Author Topic: Hmmm looking for more top-end. Piston Port 250.  (Read 3610 times)

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Offline John Orchard

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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!
Johnny O - Tahition_Red factory rider.

Offline Hoony

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #1 on: April 08, 2019, 06:42:18 PM »
it possible you were expecting too much from a 50 year old 2T?

after your 84' CR500 this 250 would be a land crab.
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)

Offline 80-85 husky

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #2 on: April 08, 2019, 08:16:16 PM »
bigger carb??

Offline Iain Cameron

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #3 on: April 08, 2019, 10:01:28 PM »
First of all piss of that huge flywheel and points . I use a 77 YZ 250 CDI system  with a internal flywheel it will rev and pick up quicker .
Yamaha tragic ; dt1, rt1, dt2, rt2, dt2mx , rt2mx , mx250, mx360,sc500, 74dt125, yzx125, yzc250, yzc400, yzd250, yzd400, yzh250, yzh80 , dt100 , xr75 ko xr80 03 , it175 82 . Not a member of any club

Offline John Orchard

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #4 on: April 09, 2019, 08:48:06 AM »
Thanks Guy's :-)

Iain am running a small rotor PVL ignition, John it's a VM38' that should not hold back power at high revs, Al l had considered that point but l can feel the power drop-off as the revs build.

Next step is to look at making the squishband narrower and lower the comp a little more. Then recheck ignition & port timings.

Then next will be shorten the exhaust header?
Johnny O - Tahition_Red factory rider.

Offline sleepy

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #5 on: April 09, 2019, 02:57:30 PM »
You need a properly designed exhaust. Every time the exhaust port height is altered it changes the pipes tuned length requirements. If the pipe is made to suit a peak at 5000 it doesn't matter what you do with the port height it won't change much except kill it down low.

Offline Iain Cameron

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #6 on: April 09, 2019, 08:33:03 PM »
I have the Yamaha tuning hop up manual for stages 1 , 2 , and 3  for the 250 . stage three is road race or US flatrack . all porting and numbers I will bring a copy to Broadford Johnny .
Yamaha tragic ; dt1, rt1, dt2, rt2, dt2mx , rt2mx , mx250, mx360,sc500, 74dt125, yzx125, yzc250, yzc400, yzd250, yzd400, yzh250, yzh80 , dt100 , xr75 ko xr80 03 , it175 82 . Not a member of any club

Offline John Orchard

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #7 on: April 10, 2019, 08:34:22 AM »
I have the Yamaha tuning hop up manual for stages 1 , 2 , and 3  for the 250 . stage three is road race or US flatrack . all porting and numbers I will bring a copy to Broadford Johnny .


Wow Iain, that would be some great info !
Johnny O - Tahition_Red factory rider.

Offline Hoony

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #8 on: April 10, 2019, 04:44:58 PM »
Stage 3 would be you Crazy John
Long time Honda Fan, but all bike nut in general, Big Bore 2 stroke fan.    http://www.youtube.com/watch?v=XJoKP6MawYI
1985 Honda CR500RF "Big Red"
1986 Honda CR250RG
2005 KTM 300EXC "The GruntMeister" ( I love that engine)

Offline John Orchard

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #9 on: April 10, 2019, 07:34:02 PM »
Stage 3 would be you Crazy John


Yes I was considering it  :-)
Johnny O - Tahition_Red factory rider.

Offline Iain Cameron

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #10 on: April 10, 2019, 07:36:48 PM »
I will bring a few copies to Broadford .
Yamaha tragic ; dt1, rt1, dt2, rt2, dt2mx , rt2mx , mx250, mx360,sc500, 74dt125, yzx125, yzc250, yzc400, yzd250, yzd400, yzh250, yzh80 , dt100 , xr75 ko xr80 03 , it175 82 . Not a member of any club

Offline Slakewell

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #11 on: April 11, 2019, 07:30:10 AM »
PVL and 34/36 Bing would fix most of that.
Current bikes. KTM MC 250 77 Husky CR 360 77, Husky 82 420 Auto Bitsa XR 200 project. Dont need a pickle just need to ride my motorcickle

Offline OverTheHill

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #12 on: April 11, 2019, 08:03:38 AM »
Could be a mis-match of pipe to porting, not that i'm an expert but have mucked around with different pipe combos on bucket racer 100/125 two strokes with a hell of a variation in performance. Had a wiseco or LA sleeve in a DT1 GYT worn out chrome cylinder & had a certain homemade underneath pipe on it which i'd had on something else & had worked great but on this cylinder it just went flat but was excellent up to that point. Anyway--was just a thought. Also stupid things that happen--yz80H, last of the air cooled [or was it a "79 never mind] came out with a D shaped exhaust outlet from cylinder liner to where the pipe bolts on. What would yamaha know lol so [on my kids mates bike] the shop made the port nice & round back to the liner but didnt touch the liner. Thing was bloody hopeless [so i didn't do it to our one lol]. Panicking as was a big meet coming up & this kid was in the running. So took it home to the farm & experimented. Cut a D shape plate [D as in flat across top D] set the plate trapped behind the gasket & fixed it 100% so got the port welded up to reshape. Things ya never forget haha. Everyone was stressing out for a bit with meeting approaching.

Offline John Orchard

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #13 on: April 11, 2019, 09:39:14 AM »
Yes l think it's the pipe Morley, all my investigations into DT1 pipes has led me to the optimum header-pipe length to be 13" long, the down-pipe that l made was not made to any formula, thinking an aftermarket chamber, made for a 250 Yam, would be in the ballpark. As l rerouted the pipe down, l kept the same tuned specifications.

Well this down-pipe has a 17" long header, so yesterday l managed to get 3" out of it, will test up the street later today, once the sleepy Dales Creek residents are out of bed :-)

Yes l have read about plates/walls at the exhaust port to stop unwanted pulses coming back into the combustion chamber, l wonder, is it a wrongly designed pipe that requires this, as the majority of bikes do not use this design?

If l can get this bike sorted in time, l'd like to have a crack at the National Champs in July.

SLAKE: It has a PVL and a VM38 !

IAIN: That would be great, l'm sure you'd sell a couple.
« Last Edit: April 11, 2019, 09:47:39 AM by John Orchard »
Johnny O - Tahition_Red factory rider.

Offline supersenior 50

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Re: Hmmm looking for more top-end. Piston Port 250.
« Reply #14 on: April 11, 2019, 05:49:41 PM »
Sounds like you
re nearly sorted, see you at Harrisville