Author Options:

Just wondering about 2stroke in a 4stroke engine... Answered

Soo, currently teh lawnz needs mowing, but there's no petrol.... for the lawn mower, that's four-stroke (straight unleaded & oil seperate) there's two-stroke stuff though(50:1 ratio or so), I did try a little in the tank, which seemed to work okay, but concerned parents are wondering it might make it choke the mower to death, can someone please confirm what can happen, and what will most likely happen. =)




9 years ago

it will make more smoke,and the exhaust gas will be blue

It'll smoke alot, but won't damage the engine. If it is mixed too rich, it could foul the plug. But worst case scenario; you'd have to drain carb and tank and start with fresh gas.

I do this all the time, I only keep one can of gas, use the 50:1 mix in all my machines, never had a problem with the 4 stroke mower. Going on year five of it's life. BTW, little small engine tip, if you find the engine on a machine running sluggish, or not at all, take off the muffler and see if it will run. They put a spark arrestor in the muffler, it will carbon up with age, had this problem with practically every two stroke I've ever owned, just pull the arrestor screen out with pliers. NMF if you burn down the countryside.

Yeah, that was always a nuisance with our compressor engines, there was one of them that blew the arrestor out, another that continued for years until the exhaust blew right off the thing, at that point we replace is with a straight pipe, it ran great and smoked alot less than before, no spark problems either, it was about a foot of pipe and an 8hp honda. The only problem that solution ever had was when something went very wrong and the engine timing got completely outta whack, misfiring, shooting flames and burning the carbeurettor to a crisp was the order of the day, soon solved by some very dodgey fix, it went on for another three years and still runs but has been retired by the coming of a fancy new one that's quiet and just as powerful.

he should be ok at 50:1 though the carb might be a bitty sticky afterwards...

It'll live. I've got three gas cans: mower, chainsaw, and who-the-hell-knows-what-I-put-in-there-last. The third one is the one I use when one of the others runs out at just the wrong time. For example, I knew I'd have to go cut down a whole bunch of trees and didn't want to run out gas, so I mixed up a gallon of two-stroke gas in the thrid can and took it with me. I didn't use it all, so I left it in there and put it all back in my shed. Wouldn't you know it, a few weeks later I run out of gas halfway through mowing my front lawn and decide to use the 2-stroke gas from the third can. No, I decided to use the gas in the can, totally forgetting about the oil. It was only after 2 hours of mowing that I realized that it was two stroke gas. When I was done and things cooled off, I took the plug out and found no oil fouling from that run time. No issue, like I suspected. Oil fouling really happens the most when your air/fuel ratio is too rich because you have a perfect ratio coming in from your carb, and then your four-stroke has worn rings or a tapered cylinder and is letting in oil from the crankcase. Then you're not mixed to burn it all. You will get complete combustion and therefore no real soot if your oil is coming in as part of the correctly mixed air/fuel in the carb. I would normally be concerned about detonation (knocking), but lawn mowers run at a low enough compression that you couldn't get it to knock if you tried. In another example, my neighbor's car (I have never seen a functional vehicle leak that much oil out that many places and continue to function) has been burning oil and belching clouds of white smoke for about a year now, and although it finally developed a miss after about 7 months, it still runs. We have a neighborhood pool on when it'll croak. Still going.

Almost guaranteed, once it runs until is sputters to a stop, the plugs will be fouled. If one "gets it going again" and runs it too much longer it could put a lot of soot deposit on the cylinder and the exhaust valve will become fouled. This means lowered compression (since it will not seat properly) and the deposits on the cylinder walls could eventually score them and then you are looking at engine replacement, if the cylinders do not come out individually.

Wow, I guess that's possible, but I've never seen the difficulty go beyond a fouled plug and a gummy carb. But then again, the most premix I've ever run through a 4-stroke, at any one time, is about a gallon.

. You're both right.
. For short runs, say one tankful, smoking and fouling are about the only problems you'll run into. Most of the soot buildup will be burnt out when you switch back to straight gasoline. There is a chance of scoring the cylinder wall if a chunk of carbon gets between it and the piston.
. With extended use, not only will you run into the problems GH pointed out, but there is also the risk of soot buildup causing a hot spot and burning valves, pistons, and cylinder walls. Seems that TUA is doing fine with 50:1, but I still wouldn't chance it. Things will be worse if you are running the "old formula" of 30:1.
. All in all, it's not a good idea to run 2-stroke gas in a 4-stroke engine, but it can be done in an "emergency." If nothing else, it causes a lot of pollution.

Well, I was talking about a "long" time :-) and under some duress.

Errr as long as the mower is running below it's capacity by a good bit it should be safe enough, it'll smoke alot but it wont destroy itself unless it's burning alot of oil and petrol, doing it for a wee while isn't too bad a thing, just go easy on it, the oil contains more energy than the petrol and will burn, at 50:1 i wouldn't be worried, I've done the same thing before with no troubles...