely modified systems (Airbox, Headwork, Exhaust), but at least on my SP500, my top end increase by 10 MPH and low end increased enough to ride a wheely. I have raced TazsGrizz since he bought his quad. He used to pull me by 4-5 lenght running down our road, now I pull him.

I am not someone that has to have the fastest and most powerful bike around, but I like to at least be competitive while maintaining reliabilty. These alterations I have made cost me very little. The reason I made this post is that I know of people that have spent money on a pipe, jet kit, and K&N and did not see any improvement at all. That is a little annoying to the guy that did not know any better, so I thought I would add my 2 cents for what it is worth.

Also, I would like to add one more thing. If a person has headwork, and exhaust work, and makes not mods to the airbox, I do not see how he can expect much improvement with out changing the airbox. If you do not increase the flow in, does it really help to inprove the flow out?

Maybe you can teach me something. I look forward to your comments.

(BTW, I am not a full time mechanic... I am a banker so I am sure there is plenty for me to learn.)

Greg

 

Ok, I have a Xpedition 425 with a 2" snorkel and it DECREASED my airflow. The Xped has the same airbox lid as the Mag, they must flow better than the SP. In stock form my plug always looked good, after snorkeling the bike it showed rich(black).
Ive installed a high performance kit(pipe,silencer,K/N) and although it was a considerable improvement in overall power, my plug is still black and you can smell gas when idling.

What can I do to further improve the airflow INTO the motor?

 

greg i have 99sp 500 with unifilter/supertrap exhaust with headpipe and a green spring.i have the stock 155 main jet.i am going to do your intake modification and was needing to know what you recommend for jetting?

 

It is hard to say. I have not tried the unifilter. I do not know what you plug looks like right now. I think that getting the correct fuel mixture on any bike is hard to do. Trial and error is probably your best bet. (I am running a 162.5 with a K&N and a 2" snorkel and may go to a 165)

Greg

 

One more consideration that explains why removing the airbox lid doesn't work. The carb is a constant velocity device that needs a certain amount of vacuum in order to work. Without enough vacuum the carb doesn't open up at all. This is why airbox mods must be done conservatively, or put a flat slide carb in.
I'm going to try taking the "horn" off the airbox and see what happens, then add a couple of holes I can easily plug if I need to.
There has to be a simple/cheap combination of tuning tricks, i.e. airbox mods, muffler mods and jetting that will get this AAEN long duration cam in the H.O. to really run.
Silly Later,,,Fourlix