Hmmm looking for more top-end. Piston Port 250.

[John Orchard]

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!

[Hoony]

it possible you were expecting too much from a 50 year old 2T?

after your 84' CR500 this 250 would be a land crab.

[80-85 husky]

bigger carb??

[Iain Cameron]

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 .

[John Orchard]

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?

[sleepy]

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.

[Iain Cameron]

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 .

[John Orchard]

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 !

[Hoony]

Stage 3 would be you Crazy John

[John Orchard]

Stage 3 would be you Crazy John

Yes I was considering it  :-)

[Iain Cameron]

I will bring a few copies to Broadford .

[Slakewell]

PVL and 34/36 Bing would fix most of that.

[OverTheHill]

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.

[John Orchard]

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.

[supersenior 50]

Sounds like you
re nearly sorted, see you at Harrisville