Aarrgghh!! it so annoying, why can’t you just idle. Yea, I know the feeling, this is a common ATV complaint.
An ATV that dies when the choke is off, is symptomatic of a dirty carburetor. Removing and cleaning the carburetor will fix the problem. Other possible issues include:
- Incorrect air/fuel mixture ratio
- Faulty carburettor
- Vacuum leak
This is a common ATV complaint, especially ATV’s that lay up for a time.
In this post you’ll learn why your ATV only runs with choke and what you can do to fix it, right now. You’ll also learn how to adjust the air fuel mixture correctly.
Air Fuel Ratio (AFR)
The simplest fix to this problem can be a quick adjustment of the air fuel mix screw, and you can read all about doing that here. However, as you’ll discover adjustment often isn’t usually the cause of the problem.
Your ATV needs an air to fuel ratio of 14.7 parts air to 1 part gas.
Any variation of this ratio and the engine is said to be either “Running lean” or “Running rich”“.
A lean condition can cause your engine to run excessively hot and a rich condition can wash the protective oil coating the cylinder and dilute the engine oil.
Both conditions will cause your engine to run poorly and should be fixed as soon as possible. If neglected, both conditions put your ATV’s engine in real danger of failure.
Lean condition – A lean condition is a lack of gas in proportion to the volume of air an engine receives. Any proportion above 14.7 parts air to 1 part gas is a lean condition.
Causes of lean condition – A lean condition can be caused by either not enough gas in the combustion chamber mix or too much air.
Not enough gas in the mix is easy to imagine, a fuel blockage or poor mix adjustment, but how could there be too much air in the mix?
As your carburetor is under vacuum, the engine will draw air from where-ever it can get it. A damaged carburetor mounting gasket or manifold will allow un-metered air in. A vacuum leak around the air/fuel mix screw is common too.
On the mechanical side, a damaged head-gasket, valve adjustment or damaged valve seats will allow un-metered air into the combustion chamber, thereby creating a lean condition.
If air doesn’t pass through the carburetor intake (Venturi), it’s not mixed to proportion.
Lean condition symptoms – These can vary, the most common include:
- Starting and dying
- Only runs with choke
- Engine surging, engine runs erratically
- Engine stalls when hit the gas
- Engine won’t idle
- Engine rpm high
- Very hot running engine
- White spark plug
- Engine misfiring
Rich condition – A rich condition is too much gas in the combustion chamber in proportion to air. Any proportion below 14.7 parts air to 1 part gas is a rich condition.
Causes of a rich condition – A rich condition can be caused by either too much gas in the mix or not enough air. Common causes of a rich condition include: Air/fuel mix screw out of adjustment, faulty carburetor, faulty carburetor needle seal, dirty air filter, air box or trunking obstruction.
Rich conditions symptoms – The symptoms vary, the common ones include:
- ATV stinks of gas when starting
- Engine floods easily
- Tailpipe blows black smoke
- Oil level higher than the full mark
- Black plug
- Engine misfiring
Adjusting Air Fuel Ratio
The AFR can be adjusted and will need to be adjusted as a carburetor and engine wears. Carburetors are set from factory and some may even have a tamper proof screw to prevent adjustment or major adjustment swings.
Incorrect adjustment can as you know be harmful to your engine.
Most carburetors will have two adjustment screws, an idle speed screw and the air/fuel mix screw.
The idle screw basically adjusts the idle speed of the engine, it simply opens the throttle butterfly flap, think of it as a door stop. It has no other function. It may be part of the throttle assembly cable on some carbs.
The air/fuel mix screw is the business end of setting the carburetor mix. The screw (brass flat head usually) will be accessible as it needs to be adjusted while the engine is running.
The engine should warm and engine off.
- Go ahead and find the brass mix screw and turn it all the way in until it seats. Don’t tighten it as it’s a delicate screw and could break.
- Now back the screw out one and a quarter turns.
- Start the engine and attempt to idle the engine, keep adjusting the screw out (anticlockwise) until the engine will idle.
- Now, snap the throttle sharply: If the engine bogs (hesitates), keep adjusting the screw out until it doesn’t. When you find that sweet spot, open the screw about another 1/8 turn. If on the other hand, after the initial adjustment, the engine doesn’t bog when you snap the throttle: Adjust the screw back in until it does, and then back the screw out again in small increments until it doesn’t, then finally back it out another 1/8 turn.
- You may need to adjust the idle screw afterwards. Turning it inwards (clockwise) increases the rpm and anticlockwise lowers the rpm. The engine should idle smoothly and not be difficult to select gears (manual gearbox).
If this hasn’t worked, then go ahead and clean the carburetor.
Dirty Carb Causes Lean Condition
As your ATV only runs with choke, it means your engine is running lean. The lean condition is most likely caused by a dirty fuel jet or emulsion tube inside the carburetor.
