Jon H Posted July 2, 2021 Share Posted July 2, 2021 Not sure about the air gap. cant see it really has any purpose ? Quote Link to comment Share on other sites More sharing options...
Mike T Posted July 3, 2021 Author Share Posted July 3, 2021 I lowered the tank last night (top now level with NVA) and flew it this (Friday) afternoon. Before mods, the tank consistently gave 13 minutes or so flying time, leaving about 20%. The first flight post-lowering I got 9 minutes but with the engine not running well (hunting at full throttle), with 20% left in the tank i.e. same as before! I restarted without refilling and re-tuned, then I refilled and flew again. The engine ran better, but I still got only 9 minutes with a 20% residue. I decided to 'go large' on the adjustments, so for the 3rd flight, I turned the airbleed in (rich) 1 1/2 turns. Engine ran fine and despite being generally richer (I fly this mainly well below full throttle) the exhaust appeared less smoky. 10 minutes on a full tank and about 15% left. I then went the opposite way and screwed the airbleed back out plas another turn and a half. I expected a lean cut from idle, but it picked up just fine. I filled the tank to about 80% and it flew for 10 minutes with 10% left when it quit. So I've got some sort of result, but gawd knows what the airbleed is doing. On balance it seems better out than in. I suspect that my modified narrow clunk bore and 3/32" silicon tubing may be limiting the effect of any adjustment, so I'll swap those for a standard bore clunk and 1/8" tube Maybe I also need to raise the tank a little? Easily achieved with my new mounting system - just need to pack the bearers by the desired amount. Quote Link to comment Share on other sites More sharing options...
J D 8 Posted July 3, 2021 Share Posted July 3, 2021 Air bleed system on carbs only affects the slow running part of throttle movement from about one third down, the purpose being to obtain a reliable tick over. More than that open it does nothing and only the main needle controls the mixture. If you are flying about most of the time at low throttle settings [ less than 1/3 ] I would say you have too much engine for the aircraft it is in. The RC glow / diesel throttle is not [ when compared to Walbro ] a sophisticated device and mostly works best at full or near full power and the twin needle or air bleed system is to get an acceptable low end tick over and transition. IC engines like to be used right up their designed power band and while most can understand that over revving, over loading and overheating are not good the same can not be said for treating them with kid gloves. I had a neighbour who never give his tractors the beans and would drive his car and Land Rover every where at 30mph or less [ even on the main road ] He could never understand why when he was "kind " to his vehicles engines they would pack up at low hours/mileages. [ gummed up and glazed ] The car, a Merc was saved by his daughter taking it out and giving it a good thrashing now and then. Quote Link to comment Share on other sites More sharing options...
Jon H Posted July 3, 2021 Share Posted July 3, 2021 with the lower tank i would expect the main needle to need opening and the slow run probably closing. as for fuel tube, 3/32 is standard and i use this tube on all my engines from 15 2 stroke to 360 4 stroke. This is not going to be a problem. Quote Link to comment Share on other sites More sharing options...
Mike T Posted July 4, 2021 Author Share Posted July 4, 2021 To clarify and summarise some of the details in my last post - I lowered the tank and got a slight improvement in the duel draw, but not an emptied tank. Adjusting the airbleed screw to quite rich and from there to quite lean had little discernible effect on running, which is a bit puzzling. Although the plane spends a lot of flying time at med/low throttle, it gets opened up quite frequently, mainly for big loops and stall turns, and it needs over half throttle to do a comfortable rolling circle, soI'm comfortable with its power and itx exercise regime ?. I'll leave the tubing for now and play around more with the airbleed setting. On winding the main needle in to establish the running setting, it took less than a turn to close, which seems very lean and implies air is still getting in via an unregulated route. This may also explain the odd airbleed behaviour... Quote Link to comment Share on other sites More sharing options...
