2000 Arctic Cat 300 Fouls plug and runs rough

[mannaserv]

I have posted about this machine before...took it into the shop and spent a couple hundred bucks... they adjusted the carb and put a new plug in. It ran well for a few weeks and now runs rough again. Today the plug was fouled, so I cleaned it well and got it started again, but after a few minutes it was running rough again. I have read about adjusting valves...would that impact my problem? If so, is there a post that describes how?

[DirtDemon]

The first thing we need to know is what is fouling your plugs, gas, oil, are they dry fouled? What does the plug smell like when it gets foulded? Gas or oil? Does it smoke at all? If so what color is the smoke? Are your plugs gapped correctly? Is your air filter clean? Provide as much info as you can, so we can better help you. As far as adjusting your valves goes, there are inspection caps on the front and rear of your valve cover, thes allow you to access the valve adjusters. There is a slotted screw in the end of the rocker arm over the valve, it has a jam nut around it. Loosen the jam nut, adjust the center screw with the proper size feeler guage between the screw and the top of the valve stem. Don't tighten it tight, you want to be able to slide the feeler guage between with just a slight amount of drag. Without moving the adjustment screw, tighten the jam nut. For clearance specs, get a repair manual or call a dealer service department.

[mannaserv]

I believe it's gas. It doesn't smoke much at all when it runs...what little there is appears black when first starting. Last year replacing the air filter seemed to help. Since then I've cleaned it regularly. It does run better with it off...are they all this touchy with the air filter? First 4 wheeler we've owned. When I tried starting it with the air cleaner off, some gas came out in the air cleaner compartment.

I'm not sure what the connection would be between valve clearance and carb issues, but have seen that conversation elsewhere on here?

There is a link on here for free service manuals, but they only go back to 2003 for my 300. Is that close enough for things like valve clearance?

Part of me just wants to throw in the towel and buy a new carb...

[TexasRapier]

I have posted about this machine before...took it into the shop and spent a couple hundred bucks... they adjusted the carb and put a new plug in. It ran well for a few weeks and now runs rough again. Today the plug was fouled, so I cleaned it well and got it started again, but after a few minutes it was running rough again. I have read about adjusting valves...would that impact my problem? If so, is there a post that describes how?

The 300 are a great little motor and and there not a lot that go's wrong with them. 90% of the problems I see with these motor are carb related.

However, the ethonal fuel is causing a lot of problems with older machines not designed for the new fuel.

First off the intire fuel system should be cleaned, fuel lines replaced and good inline fuel filter installed. The tank should be cleaned and the in tank filters/screens should be replaced. also you should check the tank valve and make sure it will turn off the fuel flow to the carb. The valve is 12 years old and probably leaking by. Also there cheap to replace. about $20.00 for the new valve and the screens come with it.

You stated that they adjusted the carb. There is only two real adjustments that can be made to this carb. First is the pilot jet, it controls the engine idle mixture and second the float setting which control the level of fuel in the bowl. Everthing else is controlled by preset jetting.

The one other thing I see with these motors is the rubber carb to head adaptor dry rots over time and causes an air leak.

First confirm that you are running the correct plug. NGK DR7EA.

Next for peace of mind do a compression test, should be around 135 psi.

Next a proper cleaning and servicing of the carb. should put everything back to proper order.

[mikeexplorer]

I believe it's gas. It doesn't smoke much at all when it runs...what little there is appears black when first starting. Last year replacing the air filter seemed to help. Since then I've cleaned it regularly. It does run better with it off...are they all this touchy with the air filter? First 4 wheeler we've owned. When I tried starting it with the air cleaner off, some gas came out in the air cleaner compartment.

.

I would not run the quad with no air filter, that will cause you nothing but big problems! My old dealer (who is gone now) did not put my intake back on right and it sucked in unfiltered air and the next thing I know I needed a top end rebuild.

Mike