OzVMX Forum
Clubroom => Tech Talk => Topic started by: John Orchard 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!
-
it possible you were expecting too much from a 50 year old 2T?
after your 84' CR500 this 250 would be a land crab.
-
bigger carb??
-
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 .
-
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?
-
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.
-
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 .
-
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 !
-
Stage 3 would be you Crazy John
-
Stage 3 would be you Crazy John
Yes I was considering it :-)
-
I will bring a few copies to Broadford .
-
PVL and 34/36 Bing would fix most of that.
-
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.
-
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.
-
Sounds like you
re nearly sorted, see you at Harrisville
-
Took the bike for a blast up the street today, much better, before shortening the header the top-end power was about 60% of the midrange, now the top-end is about 90% of the midrange.
Next mod is to dig through all the old pipes l have laying around and find a cone, with a less acute angle & shorter, than the current 'baffle' cone (the reducing one), this will make the powerband narrower but boost top-end.
-
Well l went-the-hack in the ports this week; up the street, top-end off, grind ...... repeat, garage littered with slices of piston skirt :-)
Found the top-end ;-) got so much now the clutch needed some work to stop it slipping. Yeh she needs a stab at the clutch lever to get her moving out of tight corners, in my front yard lol (if l let the rpm drop) but it gets back in the power quick & easy.
The first time l took it to the track a couple of weeks ago, l rode for much of the day, it didn't use any fuel ! even with a radically ported DT1MX barrel, she's bound to use more now :-)
Racing Classic Scramble Club's event at Eurack (near Colac, Vic) this Sunday, l'm looking forward to see how it goes, l'll enter Pre 70, Pre 75 & Pre 78, stay tuned ;-)
-
John, are you able to post a picture of your bike?
-
You could have an issue in Pre78. The rules states that it is for 1975 to 1977 machines only. Seems odd as it is the only class that doesn't permit earlier machines.
Be ok in Evo though.
Good to hear it is going better. Clutch will need Barnett springs or if they are the original glazed plate you may get by by bead blasting them.
-
I think I'll have to pay Photobucket, I have so many pics saved with them, will get some pics up soon.
This is only a club-day, they might need my entries to get full fields anyway? CSC only run up to Pre 78, pity they'd get a couple more riders if they included Evo (plus I could run my KX400A5 ;-) ); an extra 3 riders with an extra 2" travel isn't going to do jack-shit to the track ;-)
I increased the preload on the clutch-springs a touch and deglazed the metals on the concrete garage floor, plus a little more push-rod freeplay :-)
-
A mate of mine who is a hard core physicist explained to me once how standing waves develop in exhausts.
On his advice I started with an overly long pipe and kept shortening it bit by bit with an angle grinder. When I got to the sweet spot it went like a rocket. Reattached the muffler and was happy as a pig in....