tiketyboo Posted December 18, 2011 Share Posted December 18, 2011 i am flying a jk 05 with a rotax 912ul, i had the unit repainted and the motor sat for 5 months and now it starts good, accelerates good, idles good but once at cruise it runs rough, not steady but irregular rough, checked the plugs, cylinder 1, 2 and 4 are fine, cylinder 3 plugs are pure white like new, no brown or anything, you would swear they are new. any one got any ideas or history of this kind of thing.what should i be checking or looking for. tony Link to comment Share on other sites More sharing options...
facthunter Posted December 19, 2011 Share Posted December 19, 2011 You're probably getting uneven mixture. Check your carbs and connections. (manifold /carb) and adjustments. Nev Link to comment Share on other sites More sharing options...
bones Posted December 19, 2011 Share Posted December 19, 2011 3 and 4 will always be the leanest because of the way the manifold is set up, how ever should not be that noticable, as Nev said it will be a mixture thing, make sure to calibrate the carbs with a balancer, if it is only in a certain range, it could be a slightly blocked jet, ect. do you have egt's on all pipes, if no.3 if hotter it is definately telling you it is lean. You might need to go right through everything, make sure you check the balance tube between the maniflods for the slightest of cracks, it could be just enough to really lean out no.3. check and i mean really check your carb sockets(replace if in doubt), i had a 912 run like a dog shot in the a** at idle but over that beautiful, turned out to be a crack on the bottom of the carb socket(black rubber that holds the carbs to maniflods) at idle it was bouncing and leaning, at revs it was sucking enough to hold the crack closed, it is very possible it is doing the opposite here. Have fun dont you just love it when its something really crappy like this :) Link to comment Share on other sites More sharing options...
tiketyboo Posted December 22, 2011 Author Share Posted December 22, 2011 bought the carbmate and still could not synchronize the carbs or get it to run smooth so took the carbs off and cleaned them, installed them today and she is running smooooooooooth. i have put in new plugs so will check them in say 15 to 20 hours to see how they look but the one side with real clean plugs was the side that had the dirty carb. Link to comment Share on other sites More sharing options...
facthunter Posted December 23, 2011 Share Posted December 23, 2011 If you are relying on plug colour, it will change when you are taxiing . The way to check plugs is to run them for a while at high power and then a dead cut. This is not easy to do with an aeroplane unless you do a tethered ground run, if you think about it. Nev Link to comment Share on other sites More sharing options...
tiketyboo Posted December 26, 2011 Author Share Posted December 26, 2011 well the clean carbs lasted about 10 minutes, then rough running again, some one suggested pulling the choke out while flying with rough running engine, sure made it smoother so it means the carbs or carb is running lean, going to change the needle clip lower and see how that works Link to comment Share on other sites More sharing options...
jetboy Posted December 26, 2011 Share Posted December 26, 2011 Bing carbs dont have a choke so if you find the engine running better with that selected - it will allow more air after the starting fuel has drained - then your'e running too rich without the "choke" selected. More likely causes are too much fuel pressure, faulty float needle, faulty float chamber venting. Ralph Link to comment Share on other sites More sharing options...
bones Posted December 27, 2011 Share Posted December 27, 2011 well the clean carbs lasted about 10 minutes, then rough running again, some one suggested pulling the choke out while flying with rough running engine, sure made it smoother so it means the carbs or carb is running lean, going to change the needle clip lower and see how that works Reread my post above :) 1 Link to comment Share on other sites More sharing options...
facthunter Posted December 27, 2011 Share Posted December 27, 2011 The location of the bowl vent pipe affects the mixture. If it is in a low pressure area it will run lean and give the symptoms you describe. Nev Link to comment Share on other sites More sharing options...
tiketyboo Posted December 29, 2011 Author Share Posted December 29, 2011 The location of the bowl vent pipe affects the mixture. If it is in a low pressure area it will run lean and give the symptoms you describe. Nev guess i am back at the drawing board then,,,,,,, what with the choke facts, the vent pipe facts etc. will have to tie down the vent pipe and re think the jets. son of a gun Link to comment Share on other sites More sharing options...
