jetjr Posted December 19, 2015 Share Posted December 19, 2015 All correct, it seems bing float setup doesnt regulate flow too well for some reason Does make sense as we need it to flow 60 lph peak then run most of the time around 25 Higher pressure makes it worse, as said in this case flow and pressure linked closely maybe to the point of surges as float closes and opens. Could be related to suction pressure and friction loss in fuel lines hence why the need for 60lph free flow at pump. Its not a simple fix. 1 Link to comment Share on other sites More sharing options...
fly_tornado Posted December 20, 2015 Share Posted December 20, 2015 give rotax a call they seem to have those bing carbies worked out Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 True . They like them so much they use two on each engine. Nev Link to comment Share on other sites More sharing options...
jetjr Posted December 20, 2015 Share Posted December 20, 2015 What is the fuel pressure upstream of rotax carbs? Does it vary? Dont know? More garbage from FT. Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 There are some suss floats that absorb fuel and lose the required flotation ability. Is that a possibility? Nev Link to comment Share on other sites More sharing options...
fly_tornado Posted December 20, 2015 Share Posted December 20, 2015 you're well come. seems like you are measuring the wrong parameter, 1psi is almost nothing, why not measure flow rate Lph? that would indicate a problem if there was one Link to comment Share on other sites More sharing options...
jetjr Posted December 20, 2015 Share Posted December 20, 2015 No im measuring both, flow rate is static, pressure changes far faster Considering 1-3 psi is the max bing recommend, What do you suggest we run at? Probably comment on something you know about Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 The needle and float set up is pretty common design wise.. Vibration can affect seating. If you have a flow situation where there is a restriction up stream (which could be the length of the pipe total friction) of where you are measuring it, the pressure drop will be reasonably sudden as the float needle is no longer regulating the flow fully and will try to do it by dropping down to permit more flow. and in so doing drop the pressure. I'm not too happy about that as a possibility. Each PSI is nearly 3 feet fuel head..Nev Link to comment Share on other sites More sharing options...
jetjr Posted December 20, 2015 Share Posted December 20, 2015 Understood but headloss will be exponentially larger at high flow, and in that case pressure is pretty stable. Also using elect boost pump should negate this problem. Its when carb is choking flow back that theres variation Id suggest very few have measured pressure here with gear able to see changes. LSA use low pressure light, which gives plenty of problems, the same unit in a sender is 0-30 psi so not reading much inbottom 10% of range. Im using good 0-5 psi unit. Bottom line is that we know engine is highly sensitive to fuel level in bowl, the first sign of problem is possibly going to be higher egt Link to comment Share on other sites More sharing options...
fly_tornado Posted December 20, 2015 Share Posted December 20, 2015 what I said was, why bother measuring the pressure, its so low its only going to distract you 1 Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 Fuel level in the bowl variation is a concern. To a certain extent it's built into the design where high rate past the needle must have lower float position in the bowl, and corresponding lower fuel level, though realistically it should not be much, Still the effect is in the wrong direction. An unloaded pump will have a fixed limit output though. If the restriction is caused by the carb you would have more pressure as you are measuring the pressure there aren't you? Nev 1 Link to comment Share on other sites More sharing options...
OZJohn Posted December 20, 2015 Share Posted December 20, 2015 The critical measurement in any CD carby is fuel level in the bowl. 1 Link to comment Share on other sites More sharing options...
Bruce Tuncks Posted December 20, 2015 Share Posted December 20, 2015 I've wondered about putting a fuel pressure meter in, and haven't because I'm not sure the information it gives me would be worth the complexity involved. Suppose you are cruising along and the pressure reads low... so you turn on the electric pump. What else can you do? Suppose you have no pressure gauge and the engine runs rough or stops... you turn on the electric pump. Here's something I've wondered.. would a gauge that reads some small vacuum pressure put BEFORE the mechanical pump enable you to quickly diagnose any problem in the fuel supply line? What pressure (vacuum) should you see in this gauge? Of course it will go lower at full power, but should it go negative? At least with the electric pump off? I have experienced a blocked tank vent where the pressure would have been highly negative ( it oilcanned the fuel tank), but this was not normal of course. 1 Link to comment Share on other sites More sharing options...
