Motorcycle engines are relatively simple. You add fuel, ignite it, and the engine generates power. Add more fuel, and the engine gives more power.
But what causes an engine to stutter and sometimes stall when you try to open the throttle?
There are three leading causes
- Lack of fuel
- Too much fuel
- Faulty ignition system
This article looks at the causes of a motorcycle engine stalling when you open the throttle and how to fix the problem.
Carburetors Or Fuel Injection
Most modern motorcycles use fuel injection instead of carburetors to deliver the fuel mixture to the engine. Some causes below apply to carburetor engines only, so we’ll point those out as we go along.
Reasons Your Motorcycle Dies When You Rev It
1. Blocked air filter
It’s very easy to overlook your motorcycle’s air filter. You’ll often find it hidden under the gas tank, although some carburetor engines may have them attached to the carb intakes.
A blocked air filter restricts the amount of air mixing with the fuel, and this causes a few issues, notably;
- Poor running. The engine bogs down when you open the throttle, but if you open it gently, it may pick up and rev.
- Poor fuel economy. A blocked air filter will cause the fuelling to be rich – too much fuel compared to the amount of air. You can often smell the excess fuel.
Fortunately, a clogged air filter is straightforward to cure. If you have foam, or washable filters, wash them in warm water, leave them to air dry completely, and re-oil if necessary.
You cannot wash paper filters, but you can blow the dirt out with an airline to get the bike running a little better until you can get a replacement.
2. Blocked Carburetor Jets
Despite manufacturers fitting fuel filters, it’s still possible for tiny bits of dirt to get through. The jets in your carburetor have small openings and can easily get blocked by dirty fuel or airborne debris.
Prevention is better than cure, so make sure you change the fuel filter at the specified intervals and use good quality gas.
Unblocking the jets will mean removing and stripping the carburetors. It’s not a difficult job, but you must be methodical to ensure everything goes back together correctly!
3. Blocked Injectors
Similar to blocked jets in a carburetor, blocked injectors of a fuel injection system will cause erratic engine running. Running some injector cleaner through the system by adding it to your gas tank may cure the problem. If the injector cleaner fails, you’ll need to remove the injectors and clean them.
As with carburetors, prevention is the best option, so change your gas filter and use good quality gas.
4. Air Leaks on your Carburetors or Injection System
It’s vitally important that the fuel-air mixture is correct. If the mixture is incorrect, it can cause many problems, including engine stalling when you open the throttle.
On carburetors, a common cause is the rubber manifolds that correct the carbs to the engine intake. The rubber is subjected to extremes of heat and cold, and over time, the rubber can harden and split. Tiny splits can easily let in enough air to throw the fuel mixture out of balance.
If you suspect the carburetor rubbers are the problem, you can temporarily use tape to seal them. If the engine runs better, you must check the seal between the air box and the carbs.
5. Lean Fuel Mixture
A lean fuel mixture can cause many problems and ultimately damage your engine. A lean running engine will run hotter than designed, which can cause detonation, also known as pinging or pinking.
Detonation could cause throttle response issues, but it can also damage the pistons and valves more worryingly. If you suspect a weak fuel mixture, investigate the problem as soon as possible.
6. Fuel System Failure
Modern engines have sophisticated engine management systems. If there is a problem with the fuel system, the engine management may restrict the engine’s power. They often refer to this as limp mode, where the management system tries to prevent engine damage.
Diagnosis of modern engine systems will probably require the use of a plug-in fault code reader or using the built-in diagnostics codes. Failures in the gas system could include a faulty gas pump, gas regulator, or another monitoring component. The onboard diagnostics will be handy here.
7. Water in the Fuel
Bikes in regular use rarely suffer from water in the fuel unless there is a leak in the gas tank. However, if you store your bike over winter, it is common for condensation to form inside the tank and drip into the fuel.
The easiest way to prevent this is to fill the tank to the brim before storing it, but you may have the problem of the fuel going off during the winter. The alternative is to drain the tank and fuel system, store the bike in a dry location and allow the tank to vent.
