Pump gas in Rotax/TaG?

I’m entering the world of TaG this year with a Rotax Evo. I realized i don’t have MS93 and I don’t want to get to the track without knowing if the motor even runs. So I did some research and from what I’ve found the motor will run on pump gas, and some people do not even bother with the MS93 at all. I get the issue with ethanol in pump gas, but how bad is it really? Will it be that much of a pain in the ass if the fuel system is drained after every race day? What about jetting - what adjustments would have to be made?

Pump gas presents a two (or three) fold problem.

  1. Ethanol content. Possible solution: Hit up pure-gas.org and see if you can find a pump that sells non ethanol (also called non-oxy or non oxygenated) gas. A local marina is also a possible source.

  2. Composition inconsistencies. For me, this is the biggest problem with pump gas. You can’t be sure what you are getting from batch to batch. With two strokes that can make jetting fun at best.

  3. Lead.(OK moot point with MS93 as it’s unleaded, but for others) Although poisonous, lead is great for two stroke internals.

To answer the question on jetting on pump gas… I guess you could ask around. When you do, ensure you confirm the oil ratio they are running and run the same.

Draining is a good idea with fuels that contain ethanol. If it’s sitting a long time, flushing with WD40 is a good idea too.

Search around for speed shops that carry VP products. They should be able to at least order a 5 gallon pail for you. Talk to other racers at your track. They should have a source.
I ran nothing but MS93. It was a pain at first to locate until I found a dealer. A reliable and consistent fuel was important to me.

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Oil ratio shouldn’t matter. It’s a volumetric % independent of fuel composition.

When you’re comparing jetting, oil ratio matters universally.

It affects how much combustible fuel enters the cylinder. If you don’t confirm the oil ratios you’re probably going to have a bad time.

If you’re running 20:1 and use the same jetting as someone on 40:1 there’s a decent chance you’re going to run pretty lean.

Yes, I said lean not rich. Oil displaces fuel therefore the more of it you have in your mix, the leaner engine will run (if all other things are left alone).

Thanks for the replies guys. Looks like my local race shop wouldn’t be able to get MS93 for a few weeks, but has Sunoco Optima in stock whch looks compareable: https://www.sunocoracefuels.com/fuel/optima . Anyone have ant experience with this stuff? No Ethanol, 95 octane.

I mean, the ideal oil ratio for pump gas should be the same as for VP gas. It’s a volumetric dilution ratio.

You’re totally wasting you money. Run pump gas. The compression ratio on a rotax max is so slack that you don’t gain the benefits of running higher spec (higher octane content) fuels.

In the UK which had been one of the biggest rotax markets, their national championship only specified a station for the fuel, they didn’t run a race fuel provider like VP or Sunoco. Its a total waste of your money. Even in classes which do have tighter compression ratios (KZ, OK) you are looking at hundredths and low tenths of a second improvement.

@The_Karting_Channel, is correct, pump fuel or race fuel makes zero difference to your mix ratio.

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When I ran the Rotax I’d run BP or Shell Premium gas with no Ethanol if I didn’t have the VP fuel available. It ran just fine. You shouldn’t have to change the jetting much or at all from what you normally run. I used 6oz a gallon of redline oil. I’ve even when as far down as 3oz castor to a gallon. And normally around a 154-155 main jet.

If you think you need something better than pump gas get the Sunoco stuff.

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I’m not really concerned about the performance (theoretically the ethanol should make more power, correct?). I’m more concerned about the ethanol degrading the seals and gaskets.

Drain your carb after each weekend (which you should be doing regardless of fuel) and you’ll be fine. No one is running rubber fuel lines here.

I would just pull the fuel line off the pump and run the engine out of fuel. Then drain out the float bowl. Squirt some WD40 into the pump and fuel inlet on the carb and your done.


Biggest issue ethanol. Motor doesn"t like to run on fuel with ethanol as it is very hygroscopic.

If you can find ethanol free fuel should be good to go.

I tried to run my Iame x30 on small amount of premix made with pump gas with ethanol just to do a system check before a race. It ran but ran terrible. Swapped fuel at track idle perfect.

We’re taking about two different things. I’m not talking about a specific oil ratio per fuel type or the ratio changing depending on the fuel.

I’m talking about a specific oil ratio per driver/tuner when you are comparing jetting. If the oil ratio is not the same in both cases, then the same jetting may not apply. You should ensure that you are comparing notes for jetting that the oil ratios in both cases match.

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This stackexchange thread has some good chemistry talk on the first reply:

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Ya’ll are about to see someone on the internet admit he was wrong. Here’s the concerning part I didn’t know about (or maybe I slept through that chem class):

In the presence of water (pulled in by the ethanol) and oxygen, sulfur naturally forms sulfuric acid by a variation of the “contact process”; it oxidizes easily to sulfur dioxide, then more slowly to sulfur trioxide, which when dissolved in water becomes sulfuric acid. This reaction, specifically the formation of sulfur trioxide, is sped up in industrial processes by heat and by contact with catalyst metals such as vanadium oxide (hence “contact process”), but it will also happen on its own without any special treatment (i.e. acid rain).

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OK OK lets calm down. First of all there is very very little sulfur in tier 3 gasoline (10 parts per million).

Secondly phase separation of water in any gasoline is most likely with (liquid) water being mixed with gasoline. Most water in gasoline is drawn from water vapor in the air (unless you are pouring water into your fuel tanks). This does not cause phase separation and therefore is fine. The ethanol in the fuel actually makes water in the fuel less of a problem. I’ve copied the conclusion from an EPA document below, you can read the rest of it in the link.

Water phase separation in any gasoline is most likely to
occur when liquid water comes in contact with the fuel. (Water
in the form of moisture in the air will generally not cause phase
separation.) Water which is in solution with gasoline is not a
problem in any engine, but as a separate phase it can prevent an
engine from running or even cause damage. Since oxygenated
gasolines, however, can hold more water than conventional
gasoline, phase separation is less likely to occur with
oxygenates present.

For any gasoline, simple precautions to prevent phase
separation from occuring should be taken. First of all, gasoline
should not be stored for long periods of time, especially during
seasonal changes which usually have large temperature changes
associated with them. (For both oxygenated and conventional
gasolines, gumming can also occur which is detrimental to any
engine.) If it is unavoidable to store gasoline for a long
period of time, one should be sure that the tank if full to
prevent condensation of water from the air, and the addition of a
fuel stabilizer should be considered. Lastly, care should be
taken not to allow water into the fuel sytem while filling fuel
tanks or operating the engine – in the form of rain or a spash,
for example.

Good! Essentially, the same precautions you take with Methanol should be taken with Ethanol containing gasoline.

In a Rotax Max - you want the lowest octane rating and most ethanol you can get. The compression is so low and the ignition is timed so far back that it’s almost impossible to get detonation, and there is a “chemical supercharging” effect with alcohol. I ran 87 E10 for a full year without any failures.

Definitely would like to hear more about this. Don’t get me wrong, I understand you can make more power with alcohol, if the circumstances are right.

But I’m having trouble understanding how those circumstances would occur with an FR125