Briggs Opposed Twin 18hp running E85?

So on my first build, which the link is in the signature below, has an 18hp briggs opposed twin II. it has the 3 screw carb with the bigger bore. It likes gas... a lot. too much actually. Gas is 2.80 a gallon up here, because the gas station down the road is the only one for about 30 miles, so they can charge whatever they want and still get business. Well, their e85 is only a buck 30, or something along those lines. i know e85 doesnt burn quite as efficient, but its more than 90 cents cheaper than regular gasoline, so its more cost efficient to run e85 in my mower. I am wondering how difficult it is going to be to convert it to e85. I read on a forum where someone just put it on full choke and adjusted the carb and it ran fine on e85, but im kinda skeptical of that. even if i have to do say a half a gallon of gas and a half a gallon of e85, it would still be better. i cant afford to keep gas in this thing, as much as i drive it.
any suggestions are greatly appreciated!

If it's drinking gas as much as you say I'd wonder about carbon buildup in the combustion chambers, because carbon insulates heat and that breaks down fuel before it is burned properly. Might be easier to check that first. You could also just buy the low octane gas and then add octane booster.

I would just chuck some lower octane gas in. Briggs are built for lower octane fuels anyway. If I am going to be using my Briggs (maybe an hour of running) I would warm it up on petrol, then drain the petrol out and run it on paraffin (kerosene). I even think that sometimes they run better on it compared to regular petrol.

Thats what i use, is low octane. i dont pay the 50 cents extra for the medium or 93 octane. for 93 octane its ridiculously expensive. im a cheapskate, lol
And ill check the combustion chamber, carbon buildup is a possibility. I can run it for about 3 hours on one tank of gas. i think the tank in it is right at a gallon

There are also treatments made by Lucas that you could try like the upper cylinder lubricant and what not. Are you sure it's not running too rich?

I got info on this, LOL.

Ok, I'll start with E85. Ethanol based fuel mainly, mix is 85% ethanol and 15% gasoline. This is actually almost a race fuel because it has a high octane rating. Comparing to gas, pure ethanol takes little more then twice the amount burning to make the same heat compared to gasoline burning. E85 would be under the twice the amount because of the 15% gas to make a little more heat. So what this means in fuel mileage is, well in riding mower terms, a 1 gallon tank will run 1 hour on regular gasoline. Running E85 in the same 1 gallon tank would last around 35 minutes. To run E85 in an engine that ran on gas means you would need to open up the jet hole to almost twice the size to run right. Usually just changing jet size is all you have to do because E85 should be compatible with all the seals and hoses as gasoline. You can compare cost but would be cheaper to keep the engine on gasoline.

You would need a good reason to switch to E85 to justify the extra cost.

As for your engine, thinking it is using more gas then normal, changing the octane rating of the fuel won't help, any kind of additive won't help either, these motors are usually low compression and 87 octane is good. Check plugs for carbon to see how fuel is burning. If OK here, then look at timing of the spark. Timing is usually nonadjustable but 1 or 2 degrees advancement would help fuel mileage a little without causing detonation if possible.

when i got it, the spark plugs were pretty gunky. they are also the original campion j6c plugs too, which i havent seen any of them on shelves in the store. i cleaned em up and stuck em back in, because they had a very nice bright spark. better than the new champions i had on hand actually. i could imagine the inside of the cylinder looks like those plugs, and im gonna see about cleaning them out as soon as i get it home, and then im going to see about installing a secondary tank, because the one there is just so little. i have an old salamander heater 10 gallon tank that isnt super deep, but itll fit right over the belt and sit up underneath the mower. i have pipe and a cap, and i can run that out the back on the fender and have the 2 fuel inlets right next to each other, and run both lines into one with a shutoff on each line....now im getting off topic lol. so if e85 isnt going to get me any better, and im already running the low octane gas, i guess ill just pull the heads and clean em up and check the state of the cylinder walls while im at it. once i get it home and get that stuff done, ill fill yall in on how it went

Sounds good, but if you're planning on pulling the heads don't forget to buy some new head gaskets

I think i have some that will fit. i got a crapton of them at an auction in that box o' parts. Like 10 in total. We good