How does the fuel , starting and running of a 4 stroke differ from a 2stroke motor?

I use the same fuel in both - 15% Cool Power.

Starting is pretty much the same - Flip the prop a few times, add glow igniter, flip the prop. The only difference is with a 4 stroke, the prop will only reach the compression stroke every other turn instead of every turn.

As for running, they run quieter with a much nicer sound, and they don't spit gunk all over your plane.

One general difference between 2-stroke and 4-stroke glow engines is valve maintenence. After break-in, and then after every five hours or so of runtime, You will need to check and/or adjust the valves on your 4-stroke engine. If the valves aren't adjusted properly, the motor will lose power and could possibly prevent it from running at all. It's not a big deal, and most 4-strokes come with the required "feeler guages" and tools required to do this.

If you are going to use a plastic spinner on a 4-stroke (rather than a locking prop nut, e.g.), make sure you use one with an aluminum backplate rather than an all-plastic one. The added torque that 4-strokes generate can break an all-plastic spinner into tiny pieces and then throw them all over the place at high speed.

4-stroke engines tend to be a bit more sensitive to nitro levels and will respond better to higher nitro content than 2-strokes. Most 2-strokes won't behave differently if you use 10% versus 15% nitro fuel, but most 4-strokes will provide more power with 15% than 10%. Ironically, many higher-compression 2-strokes will run better with 5% nitro than 15% nitro, but novice pilots buy 15% fuel anyway thinking that "more is better."

Lastly, most engine manufacturers recommend a "4-stroke" glow-plug rather than a "regular" glow plug.

thanx guys !

What biged says holds some weight... You WILL need to use a 15% nitro fuel, and you DO need to use a 4-stroke glow plug (Which run about 10 bucks each) but I almost ALWAYS use plastic-backed spinners and never had one break. And while I do check the valve clearence occasionally (Actually "Rarely" would be a better word) I have only had to adjust one once - and adjustment is easy.

I only mention this because many people are afraid to try a 4-stroke because that have heard that they require a lot of maintanance. This simply is not true. Once you try one, well, I won't say "You'll never go back", but I will say that you'll use 2-strokes a whole lot less!

I just encountered some problems with my first 4stroke. What I was told and how that advice worked out would be worthwhile to hear.

It wouldn't start worth spit. I was told that "4strokes need 15% fuel to start good". I was using 10% so went out and bought a gallon of 15%. The engine did start better. I was at a field later where the majority use 4strokes and mentioned the 10% vs 15% deal and just about started a "Jerry Springer show" of all the guys shouting and waving their arms about how only idiots don't use 15% and how lousy 4strokes run on 10%. I was amazed, but since the engine still wouldn't start worth spit (it flooded almost every attempt) but was running somewhat better, I guess I could understand. But was amazed at the passion those 4stroke guys showed.

So I was told that 4strokes need 15%. The word was "need". I didn't notice that mine was any easier to start with 15%, but it did run better with 15%. It was easier to needle the engine and it didn't deadstick as often with the higher nitro. I'm told the higher nitro works better because it runs the engine hotter and that keeps the plug hot though the non-firing cycle. Which is also the reason you need 4stroke glowplugs. They hold their heat better when washed down with fuel in the nopower cycle.

So how do they start? My first one didn't. Not often anyway. I was at that "Jerry Springer Show" field when that crowd jumped me for not being able to start my 4stroke easily. So I let some of them have a shot at it. They didn't do worth spit either. What they did do was start up another "excited chimps in the zoo" deal when one of them noted that "4strokes need to be side mounted or upright". All of them took up the chant, so I'm guessing there is some truth to that. I know that the installation of mine was inside a cowl that gave no access to the venturi. It was a downdraft carb and being upside down would flood in a heartbeat. After ultimately swapping out the 4stroke for a 2stroke, the two stroke showed no problems being upside down, so there is some truth to "don't mount a 4stroke upside down". Yeah, I know lots of people do it. But that one didn't like it.

And while dealing with my 4stroke, the sucker ruined a couple of spinners that had plastic backplates. The engine had a tendency to backfire and a couple of backfire bouts were bad enough to strip off the serrations that help hold the spinner/prop solidly. I can't use those now even with 2strokes.

I'm very experienced with 2strokes. My experience with 4strokes so far would suggest to me that.......

They need 15% fuel.
They should be mounted sideways or upright to insure decent starting.
They might ruin a spinner or two.
Make sure you use 4stroke glowplugs.

