Can someone please advise on a problem with OS48fs Surpass

[Edgeflyer]

Bought used but looking in ok condition. I checked the valve clearances and set them to 0.04mm. Compression seemed a bit low initially but on a test stand after quite a bit of priming /choking and flicking eventually it fired and occasionally burst into life for maybe half a second but nothing more. By now compression has improved considerably.

The problem is I can only make it fire during or after choking with an electric starter turning it over and then mostly with the throttle shut.  A 12x6 prop seemed better than a smaller one. On the occasional burst the needle valve doesn't hold position and seems to have a slightly loose sealing ring (,is this a normal o ring I can change?).  Is this all basically fuel feed problems or valve timing or what? I suppose I will dismantle and look for issues . Under the rocker cover all looks nice. There is the usual black gunge in the exhaust chamber 

[Simon Lumsdon 1]

Well a few thoughts come to mind

 

1) the gap should be between .04 and .1mm so perhaps open the gaps up a tad just in case this is affecting compression

 

2) find a piece of rubber tuning that will fit over the needle valve snugly, this should prevent air leaking in and the needle moving

 

3) try a different plug, they wear out!

 

This engine should fire on the button (needle valve wound out three turns as a starting point) so something is wrong

 

Prop sounds fine

 

Good luck

[J D 8 - Moderator]

  Have you had the needle out? There may be some old gunge in the valve restricting the fuel into the carb.

[Martin Harris - Moderator]

That’s very often the case after extended storage.  Put a (clean) piece of fuel pipe on the inlet nipple, crack the throttle open so there’s a small hole that would allow a 1/16” drill to pass and blow into the tube. The high speed needle should be around 2 1/2 turns open. 
 

You should hear a small hissing - if not, you have a blockage (assuming it’s an air bleed carb) which may require the carb stripping and cleaning. If you have a twin needle version, try opening the low end needle until it hisses. This should allow a start when normal tuning should be performed.  It may be that old oil deposits will flush through and the low end needle may need readjusting after a little running. 
 

If you can get it running but suspect a blockage affecting reliable running or throttling, get it to high revs and with the throttle wide open (and with great care) block the air intake with a finger.  The suction will often pull any soft gungy blockage through. 

[J D 8 - Moderator]

  I had an old petrol Land Rover with similar issues, a bit of crud from the tank would block on of the fuel jets, engine would rev but not pull. Revving it up and slapping a hand on the carb air inlet would suck out the crud and then off to go.😁

[Edgeflyer]

Thanks for the replies.

I have checked the valve hisses when opened etc and the engine pulls fuel through the line when I cover the air inlet. It also blows it back again if I turn the prop clockwise. Is that a symptom?

I wonder whether the mount the engine inverted so as to be able to prime a few drops in the intake with the needle shut. Times gone by engines would start and run for the duration of the prime this way!

[Simon Lumsdon 1]

Dont overthink it, this engine should just run. Prime the engine by placing finger over exhaust (assuming you are using a pressurised tank) or carb and flip prop a few times, this should draw fuel into engine easily.

 

Make sure glow plug is on a separate battery to electric starter assuming you are using one and its charged. First guess its the plug. Try again and good luck.

[Cuban8]

Been here many times before with club mates' used secondhand 'bargains', so don't despair.  Obviously the engine's history is unknown so you're working pretty much in the dark - take things step by step and  keep things simple. You can satisfy yourself that the engine timing hasn't been messed with very easily and this is covered by a number of people on You Tube. Easy to do, so I'd be surprised if even the most ham fisted would get it wrong.

Make sure that the carb is completely clean as a first step - it sounds like the engine is running on the initial prime and then quitting - at least it does run...... You've already checked compression so it should be good to go once the carb has beed cleaned within an inch of its life and initial settings are made.

[Nigel R]

If it fires, runs and dies, it feels like a fuel/carb issue. My first bet would be gunk getting into the carb. Old fuel can congeal. Lumps of gunk throw the tuning.

 

I'd be tempted to pull the carb, remove needles, give it a soak for a few days, and reassemble.

 

48fs had twin needle carb IIRC, and the needles are not usually loose. They have the internal O ring, the fuel tube dodge won't work? Might be worth changing the O ring for a fresh one.

[Engine Doctor]

A few points to check .

1, The "O" that seals the carb to the inlet manifold is often overlooked and can leak air causing poor running 

2, The inlet to head flange. Some Ive had have a gasket fitted while others are a machined surface jiont. If a gasket has been fitted and over tightened this can cause a air leak causing poor running 

3, Needle valve, early examples had the thinner needlvalve with very fin thread and no "o" ring these often b3nefit from a tight fitting piece of fuel tubing over the threaded section to seal against air leaks. Later model still with air bleed carb had the thicker type NV that had an "O" ring fitted and this should be tightish fit. If when running any fuel leaks out , air can get in.

4, Check that NV will shut off fuel when fully screwed in.  Check with a lenth of fuel tubing and blow down until air stops. Replacenent needles can look similar but have different ends on them.

If it shuts off ok then set it at 2 1/2 turns out to start with and adjust accordingly.

5, When you get it running and nice and hot compression should improve as it sounds like it had a gummed piston ring. Comp will never be as good as a ABC type set up but a resistance should be felt over compression . If compression still fels low then it may need stripping and the ring groove cleaning or the ring replaceing. Also check valves are seating by turning over and listening to the exhaust and inlit , with throttle fully open an listening for air leaks.

6, if you suspect that timing has been messed with then it very simple to check. Remover cam cover, two screws on side of engine. Turn engine to TDC between compresion and look at the rnd of the camshaft you should see a small dot marked on the cam wheel. This should be inline with the centre of the csm follwer above it.

Make sure it has a FS glow plug and good fuel . 12  x 6 prop is too big for a 48 try an 11 x 5 or 6

Hope you get it sorted and keep the engine alive .

Edited September 16 by Engine Doctor

[Jon H]

10 hours ago, Engine Doctor said:

12  x 6 prop is too big for a 48

 

really? 12x6 or 13x5 are my go to props for 50 class 4 strokes and my dads old 48 swung a 12.5x6 graupner for years with more than healthy rpm. On 11x5 it would rev like crazy, but 11x6 was an ok prop for running in i guess. 

 

Beyond all that the problem does seems to be either fuel flow related or, as has been suggested, a timing issue if the engine is 2nd hand has unknown history. 

 

And as i always suggest, see if you can pop up a video showing the problem. Picture paints a thousand words and all that. 

 

 

Edited September 16 by Jon H

[Djay]

I have one of these I brought the first week they were released in UK and it is still running in my Super 60 model.

The best thing I did to it for reliability was swap out the original carb for an ASP 52fs  carb and never looked back.

It was a straight plug in and runs like clockwork.

Did the same with my OS 40FS brought when they were first released in UK and this is still singing away in a vintage plane.

 

12x6 prop for the 48 is perfect size.

 

Darryl

Edited September 18 by Djay
added prop details

[Engine Doctor]

On 16/09/2024 at 09:36, Engine Doctor said:

 

Make sure it has a FS glow plug and good fuel . 12  x 6 prop is too big for a 48 try an 11 x 5 or 6

Hope you get it sorted and keep the engine alive .

I found both my 48 Surpass' would pre ignite and throw a prop on a 12 x6  but ran perfectly on 11  inch . Jon disagrees but we can only speak as we find and if it works for you then fine. 

[Nigel R]

I found similar with the 40 Surpass, 11x6 was ok, did have one or two thrown props, it seems happier on a 10x7 or 11x5. The 40 only has the simple air bleed carb.

[Martin Harris - Moderator]

Wooden props?  My old 40 FS will throw a wooden prop at the slightest hint of pre-ignition but is quite happy on the same sized but heavier APC. 

[Nigel R]

Graupner super nylons in both sizes - I'd always considered them relatively heavy props.

[Jon H]

8 hours ago, Engine Doctor said:

I found both my 48 Surpass' would pre ignite and throw a prop on a 12 x6  but ran perfectly on 11  inch . Jon disagrees but we can only speak as we find and if it works for you then fine. 

 

I have found that fuel seems to play a big part in prop throwing. My enya 53 and os fs40 would throw props on the old mt laser 5 but not the new optifuel flavour. Equally, my dads 48 did throw props in the early days when using 20% castor but it stopped once he switched to sport 5. That said, i ran that engine on every brew under the sun and couldnt get it to throw a prop so...yea, no clear cut answers im afraid. 