Tomo Posted December 29, 2011 Share Posted December 29, 2011 Have you checked the rubber intake connections like bones suggested? Link to comment Share on other sites More sharing options...
tiketyboo Posted December 29, 2011 Author Share Posted December 29, 2011 i checked the rubber connections well, i am thinking that the plastic tube overflow lines are maybe in the wrong place as they are hanging down the side of the carbs but i do not remember where they are supposed to be, i will have to look again and see where they should be located, now i am waiting for the snow to go away hopefully it will leave soon other wise it will be spring before i will get up again as we have no snow plow at the strip. Link to comment Share on other sites More sharing options...
bones Posted December 29, 2011 Share Posted December 29, 2011 bought the carbmate and still could not synchronize the carbs or get it to run smooth Ok just so i know what you mean, i thought about this and went back and reread this post, when you say"could not synchronize the carbs " What do you mean by that? With the 912 and a carbmate you should be able to get both carbs within a bee's d*ck of each other from idle to about 3000, then depending on the throttle cable set up ect, you may get a small vairiance after that, how ever you may notice it will swap around for side to side, the 914 is a different kettle of fish though, because of the turbo. You need to start right at the start, disconnect the cables and swap the springs around, set idle speed and sync, then reconnect cables set tensions on the cables with the carbmate and then just very small adjustments from there and it will be near to perfect. Have you watched the video on using the carbmate? UNLESS THERE IS SOMETHING ELSE WRONG. You said in your first post it runs rough at Cruise? what rpm are we talking here? Also dont forget when using the carbmate, the engine will always run rough until you connect the balance tube up. Link to comment Share on other sites More sharing options...
tiketyboo Posted December 30, 2011 Author Share Posted December 30, 2011 thanks, we finally got the carbs synchronised after we cleaned them, we went thro all the steps, adjusting the cables, tieing them down etc and she was smooth, i took it for one circuit and she was just like before i put the motor in storage for 5 months. i was very happy, Next day i took it out to do some flying as i had only done one circuit the day before and it started to run rough again at cruise, 4000 to 5200 rpm, she starts well, accelerates well, climbs 1400 fpm, everything up to them is good. once i am at cruising altitude i level out and pull back the throttle and within a minute it starts to run rough, and will continue from 5200 down to 4000. the last time i had it up i pulled the choke out a little when she was running rough and that made it run a little smoother. it is not a regular rough like a propeller out of balance more like a car running on bad plugs or bad gas. i have now tied the overflow tubes to the air filters behind the carbs, installed new fuel filter and lowered the clips on the main jets one notch to the bottom notch, checked the carb bowls for dirt(none). re did the synchronisation on the carbs and hopefully will get it up in a day or so if the snow keeps away, very frustrating for sure, Link to comment Share on other sites More sharing options...
bones Posted December 30, 2011 Share Posted December 30, 2011 Boo, i had a great post written out, and it all went poof on me, and since i too tired to rewrite it. heres the simplified version, can you get it too run rough just doing a circuit? Im trying to work out if it only runs rough at Altitude? What altitude do you fly at? Couple of more options left yet, i just need to find them in my head. Link to comment Share on other sites More sharing options...
facthunter Posted December 30, 2011 Share Posted December 30, 2011 The location of the end of the vent pipe from the carb bowls, does have asignificant effect on the mixture. I have been involved with a situation where it was, so it's not just something out of a book, or so. Rotax had some reference to it, but I can't direct you to it. It makes sense because it is the pressure difference that causes the flow to occur . Nev Link to comment Share on other sites More sharing options...
tiketyboo Posted December 30, 2011 Author Share Posted December 30, 2011 the vent pipe as you say might be involved as a few times i can smell gasoline, and do not know where it is coming from. altitudes were 500, 1000 and 1500 all the same, got everything on hold now as the bloody runway is covered with snow, can not get off to try anything, happy new year to all and thanks tony Link to comment Share on other sites More sharing options...
