Exessive stall, and wont start when hot

[ttr250dude]

I recently got an 08 YZ450f. It really likes to stall when going slow in first gear unless i use alot of clutch work and rev the hell out of it. Single track woods riding isn't even managable at this point.

Also, when i pull in the clutch or drop it into nuetral and let it coast it dies unless i rev it. It wont idle whether its hot or cold and it wont start when engine is hot.

Its all stock except for a 51 rear sprocket and gytr heavy flywheel. I'm using 110 leaded gas. Jetting is 165 main, 48 pilot, 55 leak, and fuel screw 1 1/3 turns out. Using fresh 15w-50 full synth yamalube. Very rarely pops on decelleration.

Why wont this d*m thing run? Thanks for any input guys. Bill804

[grayracer513]

It won't run because you're using the wrong gas and your pilot is too big. IMO.

Try a 45 and some ordinary pump premium (91-92 AKI)

[ttr250dude]

It won't run because you're using the wrong gas and your pilot is too big. IMO.

Try a 45 and some ordinary pump premium (91-92 AKI)

I'm using 110 gasoline right now. If the top guys use it y wouldnt it work for me? I know some guys are using a blend of race/pump gas. Sorry if i sound like an ass, idk how else to word it ha

[MrBlahh]

I'm using 110 gasoline right now. If the top guys use it y wouldnt it work for me? I know some guys are using a blend of race/pump gas. Sorry if i sound like an ass, idk how else to word it ha

unless you did a whole bunch of internal engine modifications that would increase your compression significantly, do not use 110 fuel

[grayracer513]

I'm using 110 gasoline right now. If the top guys use it y wouldnt it work for me? I know some guys are using a blend of race/pump gas. Sorry if i sound like an ass, idk how else to word it ha

Your engine has no need of 110 octane by any measure. Certain blends of pump 110, where it is available, are difficult to "keep lit" in an engine that lacks enough compression to require it.

What specific fuel are you using?

[davbrucas]

Yup...higher octane means slower burn and not more power. Higher octane fuel will support more power but the engine must be capable of said power. If you want to spend money on fuel, use VP U4.2...lower octane with high O2 content. The extra O2 will give a tad more power.

[grayracer513]

...higher octane means slower burn

That's wrong, actually. It also doesn't mean it's less ignitable. Note that I said certain blends of pump ultra-premium will misbehave that way, but it isn't the octane per se that does it, nor will all high octane fuels act like that.

[davbrucas]

That's wrong, actually. It also doesn't mean it's less ignitable. Note that I said certain blends of pump ultra-premium will misbehave that way, but it isn't the octane per se that does it, nor will all high octane fuels act like that.

Okay, the higher the octane, the less easily the fuel(gasoline) ignites. The higher the octane, the more energy needed to start the chemical reaction. Of course if you blend the fuel with something else to raise the octane rating, then this may not hold. But for gasoline, a higher octane rating means what I stated. No reason to get all technical and search for ways to prove me wrong as what I said was basically correct.

[Dirtjunkie85]

The top guys use it because there bikes require it, I believe if you read your owners manual it says the bike needs 91 or higher, not 110 or higher.

[grayracer513]

Okay, the higher the octane, the less easily the fuel(gasoline) ignites.

That's also wrong. A higher octane rating is an indication of a greater resistance to ignition by sources other than a spark or an open flame, and that's all it is. This forum is a technical forum, and I'm not searching for anything, just trying to focus on fact while dealing with a subject that is immersed in 50+ years of mythological BS.

https://www.thumpertalk.com/forum/showthread.php?p=2827893#post2827893

https://www.thumpertalk.com/forum/showthread.php?p=2920255#post2920255

http://www.faqs.org/faqs/autos/gasoline-faq/part3/

[ttr250dude]

So basically the gas I'm using is killing my low end power and that's why it keeps stalling?? What about the issue of it not being able to idle or start when hot?

[grayracer513]

Not necessarily, no. The gas won't kill low end power under any circumstance, but some of them do cause "driveability" issues. You haven't answered the original question as to what specific gasoline you're using yet.

The idling and stalling and hard starting are probably due in major part to the pilot jet being too big, although the idle speed can be set upward to compensate.

Read the posts I linked you to, go back to a 45 pilot, and start there.

[davbrucas]

Semantics. What I said was not wrong. Please give us a lecture on heptane and is-octane and how they correlate to the octane rating. Higher octane rating gasoline has more iso-octane which has more branches and thus burns slower...heptane has no branches (straight chain alkane) and thus burns explosively. 100% heptane is used at the zero point of the octane rating scale for this reason...100% iso-octane would be at the top of the 100pt scale.

