Having Carb Related issues?

I’d say probably 90% of machines that come into our shop with running problems, is down to the carburettor malfunctioning.

Carbs are a way to mix the correct amount of fuel, with the correct amount of air, and pass this through to the cylinder via in intake valve. As the piston begins its downward travel following the exhaust stroke, vacuum is created in the cylinder. This vacuum sucks the fuel from the opening intake valve, ready for the compression stroke.

If a carb is out of balance, or partially blocked, this will lead to the incorrect fuel/air ratio being fed to the engine and causes all sorts of running issues. Thankfully, with enough knowledge, you can start diagnosing running issues based on the symptoms.

Lean condition

An engine with a lean condition, means not enough fuel, or too much air. A machine running in this way will run hotter than normal, and tend to pop or backfire. In extreme cases, the exhaust muffler may glow a cherry red, and can also cause the piston to melt down or seize. This is cause for concern, and should be rectified as soon as possible.

Left shows the condition of an engine running too rich. Right shows the condition of an engine running lean.

Possible causes include:

• Blocked carburettor
• Vacuum Leaks between carb and cylinder head (typically along inlet manifold)
• Clogged fuel filter
• Clogged Injectors
• Broken fuel lines
• Poorly functioning fuel pump
• Fuel tank blocked
• Decreased fuel consumption

Quick Tips: Check spark plug condition, if it has a white tip, it’s running lean. Check exhaust for excess heat, or glowing red. Is your engine “hunting”, (revving up and down) The term hunting, means it’s calling for more fuel.

Rich Condition

An engine running rich, means it’s getting too much fuel, and not enough air. This can cause your engine to bog down, and blow unburned fuel back through the carb, or even out of the exhaust. When an engine runs rich, it can also lead to a build up of carbon deposits in the mechanical valve system and cause them to lock up, especially when low on oil. If you remove the cylinder head, and find excessive carbon build up on the top of the piston, or around the valve seats, this typically means the engine is running too rich. An incorrect air/fuel ration with rich conditioning, can also cause the engine to smoke (Black) excessively. Running an engine with slightly rich condition won’t do as much damage as an engine running lean. You’ll also typically notice a decrease in fuel economy, however may also provide slightly more power. Many “experts” drill out jets in a bid to get more fuel into the combustion chamber. Whilst this presents risks, if done properly, can make an engine run faster, whilst causing no damage to the engine. If too much fuel is present in the combustion chamber, this will cause the engine to flood out, and bog, and eventually die.

Possible causes include:

• Wear inside the carburettor, typically in the jetting.
• Carb not set correctly for optimum air/ fuel ration
• Worn Injectors
• Faulty fuel pressure sensors or fuel pump computer system

Quick Tips: Check spark plugs for black sooting. Engine bogging under acceleration. Black smoke from exhaust. Poor engine performance. Fuel smell from exhaust. Decreased fuel consumption.

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