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.91 4 Stroke problem

Power loss

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Kit Morpeth21/10/2013 16:22:21
52 forum posts
17 photos


I wonder if anyone has had a similar experience to me.

Flying a rather ageing Hammerhead with a 0.91 thunder tiger 4S recently I noticed that full power could be acheived on the ground when first started but once the engine was run at full throttle for a while the power started to bleed off a noticebly. Running at full throttle with the plane held vertical showed the same problem but no significant loss. Thinking maybe the mixture was a bit weak i opened it up about 3 clicks but it made no difference. Later when flying at modest engine speed the power started to bleed off and eventually the engine cut. I should also explain that it is fitted with an on-board glo which cuts out at about 30% of throttle and which has been fine for some time. Normally this is a very reliable set-up.

Anyone got any theories? I wondered if maybe overheating could have this effect.



David Davis21/10/2013 16:47:12
2887 forum posts
489 photos

I had similar problems with the Thunder Tiger 91 in my Senior Telemaster. In my case the cause of the trouble was worn out main bearings. Has your engine started to sound like a lawn mower?

Senior Telemaster

Greybeard21/10/2013 17:27:35
726 forum posts
6 photos

Overheating could indeed have this effect.

I had a similar problem years ago when using castor oil fuel, the burned castor plated the liner and slowing down heat dissipation, at least that's what I was told by my engine man. A new liner and a fuel change saw off the problem.

Kit Morpeth22/10/2013 16:49:00
52 forum posts
17 photos

My thanks to David and Greybeard,

The engine bearings are fine and I was thinking about the problem last night. As to the cut-out that was due to a stuck clunk in the tank. As to the power loss maybe that is an overheating problem then. I shall have to delve deeper.

Thanks guys


Steve Hargreaves - Moderator03/11/2013 16:56:45
6593 forum posts
181 photos

How old is the engine Kit?? Might it simply be worn out.......??

Like most of us our engines tend to fade away as they get older...teeth 2

Bob Cotsford03/11/2013 17:04:06
7218 forum posts
406 photos

Kit, have you checked the valve clearances? If they are too tight they can close up when the engine gets hot and keep the valves open - instant overheating and loss of power.

cymaz03/11/2013 17:20:37
7598 forum posts
981 photos

New f plug? Richen it until you hear a difference then lean to full power and then back off the needle a few hundred rpm. A tacho is usefulclock

Edited By cymaz on 03/11/2013 17:20:57

Shaunie03/11/2013 19:59:08
938 forum posts
78 photos

Lacquered up piston/liner from castor oil,valve clearances, overheating. If all these are ok check the prop size, you've not changed prop size or pitch recently have you? An over-propped engine can give these symptoms.


Paul Marsh03/11/2013 20:35:51
3243 forum posts
895 photos

I have a Thunder Tiger 75 four stroke Got it new off ebay, and was ok for a bit, but had no guts, or power. Started ok, throttled ok, never stopped, but seemed to have compression.

I kept flying it like this, but eventually decided my ARTF Acro Wot with this engine was boring and wondered about fitting a OS 70.

I had a look at the TT 75 and found the intake port wasn't opening properly - hence not letting enough fuel in. I couldn't adjust the tappet, as it had worn the thread. Made a coller, stopping the nut from turning.##

Fixed, plenty of compression and snaps when starting, as still quite new, Vertical performance, and that's all it was.

John Olsen 104/11/2013 07:17:38
446 forum posts
23 photos

I had a similar problem to the OP on a fully cowled Saito 91. I haven't flown it again yet, but after some chasing around I realised that the internal lead ballast was potentially blocking the airflow a bit. It has now done a long ground run without any sign of problems, so I hope once the weather permits it will be OK in the air. Of course I realise that the engine in question may not be as fully cowled as my Fokker DVII, but it does show that overheating can give the symptoms observed. Mine was gradually losing power in flight, such that you had to land after about three minutes.


Peter Miller04/11/2013 08:37:50
9065 forum posts
1042 photos
10 articles
Posted by Greybeard on 21/10/2013 17:27:35:

Overheating could indeed have this effect.

I had a similar problem years ago when using castor oil fuel, the burned castor plated the liner and slowing down heat dissipation, at least that's what I was told by my engine man. A new liner and a fuel change saw off the problem.

Why fit a new liner? Cleaning the liner with fine steel wool will remove the varnish and restore the engine.

I have known some engines/owners who had to do this operation every few flying sessions. castor fuel and running too lean.

Braddock, VC04/11/2013 08:53:55
1464 forum posts
21 photos

Ah but Peter he had an engine man, they use the wool, over eyes wink. More charitably the liner can get scored, quite deeply. I was given an immaculate os 160 fx, well externally, the liner was scored like a scored thing and I eventually bought a new liner, ring, piston and gudgeon pin retainers and bearings - have yet to run it in. Forgot to mention the engine hadn't been run very long, possibly two - three hours worth.

It's one of the reasons why I never use castor based fuels.

William Morrison04/11/2013 22:01:02
66 forum posts

I agree, the engine man was very wooly. These are ringed engines and need a good interior coating to ensure long life. Ergo, always add some Castor to the mix and you will never regret it provided, of course, there is no wooly engine man around.

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