Why Is My Motorcycle Idling So High – (Boom Vader/Grom Clone)

Go to any FB group or motorcycle forum, and you will find this question asked so often, why is my motorcycle idling so high? 

I’m sure we all start spouting out all our ideas, trying to help out the original poster, but I take time to read around their question. Often before the motorcycle started having such high idle rpm, something was done to the bike first. If so, I check for upgrades, was the throttle replaced, was the carburetor upgraded, or was there some engine tune-up performed?

Boom Vaders (Grom Clones) have carbureted motorcycle engines, which can be very susceptible to lousy tuning or bad reassembly following tuning.   Resulting in the motorcycle idling high.

So let’s get right down to my high-rpm troubleshooting process. I learned from my own mistakes and others.

Brief List Of Causes For Engine High RPMs At Idle.

Hoping this quick summary will spark an idea of what might be wrong with your motorcycle so that you can focus you think is wrong and save time fixing it.

1.- An Engine Air Leak / Vacuum Leak.

Leaking air into an engine will provide more air(possibly fuel too) into the cylinder and cause higher RPMs. This ‘uncontrolled’ air will cause a runaway condition.

2.- Idle Speed Control / Idle Screw Adjusted Too High.

The idle screw holds the throttle body/valve open during a no-load condition to maintain a minimum speed when there is no throttle demand on the engine. This idle screw limits how far closed the throttle valve/slide can go close.

3.- Carburetor Throttle Valve Stuck Open.

An open throttle valve/slide allows too much air into the engine at idle conditions(no throttle demand), which can lead to higher-than-expected RPMs.

Did These Basic High Idle Condition Causes Tip You Off?

If so, go directly to that section.

• Fixing A Motorcycle Engine Air / Vacuum Leaks.
• Fixing An Idle Speed Screw Adjusted Too High.
• Fixing A Stuck Open Carburetor Throttle Slide / Throttle Valve.

Fixing A Motorcycle Engine Air/Vacuum Leaks.

It sounds worse than it is; finding an engine air leak or vacuum leak is easy.

The idea is to disrupt the air/fuel mixture and listen to a change in the engine RPMs. This is quickly done by introducing additional fuel to the air/vacuum leak.

With the engine running, use a can of starter fluid(or carburetor cleaner since we have those for these little motorcycle engines) in short bursts, and spray in areas where the suspect and engine air leak.

Since the Boom Vader, like its Grom Clone Cousins, has minimal parts(i.e., No vacuum hoses) spraying around the carburetor body, the engine intake would be the most likely area for an air leak.

If you spray and the RPMs change, it’s because you are temporarily disrupting the air/fuel mix ratio and changing the engine output. Using the spray and listen technique should help quickly identify and locate any engine air leak you have.

Find the leak, and fix it. Nuff said.

Fixing An Idle Speed Screw Adjusted Too High.

This engine problem should be even easier than the vacuum leak problem. A carburetor idle speed screw is designed to be adjusted.

Too often, we tune our motorcycle engines to run at a certain RPM, then make upgrades or adjustments to restart our engine and find the RPM has changed.  

After an upgrade, the RPMs are often higher due to us upgrading the engine’s performance or power.

So slowly adjust the low-speed idle screw outward till you lower the engine RPMs to your goal.

Of course, if you make the engine idle RPMs go too low, you can turn the idle screw inward to bring the RPMs back up.

This fix is easy; slowly turn out the low-speed engine idle screw on the carburetor. Best to make these adjustments when the engine is warmed up, or be prepared to re-adjust on your next ride.

Note:  If your throttle seems to ignore your idle screw adjustments completely, then the throttle slide must be stuck in a more open position. This is typical of a carburetor throttle slide turned backward on installation. Check to see if your carburetor throttle slide is backward.

What RPM Should The Boom Vader Motorcycle Idle At?

On the right side of the swing arm is an emissions plate. On my plate, it states the idle speed should be at 1500 RPMs.  

The nice thing about the Boom Vader, unlike my first bike, the TaoTao TBR7 dual-sport Chinese motorcycle, the Boom Vader has a tachometer. Using the tachometer, adjust your Boom Vader to the recommended RPMs.

Fixing A Stuck Open Carburetor Throttle Valve.