Darock--I have run 4 strokes upright, sideways, inverted on 10%, 15%, 30% and 0%--it all depends on the setting of the low end and high end needles. Your flooding problems are most likely a result of tank height in relation to the carb. I agree that four stroke plugs are best and personally prefer the OS-F plug--I run them in my Saitos as well. As for starting, I have learned on two seperate occasions that if an electric starter is not used, be sure to at least use a chicken stick. I have been bit twice tring to flip an OS 91 using my finger--one time requiring stitches.

I got a chuckle out of your Jerry Springer Show comment. You are certainly correct that there are as many opinions regarding fuel, nitro, oil, synthetic vs castor, four stroke vs two stroke, wood props, fiberglass props, glowplugs, on and on and on. Best thing is find out what works for you and go with it.

Rock, I'm just curious... What brand/size 4-stroke do you have?

<Edit> BTW, 4-strokes do NOT like to run lean! 99 times out of a hundred, spitting a prop or breaking a spinner is caused by a lean mix.

I might have missed it, but a significant difference between 2 strokes and 4 strokes is that the 4 stroke carb is often not accessable for priming.

Found out exactly why the engine had flooding problems. It wouldn't start reliably when new and especially when new and upside down. And because it wouldn't, it's "upside down" carburetor pipe caught all the fuel that came in instead of letting it drop out. That's engine being upside down and being finiky on getting just the right conditions to start were the reason for the flooding problems.

If the tank height was a problem with that engine, it wasn't with the replacement engine. Which was a 2cycle. Both engines have the carb in the same vertical position above the crankshaft right at 1" above. What they don't have in common is the 4cycle's pipe that runs from the carb to the engine. The 2cycle's carb is directly over the crank and the intake valve (which is directly underneath, in the crank itself). The 4cycle has what was working as an accumulator pipe that the 2cycle doesn't have. Two cycles don't have those long delivery pipes. And when I replaced that 4cycle with a 2cycle on that airplane, and without doing a thing to the airplane and plumbing other than unplugging the hoses from the 4cycle and plugging them to the 2cycle, the 2cycle never once flooded. Tank location causing flooding? Probably not. But if it was, then you could add another difference to the list on the 4cycle side. Need different tank locations that 2cycles to keep them from being prone to flood.

Hey, it wasn't my idea that some 4cycles don't work upside down. It was a gaggle of 4cycle guru's collective judgement. I've only had one 4cycle and it was backing up their belief in spades. And I'm passing on to a newbie what another (4cycle) newbie encountered and then was told. I've only had one and it was a pita to start while upside down just like all those experts told me. When I later ran it upright, it started almost every time and when it didn't, it didn't then fill up that collector pipe.

Having fought the "model airplane engines don't start good upside down" fight for years........ yeah, I know it's not really true. When they're upside down, you really have to have the starting technique down pat, and it starts from when you fill up the fuel tank. But what I'm saying is that what looks to me to be a standard layout 4stroke appears to be vulnerable to flooding from it's design. And they all look like that. So......... 4strokes seem to me to be even more critical to having a good starting technique down pat, and you'd better get it right for sure with a 4stroke.

OK, sounds like you got it all figured out. Good luck.

Don't usually like to dispute what other have said, but my experience runs contrary to what has been posted here.
First, I run all my four stroke engines (OS 120 Surpass III with pump; OS 70 surpass II blackhead; OS FL 70 and Magnium 91 RFS) on less than 10% nitro. The nitro percentage is around 7.5%. To further stir up the pot, i use quite a significant bit of castor oil; medicinal castor oil. This was not necessarily out of choice, but out of necessity at the time, as I as based in Dhahran Saudi Arabia where fuel was pretty expensive, so had to go for an alternative.

The Magnum 91 and 70 surpass II are both inverted and idle just great (~2100 rpm). So do the others, which are at the 7 o'clock position.
One thing though; tank height was very important. I had major issues with the magnum until I lowered the tank about 1/3". Also they all have OS F type plugs in them.

