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Sipes #412
3/9/2017 8:24am
3/9/2017 8:24am
I just bought a 2007 yz250 stock no head mods and the guy highly recommended me to run 50/50 race/pump gas but I've done little research on 100ll as a cheaper option because I don't want to spend 50 dollars on 5 gallons of race gas. First question is would I be fine running 92 octane? I have in the past on my yz's and haven't noticed issues. He told me they come from the factory to run race gas well pretty close. I wouldn't mind spending 5 bucks a gallon on 100ll. is it better to run higher octane on 2 strokes or will I be fine on 92 octane?
I run avgas in my YZ465 and in the 05 CR250 that I had. No issues and the bikes ran great. Go for it you will not be sorry.
On page 1-20 of your owners manual it say the bike is designed for 91 or higher octane pump gas, or 95+ RON (Research Octane Number).
As a private pilot, I would advise not to go putting 100LL in a bike. Theres a ton of reasons why. Main one is its just not designed for automotive and none of the additives are either. I bet pump 93 octane gas would run better.
Run pump gas, if it knocks, throw a splash of C12 in it. Stay away from 100LL. Its for planes and design for engines running at high altitudes. I would scratch it off my list.
The Shop
I do have a stock 05 KX250 that runs like crap on pump gas so we just run c12 or VP110 and it runs awesome!
While many stock bikes do not need the additional octane, the consistency and lack of crappy additives make 100LL so much better than pump swill for anything performance oriented.
I wouldn't run AV gas in a 4 stroke, but in our old 2 strokes it works perfectly fine. Look around, there's numerous cases of people running it for years with no issues. I've yet to find a single case of someone having an issue because of running it in a 2 stroke.
Greg
As for the altitude argument, consider this. On commercial airliners (I know they don't run on avgas but the explanation still holds), engines that do short runs, say from San Francisco to LA, have a much shorter service life than engines that do longer runs, say San Fran to Australia. The reason being is that the engines are taxed the most during takeoff, where the air is most dense and power is at an absolute maximum. This tells me that for any airplane, power at takeoff is incredibly crucial, and the fuel must work well sea level to provide that needed power. Sure, there may be better fuel options out there, but for just quelling detonation and raising octane there is no reason to waste money on race gas, you're just pissing it away for no benefit.
Someone above said you don't need the octane for stock engines. Well, the YZ is very on edge stock, and will detonate in extreme conditions. The 100LL is just the right fuel to ensure that doesn't happen.
Race fuel > 100LL > Ethanol pump fuel.
It may not perform as well as pump or race fuel, unless modified to do so.
Having said that, to the OP if previous owner said it needs 50/50 he might have experienced some pinging at one time.
YZ's are known to have static timing variances enough to cause pinging,
especially after rebuilding with a Wiseco crank.
Pit Row
https://www.airnav.com/fuel/local.html
My 05 CR250 was a rattle trap on pump. I ran a 50/50 mix of C12 and pump 91 and the pinging went away but I was still paying + $7 a gallon for the mix. Avgas is around $4 a gallon. I could have cut the head on the CR to run on pump but I would still be dealing with poor gas quality, short shelf life and possible oil and gas separation. Also ethanols attraction for water.
I also ran the mix in the 465. My seat of the pants dyno said no performance distance. I don't know if Avgas is a HP boost but it felt like it because both bikes ran crisper and were more responsive at the throttle.
And finally, I bought a $59 string trimmer at Home Depot for a clean up project. I thought for $59 I could throw it away when I was done with it. That thing has only run Avgas and Maxima SuperM and is running perfectly after 7 years.
Now, it is reported by MXA that the YZs will rattle and eventually break on pump gas. OP can have the head milled by RB Designs to run pump but will still have to deal with shitty gas quality unless he can find ethanol free gas. It won't hurt to try 1 gallon of Avgas.
I think VP sells 2 stroke gas that is 100 octane for stock motors so Avgas is well withing stock jetting and compression.
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