DT125R FORUM

Today me and the DT had a date with the dyno

First I convinced myself to put in a exhaust temp probe, I don't have much experience from that and thought I could learn something. I tried it on the moped once, and it was mounted too close to the port, and temp went low on full throttle. So, this time I mounted it maybe 15cm from the cylinder piston face, and it worked very good.

I started with running with the power valve in 10% steps to see the effect, and used that to make a map for it. It closes a lot more than the stock setup, and does almost nothing in 0-30% area, but I left it like that with full stroke anyway, and start opening it at quite low revs, since it seems to do some good around 3k. It also seems to make the bike slightly quieter when 'creeping'. Open power valve makes a lot more exhaust noise than a closed one, that's for sure!

After I had the PW map, I made a static ignition curve at 10deg, and then added 5 degrees per run, so 10, 15 and 20 to find where the engine responded to more ignition advance. Based on this I made the ignition map. It did a lot on the over-rev to increase the advance again after the engine was 'off the pipe'. I found the same on 23cc rc-engines I tuned before. After this I could increase the rev limit from 11 to 11,5k. Before it was 'dead' at 11k, but now it's usable to 11,5k. I could also see that the stock curve ignition retard made the exhaust temp rocket at over-rev, but with the added advance on top it stayed more constant. I had a short reading of almost 900deg once with the 'stock curve' in the Zeeltronic around 11k, that scared me a bit.. I think advance was around 5deg at 11k then. Max exhaust temp is 700-750deg now. If I try to lower it I lose power, and there is not much power anyway (it's not a high strung 2-stroke). The 1,5mm sensor seems to react very fast, faster than the meter that updates around 1Hz. Short blips on the throttle, and the temp jumps up instantly.

Finally I tried some main jets, but I could not improve on the stock main jet. I left in a slightly richer jet that gave almost the same curve as the stock setting, just to have some margin. Stock was Mikuni 125, and the slightly richer was a 'no-name' 140 (probably from china). 'No-name' 130 seemed to be same or slightly leaner than Mikuni 125.

The stock carb is 26mm, and maybe it could make some more power with a bigger carb. I have a 28mm that fits on the reed-side, but it needs another 'snorkel' to the airbox, because the connection is bigger on the carb. Maybe I could find something from some old motocross bike that I could modify. I think the original 'snorkel' is a bit small in diameter too, so putting something bigger could also help.
Not sure if it is worth the effort though?

I made s quick test drive after the dyno, and it feels just like the curves indicate, more usable rev range, pulls better from the bottom all the way to the top. I thought I heard some knocking around 8k when driving, so I backed the ignition a little bit in that range, but I need to do some more testing.

I posted the pictures in another forum for those interested: https://www.warp7.se/forum/viewtopic.php?p=36158

Please note that the rpm/torque on the graph for the 125cc cylinder is wrong (rear wheel power is correct though), because I changed the gearing after I installed the bigger cylinder.

The setup is the following:
-88 DTR
Athena 170-kit (cylinder raised 1,5mm and reworked/bigger combustion chamber, squish area 50%, 0,8mm squish)
Athena pipe
Zeeltronic CDI
Stock 26mm carb and airbox

I ended up buying a second DT, this time a tired LC Mk1, but not as easy to find parts as with the DTR in Europe (UK has some, and gets expensive with customs). Hoping for suggestions..?

I think 174 sounds very low. I have measured 190-200 on 125 & 170 Athena cylinders, depending on base gaskets etc. From memory my standard 3MB cylinder is similar in exhaust port height.
Exhaust temp is medium temp in the pipe I guess? Measured close to the exhaust port (but not too close) I bet it will be over 600.
I think you could make a pretty peaky pipe (large diameter, and relatively steep cone angles) to gain power witout losing to much of the power band since it has the exhaust valve. However, I think you need to be able to tune the exhaust valve (Zeeltronic) to the new pipe, especially if it deviates significantly from the original pipe. Maybe take a look at the TZR exhaust dimensions for inspiration?

