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SC 91fs will not stop.

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Ronaldo29/10/2019 10:59:18
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268 forum posts
21 photos

Hi all, after flying 2 stroke glow and petrol for many years, I now fancy a dabble in vintage. Can anyone tell me why my SC 91 four stroke glow will not cut when I fully close the throttle ? It just seems to be content idling away. I have set the needle settings to manufacturers instructions, also looked down the barrel when closed, and it's definitely closing fully with no inlet hole showing. Also checked inlet for air leaks, all seems ok. I am using the version where the crankcase vent recycles the excess oil back into the inlet manifold, I have even replaced the fuel tube on there to be sure of no air leaks.

Until I am ready to install in a vintage, am running the engine inverted in a Ripmax Bolero, runs great all across the throttle range, but still will not stop when I close the throttle, but I wouldn't have thought the engine position would have any effect ?

Any ideas please ?

Edited By Ronaldo on 29/10/2019 11:13:41

J D 829/10/2019 11:16:23
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1591 forum posts
87 photos

Setting to the makers recommendation is not enough, you have to tune the engine to the set up in your aircraft. [ tank position, engine position, not the orientation but the level of the carb to the tank. Centre of tank level with the carb inlet is pretty standard.

Then " tune" the engine for the set up you have. in the same way as a two stroke motor. Main needle first at full power then slow needle at tick over.

Jon - Laser Engines29/10/2019 11:48:30
5623 forum posts
271 photos

JD8 is quite right. There are no factory settings that will cover you, all factory settings do is get the thing running. Then its time to tune.

All that said, ASP carbs are not known for being that brilliant so my money is on an air leak somewhere.

As for fixing it..why bother? if the rpm is low enough that it wont impact your flying/landing then just let it chuff away. Its certainly better than have it stop on you

Ronaldo29/10/2019 18:53:15
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268 forum posts
21 photos

Ok thanks JDB and Jon. I will have a play with the needle settings, and check again for air leaks. Also as you say Jon, the engine runs well, so if all else fails I will leave it alone. The engine idles really nice and slow, so its easy enough to stop by throwing a rag into the prop.

Manish Chandrayan29/10/2019 19:18:06
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649 forum posts
74 photos

With the throttle barrel fully closed, the only way an engine can continue running is by sucking in sufficient air from some other opening. Assuming that there indeed is no air leak, it means that the barrel is not fully closed. Have you tried disconnecting the throttle pushrod and moving the throttle arm manually to see if the barrel closes any further and the engine stops? Also make sure that the set screw retaining the throttle arm is not loose allowing the arm to move without actually moving the barrel.

But have to agree with others. You problem is actually a good problem to encounter

PatMc29/10/2019 19:52:11
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4473 forum posts
548 photos
Posted by Manish Chandrayan on 29/10/2019 19:18:06:

With the throttle barrel fully closed, the only way an engine can continue running is by sucking in sufficient air from some other opening. Assuming that there indeed is no air leak, it means that the barrel is not fully closed.

It could be via the front bearing.

bert baker29/10/2019 20:00:41
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1619 forum posts
333 photos
Posted by PatMc on 29/10/2019 19:52:11:
Posted by Manish Chandrayan on 29/10/2019 19:18:06:

It could be via the front bearing.

It’s a four stroke so don’t think front bearing

Brian Cooper29/10/2019 22:39:31
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589 forum posts
27 photos

They are good engines but the carb can be susceptible to a bit of wear. . . So even with the carb fully closed, air could be leaking past the barrel. . It doesn't take much (air) to prevent the engine from stopping.

B.C.

PatMc29/10/2019 22:39:52
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4473 forum posts
548 photos
Posted by bert baker on 29/10/2019 20:00:41:
Posted by PatMc on 29/10/2019 19:52:11:
Posted by Manish Chandrayan on 29/10/2019 19:18:06:

It could be via the front bearing.

It’s a four stroke so don’t think front bearing

Doh !! I didn't notice it was a 4s. blush

I suppose though, that if oil can escape from the cylinder to lubricate the bearings etc via the crankcase, then some air could also find it's way in the opposite direction.

i12fly29/10/2019 23:00:50
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642 forum posts
22 photos

It could be bottom tolerance barrel in a top tolerance hole in the carb (or wear). I had a similar experience but on a new 2 stroke. A couple of drops of after run oil onto the barrel of the carb every month stopped the tiny air leak and cured it.

CARPERFECT30/10/2019 07:41:09
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504 forum posts
9 photos

Lots of answers about leaks, but post just says when the throttle is fully closed, First thing to check is the barrel on the carb fully closing, Could just be the end point requires adjusting, when any throttle is closed the engine should still run, until the throttle cut is activated, other wise we would all have dead stick engines while flying. ?

Cuban830/10/2019 07:45:37
3035 forum posts
1 photos

What happens if you (carefully) put your finger over the carb intake when idling?

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