Good Morning Mark,

but how should i setup the timing correctly in your opinion ?

I cannot check the timing with strobo-light dynamically by the "tap" lining up with the "stock flywheel marking" because its bend and its maybe far away from beeing perfect at 3.2mm BTDC. Even worse if I want to bring it back to 2.8mm of advance I have to guess.

My approach would be the following:
I need a reference point that corresponds to the piston at the desired pre-ignition (3.2mm or in my case now 2.8mm).
I will mark this position like i did before on the flywheel (2.8) when piston is exactly at that position.
Using this mark I can set the points statically (as best as possible).
After then I can make the dynamic timing setup using the strobe light for fine adjustments.

Thanks.

 

Best practice is right in the Yamaha manuals.  You use the dial indicator to determine the piston distance BTDC and a meter (analog ohmmeter with an RX1 scale works well or a specialized tool) to determine the exact moment the points open. 

Having marks and/or bending the timing pointer back is fine...  I use the timing pointer as a reference all the time and in a pinch will use it to set timing...

The second and perhaps more difficult challenge is determining exactly when the points open...  again, there is an accurate method using an ohmmeter with an Rx1 scale or a magneto timing tool.  I've also used the thin cellophane method or even "eyeball" method with the knowledge that cellophane will advance the timing slightly and "eyeball" ends up with the timing quite a bit advanced so I compensate by purposely setting the timing after the marks align (retarded)...  I've done this enough (and even practiced it) that I can get timing pretty darn close without any special tools. 

And even with experience I don't have the confidence I always get it right so if I bike is kicking back for example, I retard the timing slightly until the problem stops. 

 

Great write up Mark T  

Just for perspective... 
For the DT250
3.2mm BTDC = 23.2 degrees BTDC
2.8mm BTDC = 21.67 degrees BTDC
2.5mm BTDC = 20.46 degrees BTDC

In my experience, with modern fuels 23 degrees is on the edge of "too much" advance so a small error that results in too much advance can create issues.  A stock engine tends to run best IMHO with around 20 degrees advance....  Coincidentally, that's about what a 175 is timed at.

Also, ignition timing on a two stroke is a bit counter-intuitive.  More advance can give a boost in mid-range rpm torque...  at high RPM, less advance is needed.  Modern CDI's for two strokes retard timing for easy starting...  the timing advances to the low 20 degrees for mid range RPM, and then the timing starts to retard as RPM increases.  All of these numbers are far less than a four stroke and while a four stroke generally needs more and more advance at higher RPM, a two stroke needs less advance as RPM increases. 

Timing is like jetting...  you give the engine what it needs.

Hi Mark,
then my approach is not wrong.
I just use my own markings (they can't be bent so easily) or have I misunderstood something?
After that i check my adjustement by a strobe and re-adjust by fine tuning then.
Furthermore I will go back to 2.5mm BTDC as you recommend and then we will see what happens.
The keyway has no slope and is good condition. (add up error)

Next I will check the retaining bold to see if there is something missing (cam or spring, dont noticed a ball or something like that on the exploded view)

Thanks and best regards
Steffen

Hi Steffen, had the kickback problem on my DT250, too.
Timing and Ohms, everything was correct. 
Spark was there. But I found out,  Spark was too weak. Fuel maybe exploded delayed.  Mounted  the source coil from my Suzuki RV 90 just to try,. Bike never kicked back, again. 
Greetings from Germany 

Hello Wessi,

hmm... i dont know how much resistance the source wire to wire (the 2. wire isolated/not connected to core) exactly should have. my measurement was about 3.3 Ohms.
I didnt find a spec with specification of source coils exactly ohm.

If my one has more resistance that original than i could have a weak spark thats true...

Steffen,

I did not mean to say your method is "wrong".  It may not be as accurate as it could be as there can be an error creating the marks, parallax errors viewing the marks, and...  most commonly...  an error in determining exactly when the points are actually opening.  "Close enough" works great...  until it doesn't. A couple of degrees too much advance can cross the line of kickback vs. no kickback.

Not to mention the unknowns of modern fuels, etc. 

Kickback seems to be more common on the 400's...  My favorite bikes are the 175's but I have/do own 250's and 400's and have worked on them.  A legendary member of the Yamaha community, Dave Meis, suggested to me many years ago that retarding the timing slightly on a big bore pretty much eliminates kick back and in his performance testing did not have any reduction in performance.  I have used his method several times now...  on a 400 the "magic number" seems to be 2.5mm BTDC...  and it has tamed a couple of previous "ankle breakers" with no perceptible loss of performance. 

Worth a try to retard slightly in your case.  I hope it solves your problem.

 

Hi Mark, yes i will give it a try with 2.5mm BTDC and lets see!