Denis Watkins Posted February 4, 2017 Share Posted February 4, 2017 These motors are our bread and butter Nigel, they come out season after season and they perform well. I would say half the IC flyers in this country posses and fly a 40/46 range ASP, OS, SC, Magnum, Super Tigre, Irvine etc etc at some point and they are out in force throughout the season. Do not hesitate to use one Quote Link to comment Share on other sites More sharing options...
Tim Cheal Posted February 5, 2017 Share Posted February 5, 2017 How about an off the wall thought? I am making this up as I go along, but I had a similar problem high revving problem about 40 years ago with an HB40 (left the hobby for 35 years though) which I never resolved. So, don't shoot me down, just throwing a thought into the mix....... We use the exhaust pressure, presumably because the exhaust outlet is smaller than the inlet which ensures some positive pressure in the exhaust itself. Now, obviously there is flow through the exhaust, and it will vary with RPM. What if, at the point of the exhaust nipple, there was a flow past the nipple at a local speed faster than elsewhere, maybe a baffle is missing or has shifted inside, or something about the nipple placement on a non standard exhaust (not sure if yours is the standard one or not). as the RPM increases the flow increases, and at high revs instead of providing a positive pressure it induces a negative pressure (akin to the carburetor intake) and instead of pressurising the tank at high revs it is sucking pressure out of the tank, causing a reduced fuel flow at high RPM. Like I said, its a bit off the wall, but is it plausible? Battle armour fitted awaiting the inevitable....... Tim Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 5, 2017 Share Posted February 5, 2017 Ooo lots of thoughts there Gents.....thanks very much for the input.... First off let me say to Nigel & underline Denis's point that this sort of problem is very rare so don't let it put you off. IC engines these days are almost boringly reliable. Use clean fuel...take the trouble & time to run them in carefully...then spend a bit of time setting them up & you'll be absolutely fine. In my many years of modelling I've not come across a problem like this & I do believe I will solve it (or possibly die in the attempt...). I have many ASP motors & they are all great & run like clockwork so I would say buy with confidence. OS just that little bit better IMHO but are significantly more expensive so you pays yer money etc..... Before this issue I really can't remember the last dead-stick I've had in flight...apart from running out of fuel of course but that's my fault not the engines. Secretly I'm quite enjoying trying to solve this problem & I bet the other chaps are too.....it's the perverse nature of us aeromodelling types..... So to the points mentioned.... @Jim...the engine is completely un-cowled (it's the Boomerang trainer & SC46 from the group engine test in the mag a few months back) so definitely not overheating @Martin.....yep that pretty much covers it & yes I agree it sounds like a partial blockage or similar but I really can't find one. @Cymaz.....I did wonder the same thing but as above I'm pretty sure there isn't anything to cause a blockage...everything has been stripped & cleaned or replaced & nothing was found. @Tim....no need for any Battle Armour....all suggestions gratefully received. I have wondered along similar lines myself. Clearly there will be more pressure inside the muffler & hence tank at high engine speeds than at low engine speeds & I wondered if it might be taking a while for the pressure to build up inside the tank but this would give the opposite effect to what is being seen ie it would be lean at first & then richen up as the pressure built up & pushed more fuel towards the engine. All that said I'm leaning towards it being due to a vacuum forming in the tank for two reasons.....1) it seems to be worse when the tank is full....a vacuum would form quicker when there is less air in the tank 2) if you close the throttle & let it idle for a few minutes the throttle can be opened up wide again & the engine runs at high speed again before dying off once more......as though the vacuum has dissipated whilst the engine was ticking over. What, then, would cause a vacuum to develop? Well it can only be that the fuel is leaving faster the tank than the air can get in but how can that be....? It's a 46 running on standard fuel tubing which should be plenty big enough to allow fuel out & air in...I have 90s running fine on standard fuel tubing. More head-scratching, investigation & testing this afternoon I hope.....I'll report back.... Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted February 5, 2017 Share Posted February 5, 2017 Steve, have you got another silencer that you can try? It still sounds like something blocking the pressure feed. I take it that you have replaced the pressure feed pipe from silencer to tank, and that the internal clunk line isn't thin walled tube? The only two times I've seen an engine behave this way were a worn out OS25FSR and a worn out ASP 53 two stroke, both with very loose bores which is not the case here. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 5, 2017 Share Posted February 5, 2017 I have Bob & I might try that.....the pressure feed is fine though....no sign of a blockage. I am conscious of only trying to change one thing at a time. If I change a few things & the problem goes away we won't know what caused the problem in the first place..... Quote Link to comment Share on other sites More sharing options...
Tim Tolhurst Posted February 5, 2017 Share Posted February 5, 2017 Is it the tank supplied with the model? If I recall one is required to bend the plastic pipework using a little heat. Could it be that the pressure feed pipe in the bung has kinked internally in the process, restricting the flow sufficiently that the carb is pulling more than the exhaust pressure feed, thus creating a partial vacuum? There would be fuel visible in the tubing, it just woudn't flow. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 5, 2017 Share Posted February 5, 2017 It is Tim but this one came with aloominum tubes...but yes I take your point & would fully agree with you but for the fact another engine ran just fine in the model with this tank & indeed the SC has done so previously..... Clearly "something" has changed....the problem is finding that "something" Quote Link to comment Share on other sites More sharing options...
Tim Tolhurst Posted February 5, 2017 Share Posted February 5, 2017 Ah, my apologies, guilty m'lud of not reading the thread all the way through! Quote Link to comment Share on other sites More sharing options...