jetjr Posted December 20, 2015 Share Posted December 20, 2015 No it should never go negative upstream of mech pump. Thats the reason for free flow checks Id recon plenty do go negative especially with flow sensor fitted and jabs system of checking flow with elect pump on. Measuring low vacuum accurately is tough Dynon can handle low pressure kavlico senders and thats what im playing with Without fuel pressure gauge how do you know elect pump is working? I think its some standard that there be low pressure indication just before carb and thats why the low fuel press light is fitted to LSA models. Shame it doesnt work and lights flicker etc. Id suggest egt will quickly tell you of theres a serious problem. The position of the elect pump is important too. I think it should be upstream of filter nearest header tank. Many are located after filter and some are mounted in cockpit side of firewall with lines running to rear of aircraft Also consider this and throw it into Jab engine problem theory list... An engine running basically fine, at take off the fuel supply system and mech pump cant quite keep up, engine goes lean around 500 ft, running wot, high air density, its a great time for detonation. Mix in a few egt spread problems and youve got certain cylinders running hot and detonating every take off. Some schools teach to back off throttle a little around then. Some dont. Some use some mogas too Just thinkin. 1 Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 ALL of what you say is possible. Backing off the throttle may cause it to run lean. Not a good idea at above 75% power, which you still.may be at. This is all predicated on the taper of the needle being right to achieve what is desired mixture strength wise. Nev Link to comment Share on other sites More sharing options...
jetjr Posted December 20, 2015 Share Posted December 20, 2015 Maybe "less lean"than with a low carb fuel level. Ive done it after service and egt spike is dramatic, was only 1-2mm low. Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 I would be surprised if it was super critical as long as the open position allowed plenty of fuel to flow into the bowl. The height is small and it's only the force of gravity we are talking about. 2 or 3 mm height of fuel at whatever SG it is. I have found it can affect starting and idle on some other types of carb. Not recommending not getting it right though. Set up carb as maker recommends. The thing that affects it a lot is the plastic breather tube from the bowl. Where you locate it.Nev Link to comment Share on other sites More sharing options...
jetjr Posted December 20, 2015 Share Posted December 20, 2015 1-2 mm too low makes egt over 800 deg and faltering engine after 300-400 ft at wot and some serious butt puckering adjusted fuel level, and after extended on ground testing.... All normal Link to comment Share on other sites More sharing options...
Bruce Tuncks Posted December 20, 2015 Share Posted December 20, 2015 I was taught to back-off the throttle just after take-off to take some stress off the engine. It feels like you are doing the right thing but I understand the point about lean running being a risk. Would it show up on the egt if lean running and detonation was happening or close to happening? And that's good point about the fuel pressure gauge at least telling you if the electric pump is working. It would also tell you if excessive pressure was being produced when the 2 pumps were on. Link to comment Share on other sites More sharing options...
Methusala Posted December 20, 2015 Share Posted December 20, 2015 Normally carburettors are set up to run slightly rich on WOT, then to lean out a little for cruise power. This is to give some evaporative cooling at high power settings. Ergo, not too clever to back off slightly while climb power is set. Just my understanding though. Don 2 Link to comment Share on other sites More sharing options...
facthunter Posted December 20, 2015 Share Posted December 20, 2015 Two pumps should not cause too much pressure unless the back up has a pressure over what is recommended . The spring in the engine pump determines pressure, until the back up one comes into play and it then doesn't stroke because the spring never extends under those conditions.. I got caught on the logic of this some time ago, so always remember it. Nev 1 Link to comment Share on other sites More sharing options...
facthunter Posted December 21, 2015 Share Posted December 21, 2015 Don true. CASA put out a lengthy article on this way back . People were thinking they were saving their engines by reducing power as soon as possible but most pistons are over rich at take off power for extra cooling and less detonation risk. Prolonging the climb by reducing power, and leaning out the air/fuel ratio was doing more damage than the higher power setting would have. Some engines have a time limit on how long max power should be used so observe that where applicable. A general rule of thumb is not to lean out when you are over 75% power. Most of us with Bing Carbs have this taken out of our control because there is no manual leaning, but the situation is covered by the needle diameter being reduced in the travel when the dashpot is open near fully Ie full throttle on the butterfly and below a certain density altitude. Once you are away from that airflow situation the mixture becomes leaner by again the diameter of the needle changing and the mixture becomes leaner because it's restricted. This gives you leaning with altitude (more or less) automatically. Nev 1 1 Link to comment Share on other sites More sharing options...
Bruce Tuncks Posted December 21, 2015 Share Posted December 21, 2015 Thanks Nev, I have to admit that I never gave the system enough thought until your post. In systems like 2 centrifugal pumps, they double the pressure if connected in series, and I think I assumed that this would be the case here too, but you are right in saying the mechanical pump spring sets the pressure. The only time this would not be true would be if the electric pump produced too much pressure all by itself. 1 Link to comment Share on other sites More sharing options...
Downunder Posted December 21, 2015 Share Posted December 21, 2015 Normally carburettors are set up to run slightly rich on WOT, then to lean out a little for cruise power. This is to give some evaporative cooling at high power settings. Ergo, not too clever to back off slightly while climb power is set. Just my understanding though. Don That is clear on my rotax. Coming off the needle to WOT/ high rpm, EGT's cool a bit.....as it richens up... Link to comment Share on other sites More sharing options...
Russ Posted December 21, 2015 Share Posted December 21, 2015 So.........backing off power to 2900 revs, climb speed 85 is " bad " ????? Link to comment Share on other sites More sharing options...
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