Now with the Boom Vader, like standard carbureted motorcycles, a lot depends on the throttle valve(slide) position. A little open too far, and the engine RPMs are too high.

So what can cause a stuck open throttle slide on a Boom Vader?

To do these checks, best to be able to visualize the inlet side of the carburetor, so remove the air filter or air inlet to see what is going on.

Some quick checks and solutions for stuck carburetor throttles.

1.- Inspect The Throttle Cable Jacket For Pinched Sections.

If your throttle cable is pinched, like between the frame and the gas tank, the throttle cable will not slide easily. The throttle slide spring might not be able to close the throttle valve/slide fully. Check for pinches.

Looking at the throttle valve/slide, open and close the grip throttle. Looking from the inlet, did the throttle slide/valve move open and close fully and smoothly?

The throttle cable might be pinched, or another reason is that dirt inside the jacket creates a lot of friction.

Check for no pinches by grabbing and rapidly moving the throttle cable jacket, ensuring no pinches. Note, The throttle cable jacket should be secured to prevent hangups but not so much can pinch and squeeze the jacket.

Loosen up fasteners that are too tight, or route throttle cable through areas where it won’t be pinched.

2.- Motorcycle Throttle Cable Binding.

If the throttle cable is not pinched, but the throttle valve/slide is difficult to open and close via the hand-grip throttle, maybe your throttle cable is dirty.

Pull the cable, clean the cable, and lube with a good throttle cable lube.

3.- Carburetor Throttle Valve/Slide Dirty

Check for free movement of the throttle valve/slide with your hands. The throttle body should be clean, and the throttle slide should move with almost no friction, so the closing spring has very little work to close the throttle slide.

Clean the throttle body as needed.

4.- Throttle Cable Is Being Pulled On By Handle Bars.

This happens, although rare, if you watch how you route the throttle cable if replaced.

Looking at your throttle valve/slide, turn the handlebars to the left. Did the throttle valve/slide move?  

Looking at your throttle valve/slide, turn your handlebars to the right. Did the throttle valve/slide move?

If the throttle valve/slide is moved, the throttle is being inadvertently pulled when you move your handlebars.  

Easy fix, look for areas where the throttle cable might be wrapped around the handlebars, or check if the throttle cable length is long enough for your motorcycle. Fix as needed.

5.- The Throttle Slide Is Backwards.

This is a duh error and, too often, a much-overlooked problem.

The throttle slide on a typical Boom Vader motorcycle is round. There are grooves in the slide to allow the insertion of the throttle cable, and the same groove is used to prevent the throttle slide from rotating in the throttle body.

A tiny projection on one side of the throttle body rides in the throttle slides cable groove, allowing the throttle slide to move up and down but not rotate.

Opposite the throttle cable groove is a little notch, like a ramp and inclined plane. This notch is where the carburetor’s idle speed screw rests and acts against the throttle slide. This idle screw position will hold the throttle slide open to set the idle speed of the motorcycle.

I hope I’m explaining this correctly.

If the throttle slide is inserted backward, 180 degrees opposite, the idle speed notch on the throttle slide will catch on the throttle slide cable groove projection, holding the throttle slide open.

An open throttle slide will allow too much fuel/air into the motorcycle engine and cause high RPM conditions.

Simple fix, rotate the throttle slide such the notch is on the same side as the carburetor’s idle speed screw, and insert. Watch that the throttle slide inserts properly.

Another tip: If unsure, the idle speed screw will not affect a backward throttle slide. Just keep that in mind.

Read more about my TBR7 motorcycle throttle slide backwards post: High RPMs! Throttle Slide Backwards? Fix it: TBR7/Hawk 250

Conclusion:

If any causes of your motorcycle engine’s high RPMs were caused by the items I listed above, you should be happy. These were items that you can take steps to fix that high idle quickly.

Now I’ve had the dirty/stuck throttle cable problem and the throttle slide backward problem too. Other problems I put together from listening to others’ issues and testing how they worked on my Boom Vader.  

So if I missed another motorcycle engine with a high RPM at an idle problem, please leave a comment below. I’m constantly learning and hoping to share my newly gained knowledge with others here.

Ride Safe, Ride Fun!!!

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Boom Vader Gen 2 Upgrades