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SC 70 fs

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Richard Acland20/02/2020 19:23:37
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94 forum posts
31 photos

Recently purchased an SC 70 fs on e bay. I have two of these engines already which run really well. The engine I bought seems to have really low compression compared to my other two engines, so today I stripped it down. The piston ring and liner seem fine and plenty of resistance when I put my hand over the top of the liner. I stripped out the valves and checked the seats. cleaned everything up. I reassembled it checked the valve camshaft timing was correct set tappet clearances. but it did not make a lot of difference. It does run but not as well as my other two engines and setting the idle is difficult as it cuts at low revs.

Any ideas

Jon - Laser Engines20/02/2020 19:48:58
5203 forum posts
237 photos

Valves leaking. They could be scored (Look for a round groove in the head) or the head could be warped. I'm sure engine Dr will be along soon to explain that one.

To see if it is the valves remove carb and exhaust then turn it over compression with a finger over the ports. If you feel pressure on your pinkie you have your answer

Martin McIntosh20/02/2020 20:08:58
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3148 forum posts
1133 photos

It may need a couple of tanks of the dreaded castor fuel through it to re establish the piston/valve seals.

SIMON CRAGG20/02/2020 20:13:25
544 forum posts
15 photos

Having had countless ASP's over the years:

1. Are both the bearings 100% free, if they are pitted or dragging this can cause all sorts of running problems. If in doubt change both of them. About a tenner at your local bearing outlet.

2. Sometimes the timing dot is slightly out, meaning the camshaft could be fitted one tooth out. Worth moving it a tooth either way.

3. Carefully grind seats of valves and check for leakage, with thin oil or similar.

4.Worse case, you have got a duffer. ASP's seemed to fall into three groups: Brilliant, work ok, and are never really reliable no matter what is done to them.

Good luck........I know how frustrating it can be!.

Richard Acland20/02/2020 21:22:04
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94 forum posts
31 photos

Thanks for replies. I have inverted the head and filled the chamber with white spirit. The level dropped and white spirit is leaking out of the inlet port. I have had a good look cant see much wrong but there must be a problem in this area. I didnt pay a lot for this engine so I think I will buy a new head with valves fitted from Just Engines for about £55 which should cure the problem.

Richard

i12fly21/02/2020 00:06:05
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608 forum posts
21 photos

I have one, bought from new and ran it in carefully. When using it in a plane, it ran OK for 2 or 3 minutes then got very hot and compression was very poor (but good when cold. Couldn't resolve it despite lapping valve seats etc but a clubmate examined it and found that the valve seat inserts in the head were leaking. Only solution would be a new head but haven't bothered as I only fly electric now. I had another SC70FS and it was perfect, as was a 90FS, 52FS and a 120FS.

Engine Doctor23/02/2020 15:32:38
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2407 forum posts
31 photos

Hello Richard. It is what it is i'm afraid and no cure other than replacing the head as already suggested. But ..........the new head could be exactly the same and allow gasses to escape past the bronze inserts . No amount of grinding the valves will improve the compression . Look for a smashed 70 with broken lugs or crank but one you can feel the compression on and swap the heads. Although good luck on that one as folk seem to not want to get rid of old four-strokes even if they are busted ?

Anyone buying a ASP, SC or a Magnum FS engine check to see if it has compression. Even if its new and un run check it .It it has little or no comp then its as useful as a chocolate tea pot , and will not idle reliably or deliver full power .

Anyone with an old clapped Enya 90 FS ? I'm looking for a head and a crank .

E.D.

CARPERFECT24/02/2020 13:05:29
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504 forum posts
9 photos

How are the valve inserts held it ? ICan they can be removed ? if they are letting gasses by. Then remove and re seat using thread lock ?

Engine Doctor24/02/2020 13:37:06
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2407 forum posts
31 photos

They are I believe cast into the head casting and are not removable . You may be able to get some type of thread lock , even cyano into the join and fill the leak . Its worth a try . I have never bothered trying to stop the leak in the past but may try if one come my way in the future . You may be able to find the leak by blowing air into the inlet and exhaust ports having first re-fitted the valves . The SC etc are poorly built engines compared to OS etc but generally run well enough for our needs .OS use the same type of seat but I have never found any of them to leak . If you do try to seal the leak let us know how you get on .

I backed a Magnum 160 FS twin some years ago as one cylinder had no compression and kept going out. The supplier tried grinding the valve numerous times and had a perfect seal but it still blew past the valve seats once fitted on the engine . He finally gave up and fitted another head and guess what ? That had the same problem but not as bad !

i12fly24/02/2020 22:03:36
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608 forum posts
21 photos

The valve inserts are cast in. My clubmate tried to seal with JB weld (really to prove the point more than anything else), it lasted about 10 minutes before the low compression lead to overheating again.

Richard Acland26/02/2020 19:11:13
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94 forum posts
31 photos

Today I fitted a new cylinder head complete with valves and valve springs. Also fitted a new piston ring. This engine is now transformed with loads of compression like my other two SC 70 FS engines.and runs like it should. One thing I noticed was that the vavlve springs on the old head seemed very weak compared to the springs on the new head Idont know if that would have had an effect on performance or not. Anyway I am now a happy bunny.

Richard

Frank Skilbeck26/02/2020 19:40:59
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4603 forum posts
101 photos

A less stiff valve spring would mean that valve float would happen at a lower rpm, so as the revs exceed this point the engine would breath less well.

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