Going from 2 to 4 strokes

[James A]

Hello all!

 

Thinking about going for a 90 size 4 stroke in my next model (not sure what yet but something semi scale).

 

I have only ever had 2 stokes before (all SC - always found them very reliable and can't argue at the cost so will prob get one of their 4 strokes). What do I need to know/consider when operating a 4 stroke? Will I need different fuel/plugs? Any differences in starting, running in, setting up? etc etc

 

Any thoughts/advice gratefully received!

 

Many thanks

 

James A

[Bob Cotsford]

OS-F plug works well with 4 strokes, it's made a lot of difference to the tickover and pickup on a few of mine (ASP120Fs's, SC52FS).

I use the same fuel for everything, standard MoGlow 5%.

Think bigger props than for a two stroke, typically 14*6 or 14*8 for a 90.

Other than that, it's just an engine. Follow the instructions for running in, keep it rich for a few flights and enjoy

[Biggles' Elder Brother - Moderator]

I agree with Bob. These days I'm 100% four strokes - I just love the sound and the fact that you can use a more scale sized prop.

 

I use the same fuel I used previously in 2 strokes - for me Pro-Synth 2000 5%. Like Bob I use the OS F plug - costs a bit more than a lot of the others but I've never had any problems with them so I stick with them.

 

Like with 2 strokes running-in instructions vary a bit from one manufacturer to the other so just follow what they say.

 

Don't set your tick over too slow - some folks have them sounding like a tractor! 2000rpm is slow enough. And remember they don't tend to rev out so high, 9,500 to 10,000rpm is top whack for most. Just like with a 2 stroke don't pursue that last 200rpm at the cost of running too lean. Let it run richer and a little below the absolute peak.

 

One thing I would suggest, invest in a tacho iff you don't have one. They are not expensive and they are very useful with a 4 stroke as, for some reason, peak rpm isn't quite so easy to pick up as with a 2 stroke.

 

Bob's right - a 90 will easily swing a 14X7 even a 15x6 is possible.

 

Its worth checking the tappets occassionally - the clearences can go out. The instructions will explain how. But they will be set fine when you get the engine - or at least they should be!

 

That's it really - enjoy!

 

BEB

[Simon Chaddock]

 

James A

The reason that a 4 stroke can swing a bigger prop than a 2 stroke is two fold.

 

1. The power stroke is almost twice as long so more of the combustion energy is extracted. This gives the 4 stoke its fuel efficiency.

In a two stroke the exhaust has to open much earlier to give time for the induction to take place.

2. The bigger prop is heavier. Important as its inertia has to carry the engine over an extra crank revolution (exhaust and induction strokes) compared to a 2 stroke.

 

With only half the number of exhaust beats a 4 stroke may not sound like it is producing much power but at is preferred rev range each impulse is virtually twice as powerful so within reason give it some work to do!

[NigelH]

Another thing worth considering is that the four stroke throttle linkage isn't necessarily as easy to set up as on a two stroke. Some vendors sell hardware to make installarion easier.

 

[PatMc]

Sorry Simon but I think you have this completely wrong.

 

Model 4s engines are generally designed to deliver peak power at lower revs mainly due to the valve gear. Which means lower revs on a bigger prop for the same bhp. But at the same capacity a 2s is capable of being about twice as powerful as a 4s & can be as energy efficient but generally aren't because of the manner they are used. Many 2s engines can be ran with the same size (for them bigger than normal) props well below their peak but still giving as much or more power than the equivalent 4s. I used to use a 12x6 on an OS 25 in a Junior 60 - bags of power it ran for ages on the normal size tank & most people thought it was a 4s because of the low noise.

 

The prop doesn't need to be any heavier either, before I turned to all electric I used light wood props as well as plastic on both 2s & 4s engines.

[ken anderson.]

hello james-i think the Main reason that people buy and use 4st motors is the noise--they sound like an aero engine..ie spitfire...use one the same as a 2st ...but beware if you are tempted to lean the motor out too far...i've seen one miss a beat when been tried at full rev's and the prop been thrown off in the pits......apart from that - no probs...

 

ken anderson ne...1 .... 4st dept.

[James A]

Thanks all for the replies. Not such a black art after all then?!

 

NigelH - any chance of elaborating on the issue with the throttle linkage?

 

Cheers

 

James

[Gary Binnie]

On a safety note please be aware that four-strokes can spit their props off, even with the locknut arrangement that most of them have. Always stand behind the prop after starting and warn people not to stand in front while it's running.