Jimbo565 Posted February 5, 2017 Share Posted February 5, 2017 Steve, Another shot in the dark away from fuel problems,I know its a new plug but what if its dodgy,have you tried running the motor with the plug energised JC ne23 Duff plug dept. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 7, 2017 Share Posted February 7, 2017 Well I managed to stap the engine into a test stand & do a bit more testing......& it runs perfectly.... I tried it on a totally different tank....it was fine....ran it with & without muffler pressure.....both fine but it confirmed how far out the needle needed to be without pressure.....about 4 1/2 turns which shocked me. I always run my motors on pressure so this was a bit of a surprise to me. On muffler pressure its about 1 1/4 turns open. I then tried the tank from the model with the engine still in the test stand & again no issues. I was then going to try the tank with the extra clunk line on the filler pipe refitted but unfortunately rain stopped play so that'll have to wait for another day. Then I'll bung it all back in the model & see what happens.... Quote Link to comment Share on other sites More sharing options...
TIM Shaw Posted February 7, 2017 Share Posted February 7, 2017 I have read this thread with interest as I know how frustrating this sort of problem can be, although I have run OS, ASP, Magnum Pro, Thunder Tigre Pro and old SCs successfully over the years - all work well when properly sorted. IS there any chance you could be getting fuel foaming in the tank by returning the pressure line below the surface of the fuel, possibly exacerbated by vibration when the engine is fitted in the model - Prop balanced, or something loose in the mountings worth checking too. Have you removed the needle fully and blown fuel through the spray bar, checked for debris on the needle itself? I like to prove my fuel system by disconnecting the fuel and pressure lines, sealing on and blowing down the other. Make sure it holds pressure by releasing the blocked one few seconds later and listening for the hiss. Make sure the tanks empty though as an earful of 10% nitro is best avoided! Then reconnect the fuel line and blow fuel through the spray bar to make sure there is no blockage. Is the filter clean, and more subtle, is it properly sealed? I confess I used to epoxy the carbs in on the older ASPs to stop then drawing air through poor surfaces, but I guess that area has been covered already. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 7, 2017 Share Posted February 7, 2017 Thanks for the thoughts Tim...... Engine is remarkably smooth at full power....no fuel foaming at all. The tank is pretty deep so even if the fuel did have a "head" on it unlikely to cause a problem unless the fuel runs low. Tank is sealed with no leaks....I'm more of a vacuum man myself...I seal two of the tubes & suck through the other one then seal the third tube with my tongue....if I get a hiss when pulling the tube off my tongue after a minute or so I believe the tank is sealed (Tip:- it's best to perform this test in the sanctity of the workshop well away from other people who would probably call for the men in white coats were they ever to witness such a test. I know I can reveal my secrets on here as I am amongst like minded friends....) No debris in the carb as far as I can see.....I "borrowed" the carb off another engine just to be sure.... No filter in the supply line.....they are the invention of Beelzebub....fuel is filtered twice on way to tank. I'm beginning to think it must be or have been a piece of debris somewhere that I've missed. I will be very frustrated if the problem has just "gone away" rather than a cause be found & fixed....if it has just "gone away" then it can just "come back".....Grrrr!! Quote Link to comment Share on other sites More sharing options...
TIM Shaw Posted February 7, 2017 Share Posted February 7, 2017 Take your point on Beelzebub as I used to think that way too. Until I found carbon type debris in my needle valve, presumably blown out of the silencer, which was causing me similar problems to yje one you seem to be having....... Quote Link to comment Share on other sites More sharing options...
cymaz Posted February 8, 2017 Share Posted February 8, 2017 All I can suggest now is get a bicycle pump and attach the end to the fuel line on the carb. Then give the pump an almighty push to force air right though the needles. It will blow air harder than we ever can. Quote Link to comment Share on other sites More sharing options...
Braddock, VC Posted February 8, 2017 Share Posted February 8, 2017 I haven't experience this problem on SC/ASP/Magnum engines as I don't have any but I did on my OS 91 FX and OS 52 Surpass, basically the "O" rings on the low speed jet and main needle valves had hardened and were leaking. Getting the spares entailed buying them in the US via the web and they were with me inside of a week. Fitting them transformed the engines, they now have a perfect idle, smooth transition and great top ends and, tbh, can't remember when I last indulged in needle twiddling . Another problem, which only manifests itself on start up and only on engines with the rear needle valve is the needle valve seems to attract lubricant which gels over the course of a couple of weeks making first start almost impossible. Cured by pumping fuel into the tank via the carb fuel supply, bit slow but once it's clear one can use the normal fill pipe. I don't have a bike pump but I do have an old Jenni can which, when filled with air at about 60 psi at the local petrol station acts as an air duster and purges fuel lines etc with ease with a wd 40 extended spray nozzle. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted February 8, 2017 Share Posted February 8, 2017 Use a big syringe n blast it through. Quote Link to comment Share on other sites More sharing options...
Manish Chandrayan Posted February 8, 2017 Share Posted February 8, 2017 Steve Look for minute debri in the tank or the feed line. The symptoms were exactly what I had on a 40 FP, where I became butt of jokes at the field for deadsticks. My model would take off, get in circuit and and by the time I turned back into wind cut with unfailing relaibility. This continued for (I think) few weekends till I stripped off the setup and found a small piece of plastic that had liberated itself from some where within the tank/plumbing. The engine would start and check fine on ground. The time taken for the small piece of plastic to travel and lodge itself at the fuel intake nipple was about same the time to take off and turn back into wind. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 8, 2017 Share Posted February 8, 2017 "I then tried the tank from the model with the engine still in the test stand & again no issues. I was then going to try the tank with the extra clunk line on the filler pipe refitted " can't help but think you must be getting close here. the setup in the test stand is differing because: actual fuel lines are different? tank height different? silencer pressure not fitted? possible (and not visible) vibration issues? [maybe affecting o rings or needles] Quote Link to comment Share on other sites More sharing options...
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