Jon H Posted July 4, 2021 Share Posted July 4, 2021 2 hours ago, Mike T said: On winding the main needle in to establish the running setting, it took less than a turn to close, which seems very lean and implies air is still getting in via an unregulated route. This may also explain the odd airbleed behaviour... Are you sure about the logic of that statement... If there was an air leak, ie an excess of air, the engine would need more fuel for the mixture to be correct so the main needle would be open further than normal. Also, with an air leak the pressure differential seen through the fuel system would be lower than with a sealed carb so again the needle would need to be open further than normal to allow enough fuel through. Its also impossible to tune an engine by counting turns on the needle. If it is turned and happy at less than a turn then thats it, its tuned. How it compares to another engine is not important and neither is the number of turns. AIr bleed screws are quite sensitive and 1/4 turn will make quite a difference. They also impact the main needle so opening the air bleed (leaning the slow run) will require the main needle to open as well, even if it is very slight. I would recommend you remove the carb and take it all apart. Clean everything (including the air bleed hole), set the air bleed screw so it half covers the hole and then start from scratch. Quote Link to comment Share on other sites More sharing options...
J D 8 Posted July 4, 2021 Share Posted July 4, 2021 If you read my post above it explains why the air bleed screw does little to effect the general running of the motor. To set the air bleed first tune the main needle for best running at full power then close throttle to tick over. After a few seconds open the throttle to full observing the pickup. If it cuts out and stops turn screw clockwise 1/4 turn and try again, repeat until you get a smooth pick up. If it splutters and spits before slowly picking up turn screw counter clockwise 1/4 turn at a time until you get a smooth pick up. And that is it, leave it alone. Just typed the above and Jon answered first. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted July 5, 2021 Share Posted July 5, 2021 Try fitting a sintered metal clunk in the tank. Not only will it give superior fuel filtration but like a felt filter will draw fuel even if it part exposed to the air . Ripmax supply them cost around £2 to 3 . Quote Link to comment Share on other sites More sharing options...
Mike T Posted July 5, 2021 Author Share Posted July 5, 2021 Thanks again both. JD8 - I do know how the airbleed works - sorry if I didn't make that clear. I was just a bit disconcerted that the changes didn't have the effect that I expected, so (another) strip-down is the logical next step. On the subject of logic - Jon you are quite right - my comment re the main needle was completely arsy-versy ? ED - I use sintered clunks in all my tanks - except the SLECs (what's in the J60) as the aperture is too small!. If I end up swapping the tank for a conventional bunged version then it will have a sintered clunk for sure - reduces foam in the fuel as well. Quote Link to comment Share on other sites More sharing options...
Mike T Posted August 16, 2021 Author Share Posted August 16, 2021 Just to close this one off, I committed the cardinal sin of making more than one alteration at a time ? . I not only re-fettled the carb (no issues found), but also replaced the (lowered) tank with a bigger one! The tank still doesn't empty, but I get the duration I want. I suspect that to get those final cc'c out I'll have to replace the (SLEC) tank with a conventional bunged type so I can put a sintered clunk in it. Thanks again for the input, guys ?. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted September 15, 2021 Share Posted September 15, 2021 Missed this thread last month for some reason. One thing not mentioned is the possibility of fuel foaming as the level drops. It looks like the tank is mounted quite rigidly to the airframe - some foam insulation might improve the situation as would ensuring the prop/spinner are balanced. Quote Link to comment Share on other sites More sharing options...
Mike T Posted September 15, 2021 Author Share Posted September 15, 2021 I did wonder. I haven't isolated tanks for years because I 'dosed' my fuel with 'Armor-all' silicone-based spray polish. It can't be got any more and for the past year I've been using MT fuel, which I'm led to believe has anti-foaming additives. It still foams... The SM stuff I normally use produced a good head when well shaken, but after a whiff of Armor-all, it behaved like water. If anyone knows of a current silicone-based spray polish, I'd give it a go, in the meantime, I'm sure I've got some 'magic foam' or some polyprop packing that I can slip under. Quote Link to comment Share on other sites More sharing options...
david james 1 Posted September 15, 2021 Share Posted September 15, 2021 I’ve had this problem with other OS engines. Crap engineering. They should have stuck with the Woodroff key used on some OS engines Quote Link to comment Share on other sites More sharing options...
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