SC .70 f/s hopeless running

[Dean B]

Seems I am yet again cursed with my SC70 four stroke engine refusing to run. I have it mounted in a Black Horse Chipmunk. Model was chosen for the large front area and the fact that the tank can be placed below the carburator with the SC70 upside down.

I have had a frustrating afternoon, the engine runs at full throttle, no problem, and seems ok at what I would call normal flying speed. About 1/2 to 3/4 throttle, trouble starts when slowing down to landing speed. about 1/4 ish. motor runs smooth for a few seconds then cuts out dead.

Things tried.

Several different plugs.

Longer fuel tubing so it loops up over the engine before entering carb. (harder to start but once going same symptoms)

Adjusting the tickover screw. With the cowl off this can be done while the engine is running. I can get a lovely slow run this way but when opening the throttle, any more it dies. Obviously too lean to run fast.

I have blown the carb through with an airline, and a piece of fuel tubing on every orifice.

Am at my wits end with this one. I have a harmon rocket with the same engine installed only this one is at 90 degrees not inverted. That runs perfectly.

Have I just got a bad engine or has anyone got any more things I could try before I rip it out and put a reliable brushless motor in there.

[ken anderson.]

hello dean....could be the fact that when you adjust the bottom end needle---you also need to adjust the main needle....give it a go...and try a make a mental note where the slow running needle was before you start......and only adjust the slow running needle by about just less than a 1/4 of a turn each time......not massive amounts....

ken anderson ne....1 1/4 turn dept...

[Simon Chambers]

Don't inverted 4-strokes need an idle bar plug or something to help keep it alight at low revs?

Does the engine run at low revs if the glow clip is left on? Not flying of course.

Si.

[Dean B]

Hi Ken, thanks for that one, I did re check the main needle after every tweak of the slow running jet. It didnt appear to need much adjustment either way. I moved the slow jet in tiny amounts initially. Then I tried adjusting it while the motor was running. Either way nothing appears to work. The annoying thing is the engine in my Rocket is fairly tolerant of needle settings, it will still run even if either adjustment is a bit out. The new one will not.

[Dean B]

Hi Simon, the motor runs fine at tickover. Goes on forever, its just the mid point where you would shut down the engine in readiness for a landing. I have had the engine on the bench the right way up. and it runs perfectly. It just wont work upside down. its very frustrating. I dont really want to turn the motor mount around to 90 degrees as I will have to start cutting more holes in the cowling.

[Alan Cantwell]

on board glow, sub C size battery, no more problems, you could get the tick over so slow, you could jerk your finger in and out the prop, (but dont)

[jrman]

If it's a new engine it probably needs a lot more running in before you can expect good transition from full throttle to tickover and vice versa. I've seen some SC's that have taken over a gallon of fuel before anything like acceptable throttling could be achieved. Set the slow running needle back to the factory setting as per the instructions and have patience.

[Dean B]

I wouldnt risk flying a model with the cursed thing installed. Its too unpredictable. I have just removed it yet again and mounted it on my bench, it runs perfectly. Its on its second gallon of fuel so I would expect it to be run in by now. Ticks over at just over 2000rpm. As soon as you turn it over and try the same test it refuses to run. Same symptoms. I could mount it the right way up but it would destroy the scale appearance of the model.

[Alan Cantwell]

try the on bourd glow,   works wonders

Edited By Alan Cantwell on 12/09/2012 19:42:53

[Dean B]

Alan, I have tried running it with the glow starter attached. Same effect. As soon as the mid point on throttle is reached, the thing just chokes and dies. As the motor runs ok the right way up I have decided to re mount it at 90 degrees. If it works then I will cut the cowling to suit. Must be just a foible of these SC engines. They dont like being inverted.

[Ian Skeldon 1]

Hi Dean,

I would think the tank should be in line (roughly) with the center of the engine, not below it. The fuel lines should never be any longer than needed to reach the needle valve.

Get the top end running right first, then get it slightly rich, then slowly screw the secondary needle in, until the engine will tickover steadily. If it cuts on transition to full, open the main needle slightly (richer) and re-try. You may need a slightly faster tickover than usual. Try different percentages of oil content, I use laser 5 percent fuel from model technics on all my 4 strokes.

