YZ250 - rebuild - electrical inquiry

Short it. If left open, it won’t run.

TrailDigger wrote:

Short it. If left open, it won’t run.

Thank you for the info. With the bike getting spark, I'm surprised it requires to be shorted to ground. What is the reasoning behind this?

Shorted it just to see and still no joy.

Spark is good
Compression is @180
Took the carb apart- Cleaned it again, put in stock jetting, and ensured it was feeding gas when kicking the bike over. The plug will have gas residue on it after kicking it over a while.
Bike is getting air to mix with.

The only thing that I can think of is timing. I checked the stator and the timing marks. Not much to see. Does not make a heck of a lot of sense as all 3 variables are present to at least make it try to start.

When kicking it over, once in a while it will backfire a bit and the expansion chamber will get warm to the touch and you can smell premix in the air so it is trying.

Sounds like the crank cavity is loaded up with fuel mix from flooding. Remove the spark plug, turn the gas off, drain the gas from the carb, hold the throttle wide open and kick it 40 or 50 times to clean this out as much as possible.
Then put it back together and spray starter fluid in the carb and try to start it. It should run for a second or 2 and then shut off. If that is the case, I would inspect the carb for a leaking float valve or having the float height set too high.
Fix those and then restart the engine in the normal way.

Paw Paw

Thank you for the info, Paw Paw. I did a couple of things. First , i took the spark plug out and drained the float bowl. With the spark plug out I kicked it a good 70x. After this, I put the plug back in and sprayed a bit of starter fluid in the carb. Kicked it over and nothing.

I then took the spark plug out, drained the float bowl and then pushed the bike around in gear for a bit to turn the engine over even more. I put everything back together and tried to bump start it. It popped a few times and backfired a couple of times, but never lit. The plug after each occasion was wet and dark like fuel was trying to ignite.

I'm not going to try a different plug. I'm currently using what the book calls for (BR8EG) and I have heard that they can be finicky (i've never experienced this before) and to try a BR9ES. I doubt it will work, but it cheap and worth a shot at the very least.

I also checked the electrical again. Solid spark and everything else looks good to me so far as I have spark, fuel and air. You'd think a simple 2-stroke would be happy with that and work, but there seems to be a missing secret ingredient to this bike.

Have you checked the float valve to be sure it is sealing?
Have you set the float height?
Are you getting a very bright Blue spark at the plug?
Have you checked the compression ?
What about your reeds?

Paw Paw

Did you check timing with a dial indicator gauge?

Casting marks on the cases are notoriously inaccurate.

Retard the timing a degree or two to prevent detonation as well.

Paw Paw 271 wrote:

Have you checked the float valve to be sure it is sealing?
Have you set the float height?
Are you getting a very bright Blue spark at the plug?
Have you checked the compression ?
What about your reeds?

Paw Paw

The first thing I did was go through the carb and rebuilt it to OEM specs. Float height is good and I ensured it is sealing properly.

Spark is blue. Bright is subjective. I can easily see it on a sunny day when turning the bike over by hand.

Compression is @180.

Reeds are new and I checked to ensure how gaps and that they are lying flat.

I checked the timing via the marks on the stator and flywheel.
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The next thing I plan on doing is a leak down test to ensure the crank seals are good. After that, if all is ok, I plan on replacing the stator and electrical harness as the stator is Non-OEM and the harness is off a newer bike as indicated prior.

In that case I would suspect the non OEM stator or timing. They have a history of failure and also can effect the timing greatly. In most cases the trigger coil seems to be the issue effecting the timing and having the marks line up is not always correct if it is a non OEM unit.
Have you done a leak test or a vacuum test?

Paw Paw

Not to ask a dumb question but I just came across this on a guys bike not long ago that was acting this way after he did a full rebuild. He never put the key in the new crank. I asked to see the old crank and there it was. Never drink during reassembly...

Paw Paw 271 wrote:

Have you checked the float valve to be sure it is sealing?
Have you set the float height?
Are you getting a very bright Blue spark at the plug?
Have you checked the compression ?
What about your reeds?

Paw Paw

MyersAvionics wrote:

The first thing I did was go through the carb and rebuilt it to OEM specs. Float height is good and I ensured it is sealing properly.

Spark is blue. Bright is subjective. I can easily see it on a sunny day when turning the bike over by hand.

Compression is @180.

Reeds are new and I checked to ensure how gaps and that they are lying flat.

I checked the timing via the marks on the stator and flywheel.
________________________________________

The next thing I plan on doing is a leak down test to ensure the crank seals are good. After that, if all is ok, I plan on replacing the stator and electrical harness as the stator is Non-OEM and the harness is off a newer bike as indicated prior.

I would recheck timing with a dial indicator gauge. The casting marks can be off by as much as two degrees. The proper way is to check with the gauge and adjust as needed,

TrailDigger wrote:

I would recheck timing with a dial indicator gauge. The casting marks can be off by as much as two degrees. The proper way is to check with the gauge and adjust as needed,

Thank you for the info, TrailDigger. I'm doing a leak test tomorrow and if that's good, I'm going to see if I can borrow a buddy's stator (OEM) and see if it likes it. Rather borrow his before dishing out a bit of $200 on a new OEM one!

3D wrote:

Not to ask a dumb question but I just came across this on a guys bike not long ago that was acting this way after he did a full rebuild. He never put the key in the new crank. I asked to see the old crank and there it was. Never drink during reassembly...

Not a dumb question at all. I actually bought the bike after someone rebuilt the bottom end, but then dropped it and put a nice crack in the water pump. I got it cheap and had a good welder fix it up. I then found the primary drive gear was built up wrong so I had a known engine guy in the area go through the entire thing and he also found some oddities and fixed it up.

We'll see what I find tomorrow after a buddy does a leak test! If that's good and the OEM stator works fine, the bike is going to be put on the backburner for a bit as the main race bike is getting prepped as the ground is finally starting to thaw!

Paw Paw 271 wrote:

In that case I would suspect the non OEM stator or timing. They have a history of failure and also can effect the timing greatly. In most cases the trigger coil seems to be the issue effecting the timing and having the marks line up is not always correct if it is a non OEM unit.
Have you done a leak test or a vacuum test?

Paw Paw

That's what I keep going back to as well, Paw Paw. I actually went to look at it again and it seems it no longer exists!

TrailDigger wrote:

Short it. If left open, it won’t run.

This is simply not true. I have mine left open with a Zip Ty Racing cover and the old girl runs as strong as ever! I seem to remember hearing similar stories and the culprit was a bad kill switch.

Did the leak down test and it's good to go. The only thing I can think of at this point is a suspect stator (non-OEM).

A new replacement is on the way. If that doesn't work, I'm not too sure anymore!

Got a new OEM Stator. Still no go. Taking the bike over to the motor builder as there is just something I'm not seeing.

Well... it's fixed. Like I figured it was something dumb, but I don't care as it's running well now. It ended up being the woodruff key was out of place and causing the bike's timing to be way off. Got a new one installed and it started the 2nd kick.

It's basically done now and the rebuild turned out pretty darned nice from starting with a bunch of broken parts in miscellaneous boxes.