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Topics - John Orchard

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Wanted / WANTED - DT/RT 1,2,3 swingarm.
« on: June 14, 2019, 12:38:46 PM »
I want to experiment with some mods to a swingarm, but don't want to cut-up my existing item. Would later DT250, 360, 400 also fit/look the same?

Yamaha / DT1 tank badge screw size?
« on: June 10, 2019, 03:35:56 PM »
Anyone know what size screws hold the tank badges on on a DT1.  Measuring old screws I have that tend to screw in are m3.5 dia, and guessing a .60 pitch (smaller than the .80 thread gauge I tried on it).  I have no experience in such small size.

I am needing to buy some new countersunk screws for the thinner than stock badges that I have.

Tech Talk / Keihin PWK38/40 slide cutaway measurements
« on: May 27, 2019, 05:03:14 PM »
It took me a little while to compile the info, thought I'd share if anyone needs it. My carb is Chinese supplied and does not have any markings on the slides, hence the need for my homework  ;-)

I had a ping 'just' off closed throttle on my CR500, turned out to be slide cutaway.  In the PWK40 I am using, the carb came with a 7.0, it improved things going to a 6.0, then perfect with a 5.5.

Slide # -  Imperial  -  mm
5.5      -   .315"      -  8.000 (apparently PWK33 & 35)
6.0      -   .365       -  9.271
7.0      -   .385       -  9.779
8.0      -   .450       - 11.430

Tech Talk / Ohlins nitrogen needle, buy from pharmacy?
« on: May 17, 2019, 10:28:21 PM »
l've bent the shock recharging needle on my Racetech adapter, does anyone know if a syringe needle from the local pharmacy will fit? TIA

Tech Talk / Filing throttle slides, richer.
« on: May 07, 2019, 04:37:52 PM »
Anyone filed Mikuni VM throttles slides into the richer direction?  I'm thinking of filing the bottom of my VM38 slide, changing it from 2.5 to 1.5.

I found the following chart giving slide number cutaway height...

1.0 = 1/16"
1.5 = 3/32
2.0 = 2/16
2.5 = 5/32
3.0 = 3/16
3.5 = 7/32

Wanted / WANTED - Anyone got 14" shocks for sale?
« on: May 07, 2019, 03:17:56 PM »
I'm interested in trying different rear shocks, 14" or maybe 14.5".  For a 4" travel bike, either with or without springs.  If with springs, around 70 lb ~ 80 lb.


Tech Talk / Who can share 250 pp port timing maps?
« on: April 29, 2019, 08:23:29 PM »
Anyone have any port timing maps for a piston port 250 ? I guess CR250M, TM250, KX250 or Euro 250's?

I'm really interested in what the range is of what works. TIA.

Wanted / WANTED - Rear shock springs, 46mm lD, for 4" travel bike.
« on: April 25, 2019, 06:16:29 PM »
Anyone got a pair of rear shock springs for sale? Need 46mm lD, between 200 ~ 240mm long, 80 lb if linear or 70/90 progressive, or there-abouts, thanks.

Tech Talk / So what is the story with lowering transfer ports?
« on: April 19, 2019, 09:25:12 PM »
I have recently seen some two-stroke cylinders, and read some tech documents that show lowering the bottom edge of the cylinder transfer ports, below the top edge of the piston crown at BDC, some up to 6mm lower!!!

l'm guessing the only benefit to this is more exposure to a cooling flow from the air/fuel coming up the transfers, and/or maybe a more upward blast as the piston gets towards BDC?

Can anyone offer me some ideas here? :-)

What would I do without this place  ;-)

Me never had any experience with respringing a 4" travel bike, can someone give me an idea what spring rate I am after for a 90kg rider (I'd like to say 'fast' rider but time will show)?

I am wanting straight-rate springs, bike currently has 'progressive springs', not interested in the effect of little bumps when I have to hit big bumps at speed with 4" !!!

The shocks on the bike are 340mm gas Ohlins STX36 CC607's, the spring dimensions are:

Length:  228mm.
Coil ID:  46.50mm both ends.
Wire thickness:  6.9mm
Active coils:  11
Total coils:  13

Thanks.   :-)

Tech Talk / Hmmm looking for more top-end. Piston Port 250.
« on: April 08, 2019, 06:13:32 PM »
So l took the DT1 / 2 out for its first outing on the weekend, 4" of suspension travel was a big shock for me :-)

Bike went good, heaps of midrange but really flat on top, l started trimming piston intake skirt, 1mm at a time, got to 8mm shorter than stock DT1 (still idled nice and started first kick) and l retarded the ignition a few degrees, these changes helped a bit but l need lots more top-end (and over-rev).

The compression is/was very high, with the high-comp GYT head l'm running .8mm squish clearance, which l changed to 1.2mm to cut-out the pinging.

The bike has a VM38 carb, big airfilter with plenty of air flow, the DT1MX barrel has lots of port work done, any ideas?  Changes to the pipe? Would a YZ ignition change things over the PVL?

l am quite surprised that with a high exhaust port and such delayed intake timing that this thing doesn't make big power at high revs!

Wanted / WANTED - Yam DT2/3, RT2/3, RH crankcase.
« on: April 04, 2019, 10:00:51 PM »
l need a right side crank case, apparently DT2 & 3 and RT2 & 3 are the same, not sure about DT1 & RT1's as they have a different shift drum & forks ?

Will take a pair if not separating.

Tech Talk / PVL ignition, too little or too much advance?
« on: April 01, 2019, 05:45:40 PM »
The instructions with the PVL ignition states that a 250 two-stroke usually runs between 1.8 to 2.3 mm BTDC ignition advance, my DT1 seems to like it retarded to 1.5mm BTDC for a little more top-end.

What are your thoughts & settings, could l be running too high compression?

Tech Talk / Your thoughts on a carburation matter?
« on: March 20, 2019, 09:58:14 PM »
On a piston-port 250, bikes starts & idles fine but the idle adjustment screw is all the way in, having quite a high slide setting.

Any ideas why it needs a high slide setting at a low rpm idle?  Too rich needing more air?

Yamaha / Yam clutch spring specs- 90501-20122 & 90501-20124 ?
« on: March 02, 2019, 12:59:18 PM »
Anyone with a range of Yamaha Clutch springs in stock be able to tell me the difference the following two clutch spring parts?


They both are listed for  a great many model Yams, both are listed for DT1 but one is listed for RT1; if one is a little stronger I'd like to get that one, TIA (Thanks In Advance).

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