If a modern (4 stroke) glow plug is used an idle bar wont be needed as long as it is energised correctly at start up.

HTH

Ian

[Robin Kearney]

Just a thought Dean, you say it is fine on the bench, is that up the fit way, or inverted?

Might it be worth (not sure how) mounting it on the bench inverted, using the tank out of the model if at all possible, and then try it at various heights above and below the needle valve? I'm wondering if somehow the height in the model isn't quite right for it to be happy at a certain rev range. Hmm, not sure, maybe clutching at straws though, but possibly worth a try, sounds like you've tried everything else! This might also identify a problem with the tank, although if it runs at tick over and full throttle that sounds unlikely.

r.

[Alan Cantwell]

sorry Dean, i only run OS and Saito, it works great on them,

[Bob Cotsford]

Posted by Dean B on 12/09/2012 19:53:18:

Must be just a foible of these SC engines. They dont like being inverted.

I've got an inverted ASP 91 4 stroke inverted in a Chipmunk (WM Super Chippie) and it runs a treat, throttles up nicely, smooth mid-range - the works.

I've got another ASP, a 120, in the Dragon Lady with the same good manners, yet another identical ASP 120 that I had first in a Wot 4 XL then in a Skybolt would not tune for a decent mid-range even with on-board glow. I've replaced it with an OS Surpass pumper because I was just getting frustrated with it. One of these days I'll try swapping carbs/inlet etc with my no3 reserve ASP to see if I can figure out what the problem is. It just seems to run excessively rich in the mid range. The only thing I can suggest for yours is to double check the valve clearances, if they are OK then return it as unfit for purpose. That's what I would do with mine if it wasn't a)second-hand and b)several years old.

One thing I might try is to swap carb components with a good engine - low speed needle and spraybar seem good starting points. Could be it's just manufacturing tolerances and another (OS?) component will sort it out?

One trick we used years ago was to notch the top of the hole in the throttle barrel removing material from the 'top' of the out facing side of the bore so that it closed later. This was to cure over rich mid ranges with the simple air-bleed carbs. It seemed to work but needed lots of trial and error. Very much a last ditch attempt at a cure.

Edited By Bob Cotsford on 13/09/2012 10:10:59

[Biggles' Elder Brother - Moderator]

I run an inverted OS91FS in standard config in my Blackhorse Chippie fine.

Sounds like a mixture problem - when you say it chokes - does it show any symtompies of being lean in the mid-throttle range, speeding up etc. or is it more like a too rich choke-up where it just dies on you?

Onr more thing, have you tried completely changing the tank/clumk and tubing for an alternative? I'm wondering if at mid range you have a bit of fuel starvation, caused by a dodgy supply. that is cured at higher revs by greater exhaust back pressure into the tank?

BEB

[Stephen Grigg]

The answer is simple fly the model upside down--shimples

[Dean B]

Thanks for that Biggles elder brother. I have turned the engine around 90 degrees today. New fuel pipes, checked the clunk, its about 1/4 inch off back of tank. Nothing has changed. The engine runs great at full throttle. Ticks over nicely, was chugging away for 15 mins ticking over. As soon as you set the throttle anywhere between 1/4 and 1/2 throttle it starts to run rich, lots of smoke and then just dies. I'm beginning to think I have a naff engine, or maybe the carb has a fault.

[Dean B]

Thats interesting Bob, I know SC engines are what most would call a budget engine, but after my experience with the one in my Harmon Rocket I felt bold enough to get another for my Pilatus porter. That model has a very slim nose and it was not possible to get the fuel tank level with the carburator. It was slightly above and the fuel just drained into the carb constantly flooding it. Hence the engine just flooded and would not run at all.

I bought the chippie as it has a nice deep front end and the tank is sat quite nicely just below the carb. It runs fine at full throttle and tickover, just no midrange. It floods and stalls. the 90 degree turn is just an experiment. The cowl has been cut and holed to suit the engine upside down. Its going to look a right dogs dinner if I cut more holes in it to suit the engine turn.

