Roger Dyke Posted June 13 Share Posted June 13 Hi All, I have a Super Custom 61A glow engine which I purchased in the mid 90's. It has served me well and is still in excellent condition. Problem: Throttle hesitation when smoothly opening the throttle. With a full tank, the high needle set for max revs (+ slightly rich), the low needle set for best pickup, thew plane is ready to go and flies well. However, throughout the flight, especially when the tank is below half, the hesitation on the pickup from low revs is pronounced and has to be nurchered for a second or two to keep it going before back to being high revving again. it's driving me potty. I've found setting the low needle is a bit of a challenge. I have tried the 'blowing down the tube' method and also the 'pinch' method but don't seem to be able to overcome the problem. Centre of the tank position is about level with the carb and really can't be adjusted. The engine is semi inverted (135 degrees?). Fuel is 5% nitro. Any ideas would be really appreciated. Roger Quote Link to comment Share on other sites More sharing options...
Manish Chandrayan Posted June 13 Share Posted June 13 Since the problem occurs when the fuel in the tank is about half, try tuning the high and low needles to their respective best position with the tank half full 1 Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted June 13 Share Posted June 13 Those early carbs were not that great and if i remember correctly had a seal on the outside of the carb on the barrel side. Is this in good condition ? Often the best you could get was a compromise and set the high speed to max then set it a click or two rich to give it the fuel it nesds in transition from closed to WOT . Is the tank plumbing ok . When you say its ok with a full tank but starts to mess about when tank is half empty suggest a small leak in the pick up pipe. If tank is old then the nitromethane in the fuel will dissolve the copper in the brass tubing causing leaks. If new tank set up look for any pin holes in the pick up tube. Also check the pressure tube from exhaust is not blocked . 1 Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 Manish: I haven't tried setting it with a half full tank. I'll give it a go. Engine Doctor: Earlier in the year I replaced the tank for a new one due to a clunk problem with the old one (current problem same with old tank). I've also renewed all the plumbing too. There are definitely no leaks or pin holes in the pipework and nothing is blocked. Engine seals and operation are all good and clean. Could the difference in pickup be due to a different head pressure from the tank from when it's full to when it's half empty? Just a thought. Yesterday when flying I appeared to be landing long, so I pushed the thottle forward to go-around. The engine instantly spluttered as if it was going to quit so I quickly pulled the throttle back again and the engine picked up again. With a bit of careful manipulation I managed to get back in the air and flying again at normal throttle settings. The throttle and transition was checked and very good at the start of the flight. It's looking like the setting could be a compromise between a full and empty tank? Quote Link to comment Share on other sites More sharing options...
Jonathan W Posted June 13 Share Posted June 13 Are you running with exhaust pressure to the tank? If not. this might help a little and readjust the carb settings to suit. And also as mentioned already, tuning it on the rich side when the tank is full. Has this problem always been there, or has it developed recently? Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 Yes, the tank is plumbed to the exhaust so is under exhaust pressure. I have always had this problem, but up to now tried to work with it rather than find a cure (if there is one?). Quote Link to comment Share on other sites More sharing options...
J D 8 - Moderator Posted June 13 Share Posted June 13 Agree with ED above, those early SC carbs not the greatest. Tank position can be an issue more so with some engines/carbs than others. Those early SC's seem to err on the side of flat out power than tractability. As suggested set up with half or less fuel. Not slapping the throttle stick open from low to full power is good practice. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 (edited) Yes, I'll have to try that, athough I somehow feel that it will then be too rich and hesitant in the transition at idle when the tank is full. I never snap the throttle open or closed so it's nothing to do with transition speed. I do have trouble priming this (semi-inverted) engine when the tank is less than full. When full, it primes and starts easily. I also always hand start my engines as I've had bad news from electric starters doing damage to small engines in the past. Edited June 13 by Roger Dyke added text Quote Link to comment Share on other sites More sharing options...
J D 8 - Moderator Posted June 13 Share Posted June 13 1 hour ago, Roger Dyke said: Yes, I'll have to try that, athough I somehow feel that it will then be too rich and hesitant in the transition at idle when the tank is full. At least then issue will be at start of flight and you can power around a bit to use some fuel and get engine good and warm which helps with throttling. Compromise is the word I recon. PS, What plug are you using. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 Yes, that makes sense. I think the plug which is newish is an ENYA No3. Quote Link to comment Share on other sites More sharing options...
RottenRow Posted June 13 Share Posted June 13 When the problem occurs, try lowering the nose of the model, hold the fuselage at say 45 degrees. This will put the remaining fuel above the carb level again. If the problem then goes away, or at least markedly improves, that will prove that it is the head of the fuel that is the cause. If it’s still the same, then it could be that the mixture is weakening as the engine heats up during a run. Alternatively, quickly refilling and restarting would give a similar indication. It could be that the carb has a slight air leak between the barrel and the body, the effect of which would become more significant as the fuel level drops. That would also be borne out by your difficulty in priming with a partially full tank, due to the poor suction that an air leak would cause. Brian. 1 Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 There are some good points there. Next time at the field I will give the 45 degree idea a try. I will also try starting the engine with a 1/2 full tank to see if the idle setting is different to a full tank. Thanks for that. Quote Link to comment Share on other sites More sharing options...
