SOLUTION: after rebuilding the carb and ensuring it was getting fuel I put in a fresh plug. b8es isn't made anymore so I went one colder to a b9es. The old plug looked fine but can sometimes spark out of the engine but not under compression. With the air screw turned 1.5 turns out and the idle screw turned in 75% of the way, the choke off, and throttle open, it started in about 20 kicks. 


Original Question:

I just don't know what to try next....I know these "no start" threads are a pain, but I'll keep it updated to help future forum members. 

Timing: set with dial indicator and "buzz box" to begin opening at 3.2mm BTDC

Spark: new emgo coil and wire, cleaned plug, showing nice fat purple spark.

Compression: Currently reads about 110-120 depending on the morning; with the throttle wide open.

Exhaust: freshly cleaned out by putting in firepit

Carb: new jets, o-rings, and gaskets. dipped in ultrasonic cleaner several times and thoroughly cleaned. Tried starting with/without foam UNI filter and with/without tape covering carb intake side with small hole in it to restrict airflow.

Crankcase: serial is of a 74 DT250

Cylinder Head: uncertain origin, may be swapped. Cylinder head gasket in good condition.

Piston: did not take pictures or measure (because it has compression) but I removed head and it appears newer and aftermarket.

AutoLube: not hooked up but the pump is sitting on side of crankcase. The cylinder has no "port" for autolube to attach to...interesting.


Starting:

• fresh fuel and yamalube mixed to 30:1
• air screw at 1.5 turns out 
• idle screw set to just open the slide a bit
• choke "on" with the plunger in the up position and the lever in the downward position

This thing will just not start! It won't even try, just cranks and cranks.

Where should I go next?
Is my compression too low? I would think it should at least start at 110-120. 
Could my cylinder head be not matched up correctly somehow? The cylinder head is stamped "49800" and the crank case serial number is "450-211194"

I appreciate all your help!


**the forum is giving me an error message when I try to upload an image attachment, so I'll try to load some pics later



  
  

  
 

Good morning! Is the plug wet with fuel after all the cranking?

498 is a DT250B or C head.  I think it should work okay.

Forum appears to be out of space again or Mako might be working on something...  hopefully picture posting will work again soon.

Maybe try a dry plug with "Easy Start" or similar sprayed into spark plug hole?

Was the plug obtained from a reliable source? There are a lot of counterfeits out there and it's been posted on here previously that these will spark outside of the cylinder, but not under compression.

As you have said, cases are from 1974 DT250A. The 498 on the head shows that is from a 1975 DT250B. I wonder what is stamped on the cylinder?

At some point Yamaha moved the oil injection port from the cylinder to the carb. However, I don't know when that was for these models. 

Thanks for the info so far guys.

I misspoke earlier, "498" is stamped on the base of the cylinder, not the head.

After cranking several times, the plug comes out wet. It's an NGK br8es, however I'm uncertain as to where it came from or how old it is. I'll source a fresh one and try it out.

I just have to say this since you mentioned it...  the most difficult thing about these types of threads is the tendency for the person with the issue to become "combative", "defensive", and "dismissive" when someone suggests something. 

Ever since I was a little kid I've never been afraid to try to fix something and the truth is I often broke it worse and made many mistakes before it got fixed... if I got it fixed at all.    It's those personal experiences and the experiences of others we usually are speaking from. 

For example, most of us speak from experience and when we're told "carb is clean"...  and we question something about that claim...  it's not that we don't believe you!  It's that we may have been there ourselves. 

I had a issue with my MG1-T carb years ago where it had been dipped (in the GOOD Berryman's) several times...  blown out with air and spray carb cleaner several times...  Visually inspected and it looked cleaner than new.  Everything appeared to be perfect.  Yet it wouldn't run.  Finally I found another carb and ordered it from ebay.  While I was waiting for it to arrive, someone on the forum mentioned "ultrasonic cleaning"...  this was maybe 10 or more years ago and something new to me.  On a whim, I bought a cheap Harbor Freight Ultrasonic Cleaner on sale. 

I could not believe the crud that came out of that "clean" carb!  I initially left the jets in.  For the second round, I removed the jets and a whole bunch more crud came out...  especially from the pilot jet passages.

Bike ran perfectly after that.  I still have the used replacement carb I bought in a box someplace. 

That said, here are some common things I've seen and experienced in no particular order:

*Engine is just plain flooded. 

*Bad spark plug.  Spark plug can spark outside the engine but fail under compression...  more common than you think.

*Spark plug gap too big.  Should be 0.024" maximum.

*New ignition coil is crap

*Timing accidentally set ATDC instead of BTDC.  (I've done this a few times. )

*Significant air leak.  Seal forced out of case happens sometimes.





 

I believe Yamaha used the B8ES for this model. BR8ES may have too much resistance and not provide adequate spark.

I'm in Crank key Mode. But if you physically checked timing before tdc measuring down the plug hole then crank key must be ok. Possibly just flooded up. Full throttle & kick or push around the yard ignition off & plug out then try again. Note, it timing is checked just by the pointer through the slot & Not down the plug hole then crank--piston could be anywhere if crank key is broken. Are your crank seals still in place. Sorry, i'll have ta read back n catch up.

Thanks for all the replies! I'll try to sum up everyone's suggestions/questions as best I can:

- Spark Plug: yes, b8es is what is in the bike and what I need, I misspoke earlier. I can't source one locally so I'll have to order it in, It won't be in until the beginning of next week probably. 

- Crank Key: I take it that you mean the woodruff key on the crank? Yes, it is in place and the timing was adjusted with a dial indicator down the spark plug hole to 3.2mm before TDC.

- Carb Cleaning: I've been there with having to pull....and re-pull carbs several times to get it the right amount of clean, haha. I have had this one in my ultrasonic cleaner several times now completely stripped, followed by carb cleaner through the passages, followed by compressed air. I doubt it could be any cleaner. I appreciate the questioning though!

- flooded engine: I've let it sit a while with the plug out to "dry" the cylinder out, cleaned the plug, and then tried cranking again, all with no luck. Just for good measure though I'll try again tomorrow morning; the bike will have been sitting with the plug out for the night. I'll crank it a few times with the plug out and then try it all hooked up.

- crank seals: the only crank seal I can see without splitting cases would be the right hand side, correct? And I'd have to pull the right side engine/clutch cover to inspect it? I don't know the condition of the crank seals unfortunately.

- Idle Screw: what is the "right" amount that the idle screw should be set to as a baseline? I've had it set to where it's just opening the slide but I'm unsure if that's proper. 


Thanks guys!

Can't see the right side crank seal without pulling the cover & on the left side it's pull the mag. Doubt its that though so don't go there yet. Things like carb setting & timing don't have to be degree perfect to run--hell mine aren't for sure. Even compression only need a good lump to feel on the kick start. So don't get sidetracked with 'perfect' &lets get some life out of it first. Drop the pipe off & kick it a few times & any sign of life will be obvious even if only trying to catch on. If plug is still wetting up then leave fuel off, or even drain the bowl but a bit difficult on the bike unless a side mount main jet. Then kick with an full open throttle. Don't twist your wrist while kicking, just hold it wide. It'll fire up in 10 kicks--yeah right--well hope so lol. 10 kicks on mine & my legs giving up. I put a video up years ago of my old faithful DT1 after hibernating. I ran outa puff kicking 7 shoulda drained the stale fuel but she lit up in the end. Must find that vid to make you feel better kicking away.