Type of fuel to run...

[MonkeyBoy]

Need help with fuel.  I have a 250 two-stroke and need to know what fuel should I run.  I know I have to use a mix.  Owners manual says to use 98 octane.  That's a little hard to do now so let me know what fuel all you guys run.

 

[tlanders]

A month or two ago this was discussed on this forum. I use 5 gallons of 93 Octane pump gas with 1 gallon of 110 Sunoco racing fuel all mixed with 52:1 Golden Spectro Concentrate oil.

 

[Chris Brown]

I run the same as Teddy. If I end up getting 105 race gas, then I use two gallons instead of one with the 5 gallons of pump gas. And I can't get 93 from the pump out here (so-CAL, home of crappy gas)so I have to use the 91 from Chevron, Shell, or Union. I've been told over and over, NOT ARCO.
The bikes run cooler with a little more octane, and there is plenty of lube at that oil ratio. Mine is the 400MC5.
Chris Brown

 

[Chris Brown]

Guess I should clarify a little. We mix in 5 gallon cans, so if you use 1 or 2 gallons of race gas, you deduct that from the total to get the balance of pump gas.
2 gal race gas, 3 gal pump gas, etc.
If your pump gas is 93 octane, and your race gas is 110, at 20% RG the octane is raised to 96.4. 2 gals of RG would give you 99.8. At 50-50, it would be 101.5
Jetting is critical for them to run properly. That's a whole other topic...
Chris

 

[Mike Lenz]

WE all used 40 or 50 to one ratios back in the day. Why now are the oems stating to use 32 to one??

 

[tlanders]

I mix 5 gallons of 93 octane pump gas to one bottle of Golden Spectro and get 52:1. Then I mix 5 gallons of 110 race fuel with one bottle of Golden Spectro to get 52:1. THen I use a 6 gallon gas can and put the 5 gallons of 93 octane mix and 1 gallon of race fuel mix in it. I have been told that you cannot do ratio math with octane ratings. 50% 110 octane plus 50% 90 octane does not get you 100 octane, it is more complex than that. As an aside, the higher the octane, the SLOWER the fuel burns. So if you increase your octane, you also need to advance your timing more so that the fuel has time to burn and it is not just wasted and thrown out the exhaust unburnt. I put a higher compression ratio head on my Piner and am burning pure 110 octane racing fuel in it. I had to advance the timing from 3mm BTDC to 4mm BTDC to get it to run right. Have fun experimenting!!!!

 

[Chris Brown]

Mike,
Late model 2 stroke mfrs recommend a rich mixture for liability reasons. Sure as hell, some greenpea will use chain saw oil or outboard oil and sieze his new CR. They also jet them rich for the same reasons. If you lean out your oil, you will most likely need to lean out your jetting too. There's more gas so they run a little richer. Bikes are jetted to run in cool weather at sea level with lots of oil in the gas. When it warms up and/or with some elevation you are going to find that your fuel mixture is much richer.

Modern synthetic oils do not burn, so stay in your topend to lube it. A lot of racers use a richer oil mixture to guard against scuffing and lube failure from extra hot running. Less oil is need in larger CC's, more in smaller motors.

Most critical is your jetting. That's what keeps the motor running correctly and the fuel is what cools it, not the oil.

After running 50:1 GS for about 500 miles I pulled the barrel off my 400. It still looked like new inside and ring gap was still in spec. No appreciable wear. Helps to keep your airfilter clean too. Dirt is the worst thing for your piston and rings.
Chris Brown

 

[Mike Lenz]

I believe I read that even the factory mx teams are using 32 to one in the factory bikes. If thats a fact then there must be something more to it than dummy proction??

 

[Mike OReilly]

I have run 91 or 94 pump gas in my 250, with good quality oil at 40:1 since I started racing it in AHRMA in 1994. I run it at 40:1 because it provides a little extra lubrication and it is easy mix at that ratio. (here in Canada we're in metric and buy fuel in litres).

After all these years and more recently, in the last five or six years, a fairly active racing schedule, when I pulled the top end apart I was able to simply hone the cylinder and install another standard size piston.
On this basis, and on the basis that when these things were new nobody was running pure racing fuel in the 100+ octane range, I would suggest that this works pretty well. (Think about it, when these engines were new, they were expected to run hard and be reliable for six days in some extremely remote corners of Europe - I'm certain that the quality of the fuel available would have been mediocre at best).

On the subject of oil, I have always understood that the recommended ratio varied on the basis of oil quality not the particular engine. So good quality concentrates can be run at 50:1. Auto parts store generic 2-stroke oil (or even run of the mill 30 wt. oil) would be run at 20:1 or even 16:1.
For what its worth that is my experience with this.

 

[Chris Brown]

Mike L,
On factory race teams, yes some do run at 32:1. My GasGas race tech says 32:1. Keep in mind racing and recreational trail riding are two different things. A few examples:
1. Honda recommends a top-end rebuild every 20 hours on a CR. But trailriders commonly report several hundred hours between rebuilds.
2. Racers rebuild their engines before each race. They are running extra oil to make sure they finish the race.
3. Many are sponsored by racing oil manufacturers and distributors, and will quote those ratios whether they are running them or not. The mfrs want you to use LOTS of their oil, it is how they make money.
4. Indy cars and Nascars run NEO and Amsoil synthetic lubricants in trannys and diffs, while advertising Shell or Union, or whatever on the side of the car.

This is a topic of debate in many, many forums and everyone has their opinion.
I have a friend that has won the Baja 500 5 times, and has been building race bikes since the early 70's. I tend to believe him when he makes a recommendation. For the old air cooled bikes the GS at 50:1 is fine. For the new water-cooled bikes a straight synthetic at 40:1 is recommended. Fuel quality and jetting are actually more important, and will determine the useful life of your motor.

In our trials bikes we run 50:1 up to 100:1. Those motors are mostly idling when they are running.
 
Mike O's point of the type of oil is a very good one. A pure racing synthetic does not need to have the same concentration as a plain mineral based 2 stroke oil. Go by the manufacturers suggested ratios. You will notice that GS has ratio recommendations based on type of riding.

Bottom line: gather information from those that know, make your own choices, and go out and have fun!
Chris Brown


 

[Dwight Rudder]

Chris is mostly right in his accessment.
Per Dick Liechen of Maxima oils. The less RPM the engine turns the less oil it needs. That is why a trials bike can run 80-100:1 mixture and a KX500 can run 50:1 with no problems. Oil Migration time is the key. The smaller bores run much higher RPM. The old HiPoint/Spectro oils were the best in their day. We usually ran 40:1.  I found that I got about 30% more life out of an engine ( 250cc ) at 40:1 than 50:1. I run Maxima Super M oil at 40:1 in most all my 2 strokes. I don't MX them. If I did I would probably run 32:1 - 36:1 to compensate for the additional steady high RPM . ( faster oil migration )in my smaller bore bikes like 100 and 125cc machines. Maxima Super M is a blend of synthetics and Oil. It burns much cleaner than Spectro, which tends to gum up and drool out pipe. Some 2 stroke oils will not burn, some will. The key is to get one that will do its job in the lower end ( lubing the cylinder,piston and crank ) and then burn out once it hits the combustion chamber. If you want more oil in the bottom end, Use Suzuki CCI oil as a premix. Mix at 20:1. Properly jetted it will still burn very clean. When I rode for Suzuki that is what we ran exclusively . Ran great.
Cher'o,
Dwight Rudder

7 time ISDT / E medalist
7 time National Enduro Class Champion.

[sbolly]

Has anyone tried using 100LL aviation fuel?