Suzuki GN125E carburetor tuning, high idle and stalling

I ran into some common carburetor problems after buying a used Suzuki GN125E. The bike sat for a year or more. The carb was getting lacquered up and the gas tank was getting rusty inside. The bike would stall at idle. Only ran good with the choke on or keeping the throttle open a little.  After adjusting the idle screw the bike wouldn't stall, but if I revved the engine the RPM's would stay high for a while even after closing the throttle.

I discovered this little engine is finicky which gives people trouble. On other forums there seemed to be mixed info and GN owners were getting frustrated  over fairly ease problems to fix. If someone is searching for help with a GN125 carb, idle mix or carb tuning, hopefully my tips will help.

Read on...

Linkback: http://motoatak.com/forum/index.php?topic=8.0

About the GN125E Carburetor

The GN125E carb is vacuum operated. This means your throttle cable is not attached to the slide/cylinder in the carb like most small, older motorcycles. The throttle cable is attached to a butterfly valve forward of the slide. Turning the throttle opens the butterfly valve. Because the engine is running and causing vacuum, when the butterfly valve is opened, vacuum is created in the carb.

A round diaphragm is attached to the top of the slide. Due to vacuum created in a chamber at the top of the carb the diaphragm will draw the slide up. A spring causes it to return when the butterfly valve is closed and there is no more vacuum.

You may already be aware that when the slide rises more fuel and air is allowed to enter the engine, hence acceleration.

Any amount of air leakage, change in air filter or filter box, will cause a change in vacuum and cause the engine to operate poorly. Too much air getting in will cause the slide not to rise. Not enough air, easily caused by a dirty filter, will cause excessive vacuum and too much fuel being drawn into the engine. With such a small combustion chamber the proper mix of fuel and air is  crucial. The GN125E has a sensitive engine.

The basics of getting your carburetor tuned

• Non-modified airbox and filter with clean, lightly oiled, properly fitted air filter.
• Fresh, clean fuel, clean fuel tank, clean fuel filters (if any).
• No air leakage around the rubber carb manifold/tube or between the carb and airbox. No leakage around the butterfly valve shaft.
• Clean and serviceable parts throughout the carburetor. Pay close attention to pilot and main jets and slide diaphragm. The diaphragm attached to the slide in the middle of the carb must not have any pinholes of tears.
• A non-modified, serviceable exhaust tube and muffler.
• Regarding the engine, properly adjusted valve lifters, good spark plug and good compression (good piston, rings and cylinder).

Here's a diagram to see the carb parts:

Sorting out problems with the GN125E carburetor

If your Suzuki GN125 isn't running well, follow the steps to save yourself from a huge headache.

• Do not make adjustments to the carb. The goal is to fix the problem, not cause more problems.
• Inspect the air filter. Clean it with soapy water if necessary. Let dry, then oil it lightly and put it back in. Replace if deteriorating.
• Inspect the airbox itself. The air opening is rather small. Leave it that way. There should be no extra holes in the airbox and the box cover, with the arrow on it pointed up, should be in place. If not, you're getting way too much air.
• Next, inspect the spark plug. Clean with a wire brush or replace it.
• With everything back together, test your bike.

With any luck your problem may have been solved. If not, proceed to the following:

Testing for air leaks

Testing for air leaks is easy, but it does require that the engine is running.

• While running spray DW40 or starting fluid in short bursts on the front tube that connects the carb to the engine. Concentrate the spray around where the tube meets the engine. If the engine revs RPM's change, you have a leak. There is an O-ring between the tube and the engine that you will likely find hard, flat and possibly even cracked. I did! Replace it or use high temp silicone to seal the area off.
• Also spray on the main rubber area of the tube. A change in RPM's indicates the tube itself is cracked or torn. It would need to be replaced.
• Before you do this, spray more directly right around where the butterfly valve shaft enters the carb. Each side of the carb has a special seal the shaft fits through. Those seals are a harder, black plastic and may allow excessive air around the shaft. If you notice a change in RPM's, you should replace those little seals, or do as I did and pack the seals and around the shaft with grease to block any airflow.
• Some GN125E models have a EGR system with a hose leading from a black box on the right side frame to the carb. Be sure the hose is serviceable and that it isn't missing entirely. If missing, you can get your carb operational by plugging the port where the hose fits onto the carb.
• That's all at the front end of the carb. Now spray around the rubber that connects the carb to the airbox. Fix any issue you detect. This rubber can easily slip off the airbox and may simply need to be reattached.
• With all leaks fixed (if any) are you running correctly? Can you idle at 1,000 to 1,300 rpm? Turn the idle speed screw (adjustment knob on left side of carb) counter clockwise to drop your idle. If you cannot obtain proper idle or proper revving of the engine without a long delay or excessive RPM even after returning the throttle, there are more problems to sort out.

