96 Polaris SLX 780 Bogging

[dfowler1]

I just bought a 780 that was rebuilt last year and was told it ran fine. It idles fine and runs good to 1/2 throttle after that it seems to start missing and bogging. I can not get it over about 25 mph. I checked the plugs and they are all nice and brown and checked fuel into the carbs with air filters off and all looked good. Any ideas? I am a truck mechanic and this is my first ski. Thanks in advance for any help.
Daniel

[imported_Tim]

In order for your engine to run correctly, you must adjust the carburetor(s). First you must be sure that the carburetor(s) are clean. You should disassemble the carbs, clean them, and install a new carb kit. This is called rebuilding the carb(s). I have provided step-by-step guides for doing this here.

To disassemble the carbs, you need to have a clean work area. A muffin tin or egg carton will come in handy for those small parts. To clean the carbs you can cold dip them, however since most people don't have a cold dip tank in their garage, you could take the carb to someone who does, or you can try another method. Go to your local auto parts store and buy a can of carb cleaner. Disassemble the carbs. Place all the little parts in the muffin tin or egg carton. The can of carb cleaner should have a straw that attaches to the nozzle. With the carb cleaner spray through each passageway (wear safety glasses). You should see fluid come out of the other end of the passageway. If nothing is coming out the other end, then get a piece of wire (about the size of one strand from a throttle cable) and run that through the passageway, until you can spray all the way through the passageway. Repeat this procedure until all the passageways are clear. Also remove any corrosion and loose paint from the carb, inside and out.

Next, you need to have a rebuild kit. Reassembling the carb is straight forward, just the reverse of disassembly plus there should be instructions in the kit. Two things you may need to do while reassembling the carb is adjust the control arm level and the pop-off pressure. The control arm level is different for each type of carb, but flush with carb body will usually work. To adjust the pop-off pressure you must have a pop-off pressure gauge. You can order these where you got the carb kit or you can take it to a shop and have them adjust it. Pop-off pressure is different for each carb, but 20 psi will work on most Mikuni square bodies and 23 psi will work on most Keihins. Once those two jobs are done all you have to do is reassemble the carbs.

Once the carbs are reassembled and installed (always use new carb base gaskets), the carbs have to be adjusted. The proper way to adjust the carbs is to take the craft to a body of water, but before you do that you can make some pre-adjustments. Find your low and high speed screws (the low speed is lower on the carb and the high speed is higher). The low speed screw is easy to find and may have a t-handle for ease in turning. The high speed screw may have a plastic cap on it. You will have to remove the cap in order to adjust the carbs (it just pulls off). If you have a manual the carb settings should be in it or while you disassemble them you could turn both screws in, counting the number of turns until they lightly seat then just reset the screws when you are done.

Even if you set the screws back where they came from, the adjustments may still not be correct since no two engines are identical. If you don't know what the screw settings should be, put them at 2 ½ turns out. Once you have the screws set you can take it to the water. You will need a few things like a screwdriver, spark plug wrench, plug gapper, and a new set of plugs.

With the old plugs in the craft, start the engine and warm it up in the water for about 5 minutes at an idle. Once the craft is up to running temperature, remove the old plugs and install a set of new plugs that are gapped correctly (0.024" works if you don't have your exact spec.). Take the craft for a low speed ride, no more than 20% throttle, for 5 minutes. Remove the plugs and inspect them. You are looking for a chocolate brown color on the rim of the plug (the end of the metal threaded part that is exposed to combustion). If the color is tan or nonexistent, the fuel air mixture is too lean. If the color is black the mixture is too rich. Turn the screws in (clockwise) to lean the mixture and out (counterclockwise) to richen the mixture. Do this in 1/8 turn increments. You may have to run the craft and check the plugs several times to get the carbs adjusted correctly. It is the same procedure for high speed except you run the craft at ½ throttle or greater for 5 minutes.

There is a third screw on you carbs, this is the idle speed adjustment. It doesn't have anything to do with fuel air mixture, it only adjusts the RPM that the craft idles at, and this has to be adjusted too. It is the only screw that is attached to the throttle linkage. Turning the screw in will increase rpm, turning out will decrease rpm. With the craft in the water, turn the idle speed screw until the craft idles smoothly but doesn't want to take off. You should be able to stand and hold the craft stationary while idling. If you have a tachometer, it should read 1500 RPM in the water. NOTE: out-of-water idle will be significantly higher, some as high as 3000 RPM. If you don't have a tach but have a speedo, your craft should idle at 2-3 MPH in the water.