KR-1S.co.uk Forum

Hi. Due to unavailability of KR pistons where I leave had to use what I
could find. 'Yamaha YB125 super' piston which is about 6mm taller was
selected and 2 pieces of addaptors were made of aluminium. Minor
adjustment done using gaskets of right thicknesses. Compression
reading of both cylinders are 10.7kg/cm2 which is within the limit.
By the way pistons had transfer holes on them. Engine starts on 1st or
2nd kick.
I've done more modification on this bike which will be posted in due
course.

Good..

Hi everyone.

Due to leaking cylinder head gasket had to dismantle the engine.
Tall pistons had to go, replaced by shorter ones from old Yamaha DT 125 with domes.
Removed the spacers underneath cylinder & ground 1mm from the cylinder top.
Compression reading is 8, on 6000 rvs on 6th gear getting around 117 kph.
Bike is starting without problem & runs smoothly.

Next time will cut more from cylinder top to increase compression pressure.

Thanks for reading.

Good morning!

Just finished testing my KR's acceleration against h***a CB 400 ended up too far behind!
1st thing in the morning going to dismantle the head & both cylinders to increase the compression pressure. Will revert with the new pistons' condition after about almost 1000 km of low compression combustion.
Adding that I'm using local made gaskets here where I live as I'm the only KR owner so pls wish me luck, worried that my company will ask me to join a ship as my leave period is almost finished.

Thanks.

Hi

It has been very difficult finding right piston, finally after few attempts to increase compression pressure in both cylinders had to weld on top of pistons using TIG weld method. I'm not 100% sure that this will work but hopefully will.
Checking cold compression pressure before starting the motor got 10.5 reading.
Engine started without any problem thanks to modified fuel system (Reeds instead of rotary disks).
Running in period is in progress and lots of smoke rising in the air!
Few days later I'll check the compression pressures in normal running condition.

Thanks.

Saeid you seem to be getting on well with your KR. Please get some photos up on here so we can see how the finished bike looks.
Happy riding

Steve

I would like to see some pics too, especially reed conversion

Hope it running well for you !

Unfortunately this time I didn't take any photos, regarding REEDS pls refer to this topic of mine:

Reeds instead of rotary disks !

I'm happy with REEDS performance, specially when starting the engine.

Great to see you back on the KR modifications saeid! You must be a very talented mechanic, all the better for us. What model years were the DT125 pistons you used, and have you messed with the cylinder bore? Do these pistons have ports on their intake side? Are they that shorter (pin to crown) that you had to shave the cylinder top? Maybe there are other pistons that can be used as well. I'll have to go through the whole procedure when I start the engine rebuild again. Ideally my pistons will still be in great condition, but I/we have to source some compatibles sooner or later. Thanks for sharing, and I hope you'll get that h***a CB 400 nailed (I don't think it very possible though if it's the one with the CBR400RR engine )


Cheers, Fanis

Hi again

Unfortunately here there is not so many living 2 strokes! therefore spares are rare and I have to order them from other places, I'm sure this s the only KR250 around. Modifying cylinders were the only choice, I've taken few mm from the bottom as well which I forgot to mention earlier, sorry for that.
Where I got the piston they had 2 types, old type had dome and new type had flat top, on both written Yamaha DT 125.
This bike was smuggled into Iran from Japan via UAE, later was taken & sold by the authorities in an auction, many parts were missing and I did a lot to revive the bike, many thanks to Tim and his site.
I had a plan to increase the bore but the plan is on hold for the time being.
I suggest not to do as I did on barrels if you can get original spares.

Here is the dimension of the DT 125 piston I used:
Diameter is 56mm onwards
Total ht: 66mm
Top of the pin to the top of the dome: 26mm
Pin 16mm
Piston has 2 holes in its back.

I've tried the bike till 8k rpm and managed to get Suzuki bandit 250v (4cylinder) easily.