[Bob Cotsford]

Interestingly the RCV(most defintely NOT budget!) instructions tell you to sacrifice perfect running at tickover to concentrate on getting a smooth mid-range.

At a quick count I've got 7 assorted ASP, SC and Magnum branded motors out of which only one is a dog. I think it's a case where it's Chinese quality control in action - for the most part they're good but you have a higher chance of getting a Friday afternoon special than with OS.

It's just occured to me that I have a spare SC65 carb which should be the same as a 70 one, if you want to try that PM me your address and I'll pop it in the post.

[Dean B]

Bob, thats very kind of you. I have removed said engine from model and have decided its to go back to the supplier to let them check it over. I have had not problems whatsoever with the one on my rocket. it just goes and goes very well.

Thanks again.

Dean

[Steve Hargreaves - Moderator]

It does sound like a dodgy engine Dean.....like the other posters I have an ASP61 & an ASP70 inverted & they run just fine. Given the very crude carbs these engines use all the setting are a bit of a compromise but they generally work just fine.....

I wonder if there is an air leak somewhere at full throttle which you are compensating for with a rich setting that simply drowns the motor at mid revs? My ASP70 runns at about 1.5turns out on the main needle if that helps...

Be interesting to see what the supplier says....

[..]

Hi Dean, after some exelent advice on the forum I have an SC70 that runs great inverted.

But I decided before covering it inverted in the cowl of the ME 109 it was bought for I would give it a good few flights uprite and exposed in my Kyosho Calmato airframe . This allowed me to fine tune it and realy get to know the engine. I found I had to go nearly a whole turn on the slow needle and it runs at just over a turn on the main needle

I have since mounted it in the 109 and it runs like a dream, it's just an idea? But I found it a great way to set it up and prove it was a good runner before putting the invertion bit into the equation.

The only problem was it nearly cost me a new engine as I loved it in the Calmato and had to wrestle it out

hope this helps , J

[Dean B]

I was wondering about the air leak as the join above the carburator did have some small bubbles around it. I cured it by wrapping some ptfe tape around the tube to increase the seal on the rubber o ring. I have had problems with this one from new. the valve adjustment screws were found to be loose upon removing it from the box. There was no compression whatsoever, didnt take long to find the problem. The long screw on the carburator the one which holds the carb barrel in place, went missing shortly after installing and running for a short time. That must have been loose too. Needless to say the thing wouldnt run at all like this, and with the cowling on it wasnt immediately apparent what the problem was. Will see what happens next. Engine has gone back to the supplier today. Hope they can find the problem. The SC70 I have on my Harmon rocket runs beautifully in any orientation. This one must be a dud.

[Steve Hargreaves - Moderator]

It does sound like a bit of a Monday Morning engine Dean.....

[Dean B]

Update. Engine returned from Perkins, they say it works perfectly. Re installed it in my Chipmunk, found the same problems as before. Was a bit miffed for want of a better word. managed to get engine running with some help from Jeff at the model shop in West Vale. discovered I had two dodgy glowplugs in my box. Put a new taylor 4 stroke plug on it that I had had given some time ago and it fired up and ran well at full throttle and tickover. mid range was still a bit spluttery and after a short run it would still splutter and die. I took model up to flying field so the chaps up there who have more experience with engines could have a fiddle. Still no joy. Then, tried some new fuel. What a transformation. It ran fast , ticked over nicely and mid range was still a bit lumpy but it didnt stop like before. After half a tank and no stalls I took the plunge and maidened the model which flew very well. first flight was a bit twitchy and needed some tweaking on the radio. By the third flight she was flying nice and smoothly. The flaps work extremely well, I programmed about 5 degrees of down elevator into the flap switch and there was no discernible change in trim when the flaps were put down. Putting the flaps down made the model quite nice and slow, very manageable.

Then engine appears to be running fine now. My fuel bottle was getting low and was discoloured probably from sucking the fuel back into the bottle after running. maybe leaving whatever is in the tank after a run should stay there and not be pumped back into the main bottle after a few flights.

Its been a frustrating time with this one but things have been learned. big thank you  to all who have given help and advice on here.

 

Edited By Dean B on 09/10/2012 03:18:25