bones Posted December 30, 2011 Share Posted December 30, 2011 To test if it is the vent tubes, simply replace the 2 tubes from each carb, with 1 long enough to reach to both sides again, then in the middle of the loop put 1 small hole about 3mm this will be good enough for a rough test, if it doesnt run rough like that, then reroute your tubes to somewhere else, obviously you may need longer tubes. Link to comment Share on other sites More sharing options...
turboplanner Posted December 30, 2011 Share Posted December 30, 2011 and lowered the clips on the main jets one notch to the bottom notch Tickety, I have no experience on Bing, but a lot of experience on Mikuni, so you have to take this into accound. That's the second reference I've seen to altering the needle slots. You are calling them main jets. On the Mikuni: 1 fuel mix at idle is controlled by idle jets, and plays no part at cruise. 2 The needles control mixture from 1/4 to 3/4 throttle - the acceleration phase. There's no jet, just the air gap around the needle. 3 Above 3/4 throttle the needle to wall gap is more than required, so the restriction is the main jet. If the Bing uses the same principle, if you are doing as you say and the bottom notch is towards the needle taper, you are increasing richness at the 1/4 to 3/4 zone. If you cruise in that zone, needle adjustment will have an effect on rough running, and from your actions I would expect that cylinder to run a little rougher on the richer mixture. If you cruise above 3/4 throttle, on the Mikuni the needles would be playing no part, so you could adjust them back for clean throttle acceleration and focus on what the other guys are saying. I'm not familiar with Rotax, but sometimes manufacturers compensate or manifolds by using slightly different sized main jets. If they are't all stamped the same size, then you cold check that out. As I mentioned, be careful before using this information because its all Mikuni based. Link to comment Share on other sites More sharing options...
facthunter Posted December 30, 2011 Share Posted December 30, 2011 Bones is onto it. I don't think it is necessary to put the two vent tubes together (as in Tee'd) but it is important to have them both at a freestream location, ( static pressure. Check with Rotax, it's in their info somewhere). Altitude will not affect it provided the original needles are in the carbs. ( The altitude mixture compensation is a compromise anyhow). Nev Link to comment Share on other sites More sharing options...
tiketyboo Posted December 31, 2011 Author Share Posted December 31, 2011 will try the vent tubes on my next try, hopefully we will get a couple more days of flying before the snow sets in happy new year all and thanks Link to comment Share on other sites More sharing options...
tiketyboo Posted January 2, 2012 Author Share Posted January 2, 2012 i will do some studying about vents etc and hopefuly will be able to fly again soon Link to comment Share on other sites More sharing options...
turboplanner Posted January 5, 2012 Share Posted January 5, 2012 Hi ticketyboo, got a message from a mate who is familiar with your carbies. He suggests checking the float level. It is unlikely anything has changed with the vent tubes unless they get blocked. Suggests you should not be playing around with needle positions unless you have moved somewhere with a dramatic altitude change. Link to comment Share on other sites More sharing options...
bones Posted January 5, 2012 Share Posted January 5, 2012 Glad some one finally posted in this thread again, i couldnt find it, damn hard to find things on this site, and the search didnt work either. Anyway, an obvious (probably stupid)question but you keep mentioning snow ect, so your in a very cold area,,, Do you have carby heat?? The 912s will suffer carby ice in the perfect conditions, not very often but they will, and it could be that it is only enough to make the engine run rough. Link to comment Share on other sites More sharing options...
bones Posted January 5, 2012 Share Posted January 5, 2012 lowered the clips on the main jets one notch to the bottom notch I seriously doubt it will have anything to do with needles either, i must have missed where you wrote this before otherwise i would have said it earlier. If it was running fine before you sat it for 5 months, then there is no reason why it should have any machinical change unless you done it. My 914 motor sat for 2 years nd it runs just fine. Just a thought another stupid one,,,, have you checked the fuel tank vents???? it seems it only starts to do it after X amount of time flying YES? so it would lean(punt intended) toward fuel stavation cause if it were the vent tube for eg, it would do it right from the start of lift off, maybe there is a blockage in the vents of the tank and it takes awhile for it to take affect? If when it starts to run rough at the reduced rpm, what happens if you open the tap again?? continue to run rough or clear up? I dont know it is hard when your not there, just throwing ideas around here, tell me to shut up if you want :) Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now