KR-1S.co.uk Forum

Two Stroke Institute - any news on the AR engine tuning and pre 83 racing?
HNY 

A belated thanks for the scans bernie500 the AR was a bike i always wanted, i nearly bought one on ebay 2 years back, but the guy was a rip off merchant. He then abused me by private email after confessing the forks were twisted.

It's still in Japan, the bike it's self should be a shoe-in for Pre 83 racing, though I have a Waddon Rotax project as well which fits within the same era.
I have some conflicting info on the rod length some charts say 99 others say 109mm. Any ideas?
Here's a Waddon Rotax for the uninitiated



Rotax annoraks will see the top pic has a Type 128 engine and the bottom a period legal Type 124 engine.

Hi TSI

My tables are not clear either so measured an AR125 rod for you:
See pictures - standard rod seems to be 99mm.





Let me know if you need any more dimensions etc.
Cheers
Shaun

Ps liking the Waddon Rotax 

Thanks heaps for that ,I like your method there Shaun that seemed to work a treat.
So 99mm it is , my book says the AR piston has 25.4mm comp height (from the top of the pin bore to timing edge) plus 7.5 for the gudgeon gives 99+25.4 +7.5=131.9 installed height.
Hmmm pity it wasn't 109 but anyway.

56mm Mito piston flat top 17mm comp
16mm gudgeon 8
=131.9-25=106.9mm rod needed

56mm YZ 125
early KR-1 replacement 86-88 22mm comp height
131.9-30=101.9 rod

Later 90-93 flat top
131.9-20-8=103.9 rod

The 90-93 YZ with a 105mm rod and a spacer plate look good to me. Mainly as I can use a better bearing that will allow a safe 12,500 rpm.
Could also use a KR/KR-1 rod and machine a bit off the base of the cylinder.

Only unknown is the distance from the top of the piston to the bottom of the exhaust port the YZ piston is only 55mm long which could ruin everything.
Have you got your cylinder off Shaun?

Hi TSI

Got an engine in bits at the minute so no problem.

Checked some of your measurements and all good (25.4 comp ht + 7.5 for gudgeon). Only other thing is standard piston domed (extra 2.7-2.9mm)

Exhaust port 54mm from top of cylinder to lower edge of port.

Transfers - four are 52-53mm from top of cylinder to lower edge of port.
Transfer opposite exhaust port (fifth transfer) is 57mm ditto.

Piston top edge appears to stop approx 3.5mm from top of barrel (as standard)















Do you know the engine number(s) coming from Japan?
This would give me a steer on the size of exhaust and transfer ports etc you are likely to get before they arrive.

As standard spec tends to be:

Barrel to top exhaust port 29mm.
Power increases as this is reduced typically to 27 or 25mm

Transfers standard 41/42mm from top of cylinder.
Power increases as this is reduced to 40-40.5mm.

Compression most 6.2-6.5, some 7.0.
Power increases with matched increase in compression.

Combustion chamber volume most 15-15.5ml, some 13-14ml
Power again increases with reduced volume matched to compression / squish etc.

Hope that's all helpful - I'm in no way a tuning expert like you and many on this forum - I've just had a fair bit of experience with the AR.

Looking forward to hearing how it goes and learning a few new tricks


Cheers
Shaun

Thanks again Shaun.
Hmmmm 3.5mm lip at the top of the cylinder , makes things harder but a bit of maching on the head (or make a insert) and drop the volume down to 10cc will have this really ready to boogie.
I'll get onto my mate and get him to ship the engine(he's as slack as me) I think there is MASSIVE potential in that cylinder.
I'll have to crunch some numbers when I get home to know what's possible.
Have you got a spare cyl/head and any discs?

Quick number crunch
Dropping the cylinder would be the ideal solution but with 3.5mm of positive deck height, machining that amount off would expose the cylinder at TDC. That would probably be fine with the stock piston as that is 64mm long. It also depends on where the piston sits at BDC in relation to the bottoms of the transfers.
The timing with the 3.5mm of +ve deck height considered are
EX open 81.9
TX open 111.8
I would not recommend raising the ports further.
Machining the cylinder head to get the squish right and up the comp to 12.5:1 would yield great results.

Yes have a few spare barrels, heads and disks so we may be able to do a deal if your calcs work 

I will check the positive deck height at TDC on a built up engine if I get chance later today and let you know.

Can you build a matching expansion chamber?

I could easily build a chamber but I don't have a complete bike which makes it hard. I have a friend over there that might be able to do it. Are you anywhere near Lincoln?

With the disc timing changed will be a rocket and with a shitload of bottom end also. Just the thing for blowing the doors off Ninja 250's

Checked one of my spare engines and the height of the standard piston at TDC against the barrel. It stops short giving a lip of 0.3-0.5mm (excluding the dome on the piston). Think I may have mislead you with my poor use of terminology alongside the measurements.

The initial marking on the last spare barrel was 3.5mm from the top - this was actually from the top ring of the piston. The top ring is 3mm from the top timing edge. Hope that makes sense.

This means it would not be easy to deck the barrel without raising it first. Or using a shorter rod / different piston.

When you run your calcs would it work better to raise the barrel 2-4mm, then deck the top and machine the head to adjust for compression / squish. Adjusting the disk timing to suit?

I'm about an hour from Lincoln so not far away
Cheers

I never machine off the top of cylinders, with the AR you would machine away the o ring grooves, but there is no going back from that.
Would never install a shorter rod either.

Both the Mito piston/KR-1 rod and YZpiston/TZR rod look good options.
Cylinder looks bloody brilliant to me and should be a right flyer especialy with a pipe to match.
What is the diameter of the exhaust outlet at the flange?
Do the clutch springs reference with any other Kawasaki part?

I'll PM you a email/phone number if my mate is up to it.
Thanks again

Hi Guys,

Did anyone pick this little bargain up for £310.00 ?

http://www.ebay.co.uk/itm/300641855473? ... 1438.l2649

Cheers,
Bernie

Hi Bernie
Yes a bargain in that the wheels and engine would sell for £300
Does look like it needs an awful lot of work though.
Not many good ones around nowadays..
Cheers
Shaun