 

On inverted installations they can flood and suffer a hydraulic lock in the cylinder, most instructions recommend checking for this (by turning the prop by hand) before applying a starter.

 

Cheers

 

Gary

 

 

[Biggles' Elder Brother - Moderator]

Gary is right there. They are particularly prone to loosing props if they are allowed to predetonate or misfire. This usually happens when the mixture is a long way out so follow the instructions on initial needle valve setting carefully and you will be fine.

 

Best way to check for hyfraulic lock is to turn it backwards onto compression and feel for the spongy "bounce". Do this without the glow stick attached through because warm 4 strokes are very easy to start - literally just bouncing them backwards against the compression can do it! See the video for what I mean - a standard 4-stroke "party piece"!

 

 

BEB

 

 

Edited By Biggles' Elder Brother - Moderator on 04/09/2011 22:04:42

[Hamish]

Collected an old Fleet Finch bi-plane today that has a brand new MDS 52 fitted. Might be a good engine, but this type of model must have 4//// to give it the authentic noise. Have an ASP61 which will fit in nicely.

It's not all about power and hopefully with the ASP I will have the same power but the right noise.

[NigelH]

Posted by James A on 04/09/2011 19:25:18:

Thanks all for the replies. Not such a black art after all then?!

 

NigelH - any chance of elaborating on the issue with the throttle linkage?

 

Cheers

 

James

 

I'm not an expert on four strokes but I've been there. You may not get an easy straight run like you would with a two stroke linkage. Some people end up with several bends but that could result in excessive play.

 

You can buy, for example, Dubro hardware to makes things easier.

[Biggles' Elder Brother - Moderator]

I've heard others talk of this, but TBH I've never really had any problem with throttle linkages on 4 strokes - maybe I'm just lucky in the prototypes I pick!

 

BEB

[Bob Cotsford]

I just use wire pushrods and bend the end to reach the carb, using a z-bend to hold it in the throttle arm - no problems to date with lost movement . I do like that Dubro linkeage as a piece of gadgetry though.

[Peter Miller]

A few points and I have been using four strokes since they came out.

 

I use my old favourite Taylor plug, never have any trouble with them. Use them in everything. I run everything on 10% fuel. mainly because I use it on my smaller engines and can't be bothered with having more than one type of fuel.

 

For some reason none of my SC 30 FS engines will run properly on Prosynth 2000. All the other engines are perfectly happy on Prosynth but not the SC 30s and when all three engines refuse to run on a fuel but run happily on other fuels it has to be the fuel.

 

I use SC engines unless I get a real bargain on EBay.

 

I have a very simple and effective set up for the throttle cable. I use metal bowden cable and this is extended past the throttle arm. I make a short "Z" bend and bind and solder this to the cable but facing back towards the firewall.

 

The other option is to take a pushrod connector and fit that to the throttle arm and run the bowden cable through it.

 

I use both these methods regularly.

[ken anderson.]

same here as BEB...never had any problems with linking up 4st's.... just a case of bending to suit...

 

ken anderson ne..1..... 4st dept.

[Gary Binnie]

I think the throttle linkage is easier than a two-stroke myself, never even thought about it till now!

 

Anyone had an OS FS26 or the even smaller, earlier OS FS20? That must have been a little gem!

 

I find the recessed glow plugs of the OS FS engines a bit of a pain for connecting a remote glow lead, especially if it is a right angle one. I think OS's lead is not a right angle job, ought to get some!

 

Economy has been touched on, my four-stroke models fly forever on the silly big tanks I have in them and I usually land when I get bored, well before the fuel runs out!

 

GB

 

 

 

[Shaunie]

I usually start all my engines with the method shown in that clip BEB posted, twostrokes included. No putting your figer in the prop and no bending things with the electric starter either

Shaun

[Peter Jenkins]

One other thing with a 4 stroke, avoid using fuel with castor oil. That clogs up the more intricate mechanisms found in a 4 stroke versus a 2 stroke. Most OS and Saito 4 strokes need 18% oil in the mixture. I run all my 4 strokes (and 2 strokes) on 100% synthetic oil fuel and, indeed, none other than Brian Winch recommends the use of 100% synthetic oil. Some heli applications need more oil - 20-23% being figures I've seen quoted.

