You’re in the saddle, all geared up and ready to rip.
The bike started just fine; you rolled it out the driveway, it’s idling like a champ, but as soon as you hit the throttle… the motorcycle won’t accelerate.
It’s one of the most mind-boggling things to figure out, which is why we put together a quick list of the 7 most common reasons a motorcycle idles but won’t accelerate.
Table of Contents
1. ECU/CPU Control Unit Needs To Be Flashed (Fuel Injected)
This is a rare problem, but fuel injection is handled by an Electronic Control Unit on some modern motorcycles. Grossly simplified, you hit the throttle, and, whether by physical cable or electronic wire, your throttle input triggers the injector to fire for acceleration.
If your fuel injection programming is out of date or corrupted, your bike could start just fine and get to the idling stage, but when you hit the throttle, you find your acceleration hindered.
If this is the case, you’ll have to get your ECU flashed by techs equipped to interface with the ECU on your particular make and model bike.
2. Engine Timing Is Off
If you’re experiencing a lack of acceleration on a fuel-injected bike that started and is idling fine, the most probable culprit is a poor timing advance.
The quicker you hit the bricks on your bike, your ignition coil is firing your spark plugs more often.
By that same token, at slower speeds, your bike fires less often.
This harmonious spark plug-firing is governed by your engine timing method.
- On a bike with engine timing that’s out of whack, your spark plug firing gets held up, so when you hit the throttle to accelerate, your plugs don’t fire.
- If this is the culprit behind your lack of acceleration, you’ve likely noticed backfiring by now.
- Bad timing advance is generally the fault of a bad cam in the mechanical timing system. Otherwise, it’s the cause of the electrical system.
- Note: It could also be a symptom of bad ECU programming on more advanced bikes; see the previous section.
Restoring proper engine timing on a motorcycle is no job for a novice home mechanic. It’s an incredibly technical process best left to mechanics that are literate to your specific make and model.
If you insist on doing it yourself, you’ll need specific tools and possibly a way of interfacing with your bike’s CPU.
3. Throttle Cable Has Excess Free-Play
Your bike’s throttle either connects to a wire and if so, see the section above or a cable. On bikes with a carburetor, the throttle cable runs down from the throttle and to the carb.
It manipulates the butterfly valve based on the rider’s input, opening and closing the valve to speed up or slow down the bike by letting in more or less air.
If the throttle cable has too much free-play, mainly where it hooks up to the carb, the lack of tension prevents the cable’s tug from opening the valve.
The uneven ratio of air to fuel that happens due to the excess free-play can kill your acceleration.
With the bike off, remove the airbox. Now inspect your butterfly valve’s motion while a buddy twists on the throttle. Twist it all the way open; you’ll pretty quick if you’ve got too much free-play in your throttle cable, as your input should open your butterfly valve in sync with your throttle.
There’s a tightening nut if your butterfly valve is out of sync. It’s generally located above the connector at the end of the throttle cable, where it connects to the carb. Tighten that tightening nut until there’s no slack and the valve’s opening sync with the throttle position.
If they slack the cable past the point of tightening, you’ll have to replace the throttle cable to restore your acceleration.
4. Vacuum Air Leak
Another reason your bike might be riding but not accelerating is if the air is leaking into places it doesn’t belong.
Whether it’s broken intake boots or bad carb gaskets, air leaks will hinder your acceleration for sure.
- If you’ve got a vacuum leak on your air, your air/fuel mixture is likely off-kilter, meaning you’re getting too much air in place of the correct dose of fuel.
- So, when you’re jamming on your throttle, attempting to accelerate the bike, you’re giving it air instead of fuel, meaning you are losing power.
- Inspect your carb intake boots and see if they’re old, cracking, loose, or brittle; they all wear out eventually.
- Next, take a look at the hose clamps around those same boots to make sure they’re not letting any air in. Sometimes it’s as simple as replacing the air intake boots.
- Inspect the carb gaskets between the float bowl and the rest of the carb. This gasket cracks and wears in time, just like the intake boots. It needs to be airtight and in good condition to keep air out. If the seal seems broken, replacing the gasket might be the ticket.