The partially blocked jet reduces the amount of gas being fed to the engine, which in turn causes it to stumble and stall. Applying choke reduces the volume of air entering the engine, adjusting the ratio closer to 14.7 to 1 and allows your engine run smoothly again.
Gas goes stale and blended gas can go stale after about a month. Stale gas looses it’s ZING and so your engine may struggle to run and lack power. If your gas is old, drain the tank, carburetor bowl and refuel.
Carburetor gumming is the next stage of stale gas and it’s a real problem, especially with ATV’s that lay up over winter. The stale gas inside the carburetor bowl, eventually evaporates and leaves a sticky gel that blocks up the carburetor main jet.
If your ATV has been sitting idle for a time, it increases the chances that gumming is causing your problem. The only fix here is to remove the carburetor and clean it thoroughly with carb cleaner or in an ultrasonic cleaning tank.
Gumming can be prevented though, you can use a gas stabilizer in the fuel when storing. A stabilizer will keep the gas fresh for up to one years, a full gas tank helps prevent moisture buildup too.
Removing & Cleaning the Jet
The fix is simple, remove and clean the carburetor. Some carburetors will be easier to remove than others. It is possible on some models to remove the main jet and emulsion tube without removing the carburetor.
In order to do that, you’ll need room to work below the carburetor, so if you’ve got the space go ahead turn of the fuel tap and remove the fuel bowl.
The bowl lives at the base of the carburetor and is a reservoir of fuel that stands ready to feed the jet.
Grit that makes it past the gas filter usually hides in the base of the bowl. Most bowls are fitted with a drain screw, which is useful for maintenance. I’ll drain my fuel bowl regularly, it often fixes poor running issues.
Bowl Screws – The bowl is fixed to the carburetor, usually 2 or 4 screws. Here’s where you’ll need the space we spoke of earlier, to work the screwdriver. A short handled butty screwdriver and bit set is the go to tool for this job. Be sure your driver fits snugly, these screws are often damaged and can be tricky to remove when the carb is on the engine.
Gas Tap – Your gas tap needs to be set to the off position, as gas will flow as soon as you remove the bowl. You’ll find your gas tap by following the gas line between the tank and carburetor. The tap will be marked on and off. If you can’t find, not to worry use a fuel line clamp or small vice grips to gently pinch the line.
Bowl Seal – With the bowl removed, you may find a rubber bowl seal on the bowl or carb side. If it’s in place, best to leave it there, these guys can expand when removed and can be difficult to refit correctly, and especially when carb is on the engine.
Rinse the bowl with some carburetor cleaner and set aside.
Needle Seal – The plastic/metal float and needle, control the flow of gas to the bowl. Removing the float pin releases the float and needle. The needle is small so I’ll lay a workshop cloth out to help catch it.
Needle seals are a wearing part and can be replaced. The rubber seal often turns pink when old and worn.
Jet – The main jet isn’t as impressive as its name suggests. Jets are made from brass and are often just a small hollowed out screw.
You’ll need your butty driver and bit set again, brass is corrosion resistant and that’s great for a carburetor, but brass is a soft metal and an ill fitting screwdriver can easily strip the jet head. A proper fit is mission critical here.
The jet can be cleaned with a wire brush strand the screw hole must be clean.
Emulsion Tube – The emulsion tube is also made from brass, not every carb will have one, it depends on the type. However, if you have one fitted, removing the jet will also release the tube. The tube usually drops down with a little tap on the body of the carb with just the plastic handle of the screwdriver.
The tube carries multiple carefully calibrated port holes in the wall which all feed gas to the engine. Over time, these holes get smaller as microscopic grit attaches to the edges. This may not be obvious on first inspection, but as you clean a port hole you’ll see the difference.
The size of the port holes in the jet and tube are important, they’re carefully calibrated to suit the volume of air entering your carburetor.
I use some carburetor cleaner and a strand from a wire brush or copper wire strand from electrical cable. Using a more abrasive tool like a micro drill bit will make the port holes bigger, which will negatively effect the AFR.
That’s it, rebuild your carburetor in reverse order, turn your gas on and check for gas leaks before starting and testing.
Removing Carburetor – If you don’t have work space below the carburetor, then you’ll need to remove the carb. Removing is pretty simple, but you’ll need to take care with the carburetor mounting gaskets.
If they’re damaged and sometimes by just removing the carburetor damages them, you’ll need to replace them. Damaged carburetor gaskets will cause engine surging and idling issues.
A faulty carburetor is pretty common. They are precisely calibrated bits of kit, and do wear out. Unfortunately you won’t likely be able to tell a carburetor is faulty by just looking at it.
But if you’ve eliminated bad gas, fuel supply issues, vacuum leaks and mechanical issues like head-gaskets valves leaks, go ahead and change it out for a new one.
Is it bad to run an engine with the choke on? Yes, running an ATV with the choke on risks excessive gas washing the protective engine oil from the cylinder. The gas makes it’s way to the crankcase and dilutes the engine oil.