 

The valve clearance is another interesting issue. Saito provide a gauge but this is so thick that you are better off using car feeler gauges (that dates me!). My Saito 180 has a recommended gap of 2-3 thou whereas the Saito "feeler gauge" is almost 3 times thicker. Remember that the engine crankcase is usually allow whereas the pushrods are steel so the gap will tend to get bigger as the engine warms up.

[Frank Skilbeck]

Worth using a tachometer to set them up, they slow down a little when you lean them out too much and lose a few revs which is a bit hard to pick up by ear

[Peter Beeney]

It would seem as though we all have slightly different ideas on running 2 strokes and 4 strokes and generally speaking they all seem to work. I’ve used both flavours right from the time of the first OS 60FS; when I obtained mine the design was so new it didn’t have an oil drain nipple in the crankcase, I had to fit one, admittedly soon after I’d bought it. Like some other engine users, I’ve used synthetic oil for everything, right from the day I was able to buy fuel that contained this. Mine is slightly thinned down, to around 15% oil and as I like to add a smidgin of nitro to 4 stoke fuel, say 10%, that lessens the oil content a touch more. However, it seem to me that there is always plenty of oil kicking about, even with this reduction; and I believe one line of thought says that the unburned oil, and there’s plenty of it, even so, which gets ejected in the exhaust gas helps to carry the heat out. I’m sure this is correct, but I’ve never been aware of having an overheating problem as a consequence. These little drops of oil in the exhaust can get pretty hot, though!

I too have some ideas about running in, I’m also not struck on the idea of running in very rich, either, so I don’t do this very much. With 2 strokes I tend not to bother too much , they just get standard attention, but with 4 strokes I usually add a little extra oil in the first two or three tanks, that’s the medicinal Castor variety, from Boots. As as been said before, if it goes wrong then on my head be it. Which I fully accept, but in many years this has never happened, so I’m keeping my fingers crossed for just a few more trouble free!

At the moment I’m expecting delayed delivery of a SC 1.80FS, which will take up a long awaited residence in a Seagull Sea Fury. I quite like a bit of hurry-up, and if I ever get it I’ll bench run it for a while to get familiar with it. I’m sure we’ll get very friendly! This will be with a bottom of the prop range size, run it in fairly fast but lightly loaded, and then later with a 10 inch pitch on, if I can get 10,000 rpm this theoretically gives it a speed of around ninety miles an hour. That should at least be adequate for some nice vertical and wingover stuff, for starters.

I’d agree with the reasoning that says it’s difficult to just judge the steady speed of the engine, I often use a tacho, it gives you a climatic comparison, and also a rough idea of the model’s possible speed. However, I think it’s possible to hear the transition time, the short period when the note changes frequency slightly as the revs change. As Frank says, when you lean it out too much the revs can very slightly fall and I would also say to ‘harden’, for want of a better description, the ‘timbre’ of the sound can change a touch, but you have to listen carefully and it does take practise. When I hit the hot spot I tend to leave well alone, I’d adjust it from then on only if absolutely necessary.

I don’t think that I’d entirely agree that the engine has to be way out of tune to detonate, or backfire, if you will. I remember an early OS 90FS, (not mine) that did this a few times, always in the air. If the mixture was made any richer the performance started to suffer. I think the margin here between the correct mixture and a ‘quick-stop’ lean time can sometimes be very narrow. But it was back in the times when we didn’t fully understand what was going on; I’m not sure that I do even now but I did relate what I thought was happening when this happened in another thread recently so until I see a better explanation I’ll stick with that.

I think some of this is down to design. The glow engine is pretty basic, perhaps one good reason is to to keep the price down. But the manufacturer will also be looking to maximise performance, an important benchmark. So on one hand we have ASP, SC and even OS, for what they are, and on the other we have Laser, which by design will not detonate but perhaps does not have the sheer out and out performance of the others; and, indeed, may well be a bit more expensive; or even more than a bit; but the quality is superlative, too, and it seems that anyone one who wants everlasting reliability buys a Laser!

Running in can be a bit different. Once upon I time I was sent off to the Rolls Royce Factory at Shewsbury to learn about maintenance on their big V diesel engines and at one stage someone asked the course lecturer about how RR would run these engines in. The reply was instant, he was an extremely knowledgeable and amicable chap, and he said they threw everything they could find at it. Flat out with maximum load, all the way! We were a bit surprised, it was not quite what we expected. But, of course, it’s what we used to do anyway, as soon as we got our hands on them!
So it’s not always the same procedure, for instance, ABC two strokes tend to be run in a bit fast too, indeed flat out, as I’ve always understood it.

PB