My brother and I have switched over and now run four strokes on almost all of our planes.
(1) 61, (3) 70's, (6) 91's they are a mix of OS and Magnum.
We have never even checked the valves on any of them and unless one of them gives me a reason I don't plan to.
I used 10% fuel for the first couple of years and was completely happy.
Then we got a deal on a couple of cases of 15% fuel and we got a 700 - 1000 rpm increase depending on the engine.
I didn't expect the power boost but it was a very nice bonus and I will stick with the 15% nitro.
I use a tach to set the high speed needle and check that setting every 3 or 4 trips to the field.
These engines have all been great performers, very reliable, the only deadsticks i can recall were caused by a clunk being stuck and a cracked fuel line.
I see a lot of people constantly fiddling with anything that can be adjusted and they always seem to be having problems.
We try to set them up carefully the first time getting the low end needle set so the idle and transition is excellent, after that we pretty much leave that part alone.
If we are flying a lot we don't mess with the high end much either.
When we were using the two strokes we pretty much followed the same pattern.
The Magnums have worked very well for me, if I was making a recomendation on a "first" four stroke I would recommend OS.
Oh yeah, an electric starter is an excellent investment.

Actually, 2-strokes have the worlds longest "delivery pipes" commonly known as the engine's crankcase. When that crankcase gets loaded up with raw fuel from excessive choking, it can take five minutes of flipping the prop with the fuel line removed and having the engine run for short bursts before it finally cleans out and runs for several seconds with only the fuel that's in the crankcase. No wonder electric starters are popular.
You do the same thing with a loaded up four stroke, you flip it a couple of times and it runs for a short burst and guess what! It is now cleaned out and actually needs to be reprimed. Four stroke engines pump the excessive fuel through so fast that they clean out almost instantly.

Hand starting four strokes is not only easy but quite safe if you use the right technique. First of all, forget completely about flipping the engine through its compression stroke. That results in violent kickbacks that can really injure you or break chicken sticks. First, you want to prime the engine and get it really wet inside the cylinder. If you turn the prop by hand slowly with a firm grip on the blade, you should feel a "bump" almost as soon as the compression starts. That's what you want, good and wet. Now, close the throttle to a normal or maybe just a slightly fast idle position, light the glow plug, and then grab the spinner, not the prop, and spin the engine backwards. As the compression starts, the mixture will fire, reverse the engine and continue to idle forwards, usually on the first try. Remember, the idea is not to spin it backwards through the compression stroke but to throw the prop backwards into the compression. Your fingers should be off of the spinner before the reverse compression stroke even begins. Pure momentum against the compression stroke is what starts the engine. If using a chicken stick, position the prop so the chicken stick is off of the prop before the compression stroke begins and then give it a gentle slap backwards............VOILA.....a running engine.

One other thing. When using a 4 stroke engine, make sure you use a double jam nut arrangement to hold the prop on. This will keep the prop from going bye-bye.

Actually, they don't.

The crankshaft is immediately under the carbs of most 2cycle engines that're sold today. And the crankshaft has a hole in it. When the crankshaft turns it opens the hole for some degrees of rotation and that's the intake part of the cycle. When the hole isn't open, it's closed. And that "delivery pipe" isn't filling up. Matter of fact, it's not even collecting residual from momentum of the gasses.

With the 4cycle and it's long delivery pipe, there is an actual delivery pipe that is obvious for all to see that goes from the carb all the way to the head of the engine. It's a pipe with no flow control that stops gas flow for the majority of each rotation. With the 4cycle that long pipe goes to the head and when it gets there there is also an area inside (pull your carb/delivery pipe and look, you'll be amazed) that has more volume itself than the very small area between the carb and crank of any 2cycle. And THEN you get to where the valve sits, closing off that long pipe and large intake area under the head.

The difference in available flooding area that's shut off by the intake valve is huge. One is almost nonexistant and the other must be what? 10X? 20X? larger.

Thanks for focusing on this, because I'd only mentioned the pipe, and the area inside the head also contributes to "flood holding area" and is large and should have been mentioned.

BTW, lest we forget, when the 4cycle is turned upside down, that pipe and head is acting as a receptical roughly TWICE as often as the 2cycle. In fact, the 2cycle is open then closed each revolution. With the 4cycle, it's open then closed one revolution, and then closed an entire revolution before opening again.

Yeah, there really is a large difference with 4strokes. Both the "floodable" area AND the amount of time it's collecting "flooding".

Actually, OSMax suggests in some of the 4stroke manuals that their engines that do not have chokes will start best with electric starters. That suggest they know how hard it's going to be trying to hand start them. Especially when the engine is inside the cowl and can't be finger choked.

