YZ250 Blaster Build Log

[armengcorp]

Okay, sealed up the base gasket, there's definitely no other air leaks besides the power valve.

It keeps backfiring, I'm really at a loss, if it's not timing and not jetting could it possibly be the CDI?

Any suggestions?

It is possible, but generally, the 2T CDIs either work or don't. Spark quality is a very near second to spark timing. Check to see if you have a good hot (blue not yellow) spark first. If you were running really rich, you can foul a plug in no time. Plugs are cheap, CDIs, not so much.

 

[Tbrock77]

Spark is strong blue color, even tried different plugs. I bought the tool to check timing and set it at stock and +1 with no difference, still just have a soaked plug. I've pulled the plug to watch it spark and it goes fires every time it turns over.

I guess I'm just going to have to keep doing down with pilots. Stock is 45 and I'm at 40.

Could it be possible that my k&n filter is too restrictive? It's brand new. I'm not really sure what else I could run with the set up I made and obviously running it with no filter isn't going to help me realistically jet it.

 

[82cutty]

Does it run at all ? Did you over oil the air filter ?

 

[Tbrock77]

Does it run at all ? Did you over oil the air filter ?

I've got it running 3 times. After kicking it for a while, just seems to be luck of the draw. The filter is unoiled as of right now.

 

[82cutty]

You said spark is good...you fixed your air leaks and checked the flywheel and timing . Does it have good compression ?

 

[Tbrock77]

This is what I've done/checked
- timing set to .006" and .010" BTDC
- spark is a bright blue
- no air leaks
- tried pilots 30-45
- checked reeds
- compression is 180psi

 

[Tbrock77]

Here's a video of the spark. Looks okay to me. Not the best but certainly enough.

I should note that the spark seems erratic, it throws a spark randomly through the cycle but always at TDC.

 

[Larry's Shee]

If at all possible try another CDI unit. I think yours is getting ready to take a finale puke.

 

[Tbrock77]

If at all possible try another CDI unit. I think yours is getting ready to take a finale puke.

I'd have to order a used one off eBay, I don't have access to another one and no one I know has a bike this old.

 

[Larry's Shee]

That would be a real shot in dark, could end up with two bum units.

 

[Tbrock77]

That would be a real shot in dark, could end up with two bum units.

It's a 1989 engine, probably impossible to find a new unit and I haven't found any aftermarket ones either

 

[Tbrock77]

This project is dead in the water until I save up to bring it to a shop. I'm really at a loss here. There's a few things that need to be buttoned up but the build is basically done. I'll update when I get it running properly.

As of today I have 4 other 2003+ blasters, 3 of which need to be built and 1 that runs and drivers, not sure how I came across 5 of them total

 

[best]

Ha! It is a terrible addiction. I suffer it too. 3 Blasters, 3 KTM bikes, 4 other motorcycles. We need therapy, but you know, I really don't sense a problem...

Plug on brother.

 

[Tbrock77]

Just a little update. I've had it running a few more times now, still having the hardest time getting it started. My dirt bike buddy is the only one who can do it, his kicks flow into each other. Even then it takes a lot of kicks to get it going. The way it goes is it gets kicked 3-10 times then pops and then once the excess fuel is burned off 2-3 kicks it starts. I've gone way down on the pilot jets and still have the same issue, there is some underlying problem I just can't figure out.

If anyone has any suggestions please shout them out.

 

[Tooloose]

Your spark is not constant! It's white, not blue, probably bad ground, or sheared flywheel key, weak stator output, or failing cdi unit, any or all will cause this hard to start condition. Get with snap on tool rep and purchase a spark output tester. Very easy to use then as you perform repairs I've indicated you can check the ignition system for proper amount of spark. It's not enough high voltage that is your problem. Peace

 

[Tooloose]

Check all grounds first !!!

 

[Tbrock77]

Your spark is not constant! It's white, not blue, probably bad ground, or sheared flywheel key, weak stator output, or failing cdi unit, any or all will cause this hard to start condition. Get with snap on tool rep and purchase a spark output tester. Very easy to use then as you perform repairs I've indicated you can check the ignition system for proper amount of spark. It's not enough high voltage that is your problem. Peace

These old yz's didn't have a ground, it has connections for the coil, stator and killswitch. The stator might be grounding to the engine, but I already checked that, I sanded the case and the back of the plate to have clean metal showing.

It could be inconsistent spark but wouldn't it not run at all if that was the case? Once it's started it runs great, sounds good, I don't hear any pinging or misfires.

I also swapped out the coil, made no difference.

 

[armengcorp]

So it's extremely hard to start but runs just fine....a few other things I could think of:

Depending on the angle of the carb, it is possible for fuel to leak into the crankcase if the float isn't adjusted to compensate...no bueno starto.
Checked your reeds for cracks?
Still running the race fuel blend? If it's not pinging try straight 91, it it ignites easier.
Does it start easier once the engine has run for a few minutes and is good and warm?

 

[Tbrock77]

So it's extremely hard to start but runs just fine....a few other things I could think of:

Depending on the angle of the carb, it is possible for fuel to leak into the crankcase if the float isn't adjusted to compensate...no bueno starto.
Checked your reeds for cracks?
Still running the race fuel blend? If it's not pinging try straight 91, it it ignites easier.
Does it start easier once the engine has run for a few minutes and is good and warm?

Thank you for the ideas,

It is angled slightly toward the crankcase, maybe 5 degrees. How should I compensate that with the float?

I put a fresh set of reeds in when I put it together, no cracks, I double checked

I'm running straight 91 in it now, made no difference with starting or running. Haven't noticed any pinging.

Starting toughest doesn't change between cold and hot engine.

 

[armengcorp]

Five degrees shouldn't cause that problem. You'd know because after sitting a couple of days the fuel leaking into the case would evaporate leaving mainly oil. When it finally starts it would belch white smoke and run like garbage for a while till the excess oil is purged.

Hot or cold doesn't matter...now that's really strange, but try this and see if it helps any:
If your jetting is in the ballpark, then you will have to choke it when cold. Kick Slowly two or three times to get some fuel and find just barely past TDC. Crack the throttle, reset the lever and kick once but kick FAST. If it doesn't work, ease past TDC and hit it again. One problem is that in a dirt bike the kick shaft can rotate almost 180 degrees but in a repowered blaster, it rotates less than half of that. For every 90 degrees, your motor will barely make one rotation. Inirtia is not our friend and half kicks will almost never cold start any bike, new, old or otherwise. I'm about a buck 75 and have to jump on it and kick down at the same time.