2-stroke fuel in a 4-stroke motor

[blackduc98]

I did a stupid thing this weekend: I accidentally put about 3/4 gallon of 2-stroke premix into my son's ttr-125 after he ran out of gas. It fired right up, but would not idle too well with the choke off. So before I realized that I used fuel from the wrong can, I gave it a quick tweak of the pilot air screw and the throttle stop screw, and now she idles fine. While I was making these adjustments my buddy commented that the bike sounded like it had some gunk in the carb, and his words made the light bulb in my head go on, and I finally realized that I used fuel from the wrong container! Doh! So I shut off the petcock and let the engine run until the float bowl ran dry. I was about to drain the tank, but then I got to thinking... what's so bad about running 2-stroke premix in a 4-stroke engine? I suppose the oil leans out the mixture a bit, which explains why I had to mess with pilot air screw a little to get a nice smooth idle. Is there any other problem, for example, will I have to tear down and clean the carb? I'm sure I'm not the first person to make this mistake so somebody out there has got to have words of wisdom and experience to offer.

 

[biglou]

I burn my old 2stroke fuel in my TTR all the time. Lawnmower, too. :nod: Never had any problems.

 

[BadgerMan]

I burn my old 2stroke fuel in my TTR all the time. Lawnmower, too. :nod: Never had any problems.

Yup,

Left over 40:1 (MX2T) pre-mix goes in the XR, TTR, lawnmower, truck, etc. If it bothers you, drain the tank and put in fresh fuel. I should be fine though, even if you don't drain the tank.

As an aside, I once had a lawnmower that I had bent the crankshaft on (hit an exposed root). As a result, it vibrated and started using oil pretty badly. I was planning to buy a new one but thought I would run the old one until it quit. I put every type and weight of leftover and used oil in that thing in an attempt to kill it, including atf and fork oil. It never did quit so I gave up and got a new mower.

 

[AssistSuper]

I dump it in my truck. Doesn't hurt it.

 

[john3_16]

O.k....It seems fine to run 2 stroke premix gas into a 4 stroke motor...But, how long would a 4 stroke motor last if you dumped the oil used to lube the engine and just ran 2 stroke premix gasoline for it's source of oil ?

 

[TimberPig]

O.k....It seems fine to run 2 stroke premix gas into a 4 stroke motor...But, how long would a 4 stroke motor last if you dumped the oil used to lube the engine and just ran 2 stroke premix gasoline for it's source of oil ?

It would die a quick death as soon as the residual oil drained off the crank bearings. Since the oil from the premix would only enter the top end, the crank would get no lube and would seize up or throw a bearing quickly. On most 4 strokes, the engine and transmission use the same oil for lubrication, so your transmission and clutch would also be running dry.

 

[ellandoh]

As an aside, I once had a lawnmower that I had bent the crankshaft on (hit an exposed root). As a result, it vibrated and started using oil pretty badly. I was planning to buy a new one but thought I would run the old one until it quit. I put every type and weight of leftover and used oil in that thing in an attempt to kill it, including atf and fork oil. It never did quit so I gave up and got a new mower.

dat right dare is scientifical :laugh:

 

[oldfrt613]

Actually, Stihl is doing exactly that with their 4-stroke weed eaters and chain saws. They call them 4-mixes. Thay have no crancase oil bath, all lubrication is accomplished with premix.

 

[krosser]

Although i have used premix in almost everything, mostly the generator as it's convenient, your cat wont thank you if you make a habit of it. They aint cheap to replace either.

 

[TimberPig]

Actually, Stihl is doing exactly that with their 4-stroke weed eaters and chain saws. They call them 4-mixes. Thay have no crancase oil bath, all lubrication is accomplished with premix.

If you look into it a little closer, they have passages to allow the oil to lube the crank and bearings. A 4 stroke dirtbike doesn't have this.

As of right now, Stihl only has the 4 mix on a couple models of trimmers, no chainsaws. Their chainsaws are all still traditional 2 strokes, although shortly a new one is being introduced that is stratocharged. Still a 2 stroke, but uses air rather than air/fuel mix to scavenge the chamber. Husqvarna already has this out in the 575 XP, which is a bit porky compared to the 372XP it replaced, but the torque output is higher on the 575. Still a few issues to work out there.

 

[Masterphil]

Hey, at least a premix 4-stroke will still make that good smell...

 

[john3_16]

Maybe it's even better to run premix with your gas....Extra top end lube ? lol

 

[MOTOX888]

Yea, only premix will NOT work in a fourstroke. The gas/oil muxture enters the bottom end first and then works its way up in a 2 stroke. There is no way for the lube to get to the bottom end of a fourstroke without transfer ports.

 

[TimberPig]

Yea, only premix will NOT work in a fourstroke. The gas/oil muxture enters the bottom end first and then works its way up in a 2 stroke. There is no way for the lube to get to the bottom end of a fourstroke without transfer ports.

That is what I was already trying to say, was that you cannot just dump premix into a 4 stroke bike and ride it. However Stihl has developed a 4 stroke engine that they are using on some of their string trimmers and brushcutters, that is a 4 stroke engine lubed by premix. It has passages to allow the oil to lube the crank and main bearings, so a premix 4 stroke is possible.

The transfer ports aren't the reason why the oil can lube the bottom end in a 2 stroke. They serve to channel the air/fuel mix from the crankcase into the combustion chamber. By the time the air/fuel mix goes up through the transfers, the crank has already received its lube as most of the oil has dropped out due to the rotation of the crank. The fact the air/fuel mix, which contains the mix oil passes into the crankcase where the oil separates out of the mix, is what allows a 2 stroke to be lubed with premixed fuel.

You can read about it here

Stihl 4-Mix

 

[john3_16]

Yea, only premix will NOT work in a fourstroke. The gas/oil muxture enters the bottom end first and then works its way up in a 2 stroke. There is no way for the lube to get to the bottom end of a fourstroke without transfer ports.

We kind of already established that you can't run premix in a 4 stroke motor without crankcase oil...That's why I said extra top end lube.

Just say you have your crankcase oil filled up and decide to use premix gas in your 4 stroke...I wonder if it will provide better lubrication or just end up sludging things up in the long run.

What do you guys think ?

 

[TimberPig]

I'd have to say that it will likely add to the carbon build up if you did it all the time. It isn't necessary for the 4 stroke, and we've all seen the kind of spooge that can build on 2 stroke powervalves and in their exhausts. In the long term, a 4 stroke would probably see similar issues if burning premix. The type of oil, and how completely it combusted would probably play a role in how much and how quickly it happened.

 

[Vic]

TimberPig's got this one covered.

:nod:

:cool: