OS 70 four stroke

[Tony Read 2]

Being mainly an electric flyer I very rarely venture into the world of glow power but decided to dust down a Flair Fokker D7 I acquired a couple of years ago. Its powered by an OS 70 FS which had been oiled up and turns free. The fuel tank is a nicely made tin plate job, which I ran some clean fuel through a couple of times.

The engine fired up after a couple of tickles with the starter and ran well, however at the top end of the throttle range it has an occasional mechanical 'clacking' sort of sound. Could this be the tappets or something more serious?

Whilst the engine would still run pointing up, it would quit when pointing down, however this may be a fuel leak problem, as I noticed some raw fuel running down the model.

Anyone know what the tappet clearance should be for this engine, and do you check them hot or cold?

If its more involved than the tappets I would like to get it checked out, who's around that could do this? I'm thinking 'Just Engines' perhaps?

[Tony Read 2]

Something else, what size wooden prop would be ideal for this type of model?

[john stones 1 - Moderator]

Google O.S. specs...manual Tony

All the info you need will be in it

John

[Alan Gorham_]

Tony

You may be running too lean at full throttle giving the mechanical sound. However it's good practise to check/reset tappet gaps after long storage. At least you will have ruled out a potential snag doing this.

One small point re wooden props on four strokes: they are often not recommended as they are low in inertia and can cause detonation which will kick the prop loose worst case or at least can cause uneven running at idle or lower throttle settings. You may have better results with a heavier glass-filled nylon prop.

As John says, get a look at the kosher OS manual and you will know everything you ever wished to know about your engine.

[Tony Read 2]

Thanks guys.

Alan - Interesting about what you said concerning wood props. It did kick the prop loose once, so maybe I need to change it. Wood does look good though on a WW1 type.

[Alan Gorham_]

Kicking the prop loose can also be a sign of a too lean setting at full throttle Tony.

Give the tappets a check, fit a new glow plug and a decent non-wood prop and tune it 3-4 clicks rich of best setting at full throttle and you will be there or thereabouts.

[Braddock, VC]

This is just me but I'd get rid of the metal tank and replace it with either a slec square one or a dubro square tank. 8 ounce size would be ideal.

You will then be able to see the gubbins inside the tank and know for sure that the pipes and fuel tube are new and airtight.

Connect it up so that you can fill and drain through the engine feed line and connect up a vent to the exhaust for pressurisation, if it's a 3 line tank, plug the other one.

Then tune as per the other chaps described. I use a 13x6 graupner g-sonic prop on my saito 62 and hat will probably suit the 70 but if you can get a 14x5 it'll be so much better. I bought a few at Jim Davis models in the early 1980s and they really help high drag planes fly well.

Edited By Braddock, VC on 02/07/2014 14:54:15

[cymaz]

The sound could be pre-detention of the fuel. Either because it is too lean or being well under proped. Check the barrel is fully open at WOT on the Tx.

[Tony Read]

Thanks again.

I have now removed the engine and after a lot of effort, removed the fuel tank, it was well siliconed in. I'll fit a slec one in its place. Thanks for the size Braddock, much appreciated.

I'll have to measure the prop but would not think that its as big as a 13x6. It might well have been under-propped.

Now the motor now its out I'll see if I can detect any issues with it.

As I say, thanks for all your comments.

[YakMad]

I had 2 OS 70's with 13X8 props and they were very happy, I always felt that the OS four stokes have more torque but not were not happy being under propped and sound harsh on smaller props unlike the my Saito 62 and 72's which are happy to rev alot harder.

[Tony Read 2]

The slec square tanks have two fittings in the middle, a fuel inlet and an outlet to the engine. To pressurise the tank you are supposed to run a T off the former. Is there any reason why you cant simply run a pipe from the higher of the two 'nipples' moulded near the edge of the tank?

[Former Member]

[This posting has been removed]

[cymaz]

I loosen my wooden props after the days flying session or if I'm not going to use it for a while.

[Braddock, VC]

slec tank. The two spigots in the rectangular slot are for exhaust pressure and fill (or plug if you follow my advice as you have to invert the a/c to draw the fuel out of the tank)

The two spigots or nipples in the round holes are plugged and are intended to connect with silicon tiube taking it outside the a/c when you can check the level of fuel in the tank. Before you do this you must drill out the nipple witha 1.5 mm drill.

[Stevo]

Great discussion. I run nothing but 4S anyway.

Regards tanks, I've used a few, and went almost entirely to SLEC. I had an old W4 that refused to start once, and when I looked into it all the brass pipes had dissolved. I've never had a problem with a SLEC, but I do see that many have, particularly a failure in the joint. A couple of my ARTFs (Blackhorse) have the usual 'bottle' tanks but the pipes are plastic so I tend to keep them that way. I always block off one of the top two spigots, and use one for the exhaust pressure. I drill out the lower one for filling and draining and this works well.

Throwing a spanner in the works, I got a S/H Pilatus at a bring and buy (bargain!) and it had a SLEC tank. And, yes, folks, it leaked around the seal!!

Props - yes wooden indeed for the WW1 types. But for looks only, particularly for a 4S. The 'plastic' types have more mass creating more inertia keeping it all ticking over - they run far more happy that way. What would I run on a 70FS? my Spacewalker has a Saito 72, 13x6, very happy on that one. The DVII has a laser 70, again 13x6 and once again, delightful.

[Stevo]

Ha!

And I never answered any questions!

The tappets are adjusted with the engine cold, and really are for guidance. I've never had any problems with an engine's performance if these are slightly out, as long as there is clearance and it's not way too much.

What would I do with that engine? Change the pug, check tappets, run with correct size prop and DON'T go full bore with the glow driver attached else it will pre ignite and may throw a prop or worse. That's about all you can do, really... What fuel are you using?

[Tony Read]

Thanks again folks! Great common sense advice.

Stevo, I'm using Model Technics fuel with 10% nitro.

[Stevo]

Tony - exactly what I use.... you'll be OK with that

[GrahamWh]

Tony I have an old OS 70FS and a new ASP FS70. I find that 13x6 is okay but it's easier to slow my biplane down with a 14x5 which I prefer. But it seems to be very hard to find a GRP 14x5 so I am using Turnigy Cherry wood 14x5's from Hobby King and they are a bit light. If you can get a GRP 14x5 I think that would be the best choice if you're flying a slow plane. Does any one know where one can buy a GRP 14x5?

The tappet setting for the os fs70 surpass is between 0.04 and 0.1 mm according to the manual for both inlet and exhaust valves.

If you pm me your email address I can send you a copy of the original OS 70FS manual I downloaded from somewhere on th'web.

[Steve Hargreaves - Moderator]

I'd be wary of using a wooden prop on a 4 stroke...they can be a bit too light & lack the "flywheel effect". In Grahams case a wooden prop seems to do the job.....probably because its a fairly big 14" diameter....

[cymaz]

Al's Hobbies do a 14x5 narrow APC.

[GrahamWh]

Thanks Cymaz.