 

5 hours ago, Martin Harris - Moderator said:

Wooden props?

 

my saito 45 purrs like a kitten on a wooden prop, and i know os52 surpasses are happy enough. 

 

[Engine Doctor]

Hi Jon Something you posted about you dads old 48 got me thinking. Old being the operative word. An engine once run in and with everthing at perfect tolerance may be oved compressed . As it gets a bit worn it will lose a touch of compression making it an easier to use engine. The 48 Surpass was still early days for fourstrokes and OS were trying to push the boundaries a bit so possibly made it with too high compression ? Move onto a  52 Surpass etc and OS have got it right.

My old Enya 60 4c was a pig when I bought it many years ago. I tried all types of fuel, every different plug I could find but it continued  pre igniting and  throwing props to the extent that it bent the gudgeon pin and damaged the piston and con rod! I fitted a new piston , gudgeon pin and rod but still the same . I then cut a head shim from some anealed Litho plate and hey presto engine transformed. The head shim was the the fix and have used this fix on a few engines over the years , sometimes in addition to original head shim.

Its still fitted in my x2 LadyBird and purrs but can still do the busness if needed

Saitos are the tricky ones being fixed head but fortunately dont suffer with preignition much unless really pushed or run too lean.

[Jason Channing]

Saitos never had a problem, All you need to do is add an extra gasket to the barrel and reset the rocker clearances,

[Cuban8]

Ages ago a mate had an Enya four stroke, the size of which I forget now. It was a terror for throwing props and many a time I had one whistle past my ear....pre health and safety you understand.  Don't recall him ever really solving the problem, but I suppose we were much less experienced in such matters forty plus years ago.

[Paul De Tourtoulon]

2 hours ago, Jason Channing said:

Saitos never had a problem, All you need to do is add an extra gasket to the barrel and reset the rocker clearances,

My two Saito .100 did, like my Enya .90 and 120 4c, all needing 0.4mm head gaskets.

[Jon H]

6 hours ago, Engine Doctor said:

Old being the operative word

 

Actually there's a bit to go through here. My dads first 48 surpass threw props like it was going out of style. These were graupner grey jobbies and the model was a flair pup. That engine bit the dust as its crankcase decided to explode randomly one day. The next engine was nowhere near as bad, but still did it from time to time. Changing fuel away from 20% castor certainly helped, which makes sense as a reduction of oil will slightly reduce compression ratio, but i think my dad also got better tuning it, and he lowered the tank preventing progressive leaning in flight. So certainly a combination of factors there. 

 

In any event since that time the engine has never wanted to throw props

[Cuban8]

20 hours ago, Jon H said:

 

Actually there's a bit to go through here. My dads first 48 surpass threw props like it was going out of style. These were graupner grey jobbies and the model was a flair pup. That engine bit the dust as its crankcase decided to explode randomly one day. The next engine was nowhere near as bad, but still did it from time to time. Changing fuel away from 20% castor certainly helped, which makes sense as a reduction of oil will slightly reduce compression ratio, but i think my dad also got better tuning it, and he lowered the tank preventing progressive leaning in flight. So certainly a combination of factors there. 

 

In any event since that time the engine has never wanted to throw props

Thinking about my mate's Enya and its delightful habit of throwing props at unsuspecting bystanders, considering all this was happening forty years ago and we were almost certainly still stuck in the dark ages with fully 20% castor based fuel, then that brew certainly wasn't helping matters, I suppose.

Edited September 21 by Cuban8

[Engine Doctor]

20 hours ago, Cuban8 said:

Thinking about my mate's Enya and its delightful habit of throwing props at unsuspecting bystanders, considering all this was happening forty years ago and we were almost certainly still stuck in the dark ages with fully 20% castor based fuel, then that brew certainly wasn't helping matters, I suppose.

Yes and i seem to remember one manufacturer made a Fourstroke mix with extra oil ? 

The head shim on Enyas works a treat and I currently have a Enya 120 waiting to be done. It was aquired S/H in an airframe felt good so decide to fly it. It threw a prop on 15% fully synfhetic oil fuel while running up  so its in the box waiting .

 

[Mike T]

An idle thought: couldn't you achieve much the same effect (decompressing) with an extra washer under the plug?