Yes, what you said was true as well. The octane rating of gasoline simply tells you how much the fuel can be compressed before it spontaneously ignites. But, higher octane fuels have more iso-octane and thus resist autoignition better...93 octane gas has 93% iso-octane and 7% heptane...of course there are other additives such as ethanol or aromatic hydrocarbons which further increases octane. But you get the picture. 80 years ago lead was added to gasoline which effectively increased the octane rating above the octane/heptane combination by increasing the compression level.

Before I went to medical school I got a degree in chemistry so this stuff isnt foreign to me.

[ttr250dude]

Not necessarily, no. The gas won't kill low end power under any circumstance, but some of them do cause "driveability" issues. You haven't answered the original question as to what specific gasoline you're using yet.

The idling and stalling and hard starting are probably due in major part to the pilot jet being too big, although the idle speed can be set upward to compensate.

Read the posts I linked you to, go back to a 45 pilot, and start there.

Thank you! You seem to be a wealth of knowledge hah. I'm using 100% Sunoco 110 octane leaded gasoline, i think its called Turbo Blue.

So here's my thought, go back to the 45 pilot, fuel screw 2 turns out, and raise the idle a bit. I might also run a blend of 50% race gas and 50% 91 octane no ethanol pump gas. (i still have $50 worth of race gas that i dont wanna waste)

Does that sound good enough for now? idk when i'll be able to work on it and test it out yet. Race is sunday so hopefully its ready by then. ?

[ttr250dude]

Side note: On 2t race snowmobiles you have to use smaller jets when using race gas than if you run pump gas. For example, my 07 mxz440x (snocross sled) uses around a 260 main with leaded race gas but uses about a 330 main with pump gas. Why is it not the same for 4t dirtbikes?

[grayracer513]

The Sunoco is probably not hurting anything, but it's a waste of money. Likewise, if you can get a decent low to no ethanol 91 octane premium pump fuel, run that straight until you get it running right. Don't get involved in trying to blend race gas with pump gas until you get it basically figured out.

[grayracer513]

Side note: On 2t race snowmobiles you have to use smaller jets when using race gas than if you run pump gas. For example, my 07 mxz440x (snocross sled) uses around a 260 main with leaded race gas but uses about a 330 main with pump gas. Why is it not the same for 4t dirtbikes?

It entirely depends on the fuel in question. Blends like the U4(x) fuels from VP use oxygenating agents. Pump fuels use this trick to add oxygen to the the combustion reaction to reduce emissions of unburned hydrocarbons. Race fuels that use oxygenates do so for the purpose of getting more energy from the combustion. Larger than normal jets are usually needed with these fuels to make up for the oxygen content.

Non-oxygenated fuels can go either way, depending on specifically what they are formulated from. The term "gasoline" is somewhat like the term "plastic". It does not in any way define a certain recipe of fuel made from a single set of components.

[grayracer513]

Higher octane rating gasoline has more iso-octane which has more branches and thus burns slower...heptane has no branches (straight chain alkane) and thus burns explosively. 100% heptane is used at the zero point of the octane rating scale for this reason...100% iso-octane would be at the top of the 100pt scale. ...

But, higher octane fuels have more iso-octane and thus resist autoignition better...93 octane gas has 93% iso-octane and 7% heptane...

You should read the complete article by Hamilton that I linked to. The basis of the octane rating system (about 1912) was in fact based on a comparison to a blend of heptane and iso-octane, and a fuel that produces an octane number of 93 does in fact resist detonation as well as a blend containing 93% iso-octane and 7% heptane, as you say. What it does not indicate is that the fuel contains any of either one of those two. Gasoline is not a simple blend of those two components any more, and hasn't been for more than 50 years. Heptane is seldom used in gasolines at all any more. Iso-octane is more commonly used as additive to raise the octane number than as a basic fuel component these days.

Note the contents of U4 on the third link I posted.

Octane number is an index of the fuel's resistance to auto-ignition, and has no direct relationship to heating value, burning speed, ignitability, or any other attribute. To assert that it does is to promote some very old and erroneous beliefs.

[ttr250dude]

So from what I'm hearing it sounds like the gas I'm using is not the problem, the problem lies in my jetting. Correct?

It also sounds like the sunoco 110 leaded isnt hurting anything but its not gaining anything either (compared to quality pump gas). Right?

[grayracer513]

Yes, the problem sounds like it's mostly your jetting. The gas could be a contributing factor, though.