Nigel Dell Posted January 2, 2017 Share Posted January 2, 2017 Had a very similar issue with an early SC 46 turned out to be the O ring on the needle valve had shrunk and more importantly hardened, no amount of faffing, including putting fuel tube over the needle/spray bar join cured it but once the O ring as replaced all was good again, it has to be said this was after the model had been laid up for a few months. Worth a look at that, could also be damaged. Edited By Nigel Dell on 02/01/2017 18:17:15 Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 2, 2017 Share Posted January 2, 2017 Gangster...pressure nipple free and clear. Percy.....checked the fuel level after the cut outs, only 1/8" level used. So clunk and feed pipe still well under fuel. Donald , Percy...like your thinking but why doesn't the engine not cut after 3-4 mins on the ground ? Thanks for the suggestions. Tricky one this ! BTW fuel tubes nice and tight in the rubber bung. Bung tight in the tank neck Edited By cymaz on 02/01/2017 18:24:42 Quote Link to comment Share on other sites More sharing options...
Caveman Posted January 2, 2017 Share Posted January 2, 2017 I had a similar problem. There were air bubbles in the fuel line, but not always. The answer was the 'brass' used in the fuel tank, which had been badly corroded by glow fuel, allowing air into the system when the tank got below approx. 50%. Now replaced with aluminium and no further problems. GDB Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted January 2, 2017 Share Posted January 2, 2017 Brass is wonderful stuff, it is malleable when needed to be, yet it machines beautifully, but is very weak in any form of corrosive liquid. The SC46 like many motors with his carb, has a very sharp bend at the fuel inlet nipple, and if you remove the nipple, inside that bend you will invariably find a "fluff like" mess that needs removing Edited By Denis Watkins on 02/01/2017 18:42:49 Quote Link to comment Share on other sites More sharing options...
Keith Berriman Posted January 2, 2017 Share Posted January 2, 2017 Have you tried a new plug? , after a winter stand the plug could have got a slight coating on the element and all is okay on start up then after a while it looses its chemical action and no ignition Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 2, 2017 Share Posted January 2, 2017 RC...plug replaced, one of the first jobs! Denis...new pipe work Looking forward to the new carb... Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted January 2, 2017 Share Posted January 2, 2017 If the tank is not packed in foam to isolate it from airframe vibration, do it now. If it is packed in foam, check that no bolts, tank hatch or anything else can touch it. Also check that the outlet pipes can't touch the front bulkhead. If the foam is very soft, the tank may squash it when the g level the tank sees increases in a turn. Airframes vibrate in different ways when in the air to sitting on the ground. I saw this on a test of an airliner fuel measurement probe which would get 'black holes' in the fuel when tested in a transparent cylinder of fuel on a vibration rig. The 'black hole' attracted all the dissolved air in the fuel like an 'air magnet' and caused it to give an incorrect reading. If the fuel tube is touching the engine or exhaust, the heat may be enough to vapourise the fuel preventing a steady fuel supply. Also an inline filter, if fitted can leak at the gasket where the two halves screw together. Better to filter the fuel when filling. All the comments regarding glow plugs, pin holes in tubes, kinked rigid tubes and blockages and airleaks in needle valve assemblies are issues I've had over the years. If there is an O-ring between the spigot on the throttle body and the socket it goes in on the front housing, check the carb is fully home and squashing it. A lot of MDS problems were due to people having this O-ring in the wrong place (around the spigot, not below it), which caused air leaks. Edited By Robin Colbourne on 02/01/2017 21:25:33 Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 2, 2017 Share Posted January 2, 2017 Tank supported in foam and hard foam around the neck to stop the pipes kinking. The O ring on the carb body was down tight and in good condition ! Quote Link to comment Share on other sites More sharing options...