The spacer I used was just a 1,5mm thick alu sheet that I cut to the shape of a base gasket. Remember the head has to be 'moved down' too, to keep the squish nice and tight. Can be done by machining the top of the cylinder down, or machining the head.
IMHO the head volume is too low from factory, and combustion chamber needs to be enlarged to have a safe CR if squish is below 1mm as I want it (0,8mm now). The 125 and 170 inserts seem to be the same from Athena..

No porting done on mine. I think widening the upper part of the exhaust could add some power at higher revs and keep the low band pretty much the same. Pretty easy to do a portmap on paper, but I did not do it before I installed the cylinder. Just measured the port height and calculated the port timings to get a rough idea what height I should try. Raising it the full 3mm to get the 'full opening' of the transfers gives pretty wild port timings, but it would probably work with custom ignition curve and pipe to make it a screamer (power at high revs). Not what I'm after though. Pretty happy with it as it is now.

I think there could be some benefit of a bigger carb too. The 28mm Keihin I have is 50mm too, so if the TM28 is the same, I should be able to use the newer 'snorkel' to the airbox.

I wrote down my settings as they are at the moment, so I thought I would post them here too

Ignition:
1000 17
2600 25
5000 18
8500 16
9000 12,6
10000 14
11000 24

PV:
2300 0
3900 30
5900 50
7000 75
8000 100

Will not apply to any engine for reasons I mentioned before.

@HOTSHOT-III Sounds good if the new version would fit the other carb I have! What's the diameter of the TM28SS connection on the intake/airbox side? Maybe somebody has a spare carb and could measure? I used 'car windshield glue' for doing similar glue joints before, that held up well too.
Basic spec is in the end of my first post. Reeds are carbon I made myself from 0.3mm sheets (Polini I think). I bought some online, but I thought they were too thick and stiff. I always want as thin as possible (without getting flutter) for throttle response. Reed cage is standard, I thought it looked like it would flow good enough. I'm not sure changing the reeds made any difference either.

@Calum For sure, max power is not a lot, but pretty happy with the wide band on the engine. Getting a higher revving pipe, widening the exhaust, raising the cylinder more etc would probably get the power up, but would probably sacrifice some of that power band too. I need to drive it for a while to make up my mind on that.. The dyno gives realistic to low readings, and this is rear wheel power, so crank power will be a couple of horses more.

Please be aware that the Zeeltronic tunings depends on the 'calibration' of the power valve and ignition. I tuned the end points of the power valve to maximum stroke, so that can vary from servo to servo, and bike to bike. Zeel recommends some other end point values, but I prefer doing things my own way

When checking the ignition advance with a (known good) timing light, I found I had to change the 'trigger angle' from 80 to 67deg to get the actual ignition advance to correlate with the set value in the Zeel. Seems this can vary from bike to bike too. According to Zeeltronic, this should be 80deg on the DT, but not on my bike, this means set map values would be way off from reality. I made a static advance map (constant 10deg) and made markings on the flywheel to check the actual advance with a timing light.

Well, I was too exited to test when I put it together. I had knocking at light loads with a hot engine (hard to hear what happens on high load/high revs), so it had to come apart again. I measured squish as it was, and it was close to 0,5mm.. Anyway, compression had to come down further, so I increased combustion chamber volume (diameter), and reduced squish area. It was almost 60%, and from memory 50% should be good for a wide band engine, so this is what I made it.
Also did some stuff to try to improve cooling of the head insert, since it seemed there was deposits of glycol on the insert where it was in contact with water. Like there was local boiling leaving glycol deposits on the insert. I sand blasted these areas to increase surface area a bit. I also blocked the holes in the head gasket closest to the thermostat, to force more flow through the head, and not going straight from the cylinder to the 'exit'. From what I have learned, exhaust side of cylinder and head is what needs most of cooling.
Need to do some jetting and driving in hot weather to see if this has solved the knocking.

For you guys in the UK: https://www.gpxmotouk.com/shop-gpx.html
They have the parts to convert the DTR to 230cc, can probably get the complete engine too. If I was thinking of a engine rebuild and/or Athena 170, I would go for this instead, based on what I learned from the Athena 170 kit.