Two Stroke hierarchy, some advice please.

VonYinzer

VonYinzer

Over 10,000 Posts
Recently picked up a neat little DT250 from good ol' Jussy. It's on the back burner till he and I settle up and the XS is done, but it doesn't NEED too much to get it on the road. That said, I didn't NEED to order that last beer but I did it anyway. See what I'm sayin?

Here's the skinny. How do I make it better? I understand that a 360 swap would be a massive upgrade, but I kind of want to see what I can do with the lump I have. An expansion chamber has already been discussed and will happen. Larger carb? Check. Couple options available. So what else? Anyone have a good port map for these things? I want it to remain streetable, but have a bit of "oh shit" on tap.

Info for these bikes is somewhat limited on the innerwebs, and I don't trust most of that (outside of this joint, obviously) anyhow.

So, if anyone has any input, it would be greatly appreciated.

Thanks in advance.

-Mike
 
What year is your bike?
Reed Valve or Piston Port?

There is a TON (no pun intended) of information available as well as some serious experts on tap at www.YamahaEnduro.com. It can be a pain to get registered as the site owner has it really locked down due to hacker activity, but worth the effort. Let me know if you have difficulty and I can see if I can help.

I have a copy of the factory racing tips manual from back in the day and it has a lot of the engine hop up information, including chamber specs and porting information.

Download a copy Here: s93795793.onlinehome.us/pdf/DT2_Racing_Guide.pdf.

You can also play with sprocket gearing for some extra kick.

Two Stroke Tuning Guide Link: s93795793.onlinehome.us/pdf/2stroketunershandbook.pdf

DT/RT Factory Manual Link: s93795793.onlinehome.us/pdf/dt1_2_3_RT1_2_3_manual.pdf

Color Wiring Diagram Link: s93795793.onlinehome.us/pdf/DT_RT_Wire_diagram_Color.pdf

I have a 1971 with a 1972 reed valve motor which is the sun tank bike in my signature, as well as about a dozen other old DT's.

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Rode a '75 DT400 across the country on the Trans Am Trail. Seriously fun bikes.

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In Full ADV Trim:

Day1005.jpg
 
Wow, thanks man! I'll sign up over there (or try to :) ) asap. Mine is a 75. Like I said, not looking for a race bike. Just want to pull I can out of it without ruining the motor.
 
I had an old fubar'd DT250 that I sold to a collector (he was just after the OEM exhaust). He told me the 400 and 250's share the same bottom end, so a 400 top end swap was the quick / easy way to get more grunt.

Not sure what years we're talking here, but the one I had was a monoshock one, so late 70's / early 80's??
 
The 75 is a different setup than the racing manual I sent you. It is for the 72-73. The motors changed significantly for '74, then again for the '75-76 models, then yet again for the '77-79 models. 1977 was the first year for the enduro monoshock bikes, though the MX monoshock models started with the '75-76 models. I am quite familiar with the '75-78 model 400's but not the 250's. I am not of the belief that the top ends are interchangeable between the 250 and 400. I have not seen that conversion done anywhere, though I would not rule it out. The cases mount up the same, and you can bolt a 250 motor into a 400 frame, and vice versa, as the mounting setups are the same. They changed in '77 so those years are not.

The 360 motor ended in '74 as a one year only motor/model. It, like the 400's, was a CDI ignition, while the 250's remained points up until after '77. The ignitions are interchangeable but you need everything. Stator, flywheel and coil of the type (cdi or points) that you are going to. The systems are pretty reliable, but if your pulsar coil goes bad they are quite expensive and very, very hard to find. I have another contact in North Carolina who can restore the stator to better than new if your's needs that. Let me know.

Up until '76, ('75 if it is a Canadian model) the bikes did not have voltage regulators and are prone to blowing bulbs. You need a good battery to act as an electrical sink or add a 6 volt voltage regulator. The latter is in a Yamaha factory service bulletin.

There is a really neat 12 volt/CDI conversion which is available from Germany from Powerdynamo (http://www.powerdynamo.biz/eng/systems/7238/7238dcmain.htm)which a friend of mine in Idaho helped develop. It works well and is fairly bolt in.

You can bolt an MX motor into an enduro frame, but they lack the oil pump which will mean premix, though it is possible to add one to the MX motor I believe. That is part of my build plan for mine.

Chambers for these bikes are pretty limited as they are a through the frame design. You could build a down pipe or add something. Bassini made a great chamber, but they are kind of spendy and hard to find. I have an interesting NOS through-the-frame chamber which is slated for an MX based street tracker on my to do list.

Here is a link to the correct wiring diagram for your bike year: s93795793.onlinehome.us/pdf/wiring%20diagrams/dt250b%20wiring%20diagram.pdf[/b]]s93795793.onlinehome.us/pdf/wiring%20diagrams/dt250b%20wiring%20diagram.pdf

I have several paper manuals for the 75-76 models, but none have been scanned to PDF. Let me know if you need anything in particular.

