A little choke helps your engine start from cold, some bikes just seem to run better with the choke on permanently. But that’s your bikes 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 how to fix it.
1 Engine Runs Rich
Your carburettors job is to mix gas and air together to a precise ratio of 14.7 parts oxygen to one part gas. This is the ideal ratio, it’s the mix that produces the most power and burns more complete, 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 normalise, 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 negative effects on your engine, some of them can be serious. we cover them in more detail below.
2 Dilutes Oil
A rich running engine, has a hard time burning all the gas. Instead, some of the excess gas makes it 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 watery and stink of gas.
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 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 engines 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 and then ignited by the hot manifold or exhaust pipe.
Consistent backfiring can cause damage your motor, explosions are designed to occur inside the combustion chamber. Valve and exhaust damage is usual, if the the root cause isn’t fixed.
Engine Won’t Run Without Choke
What if you have your choke on permanently because the engine won’t run without it.
That’s a common problem, it means ironically that your engine is running lean (lacks gas) and as you’ve learnt applying the choke adds gas.
If your engine won’t idle without choke, it’s because your carburettor is likely blocked by dirt.
Your carburettor isn’t hugely complex but is a precision instrument, even a little speck of grit can block important fuel passages or orifices.
Your carburettor uses brass hollowed out screws with very precise holes to direct the right amount of gas, they’re 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 which is responsible for idle to 14/ throttle circuit.
- Needle which is responsible for 1/4 to 3/4 throttle.
- Main jet which handles the fuel for 3/4 to full.
If your bike won’t idle, it’s 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 carburettor fuel bowl and remove the pilot jet.
Some carburettors are a pain in the ass to access, but some can be rotated enough to gain access.
The jet simply unscrews, use a strand of fine wire (strip some electrical cable for fine wire) and probe the jet.
The main jet lives right beside the pilot and could likely do with some love too. As an alternative, go ahead and take the carburettor off the bike, strip the carb and clean it in an ultrasonic tank. You’ll notice the difference in performance.