12th Oct 2017, 20:14

Blimey, major lack of AR knowledge.

I'm just writing this on the hoof, but I've got a garage full of AR stuff.

Running the KH400 piston after trimming the skirt and removing about 3mm from the barrel bottom (will need to check) and a head reprofile plus a fair bit of port work sorts the power plant out, even with the restricted cool. The coil seems to be a red herring. Portwork is not hard, but you need the tools to work the transfers properly.

You need to extend the disc valve closing by a good amount and the opening by about 2mm.

Reprofile the disc cover and open the reed stops by 2mm and flow the inlet tract. Forget the 26mm carb, use a KR1s Keihin PWK 28mm flat-slide as you get a better throttle response and it's bigger oh and hassle Padgetts - they made a pipe called 'top speed' that was a chrome chamber and stinger with an alloy can. Nice.

You will need to gear up by (at least) 1 tooth as it will rev clean out in top after all this and you will probably kill it, so go premix and a good fuel - 4 star prefs.

The air box is optional, but unless you're racing I'd keep it. Strokers like airboxes from a health and reliability point of view.

The problem with handling is the the top yoke and chassis - it flexes like a jelly d*ldo and will lowside you, so plate it up... don't be shy, good thick steel is needed and get a steering damper / fork brace and slightly heavier oil. Go 1 bigger on the rear boot and leave the front stock.

You don't have to throw a fortune at it, just find the right people.

Peterborough Kawasaki and Padgetts were great as was Uncle Bob... Bob Farnham of course, the god damn genius!

You can easily see 25hp as an AR145; the extra 20cc'c make the PWK28 work well and it retains a good induction pop when the disc valve opens.

You can make reasonable gains from a stock engine / pipe and carbs, but it struggles to shine. Get an RDZ or a TZR instead if you want easy power.

20th May 2018, 09:55

Has anybody out there got a set of nos exhaust to cylinder split collets that they would part with or know the whereabouts of some for sale?

For a B7, any info would be gladly received.

16th Jul 2018, 10:37

I have a 1988 AR125A and the disk valves for both the (A) & (B) models looks to be the same; they share the same part number 12021-1017.

5th May 2020, 22:12

Any new developments with tuning? Always wanting to hear new ideas that have been tested and work?

3rd Jul 2020, 22:38

I had an AR125 import I bought off my mate who bought it from new. It was 23 bhp, full fairing, not standard. I got 105mph out of her; what a bike. B reg if I remember correctly.

21st Aug 2020, 19:11

What, don't think so matey.

18th Oct 2020, 04:07

Completely possible regarding the 23bhp, I have a Japan domestic market version and a dyno print out showing that my bike produced between 22-23bhp and on a bike that is now 32 years old.

I really wouldn't be surprised if that fella's bike reached 85-90mph, but not heard of one reaching 105 mph, although not impossible with the correct gearing and engine work.

16th Nov 2020, 14:31

Yes 85-90 is possible, but 105 from stock, no way. My AR with tuning will do a little over 85, quite a good power band for a 30 year old. I want to push the tuning some more over the winter, so if you have made some good gains, then please exchange the MODS. Cheers.

4th Jan 2021, 23:51

Hi, you seem to have quite a lot of experience with tuning, would you be up for chatting some more about it?


11th Jan 2021, 04:09

[diffrentstrokers.com](http://diffrentstrokers.com) is brilliant resource for tuning advice.


12th Jan 2021, 23:33

Thanks, I will have a look at that site, cheers.

21st Jan 2021, 19:28

Hello, can I ask how the kh400 piston actually works, how does it make 20cc difference, would like to learn?


24th Feb 2023, 22:11

How could you ride one to school at 16?

28th Apr 2023, 15:24

I didn't pick up on that one, ha ha, naughty boy or day dreamer?

7th Jun 2023, 11:09

Any new tuning info out there to be shared?

14th Jun 2023, 19:53

Re. Any tuning information?

I used to be foreman-mechanic at a garage. In some ways a dream job as long as a profit was coming in, we did just as we pleased. So the apprentice had a nearly new one, but it was the 12bhp restricted version. Spending several afternoons, we derestricted it.

We matched port timing to the non-restricted version, I'm pretty sure timing details were in the Haynes manual?

Cut the rotary valve disc to increase inlet duration, also from the manual, and the bike absolutely flew. I road-tested it, then the owner. He loved it for all of 20 miles. Some yobs chased him causing him to crash. £1300 in damage. A write-off.

If you cannot get unrestricted port/valve timing figures, get 'Two Stroke Performance Tuning, Graham Bell' in free PDF format online from 'Academia.edu'. Read the bit on rotary valve timing to begin with.

Buy some decent metal shears from CK and carefully cut to match his figures for your type of engine. Then read the rest of the book. Six times. Try a 140/85deg open/close timing to begin with and 115deg transfer port closing figure. Bigger main jet on the carb and raise the needle by one or two notches.

Get the exhaust port dimension from the PDF.

Also carefully adjust piston to squish clearance to 1mm.

Read the full book! It's quite brilliant...

30bhp following the text is quite easy. It is a very restricted engine, so big gains are easily achieved. I don't know if there was an exhaust restriction washer, but if there was, it was easily spotted/removed. This was in 1984 or 85.

BTW I'm typing this on a keyboard with no 'A' so I've also got an onscreen keyboard. I'm trying to use words without too many 'A's - not easy...

Good luck!

15th Jun 2023, 18:42

At 57mm diameter, it's simply a larger piston, with the standard piston an oddball 51.8mm. This gives a new displacement of 132.2cc. The AR125 for the UK market is heavily restricted to comply with our learner laws. The simplest derestiction starts with cutting the disc valve to give a longer opening period. The porting as mentioned isn't too difficult. The one I did took two days from start/finish.

As the bike is so heavily strangled to 12bhp, the results are really rewarding in comparison to, say an Aprilia RS125. Intelligently tuned, it should make 30bhp whilst still having decent reliability.

8th Aug 2023, 20:22

Hi, I have a 1990 bike that I fully de-restricted a few years ago; even cut open the exhaust and took the junk out. The bike really went well with a strong power band, and over the winter I bought a NOS micron pipe and excitedly fitted it, now I just can't get it to run correctly. I'm pretty good at jetting, but it seems strangled half way through the power band. Is the pipe only good for the 12hp? Think the old pipe will be going back on if I can't find the answer!