Help Acceleration Issue - 1990 ZXR250R

How much is it costing you to get the liners plated? New OEM liners (For C models) were available at about $100USD each last I checked. They are press fit into the cylinder block, with an o-ring to seal them from the cooling passages (I had an FZR 250 with a broken o-ring that would get coolant in the oil).

Glad that you have found the problem.

It's worth cleaning your cylinder head combustion chambers to check whether the alloy is cracked between valve seats and spark plug threads. My C model had cracks on every pot. 2nd hand head was garbage so I ended up pressing the original into service, just lapped valves, light gone on the cylinders and new rings - and a lot of cleaning to remove all the carbon.

 

If you're going Nikasil have a look at advanced sleeve for alloy sleeves - much better thermal transfer - you won't find them via a google search and for plating NZ cylinders comes highly recommended although you're in the US so Millennium are the business over there

There is also a host of bigger bore pistons which might be suitable if you know where to look - ask away - need, pin diameter and compression height

 

OK, plating will likely work to get you back to spec clearance - have you considered L.A. sleeve, or Advanced sleeve for iron liners, alternatively you could bore it to 49mm, which is I think C model spec bore

C Model pistons
https://en.impex-jp.com/catalogs/mo...-36-ke-japan-8469/cylinder-piston-518778.html

https://en.impex-jp.com/parts/new/search.html?partNo=13001-1366 - listed as available from Impex

The cylinder head part # is 11008-1260 which is listed for both the A and C model

A model:
https://en.impex-jp.com/catalogs/mo...white-dz-japan-8431/cylinder-head-515749.html

C model:
https://en.impex-jp.com/catalogs/mo...white-hg-japan-8427/cylinder-head-515423.html

So the combustion chamber and corresponding piston crown shape should be consistent between them

 

I can not imaging that 0.005" off the head is going to make a significant difference.
Sure the CR will go up a small bit, but I doubt that it will cause any issues with clearance to the piston/valves/etc.
If the engine has some miles on it, the valves have probably settled a little further into the head (particularly if they have been lapped in at any stage) which will reduce the CR a little and give a little more valve to piston clearance.
I would expect that Kawasaki would allow some clearance for general engine refurb work during the life of the engine.
Provided the rest of the engine is in good shape, I doubt that you will run into clearance issues.
Just run good oil and fuel.

 

0.005" is 0.127mm, which is a lot when you consider the fine tolerances required in these engines.

The only way to know for certain is to assemble and test with some plasticine on the pistons and then measure your piston to valve clearance. I would also fit spark plugs while doing this to make sure the pistons aren't going to kiss the plugs either.

These pistons are not flat topped and already have recesses for the valves, if clearance is insufficient you could easily have the recesses machined further to provide some more clearance, if necessary.

 

Disclaimer, I don't own a ZXR250, never ridden one either, so I'm shooting in the dark.

I was chatting with @Frankster about his ZXR250 turbo project, anyway looked at the ignition curve [from the service manual IIRC] and there is a sharp rise to ~50 degrees @ 5K RPM [it lowers back down] - I am assuming that is to alleviate something off resonance at that RPM, airbox &/or cams

So with that said, a 6K RPM hesitation may be down to carbs.
Are the needles adjustable?
I'm wondering if it's a transition point on needle taper and too rich or too lean.
@Linkin has been through thoroughly these bikes.

 

The hesitation on mine was at 7k RPM with a large, open exhaust. at 7,000 you are at half of peak torque (14,000) which I put down to the usual torque dip experienced with a 4-into-1 exhaust made worse by 'extreme' cam profiles and an open exhaust. I was toying with the idea of grafting an EXUP from a Yamaha onto it but never got past the idea stage.

Looking at mine on a dyno, it made only 37HP and was ultra rich at 7,000, and rich in general.


As for noises, I was never able to sort out the rattle mine had. Tuned, balanced and adjusted to within an inch of its life. Brand new clutch basket with back gear and all (mainly for the rubber/spring dampers). The last thought was it could be the positive neutral finder mechanism rattling without load, but never tore down to confirm. But @Frankster s A model experienced the same thing, no 2nd gear lockout on A models.

When put in gear and engaging the clutch, noise disappears.

Timing chain noise is more of a scraping noise that gets better the higher the RPM goes when unloaded, and comes back with a vengeance on deceleration / throttle down. The tell is marks on the inside of the cam cover where the chain runs.