Snap throttle engine stalling, but bike runs fine

[Matt6288]

Hello,

I've got a brand new '24 Hawk 250 with less than 600 miles on it, and I installed a brand new PE28mm Nibbi carb to upgrade it. I didn't notice this for the first few rides but not long after upgrading the carb, I noticed a weird problem.

The bike idles and runs perfectly. On the highway, it has healthy acceleration and will reach top speed no problem. If I rev the engine gradually up to full throttle, it revs up just fine. But if I snap the throttle, from closed to fully open instantly, the engine stalls.

Now I have no prior experience working on carburetors, but I'm learning. I've read lots of things and watched lots of videos, and have been playing with my adjustment screws, but no matter what I do, the problem isnt going away.

Any suggestions?

[Dusman]

This can be normal with carbs, especially those without accelerator pumps and are a “round slide” type of carb, which the Nibbi PE28 is. You may be able to remedy this with going up one size on your pilot jet, as an off-idle bog is usually caused by a pilot jet that’s too lean. If your bike pops a lot on deceleration, then you definitely have a lean pilot jet and I would look there first.

[red2003]

Raise the needle (drop the clip). This is the most likely cause. If you don't know what that means, there are tons of videos on the tube to show you how to do it.

[Thumper]

I jetted a bike too rich and had those symptoms. It tolerated gentle changes, but coughed with a quick blip.

Take a look at your plug. Blackened?

[Matt6288]

it looks pretty normal to me, but again, I'm no expert.

Also, I've never heard any deceleration popping. But if I had a rich condition, shouldn't the problem go away if I were to back the air screw way out? No matter how I adjust it, whether obviously way too far out or in, the engine still dies with a snap throttle.

[JerryHawk250]

The idle mixture screw only effects the idle mixture. As red2003mentioned. You might try raising or lowering the needle to see if that clears it up.

[Thumper]

it looks pretty normal to me, but again, I'm no expert.

Also, I've never heard any deceleration popping. But if I had a rich condition, shouldn't the problem go away if I were to back the air screw way out? No matter how I adjust it, whether obviously way too far out or in, the engine still dies with a snap throttle.

It seems sooty, which suggests too rich.

If the airscrew is on the intake side of the carb body (engine side), it will probably enrich when you open it (counterclockwise). If the airscrew is on the air filter side of the carb body, it is likely a true airscrew (opening it leans out the mixture).

From there, try returning the air screw to ~1.5 turns out. If that leans it out (based on the above), maybe it will behave a little better on quick turn. If you still have the basic symptoms, I'd lean out the needle setting (drop the needle a little, or move the clip up).

You will have to do minor adjustments and observations to figure out what's going on. But that plug definitely has black soot on it. If it was too lean, it would have a white powdery deposit.

[ProDigit]

You can either lower the Jet Needle, or turn in the AF screw (less fuel).

It's typical for carburetors to when they work stoic at a fixed speed, that they add more fuel when opening the throttle. This causes a perfectly stoic mixture, to still run rich during acceleration.

[bigdano711]

Try cracking the throttle to half instead of full and see if you get the stall. This will point directly at the needle. I also have a Nibbi PE28FL and have battled this condition. I tried every position with the stock needle, no noticeable change at all. I should mention I am at 6000+ elevation. I had ordered a needle/jet kit from Amazon, mostly going for the needles. I did not realize they were Keihin clones. I read some forum posts elsewhere about how the JJH needle did wonders, so I tried it. I still have the issue of it stalling and have only tried the middle position and maybe one other, can not remember. Got tired of pulling the carb off/on/off/on, etc. I recommend the JJH needle.

[Megadan]

You can either lower the Jet Needle, or turn in the AF screw (less fuel).

It's typical for carburetors to when they work stoic at a fixed speed, that they add more fuel when opening the throttle. This causes a perfectly stoic mixture, to still run rich during acceleration.
If he turns in the air screw, he gets less air. Nibbi PE carbs idle mixture is an air screw.

Also, if the idle mixture is after thr slide, it's a fuel screw.

To OP. reading the plug only tells you about the fueling under that specific throttle position. If you need to check fueling of a specific jetting scenario a plug chop in that throttle position is necessary. Your plug shows a healthy side of slightly rich, which is great, but that is likely only telling us about the idle mixtures.

[Notjoemechanic]

Aside from plug readings there is another way to help you diagnose the difference between a lean or rich bog. lf snap the throttle and it acts like you hit the kill switch or has a hollow sounding drone before the rims pick back up then it is likely lean. If it chokes, pops and sputters then it is likely to rich. Even if your pilot and main jets are good you may need to move your needle clip up or down to get your mixture correct at part throttle just comming off of idle. It can be a long process getting it tuned just right because the pilot, needle and main jet all overlap and one can affect the other. Typically the pilot will mainly affect idle to 1/4 throttle, the needle will affect 1/4 to 1/2 and the main jet 1/2 to full. Like I said though they all overlap and affect each other. Some carbs are more complex than others but this is a good place to start with these simple carbs. I would start on the rich side and lean it back.

[bigdano711]

That's why I recommend the JJH needle with the Nibbi carb specifically. I had a 40 pilot and I believe a 110 main. I tried all the clip positions on the stock needle, no noticable change from one position to the next. When I put the JJH needle in and tried to tune the air mixture screw as well as the idle, no matter how far I went to the lean side I couldn't get the idle to come below 2000 rpm. Clear indicator I need to come down a size on my pilot. Went to the 38 pilot and a 108 main, she's running pretty good. I will get some new video up soon.

[Notjoemechanic]

Another piece of basic tuning information I forgot to mention and is easily overlooked or forgotten about. The first thing to check before tuning your carb is to make sure you have the proper fuel level in the bowl.

[zero_dgz]

...And that it ain't half full of water from repeated riding and parking in the rain, plus bike washes. I just that that problem, in fact.