Jon H Posted June 13 Share Posted June 13 Sounds like tank too high, main needle to rich, slow needle too lean. Lower the tank so the top of the tank aligns with the middle of the carb. If something is in the way cut it out. Tune for peak revs, do not run rich as there is no advantage. Tune slow run needle by setting the engine at idle and firewalling the throttle. If the engine stutters to full power its too rich. If it bogs then its lean. I aim for a miniscule amount of bogging as the engine to richen slightly in flight and be spot on. The full/tank low tank compromise is caused by it being too high with one half of the tank assisted to the engine by gravity and the other half resisted by it. The leaky carb/O ring chat is all perfectly valid and worth investigating, but until the base setup of tank placement is sorted its never going to be right. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 As mentioned in my original post, the tank height cannot be adjusted. It is where it was designed to go (due to profile of the nose). The centre of the tank is about level with the carb needle. I take your point about gravity feeding when the tank is full but there is not much I can do about that I'm afraid. I can set the tick-over and transition quite well on a full tank. I need to do more experimenting with 1/2 full tank etc., Or maybe it's something I've eventually got to live with. I note your point about a leaky carb 'O' ring. 1 Quote Link to comment Share on other sites More sharing options...
Jon H Posted June 13 Share Posted June 13 4 minutes ago, Roger Dyke said: the tank height cannot be adjusted Yes it can 11 minutes ago, Roger Dyke said: It is where it was designed to go Model designers often forget about the requirements of their power plants. 4 minutes ago, Roger Dyke said: there is not much I can do about that I'm afraid Yes you can. The answer is power tools. Its only wood, just whip out the chainsaw (more delicate options do exist im told) and have at it. A few hours work for infinite years of trouble free engine operation is a good trade. You may be able to minimise or even eliminate the work with a smaller tank, as most of the time we use tanks that are way too big, but the engine is simply never going to work if the tank-carb relationship isnt right. You can give it the half arsed British Leyland 'that'll do' if you like, but its not worth it long term as it will never be right. Bite the bullet, do the work. Its been 10 hours since you posted this thread and the problem is not yet solved. In 2 hours the tank could be moved and the problem cured forever. If its not cured, and its still rubbish, you at least have the tank eliminated as a problem and can look at the O rings and all the rest of it. Right now if you change the O rings, faff with carb and all that stuff it might still be rubbish because of the tank. Working at laser i came to the realisation that probably 90% of engine reliability and tuning problems stem from tanks being as little as 3/8'' too high. Its mind boggling. What model is it anyway? Unless its something with a nose leg retract i find it highly doubtful that the tank cannot be moved. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted June 13 Share Posted June 13 The carb "O" ring is now very old and probably rock hard or cracked try swapping for a new one. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 13 Author Share Posted June 13 I put this plane (Dalotel) together in about 1995. It has had 100's of flights and has great sentimental value to me. I really do not wish to start hacking it around like I have done with a few of my other models. But not this one. I will try the air leak/carb seal investigation first and the other things above, and if no good then I will have to make a decision whether or not to live with it. 1 Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted June 13 Share Posted June 13 I had similar problems with this engine and would recommend checking the carb and needle valve o rings first, also try a different plug (they wear out) this carb is very sensitive to the low speed needle valve only small changes are needed and a lot of patience good luck Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted June 13 Share Posted June 13 Any other carb a fit for the S.C ? I accept that correct tank height is the ideal we look for, however thousands of models are flown without the ideal being met (myself included), and flown without issue, never in my experience have I met a glow motor that intolerant. Quote Link to comment Share on other sites More sharing options...
J D 8 - Moderator Posted June 13 Share Posted June 13 Not done so myself but have known others to slim down the venturi with thin alloy tube, a couple of petals over top edge to keep in place. This was with old MDS engines [ More Dead Sticks] to keep them going. Smaller venturi greater draw. Would it help here I don't know. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 14 Author Share Posted June 14 My thanks to you all for all your valueable replies and suggestions. I think I am now armed with enough information to try to overcome my problems. Thanks again Roger Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 14 Share Posted June 14 Can you rotate the engine to 90 degrees instead of 135deg? Might be worth thinking about a different carb, as mentioned. Or, Sleeving the existing one Might make all the difference. Another option, add a Perry pump to the setup. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 14 Author Share Posted June 14 I cannot really move the engine position from where it is due to the nose of the aircraft designed around it. I have another new carb on the way as we speak. This morning I have measured the tank position exactly:- It is a yellow SLEC 9oz tank and the centreline (carb feed) is 1/4" below the middle of the carb needle. Quote Link to comment Share on other sites More sharing options...
Jim Hearnden 1 Posted June 14 Share Posted June 14 No one has mentioned the plug. Years ago when I used to sort out quite a few engines, I'd ask how old is the plug & get a blank look! A shiny new Fireball or whatever I had spare would solve the problem instantly. When I raced boats I carried a magnifying glass. If you look at a dodgy element it would be crystalline white, not shiny. Quote Link to comment Share on other sites More sharing options...
Roger Dyke Posted June 14 Author Share Posted June 14 The plug is an ENYA 3. About 2 years old (approx 10 flights) and looks fine to the eye. I will probably try a new one next time out with the plane. Quote Link to comment Share on other sites More sharing options...
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