- Some bikes have a vacuum port that runs from the top of the engine to the carb. Sometimes these hoses leak.
- You might have an extra hose for an accessory on other bikes, or perhaps the part is cross utilized. If so, new hoses need to be capped off. If a hose cap pops off, your acceleration will suffer.
5. Clogged Carburetor Jet
If you don’t have an air leak and your throttle cable is in good condition, but your bike is idling but won’t accelerate, this is probably your problem.
Carb jets are the passageways that let the fuel run into your car and combine with air to power your motor with the spec air-fuel mix it needs.
Your carb has pilot jets and main jets, and a clog in either one will dampen your acceleration.
Whether it’s debris, weather, or ethanol in some harmful gas, your fuel isn’t being delivered when you’re hitting the throttle if your jets get clogged.
Inspecting, cleaning, and rebuilding your carb is part of standard motorcycle ownership if you’ve got a carbureted bike. You’ll also have to pace your jets from time to time and make adjustments when riding in different altitudes.
To clean your carb:
- Remove your float bowl; some bikes have multiple and all your jets. Use Google or a service manual for your specific make and year model moto to find where they are.
- Spray carb cleaner through your jets until you see it coming out of the other end. Blow any leftover debris out with compressed air, repeat for all jets.
6. Busted Carburetor Spring
As its name implies, your carburetor spring is a vital component for carb operation and is critical to your motorcycle’s acceleration.
The carb spring might be a small and unnoticeable piece, but it if breaks, you’ll have problems accelerating, whether or not your bike is staring and idling just fine.
- Carb springs are located in the connector piece where the carb meets the throttle cable. This spring manipulates the butterfly valve we dealt with when we were checking out the free-play of the throttle cable.
- If the spring is broken and the valve can’t open, it has the same effect we talked about earlier; your acceleration will suffer when the butterfly valve can’t open.
- The valve lets air in the same way your jets let fuel in. So, while clogged jets means too much air and not enough power, a restricted butterfly valve means you’re running rich on fuel.
- Regardless of whether your air/fuel mix is lean or rich, it causes power loss.
- Make sure a faulty carb spring is a problem by killing the bike and removing the airbox.
- Now you can look inside the carburetor, and if it is your butterfly valve opens and closes when you twist the throttle,
- Just like before, when your throttle is opened all the way, your butterfly valve should be wide opened too.
- Take your carb off all the way to replace the spring. We don’t suggest attempting to fix the spring yourself, as it’s a cheap part to replace. Just like any busted slinky, a broken carb spring is never the same again, even if you try to fix it. Best to call it dead and swap it for a freshie.
- Anytime you take your carb apart, we suggest cleaning it all out and starting fresh.
7. Carb Spring Installed in Reverse
This next bit is more common than you think, so don’t feel insulted. This honestly happens often, as even some decent home mechanics I’ve heard of think that carb springs can go on either way.
If the carburetor spring is put on backward, either the last time you rebuilt your carb or the last time you replaced the spring, as mentioned above, it will affect your acceleration.
In this case, you’d be able to start your bike and get it idling just fine, but you wouldn’t be able to accelerate when you were hitting the throttle.
If your carburetor spring is installed upside down, it will do the opposite of what it’s supposed to when you hit the throttle. So, when you attempt to accelerate, it will close the valves, and when you try to decelerate, you’ll open the valve.
See, I told you this backward little spring could be a big problem.
Once the butterfly valve is forced to close while you’re trying to accelerate, not only will the acceleration fail, you’ll experience a loss of power at times.
This one’s easy:
- Just like we did twice earlier, with the bike off, pop off the airbox and look at your butterfly valve (see, told you to read the whole article first).
- Twist the throttle, again it’s easier with a friend, and see if the butterfly valve is opening or closing along with the throttle.
- If the valve closes when the throttle is wide opened, your spring is backward.
- Remove the carb spring, inspect its quality, then flip it upside down and re-install it so that the valve opens and closes in sync with your throttle input.