The reason why your ATV will only run on reserve is that your higher inlet in your gas tank is clogged. Let me go into more details about this and what you can do about it.
The Point Of Reserve
In a carbureted engine like what many ATVs have there is a petcock or also called a fuel selector. This fuel selector will have 3 different positions – ON, OFF, and RESERVE.
Due to price, many ATVs won’t have a gas gauge and instead will use this petcock as a way to let you know you’re almost out of gas.
When you have it on the “ON” position, the carburetor is sucking gas from a higher place from inside the gas tank. When you get below that spot, you flip it to reserve which sits lower and allows you to get the last bit of gas. When you get to this lower spot its time for you to get some more gas or you run the risk of being stuck.
Why It Gets Clogged
Since you use the “ON” position of the fuel selector more, it’s going to get clogged a lot more. You’ll usually have a pre-screen or something to stop the big chunks, but over time it will get plugged anyway.
This becomes especially true if you let your ATV sit without any fuel stabilizer in its tank for months without riding it. You don’t want gas to sit for too long especially in your carb.
It’s best to run the engine and flip it to “OFF” to let it use whatever gas is in the carb if you’re not going to ride for a month or more. Then place some fuel stabilizer in the gas tank to keep it fresh.
Will It Hurt To Run on Reserve All The Time?
No, it won’t hurt to run your ATV on the “RESERVE” switch but its best not to do that. The Reserve is supposed to be your back up plan and just enough to get you back to refuel.
Unless you carry extra fuel tanks with you on every trip, you don’t have a plan-B. So its best to get this issue fixed.
How To Fix It
Your biggest issue now is that you might have junk in your tank. This junk that is clogging the inlet can lead to some problems that could be dangerous. For example, this could cause your carb to overfill and dump into the overflow and spill gas (fire hazard).
And Whatever clogged it in the “ON” position is sure to clog it in the “RESERVE” position too, so its best to get this fixed right away.
So, how do you fix an ATV that only runs on Reserve?
The answer is to get the carburetor rebuilt and a new fuel filter if you have one. Even a proper cleaning of the fuel tank would be wise too. Add some new fuel lines, and you should be better than new.
Here is a great video showing you how to clean the carbs on an ATV
What I have Done Before
I can’t guarantee this will work for you, but I have had some ATVs that even though I clean the carbs and do all I can it still will not run right.
So one day I decided to explore a bit to see what could help. One thing everyone likes to mention is SeaFoam and to be honest, it did not help me. SeaFoam has helped me with my fuel injected machines but not so much with my carbureted machines.
What I did find was Fuel System Restorer From Restore that made my old Suzuki Eiger 400 run like new again.
I got some fresh 93 Octane Gas and filled up the tank. Then I poured in the recommended amount of the Fuel system restorer. I let it sit for an hour and then started it up and played with it till I got it to purr like new again.
As I said, I don’t know if this will work for you, but this stuff has become my go to. I’ll even try it before cleaning a carb just to see if it helps since its so easy to do. It’s no magic bullet but its worth a shot.