Help FZR 250 R 3LN1 - Unusual sound and struggle between 5 to 9000rpm

Progress! I think you have identified the major cause of your problems. Split diaphragms will 100% cause issues, the slides will not lift, you will get no fuel flow through the main jet or emulsion tube.

Luckily, the diaphragms are available. You have two choices - OEM Suzuki (used on the GSXR250/GSF250 as they use very similar Mikuni carbs) or Chinese aftermarket, which are cheaper and do work.

Suzuki OEM part number for diaphragms is 13507-32C02, which supercedes 13507-32C01. Search either of those part numbers on ebay, or try your local Suzuki dealer. The aftermarket ones are available on Aliexpress: https://vi.aliexpress.com/item/1005007515543175.html - you should replace all 4 diaphragms at this point because the ones that haven't split may do so very soon.


All the other information you need is in this post:

 

While you have the carbs apart, you should split the banks into individual carbs and check the fuel seals between each carb body. It is likely that they are old and brittle and will need replacing. If you ignore this, they may leak in the future.

Once again, I recommend Litetek parts as they are guaranteed to work. I don't know about NRP in the UK. They sell the Keyster kits which are fine, but they do not contain the fuel T-piece seals. Only OEM and Litetek, if OEM is even available any more.

https://litetek.co/Carb_Kit_Yamaha_FZR250_3LN1.html

Parts FS-001 and OR-20 (I think) will be needed, but once again you should just get the whole kit. The choke plunger seals are handy too as no one else does them and they can be a cause of air leaks too.

For removing all your rust/dirt buildup, you should use the Threebond Engine Conditioner (Petrol) to soak all affected parts. Be aware that this stuff will destroy rubber and plastic, so it is for use on metal parts only. That's why disassembling the carbs is so important.

What I usually do is clean out all the bowls with Threebond first. It will bring them up clean and shiny like new. Flush with regular throttle/carb cleaner or brake cleaner. Then I place all the jets and brass bits for each carb in their respective float bowl and fill it up with Threebond and soak for a few hours. Leaving them out in the sun while you do this works wonders, the extra heat has a good effect on the cleaning.

 

Nice work!

You should remove the jet housings from the carb bodies. Just pull and twist. There is another seal under them that will need to be replaced. Then you can carefully remove the needle jets for threebond soaking.

 

FZR's only have mixture screws, there are no air screws.

Start with 3 turns out from lightly seated as a baseline.

For carb balance, you want the vacuum figure to be the same between all four carbs. For multi cylinder bikes the adjustment screws adjust each pair of carbs. 4 cylinder bikes will have 3 balance adjustment screws. One screw for carbs #1 and #2, another for #3 and #4, and the third is for #2 and #3.

You adjust one and two together, then adjust three and four together, then adjust 2 and 3 three together.

Between each adjustment, rev up the bike and let it return to idle.

The bike should be running at operating temperature with the thermostat open.

When you make adjustments in one place, it can affect the other. So you will need to repeat the procedure until all four carbs are at equal vacuum. Do not forget to adjust idle speed (reference = 1800rpm) as this affects things too. You will hear it when it's running right, it will sound nice and smooth, respond cleanly to throttle applications and return to a consistent idle.

If you get the idle hanging up before dropping down, or bogging down below idle speed and returning, you need mixture screw adjustments.

These videos are a before and after. The gauges are digital but it's the same principle with analogue gauges. I can't see your balance gauge image for some reason.

 

This is the correct mixture screw arrangement:



They live here:




These are your balance adjustment screws:

 

Don't worry about the numbers or green zone, as long as all gauges read the same it will be right.

 

Sorry about that, Australia hasn't yet worked out the meaning of 'express' when it comes to postage.

If you want a shining example, look at Japan.