The important aspect above is that you corrected any leaks around the carb. NO LEAKS! I can't stress it enough.
If you detected a leak around either end of the butterfly valve shaft, you'll need to tear the carb apart to replace the seals or pack with grease.

If you're still not running right, now we'll get into cleaning and adjusting the carb.

Hi Taba. Is the second part about cleaning and tuning the carb available please?
Great post and with fantastic detail. I have a carb problem now. 
How finicky is the pilot jet size. I reamed it out with a slightly larger drill bit as I could not clear it.  It has been parked for over a year.
There is I believe an air screw towards the front wheel on the idle adjust side. With the larger pilot jet should I turn it in or out. In the past it sat at 2.25 turns out.

Do to the size of the engine (on the Suzuki GN125), the pilot jet size needs to be pretty close to original size. If you didn't notice any brass getting reamed out when you used the drill bit, then you're probably ok. But that would have been a really tiny drill bit. If you think you made the pilot jet bigger, but not too much, then you SHOULD be able to compensate by adjusting the idle air/fuel mixture screw as you suggested.

If your pilot jet is now letting more fuel in than stock, then you'd want turn your idle mix screw IN or clockwise just a little, to cut cut the mix back down. Turning in 1/8 turn (.25) may do it. Depends on how large you made your pilot jet. If it were me I'd do the following:

1. Use fresh gas only and drain the bottom of the carb. Make sure you have spark and the the spark plug is clean or new.
2. Get the bike started and able to idle at 1000rpm or as close to that as you can. Leave that idle mix at 2.25 turns out. Let it warm up.
4. Twist throttle a little and let it back. If the idle drops back down, good. If not, or if it stays high for a long time before dropping back down, there is probably an air leak (see previous post) or the idle speed adjustment is screwed in too far (throttle valve open too much). Back it off and try again. Once you have throttle/idle under control, continue.
5. Turn the idle mix IN 1/8 turn. Wait 10 seconds. Did the engine speed pick up? Or no change. If no change, repeat. If the idle speed starts going down, then turn the idle mix OUT 1/8 turn. Wait 10 seconds. Repeat. If it starts getting bad going the other direction, put it back to where it was best. You want the engine running as smooth as possible (you have to listen very carefully) and the engine speed should pick up a little. That is okay.
6. Each time you get the engine running smooth, adjust the idle speed adjustment out to get closer to 1000rpm. The goal is to have the butterfly valve (throttle valve) nearly closed (but you can't see it with the air filter on unfortunately) so that the engine runs off the pilot jet and idle mixture circuit entirely and NOT THE MAIN JET.

Goals to obtain:
1. Throttle Valve hardly cracked open at all, just a tiny bit.
2. Engine running smooth at idle, 1000rpm.
3. Spark plug looks clean or light tan after running idle for a while.
4. When you blip the throttle, the engine speed should pick up and drop back down quickly. Once you get the idle going well, if you open the throttle and get sputtering, backfire, or engine speed stays up too high for too long... there are more problems to sort out.

If you can't get it, I would look into cleaning out the entire carb and all circuits/passages. Also buy a cheap carb rebuild kit so that the fuel inlet valve and jets are new and clean. Check the adjustment on the float. If your float is closing off the fuel too soon.... your engine is starving. Make sure you spark plug is clean or new.

Do you have the manual?

Believe it or not, you can buy a new carburetor for $25-$30 on Amazon here: http://amzn.to/2l5e7CA
This is what I did when I decided to customize my girlfriend's GN125. I coated hers with a ceramic, faded gold color. See the photo.

I took out quite a bit of brass. I never thought it would be so sensitive.
I ended up picking up a totally new Carb for 47 US in Ecuador ( bike is in northern Peru )

Just a question. With the pilot so darn small it's hard to find anything that will pass into it.
I boiled it. No luck.
With the new carb it's fine but I am sure it will plug again. I leave it sit for a year.
This time I drained the carb hoping that will help.

A little trick is to take some stranded wire and pull out one strand. Use that to poke through the jet. I've done it many times. You can even rough the trad a litle with sand paper first so it will scrub the jet out a little better.

Or get one of these tools that will ream/clean jets and passages in the carburetor. One of these things: http://amzn.to/2kUXej6

When you store your bike for longer than a month, turn the gas off and drain the carb. Your bike will go for years without getting plugged. You can also add a little gas treatment to the tank or drain the tank.