Help Acceleration Issue - 1990 ZXR250R

On two FZR250 transmissions that I have, both have an amazingly wobbly first gear on the output shaft, which rings at that "bag of spanners" note that was described to me when I first got the bike.

The first gears don't show wear on either the engagement dogs of the bronze bushing which runs on the shaft - they're just loose by design IMO.
It's companion gear on the clutch shaft isn't loose as its running on splines on the shaft.
I think a bike that's idling is jerking not rotating smoothly, those pulses are transferred to the transmission and the first gear rings.

 

I won't recommend it, only that I've read about it and I've not done it, however on CV carbs getting the slides to lift faster/earlier can be achieved by doing either or both drilling the holes in the bottom of the slides very slightly larger &/or clipping the springs slightly shorter.
I've read those references in respect of racing, so low RPM manners are less of an issue.

BUT both of those mods are irreversible, which means that if it makes it worse it could be a difficult and costly affair to remedy, if parts are available.

I think the explanation @Linkin gave concerning the exhaust influence is instructive.
There was another forum member that removed the EXUP valve on his FZR250 and put different headers on that was experiencing a hollowness in the power curve below 10K RPM as a result.

Is the exhaust stock standard?

 

Hi Guys,

I have played around with carb mods in the past. To be honest, with limited success.
Lifting the slides faster does not just magically solve lazy performance. In fact it can do the opposite. On CV carbs, the slower lifting providers a slightly richer mixture to help with acceleration. Modern CVs lift pretty quickly anyway. Not like the old SUs of British sports cars. Playing with slide mods, can make a difference on them. Even running a different grade oil in the daspot can help. No dashpot on bike CVs.

To solve this sort of problem, you either go back to stock everything or spend many hours on the dyno or at the track with a stop watch. Lots of trial and error I am afraid.

 

Had one guy with 9K stumble on U.S. carby 2008 Ninja 250. Also had 2nd identical bike without stumble. He was hesitant to do work to tear apart carbs and compare. Finally got him to do it. Here's what we found.


#1. '88-07 Ninja 250 aftermarket FactoryPro needle
#2. '88-07 Ninja 250 stock needle
#3. '08-12 Ninja 250 stock needle
#4. '08-12 Ninja 250 aftermarket DynoJet needle

On his working '08, he had correct #3 OEM needle. On stumbling bike, someone had installed #1 needle from earlier generation bike!! Stumble occurred at transition to thinner tip. The earlier needle got way, way too rich too quickly and bogged engine down. Until it could rev little bit more and increased airflow leaned out mixtures enough for better combustion and bike took off!

The earlier engines were tuned more for high-end power while later bikes were designed for low-end torque with about -15% less peak-HP in high-end. So with similar carb specs, newer bike ran way too rich in high-end.

DynoJet needles for newer bikes had thicker tips to lean out high-end mixtures for more peak-power. Installing one of these Dyno-Jet needles solved his 9k stumble and gave nice screaming top-end.

Maybe compare shape of your slide-needles with one from known bone-stock set of carbs. I have several MC19 carbs laying around. I'll take some photos for you.

 

Those variations between those needles are huge, amazed it made it to the upper rev range with the ones that were incorrect.

Here's a thought for a simple test for carb needle settings, the Mikuni clip notches are IIRC 1mm apart, you could add 0.5mm washers below the needles to richen it up, if it makes it worse you know you're moving in the wrong direction, but it can be easily reverted back and nothings lost, if it improves it's an easy solution.

 

Yes exactly! Experimental data to track down issue!!

 

CBR250RR MC22 has different length intake trumpets [longer inner pair, shorter outer ones - people racing them swap for all four short], wonder if it's to deal with this off resonance issue and ease it through a flat spot more smoothly.

I know in 2008 the Yamaha R6 got the variable length intake trumpets which just have the longer back moved out of the way at higher revs - pretty nifty solution that one IMO.

 

One solution I heard of was replacing the air jets in the throat of the carbs, however they are pressed in and the risk of damage is probably high.

I still have a set of ZXR250 adjustable needles I never got around to using. Let me know if you want them, because I have no use for them now.

 

When I was researching tuning CV carbs, it occurred to me that if we were able to remove main air jets it would open up the tuning possibilities.

Mostly it's just the idle air jet which is screw in.

I wonder if removing the pressed in ones and replacing them with threaded ones is possible, it's a potentially risky venture though.

 

OK something which may help with using Mikuni needles in the Keihin slides, on the CBR250RR carbs I have here, it's the upper diameter in the slides that are larger on the Keihin needles ~2.9mm.

FZR250 needles are 2.515mm diameter at the top.

Hobby store thin wall brass tube may be able to be pushed onto the Mikuni needles so they don't flop about.
The CBR250RR carb slides have a ~3.1mm opening from the inside top then a smaller hole at the bottom, that means that the tube on the needle wouldn't be able to fall through.
The ZXR250 carb slides may be the same.

 

Reading this thread got me thinking

https://www.2fiftycc.com/index.php?threads/zxr250-carb-mod.8606/

Looking at the tidy up @Frankster did on his intake trumpets, what about getting some aluminium [aluminum for the US counterparts] flashing and fashioning up some extensions.

I've got some in 0.3mm thickness, it's soft and malleable, you could use the existing trumpets as a mold and use a hammer handle to run around to make the trumpet shape.

It also comes in 0.7mm here

Than perhaps hot glue them in place, outside the aluminium to the plastic trumpets and see if that makes an appreciable difference.

That would indicate if it's airbox resonance playing havoc.

It's not quite a solution as such as it will adversely affect high RPM, but you may have the flat spot being exacerbated by something else fueling wise.

Does anyone know if the Balius intakes trumpets in the airbox are longer?

 

Haven't been on here for a long time, sad to see you're still having tuning woes. My input is being that the engine has had a freshen up i would still suspect it's the carbs. These things are extremely fussy to the point where I've even had running issues with the springs in the float needles being weak from age.

What are the exact carbs that are on it? Jetting ? Needles? Pilot screw setting? Whats been replaced? Have you verified there's no air leaks ?
Sorry to sound like a broken record