Yz250 won't start

If the bike is jetted for 50:1 and you're running it 32:1, the bike is probably really lean. So you are going to want to see what's in the carb and go from there. No way for us to know.

Most likely the reason it won't start is a fouled plug.

cc304 wrote:

Okay... so I bought a yz250 today and ran like a raped ape and now won't even try to start. Has compression, and is getting gas but ran good earlier... so is it just a plug? I ran 32:1 as manual says but guy claims he ran it at 50:1. And oil coming out of head so instead of burning the oil it's blowing it out. So is my mixture the problem.

Oil comming out of the head?. Where exactly? If its not from the power valve breather then there is a big problem with the head, dont think its oil.

Mixture is the issue as well as a fouled plug.

When the premix isn't burned it goes out the powervalve breather hose and drips onto the floor.

Try 40:1 or rejet.

If oil is coming out if the head you have a serious head gasket/oring problem. And youre running rich. A fouled plug is likely the culprit. Run that thing at 32:1 and rejet accordingly.

Flooded too probably. OP did you shut off the gas when you parked it? A fouled float valve can let fuel in and flood overnight.

Acidreamer wrote:

If oil is coming out if the head you have a serious head gasket/oring problem. And youre running rich. A fouled plug is likely the culprit. Run that thing at 32:1 and rejet accordingly.

If the bike is jetted for 50:1 and you run it at 32:1 you are actually lean. Yes there is more oil in the mix, but your air to fuel ratio is lean. The more oil you run, the larger the jets need to be.

Acidreamer wrote:

If oil is coming out if the head you have a serious head gasket/oring problem. And youre running rich. A fouled plug is likely the culprit. Run that thing at 32:1 and rejet accordingly.

JBecker 72 wrote:

If the bike is jetted for 50:1 and you run it at 32:1 you are actually lean. Yes there is more oil in the mix, but your air to fuel ratio is lean. The more oil you run, the larger the jets need to be.

It will not have enough of an affect to cause a none starting issue..

Acidreamer wrote:

If oil is coming out if the head you have a serious head gasket/oring problem. And youre running rich. A fouled plug is likely the culprit. Run that thing at 32:1 and rejet accordingly.

JBecker 72 wrote:

If the bike is jetted for 50:1 and you run it at 32:1 you are actually lean. Yes there is more oil in the mix, but your air to fuel ratio is lean. The more oil you run, the larger the jets need to be.

BobPA wrote:

It will not have enough of an affect to cause a none starting issue..

I agree, just trying to give OP correct info on jetting.

FWIW, I run my Y250 at 32:1 and use FMF's recommended jetting. Very close to stock just one size larger on the main.

JBecker 72 wrote:

If the bike is jetted for 50:1 and you run it at 32:1 you are actually lean. Yes there is more oil in the mix, but your air to fuel ratio is lean. The more oil you run, the larger the jets need to be.

BobPA wrote:

It will not have enough of an affect to cause a none starting issue..

JBecker 72 wrote:

I agree, just trying to give OP correct info on jetting.

FWIW, I run my Y250 at 32:1 and use FMF's recommended jetting. Very close to stock just one size larger on the main.

My 125 and 250 also get 32:1. I 100% agree you can see a jetting difference between 32:1 and 50:1.

BobPA wrote:

It will not have enough of an affect to cause a none starting issue..

JBecker 72 wrote:

I agree, just trying to give OP correct info on jetting.

FWIW, I run my Y250 at 32:1 and use FMF's recommended jetting. Very close to stock just one size larger on the main.

BobPA wrote:

My 125 and 250 also get 32:1. I 100% agree you can see a jetting difference between 32:1 and 50:1.

Rich/lean is air to fuel ratio. If your bike is spitting oil instead of burning it its getting too much gas and not enough air.

Compensating for bad jetting by changing the gas ratio is apparently what the previous owner did. Theres no reason to run 50:1 on a yz250 unless you want it to blow up due to lack of lubrication.

JBecker 72 wrote:

I agree, just trying to give OP correct info on jetting.

FWIW, I run my Y250 at 32:1 and use FMF's recommended jetting. Very close to stock just one size larger on the main.

BobPA wrote:

My 125 and 250 also get 32:1. I 100% agree you can see a jetting difference between 32:1 and 50:1.

Acidreamer wrote:

Rich/lean is air to fuel ratio. If your bike is spitting oil instead of burning it its getting too much gas and not enough air.

Compensating for bad jetting by changing the gas ratio is apparently what the previous owner did. Theres no reason to run 50:1 on a yz250 unless you want it to blow up due to lack of lubrication.

It's actually possible to have the bike running lean, and spitting a ton of oil. Try running a stock KTM 250SX that's jetted for 60:1 on 32:1.

Plenty of people run their 250 on less oil. I'm not one of them, but it's quite common with woods riders.

BobPA wrote:

My 125 and 250 also get 32:1. I 100% agree you can see a jetting difference between 32:1 and 50:1.

Acidreamer wrote:

Rich/lean is air to fuel ratio. If your bike is spitting oil instead of burning it its getting too much gas and not enough air.

Compensating for bad jetting by changing the gas ratio is apparently what the previous owner did. Theres no reason to run 50:1 on a yz250 unless you want it to blow up due to lack of lubrication.

JBecker 72 wrote:

It's actually possible to have the bike running lean, and spitting a ton of oil. Try running a stock KTM 250SX that's jetted for 60:1 on 32:1.

Plenty of people run their 250 on less oil. I'm not one of them, but it's quite common with woods riders.

Not saying its impossible. The point is, yamaha engineers know their engine better than anyone else. They spec 32:1 to provide the correct amount of lubrication in the crank and cylinder. youre asking to blow the thing up by starving it of oil.

If you like the way the bike runs on 50:1, run your gas at 32:1 and jet for a leaner air fuel ratio to get the same effect. That way you get the lube and power.

When you get a 4 stroke remapped, they dont change the oil in the cylinder and crank. They adjust air/fuel ratio. Same principal for carb tuning.

IMO changing the fuel ratio is the lazy way out for trial and error testing to get good carb settings for your specific bike and track.