If you suspect there is water in the fuel, drain the tank, drain and blow through all the fuel lines, and allow them to dry thoroughly. Fill the tank with fresh gas and test the engine to check you’ve cured the problem.
8. Choke Permanently On
Carburetor engines have a manual choke lever that makes the fuel-air mix richer to allow easier starting in cooler conditions. The Engine Control Unit (ECU) controls the mixture of fuel injection engines.
Too much gas in the mixture will cause the engine to “bog down” and stall as you open the throttle.
The manual choke lever uses a cable; over time, these will dry out and may stick. Either lubricate the cable or replace it to cure the issue.
On injection engines, you will need to have the ECU checked unless you can find a working ECU to borrow and swap out to check if it is working.
9. Blocked Fuel Filter
A blocked fuel filter will restrict fuel flow, so when you open the throttle for more power, the engine cannot provide it and stalls.
Fuel filter positioning varies from bike to bike, with some inside the gas tank and others fitted externally.
External filters are simple and cheap to replace, but internal filters can be difficult and expensive to change. Owners of motorcycles with gas tank filters often bypass them by fitting an external filter.
10. Blocked Gas Cap Vent
A blocked gas cap is a relatively rare problem, but if you have stored your motorcycle for a while, it may be a cause.
Gas in your fuel tank expands and contracts as the fuel heats up and cools down, and the vent prevents a vacuum or pressure buildup.
While riding, the fuel level reduces, of course, but a blocked vent will cause a vacuum to form in the tank, which could prevent fuel from getting to the engine.
11. Ignition Timing
Correct ignition timing is critical to the smooth operation of your engine.
Advanced ignition causes a loss of power and possibly knocking, as it ignites the fuel before the piston has reached the top of its travel. The ignited fuel slows the piston down as it tries to reach the top-dead-center (TDC).
Retarded ignition occurs when the piston has already started traveling back down the cylinder, igniting the fuel too late. A loss of power and poor fuel economy are symptoms of retarded ignition timing.
Modern motorcycle engines have electronic control units (ECUs) that ensure accurate ignition timing, which is generally non-repairable, and you must replace it if faulty.
Older motorcycles used breaker point ignition, a mechanical system that switches power to the ignition coil at the correct time. A breaker point system requires careful setup to work correctly, and over time, the contacts of the breakers wear away, and you must replace them.
12. Low Compression
An internal combustion engine relies on a good cylinder seal, ensuring all the explosive power of the ignited fuel pushes the piston down. Low compression can have several causes, but the main problems are gasket failure or poor seal between the piston rings and the cylinder bore.
You must dismantle the engine and replace the damaged parts in both cases. You can replace failed gaskets; fitting oversized piston rings will improve the seal between the piston and bore.
If the cylinder bore is damaged, a re-bore may be required, with oversized pistons and rings.
13. Weak Ignition
Your motorcycle ignition system needs to be in good condition to ensure a strong spark in the engine. Replace your spark plugs at the specified interval, check the ignition leads for any damage, and ensure a good connection between the spark plug and the ignition coil.
If you suspect this is the problem, spark plugs, ignition leads, and coils are easy to replace.
14. Unbalanced Carburetors or Throttle Bodies
The carburetors or injectors must feed the correct and same amount of fuel to each cylinder. Different amounts of fuel in individual cylinders will cause rough running and poor fuel economy.
Balancing a set of carburetors requires a set of vacuum gauges, so it may be a job better left to a motorcycle mechanic. Throttle bodies can also be balanced using vacuum gauges, but usually, you use an electronic device connecting to the ECU. Again, probably a job for a motorcycle mechanic unless you want to spend money on the correct kit.
We included a range of possible causes for your motorcycle engine dying when you open the throttle. Some are obvious, and others may cause different issues as well.
If you’re a confident mechanic, you can work through most causes quickly and hopefully find the fault. The more complicated problems may be better left to a trained mechanic.