OK, I've posted a few different threads about getting my Rappy running right since installing an HMF Sport Full System & UNI Filter. It's running DECENT. But still needs some tweaking. I started with changing the main jets to 150 & 155 (l/r), 3 turns out on the air/fuel mixture screw, and 3rd clip position. It ran OK, but had some nasty deceleration backfire. I was told to up the pilot jet to 27.5, which I did. I wasn't sure what to do with the air/fuel mixture screw, so I set it at 1-1/2 turns out. I thought the pilot jet and the air/fuel mixture screw worked hand-in-hand. Perhaps I'm confused. Well none the less, it still needed a little tweak so I turned it out to 2-1/2 turns. So now I'm running the 150/155 main jets, 27.5 pilot jets with 2-1/2 turns out on the screw. It runs OK, pretty much stopped the deceleration backfiring, but I still seem to be getting some cutting out at top end. When I have it pegged, top speed, it has some intermediate hesitation. I'm starting to think maybe the air/fuel mixture screw needs turning out maybe? When I had it out to 3 turns with the 22.5 pilot jets I don't remember it being that bad at top end. Please explain what this screw does and what I need to do.

 

The air mixture screw regulates the amount of air to fuel for the idle circuit in the carb - on most 4 stroke carbs the screw controls the amount of fuel being delivered through the idle circuit - most 2 strokes regulate the amount of air being supplied to the idle circuit.
The pilot jet (idle or slow speed jet as called by some) controls the maximum amount of fuel allowed into the idle circuit through the carburetor fuel bowl. Since every quad reacts a little differently, the mixture screw tweaks the amount of fuel being delivered.
The most important roles of the idle circuit are starting and deceleration.

The idle circuit plays very little part in your top end performance - I would venture to guess you need a needle clip change or different main jet to correct your top end problem.

 

Great explanation! Thank you... now about the needle clips, that was another thing I wasn't completely sure of. What is effected by altering the clip position? and when you guys say 1st clip position, is that the position closest to the point of the needle or the top of it? What is changed by moving it up or down? and when you say up, i'm assuming you mean closer to the top of the carb, and down would be closer to the bowl???

 

The needles are tapered, so if you have the clip in the top groove, you have dropped the larger part of the needle's taper down into the needle jet - reducing the amount of fuel. If you place the clip in the bottom groove, you have raised the needle's taper to a smaller diameter on the taper - increasing the amount of fuel.

Most commonly needle grooves are numbered from top to bottom, Top groove being position #1. What can get confusing is moving the needle up means moving the clip down, and moving the clip up means moving the needle down. Whenever you make a needle change, think of the direction the needle is going to move in relation to the direction you are moving the clip - if you need more fuel, move the needle up - if you need less fuel, move the needle down.

What is effected by a needle clip change? Many people will tell you that the clip position will only effect the mid-range throttle positions - this is not entirely true, as the location of the needle's taper has some effect on the idle fuel flow and full throttle fuel flow.

Also, when you are making changes to the "tuning" of the carb - write everything you do down on paper (jet size, clip position, mixture screw position, etc - and how it runs in that configuration) - AND, only make 1 type of change at a time. This way you can look and see what you have done, and what the results are with that change

 

You sound like you know what you are talking about. I have a similar problem with my 02 660R. I just had all of the gaskets and seals replaced in the carb and a stage 1 hot-cam put in the quad. It runs fine but backfires on a decent and feels like I am riding a 400cc. Lower end seems to have more power than top end. I took it to Sand Mountain and could not make it up the face where as before I could make it up in 3rd. The jets are stock and I have not messed with the air/fuel ratio. What should I do? Thanks.