Help Cylinder 1 sputtering and wetting plugs - CBR250RR

leaking needle valve?

 

Any ideas to test this?

 

Easy, take the carbs off and check if the inlet port is full of fuel. Drain the carbs, take them off, take off #1 fuel bowl and supply fuel while you have the carbs at angle such that the float fully seats the needle.

The CBR's and ZXR's run keihin carbs, from my experience on the ZXR, the float needle seats have o-rings which go hard and brittle, and leak. The rubber tipped needles can also wear.

 

My bad, it still is just the 1 cylinder. Force of habit to say plugs

Ill be working on it tomorrow so ill give an update then.

 

BIG UPDATE: [Its a bit of a read but i thought i would get all the info i could into it for future forum browsers]

To start off with I've tried every suggestion so far to no avail, I still appreciate all the suggestions though.
I had an idea that maybe Cylinder 1's balance might be out even after the carb balance with gauges. Off this hunch i decided to try to create an intentional vacuum leak by taking the fuel vacuum line off Cyl 1 and moving it to Cyl 4. I left the vacuum port of Cyl 1 open as seen in the photos below. After that i took it for a ride around the block and for the first time since owning this bike it hasn't sputtered or fouled the plugs which was an amazing experience. I could feel it was running lean as hell but it wasn't sputtering.

After i got back i put that little rubber vacuum port cover onto the vacuum leak and took it out again for a hard ride, it sputtered and did all the usual BS. Once i returned i took a temperature measurement using the thermometer i got off eBay ( Thanks my67xr ) and as expected Cylinder 1's header was much cooler than the rest which were all the same give or take a few degrees ( i forgot to take photos of the headers with the vacuum leak for comparison).

So given all this i believe the gauges used for the balance were themselves not balanced so Im going to make one using that setup with the 2 bottles with hoses and liquid, I don't know the name of it but Im sure you know what Im talking about. If anyone has a link to a good guide that would be awesome.

Thanks everyone for the help so far!
TL;DR VERSION:
Cylinder 1's balance is way out causing an extreme rich condition which explains the sputtering a low throttle only.

 

To get accurate readings, you want to measure the temp as close to the exhaust port as possible. The temp gun I have only reads up to 260*C so milage may vary

 

Great to hear you might be making some progress.
How does the bike actually run at idle? Carb balance issues will show up mostly as rough idle.
I must admit to never having used a balance gauge. I have a number of vehicles (race car and bikes) with multiple carbs and have managed to set them all up just fine using a trusty bit of hose and my ear as well as setting them up on the bench using a small drill bit as a gauge under the throttle butterflies.

 

How long did the bike sit for ?

I wonder If the passage that runs from the a/f mix screw to the slow jet and the air jet on the front of the carb is blocked .

Easy way to check is to remove the af mix screw and the slow jet then use carb cleaner or threebond and spray it down the af mixture screw hole, it should come out the slow jet hole, if it does block the slow jet hole and it should come out the front of the carb

This is what happened we with @Phil zzr600, once it was Cleared , it was a different bike and used heaps less fuel

 

The fuel petcock vacuum line goes to Cyl #1.
If the petcock diaphragm is damaged fuel will pour into the cylinder, fouling the plug.

 

I'm not familiar with Keihin carbs, however what about measuring the fuel level from the fuel bowl drain holes to rule out a leaking needle seat valve assembly, good thing is that it can be done with the carbs installed on the bike

BTW it doesn't matter if you don't know the fuel height measurement because you're just looking for a difference between cylinder 1 and any other cylinder.

https://2fiftycc.com/index.php?threads/my-fizzer.872/page-32#post-15537

Then the excellent suggestion from @C58 is worth considering as the other potential source of excess fuel

 

The problem was solved! thanks for everyone's suggestions. It turned out to be the carbs badly out of sync causing cyl 1 to be completely closed at idle.

 

Kudos for reporting back as that adds to the bank of knowledge to help others in the future