The stock 75 DT250 was rated at 24 hp at 7,000 rpm while the DT400 of the same year put out only 27 hp but at 5,000 rpm. There is a huge seat of the pants torque difference in the 400. There was no MX 250 of those years I can find, but the MX400 motor for 1975 put out 38 hp at 8,000 rpm. That motor would give you the biggest HP increase, but I am sure running a streetable exhaust would cut down HP somewhat.

They are not bad streetable enduros in stock form, having ridden mine about 6-7,000 mile on the TAT trip(s). We did about 2,200 miles of street/gravel/trail/desert loaded heavily with supplies and extra fuel. I maintained an average of about 40 MPG on the road with my 400 and it was not ridden easily.
 
Wow. Thanks again.

As far as the chamber goes, thanks to Brad and his slip roll/welding skill making a chamber is no problem. Just need to nail down some numbers.

I'm wondering if perhaps some basic porting, a big pipe and upgraded carb will do the trick for now.

As far as electrics, I'm led to believe that the bike puts out 18v. It's kicked down to 6. Not sure how all that works at the moment, but only b/c I haven't looked into it at ALL. If that's the case, I'll be adding a 12v reg/reg/rec and lighting.
 
there was an mx250 in 73 74 75
first thing with any 2 stroke is get the squish set where it needs to be
most of those old jap 2 strokes have too much clearance and end up just wasting fuel..its not just a matter of milling the head the chamber volume should be checked and recorded in your notes before doing any combustion chamber squish band work
with the squish set correctly the spark does not need to be so far advanced either which is a good thing
eric gore has good guidelines for all this
 
i also have an old school aftermarket pipe for the DT1 not sure if it fits the DT250 but it would be a good start to a pipe missing the glass pack core but otherwise a really nice condition up pipe
 
xb33bsa said:
i also have an old school aftermarket pipe for the DT1 not sure if it fits the DT250 but it would be a good start to a pipe missing the glass pack core but otherwise a really nice condition up pipe
Click to expand...

Yeah? Got a pic? Price?
 
Two Stroke hierarchy, some advice please.

Damn. Saving this one for later.
 
While Brad probably wouldn't agree, I don't think you need do too much to one of these to get a fun ride. A good pipe and carburettor set-up could even be enough. Up-sizing to 360 is an option but the 400 doesn't gain you that much extra.

Whatever you end up with will be grin-making!

Crazy
 
stroker crazy said:
While Brad probably wouldn't agree, I don't think you need do too much to one of these to get a fun ride. A good pipe and carburettor set-up could even be enough. Up-sizing to 360 is an option but the 400 doesn't gain you that much extra.

Whatever you end up with will be grin-making!

Crazy
Click to expand...
I kinda told him the same thing or maybe the exact opposite can't remember
 
As far as electrics, the stator very likely can put out 18 folks - I have seen one project where the fellow went 12 volt with the stock stator.

All of the old enduros, save the electric start 125 models use a split AC/DC system. The headlight ONLY works with the bike running and it runs off of the AC circuit. I believe it is the yellow wire off the stator. It goes to the high low switch then to the headlight. It also runs the gauge lighting, so they don't illuminate until the bike starts.

Taillight, blinkers and horn run off the battery, which is charged after the rectifier conveerts to DC. The system is set up with the battery, in good condition, acting as the sink for the over voltage at higher revs. If the battery is bad or missing without a regulator, you blow the lighting.

The only issue with the 6 volt system is the headlights are getting difficult and kinda spendy. Yhey put out plenty of light in good operating stock form actually, bur accessorizing is limited.

If you go 12 volt with stock component ray, consider LED lights for everything but the headlight, as the system doesn't put out a ton of amps. Also, either way, consider a voltage regulator. Yamaha started installing them from the factory in 1976 (75 for Canadian models) and had dealers retrofit if bulb blowing was a customer problem.

You cannot install the 125 system on the other bikes, save for the 175 models which use essentially the same bottom end. To do so requires changing the left crank half and all of the electrical system . . .

If you do the 12. Volt with stock components, let me know and I can try to find the fellow on the yamahaenduro.com site who did that.
 
not to thread jack, but do you guys know if a later model dt250 cdi will work on a dt3? (essentially a 1973 Yamaha dt250)

Also, oddly enough part of the serial number on my dt3 engine case reads dt1, but I do have reed valves. Is the Dt1 bottom end the same as the dt3?

Thanks!
 
I can't guarantee the '77 CDI setup will work, but I have successfully installed a '75 DT400 CDI setup on a '72 DT1. You need the CDI box, the coil, the stator and flywheel from the donor bike and use them all. Crank taper is correct. The later flywheel fits in the earlier case, but the early flywheel is too big for a later case.

The CDI has a small initial timing retard, so getting it dialed in is somewhat different.

There are some case differences, but I can't tell you if you can interchange them.
 
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