A little choke helps your engine start from cold. Some bikes seem to run better with the choke on permanently. But that’s your bike’s way of telling you it has a problem.
The top 5 reasons you shouldn’t ride with your choke on include:
- Engine runs rich
- Dilutes oil
- Washes the cylinder
- Burns valves
- Causes backfiring
In this post, you’ll learn why you shouldn’t ride with the choke on, what it means if your engine runs better with it on permanently, and what you need to do to fix it.
1 Engine Runs Rich
Your carburetor’s job is to mix gas and air together to a precise ratio of 14.7 parts oxygen to 1 part gas. This is the ideal ratio. It’s the mix that produces the most power and burns more completely. It’s known as stoichiometric. Changing this recipe changes how your engine performs and how your bike handles the gas.
Your choke lever, as you know, causes extra gas to enter the engine, and that’s great for a cold engine. That’s because cold air contains more oxygen than warm air, extra gas (choke) is needed to start a cold engine. It brings the ratio closer to stoichiometric.
As the engine warms, oxygen levels normalize, and if the choke is still on, your engine is now getting too much gas and is described as running rich. A rich engine, apart from not running at its best, can have some adverse effects on your engine. Some of them can be serious and we cover them in a little more detail below.
Symptoms of a rich running engine include:
- Black smoke
- Spark plug fouling
- Black sooty spark plug
2 Dilutes Oil
A rich running engine has a hard time burning all the gas. Instead, some of the excess gas makes its way to the crankcase, where it dilutes the oil. Diluted oil is thinner and won’t coat, cool, and protect internal engine components like fresh oil. There is a real danger of engine seizure when riding with diluted oil.
Contaminated oil will look thin, watery, and stink of gas. In addition, you notice your oil level higher than normal
3 Washes The Cylinder
Un-burnt gas inside the combustion chamber, as you know, makes its way to the crankcase, but to get there, it must slip past the piston and rings. As it does, it washes the protective oil coating from the wall of the cylinder. The rings make direct contact with the wall of the cylinder and need the oil coat to lubricate and cool.
A washed cylinder will cause the rings to wear prematurely and possible cylinder damage too.
4 Burnt Valves
Your engine’s valves open to allow fuel in and spent gases out. When closed, they create a tight seal. Extra gas that hasn’t been burnt and hasn’t slipped past the rings to the crankcase gets expelled out the exhaust valve. The constant washing of gas past the valve and seat causes the face to erode.
A worn valve face doesn’t create a good seal, and engine compression drops. The only fix here is to replace the valves.
5 Causes Backfiring
Backfiring is a loud bang from the exhaust of your bike. It’s caused by the un-burnt extra gas in the cylinder, which is pushed out the exhaust valve (as mentioned) and then ignited by the hot manifold or exhaust pipe.
Consistent backfiring can cause damage to your motor. Explosions are designed to occur inside the combustion chamber.
Valve and exhaust damage is usual if the root cause isn’t fixed.
Engine Won’t Run Without Choke
An engine that will only run if the choke is partially on is a very common issue. It means your engine is running lean (lacks gas), and as you’ve learned, applying the choke adds gas and that’s why it runs better with coke on. The problem is, it’s an imprecise solution and although, your bike needs the extra gas, adding choke will likely at times add too much which as you already know causes other problems.
The root cause of the issue is most likely your carburetor, it’s likely partially blocked. Your carburetor isn’t hugely complex but is a precision instrument.
Even a little grit can block important fuel passages or orifices. Your carburetor employs brass hollowed-out screws with very precise holes to add just the right amount of gas for a given throttle demand. These hollowed out screws are known as fuel jets.
As the jet holes are tiny, they block easily. Your bike will likely have two jets, maybe more. Each jet is responsible for different fuel circuits.
- The Pilot jet is responsible for idle to 1/4 throttle circuit.
- Needle jet is responsible for 1/4 to 3/4 throttle circuit.
- Main jet is responsiple for 3/4 to full throttle circuit.
If your bike won’t idle, it’s likely the pilot (aka idle) circuit that’s causing the problem. The most common cause is a plugged-up pilot jet. Removing the pilot jet and cleaning will fix the issue.
To remove the pilot, you’ll need to remove the carburetor fuel bowl and remove the pilot jet. Some carburetors are a pain in the ass to access, but some can be rotated enough to gain access.
The jet unscrews, but use a good fit screwdriver. Jets are made from brass which is a really sodft metal and easily damaged. Use a strand of fine wire (strip some electrical cable for fine wire), and probe the jet. Don’t be tempted to bore the holes bigger, that will cause you no end of problems.
The main jet lives right beside the pilot and could likely do with some love too. As an alternative, go ahead and take the carburetor off the bike, strip the carb and clean it in an ultrasonic tank. It’s worth the extra effort and you’ll notice a big lift in performance.
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