flight1 Posted January 2, 2017 Share Posted January 2, 2017 Also to check o ring on the low end idle ajustment and the seal of carb to crankcase? Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted January 2, 2017 Share Posted January 2, 2017 Runs on floor but won't have it airborne ? on floor it vibrates more so i reckon there's a bit of crud settling and blocking somewhere, strip carb and all nipples ? John Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 2, 2017 Share Posted January 2, 2017 I had a similar problem recently Cymaz & coincidentally with an SC46 too......the model had been fine but this particular day started playing up. It deadsticked on me (my first for a very long time) & running the engine in the pits afterwards it was fine until run for a minute or so at full throttle when it seemed to be being starved of fuel. This model had a tank hatch which I removed & i could see a "plug" of oil forming between the top of the vent/pressure pipe & the internal surface of the tank.The gap between them was really quite small when I checked. It seemed that this causing a slight vacuum to form in the tank thus starving the engine. As I mentioned it seemed odd as this model had been fine before. My theory is that the very cold weather that day meant that the oil was that much thicker & this allowed it to "seal" the vent pipe more effectively. Removing about 5mm from the top of the vent pipe prevented this plug forming & a test run in the garden yesterday had the engine holding full power for several minutes (oh yes my neighbours love me...) with no problem at all. Might be worth checking the vent pipe to tank clearance in your model too..... Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 2, 2017 Share Posted January 2, 2017 Good idea Steve..it has been cold and the engine was faultless all summer. Oily stain on the tank roof would indicate the gummed up pipe I assume? Edited By cymaz on 02/01/2017 22:00:45 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted January 2, 2017 Share Posted January 2, 2017 Mmmm not sure it would stain it Cymaz.....the oil is fresh from the exhaust so is fairly runny & as any fuel sloshes over the area any oil would be simply washed away.... can you see the top of the tank where the vent/pressure pipe is? Can you see the oil exiting against the top of the tank...(appears as a dark patch)....can you see how close the pipe is to the top of the tank? If it's very close maye just shorten the pipe a tad anyway to eliminate this as possible cause... Quote Link to comment Share on other sites More sharing options...
flight1 Posted January 2, 2017 Share Posted January 2, 2017 That Oily stain on the tank roof was from the original brass pipes being badly corroded or even too much after run oil blowing in on first run? Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted January 2, 2017 Share Posted January 2, 2017 Thought you said you changed all plumbing and it still do's it ? John Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 2, 2017 Share Posted January 2, 2017 Posted by john stones 1 on 02/01/2017 22:10:41: Thought you said you changed all plumbing and it still do's it ? John Yes it does. Different tank, pipe, clunk, stopper the lot! I will look at the tank again and cylinder liner all tomorrow. Crossed fingers! Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 3, 2017 Share Posted January 3, 2017 Took the engine apart and the tank out before work. So from clunk upwards Clunk ok and free to move Pressure feed pipe in the tank is 5mm from the tank top and is in good condition Stopper good Pipework good Piston and liner inspected, shows wear but OK engine backplate tight Head tight Engine has good compression and good seal. The only thing it can be is the carb. It did have a needle extension ( not a long one ) that was taken off at the weekend. I would think this could have worn the threads over time and this is now the failure point. Now waiting for the new £20 carb to appear from JE - much to the disgust of the club treasurer Quote Link to comment Share on other sites More sharing options...
Robert Foreman Posted January 3, 2017 Share Posted January 3, 2017 I put a new ASP46 in a plane last autumn and flew it several times without any deadsticks. But when I flew it a few weeks ago when the temperature was only just above zero, each flight lasted for just a few minutes before going deadstick. The engine was running at about 1/2 throttle and just suddenly stopped without speeding up so I think I can rule out lack of fuel. It was running with an OS No8 plug. I've now fitted a hotter (No6) plug and am waiting to try it again. With other engines I've noticed less deadsticks with a hotter plug even during warmer weather. Quote Link to comment Share on other sites More sharing options...
Frank Skilbeck Posted January 3, 2017 Share Posted January 3, 2017 Just a thought, could there be some carb icing happening once the plane is in the air and in colder damp air. At ground level the temp is a few degrees warmer but start flying and at height the air is cooler, leading to colder air entering the carb and water vapour icing out, by the time you land any ice in the carb has gone as the engine has stopped and the residual heat from the engine with no cold airflow through the carb has melted the ice crystals. You would only need a small amount on the spray bar to cause the problem. Quote Link to comment Share on other sites More sharing options...
Ian Mat Posted January 3, 2017 Share Posted January 3, 2017 I had similar issues with an ASP 46, which I believe is the same engine. I found that there is no decent seal between the 2 screws that secure the carb to the main engine. Air was leaking past both screws and messing up the mixture. I used some high temperature sealant on both screws and the engine was perfect afterwards. Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 3, 2017 Share Posted January 3, 2017 Thank you so much for all the feedback. Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 4, 2017 Share Posted January 4, 2017 Carb arrived today. On closer inspection the old main needle is showing a little wear in the threads. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted January 4, 2017 Share Posted January 4, 2017 Hope it sorts you Cymas, but there's an O ring after the thread so hows that at fault ? John Quote Link to comment Share on other sites More sharing options...
cymaz Posted January 4, 2017 Share Posted January 4, 2017 Straws , at them I clutch ! Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted January 4, 2017 Share Posted January 4, 2017 It'll be the carb you'll be good to go John Quote Link to comment Share on other sites More sharing options...
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