OK, the subject of this thread is what's different about 4strokes. With the design of most, if you cowl them in completely as the majority of our models do, the location of their intakes are going to be completely out of reach. Totally inaccessable. No way to finger choke or to squirt prime into. And if you plan to get the cylinder really wet inside you best figure out something special to do that trick.
However, if you've got the engine upside down inside that cowl, you could just run your electric starter for awhile, and the delivery pipe will provide all the wet cylinder you could desire. (newbies, that actually is a joke, don't plan on that doing anything other than flooding the sucker)

As it is with one of my OSMax's, the OS manual suggests if you plan to have the engine inside a cowl, the way to get it cranked without a prime or any ability to choke it, would be to hook up your starter battery, and simply apply an electric starter. They tell you that in their manual. And it works great. It didn't work too great with that engine for me until it was broken in and the needle settings were set. But it did work ok then. And until it was adjusted, while it was upside down and not broken in, it was a witch to get the sucker started without flooding it.

Any engine, two or four stroke that uses muffler pressure to the tank can easily be choked by merely holding your finger over the muffler outlet and hand flip the prop a couple of times with the throttle fully opened. Just make sure the glow lighter is not on. If you are not using muffler pressure then just blow into the tank vent line. Easy to force a little fuel into the carb.

if the intake on your four stroke is filling up with fuel, then your tank is too high--plain and simple. lower the tank and the engine will run great inverted. In actuality, four strokes tend to do better inverted than two strokes because most two strokes have the glow plug right in the middle of a domed cylinder head--so fuel will run right down to it before it gets vaporized. four stroke plugs are canted, and usually not where fuel can collect on them during the intake and compression strokes.

I prime four strokes for starting in one of two ways, by stopping up the exhaust to create tank pressure while cranking or sometimes I will use a long piece of fuel tubing and just blow air into the tank vent until the fuel reaches the carb, and it is very important to have the fuel line to the carb filled with fuel. Flip forward a few times to get the cylinder primed, light the glow plug, set the throttle to idle, and then flip the prop backwards as I mentioned before, and I have a running engine, usually on the first try.

Notice the emphasis on throttle position, it is that important.

I'm well aware that OS says to use an electric starter on all of their engines now. I think it is a CYA thing and who knows, they may have a financial stake in electric starter sales. In the older days, they detailed how to hand start engines.

Swapped out that 4stoke with a 2stroke. Didn't change a thing including the fuel tubing. Simply pulled the fuel tubing off and unbolted the 4 and bolted on the 2stroke and stuck the fuel tubing on it. I'd hazard a guess that process didn't move the tank at all.

And the 2stroke cranked right up without flooding. Plain and simple.

if you are looking for reliability. a 4 cycle will do the trick. Price?-2 cycle

you fixed the symptom, not the problem--plain and simple. There is nothing wrong with two strokes--i have both. But four strokes run extremely reliable when inverted--almost all of mine are set up like that.

If the tank was too high, it should have affected any engine equally. The carb on a 2stroke is about 1" above the centerline of the crankshaft. The carb on that (and most) 4stroke is about 1" above the centerline of the crankshaft. The carbs on the two engines wound up in exactly the same relationship to the tank in the airplane, so if the tank had been too high for one engine it would have been too high for the other, pure and simple. And the fuel tubing was routed roughly the same. The tank was in the same place in the airplane for the 2cycle it was for the 4cycle.

The 4cycle flooded for a number of reasons that were combined. The 2cycle didn't flood for another set of reasons. None of the reasons for either engine included the tank being too high.... or low.... or too far back.

That 4cycle flooded probably because of a number of reasons. All of them are worth listing for the guy who asked if there were differences he should consider for 4strokes. That engine was not broken in yet. It's high speed was very sensitive and the lowspeed wasn't properly set. I was not using whatever choking technique the engine might or might not have liked. The design of the "downdraft pipe" compounded the problem greatly. The downdraft pipe is a unique feature of most 4strokes. When it's upside down it sets up conditions worth considering and worth understanding.

The key to reliable inverted running is a correct fuel mixture at idle. My Saito needed to have the low speed needle turned an entire turn before the idle was correct. Unlike two strokes, a four stroke engine's low speed fuel/air mixture can be stupid rich and the engine still will have almost instant throttle response. The only clue is that it wants to quit at idle and has a voracious thirst for fuel.

I have also seen people who couldn't keep their inverted two strokes running for love or money. Unfortunately, I am usually helpless to help them because they have flown model airplanes for (fill in the blank) years so they obviously know what they are talking about.