ricky m Posted August 2, 2015 Posted August 2, 2015 I am getting an ocasional misfire from my jabiru when on full power climb.. Thought was carb ice first then found choke not fully shutting off so thought that be prob, but happend again today ok on first take of but presented problem on 2nd take seem to be second take off as first time flew to place departed miss... Second time took off done a pfl missed on climb.. 3rd time today flew to place took off 2 hours later missed... Cht all good rpm good oil temp good electric pump on... Thanks ricky.. Enginge 78 hrs since overhual run on bp 98..
Camel Posted August 2, 2015 Posted August 2, 2015 Just saying a fact, an engine missing under load ! Spark plugs is my first pick, fouled or unserviceable. 2
ricky m Posted August 2, 2015 Author Posted August 2, 2015 Just saying a fact, an engine missing under load ! Spark plugs is my first pick, fouled or unserviceable. Would b unlucky to get two on same cylinder faulty??? But spose poss 1
Dreamliner Posted August 2, 2015 Posted August 2, 2015 Recently had a misfire on shutting down, RH mag to be precise. Plugs all OK, fault traced to faulty connection on the spark plug lead at the distributor end.
JEM Posted August 2, 2015 Posted August 2, 2015 Hi Ricky Could be a sticking valve. Try a compression test 1
billwoodmason Posted August 2, 2015 Posted August 2, 2015 Could be electrical, fuel related or mechanical, methodically work your way through each and eliminate each possibility as you go. Speak to an L2 if you run out of ideas - these things don't happen for nothing. There will be a cause. Good hunting!.
Nobody Posted August 2, 2015 Posted August 2, 2015 A full power miss is unlikely to be carb ice and more likely to be something ignition or valve train. Is it s stumble for just a few cycles or does it continue for a period? Do you have EGT monitoring on all cylinders. If so does the cylinder go "cold" indicating complete lack of firing or does the engine just run rough but without a large EGT temperature drop on 1 cylinder. A completely cold cylinder is likely to be valve but rough running could just be 1 plug not firing.
ricky m Posted August 2, 2015 Author Posted August 2, 2015 A full power miss is unlikely to be carb ice and more likely to be something ignition or valve train. Is it s stumble for just a few cycles or does it continue for a period? Do you have EGT monitoring on all cylinders. If so does the cylinder go "cold" indicating complete lack of firing or does the engine just run rough but without a large EGT temperature drop on 1 cylinder. A completely cold cylinder is likely to be valve but rough running could just be 1 plug not firing. It stumbles for about 1 minute then clears.. Unfortunatly no egt sensors just the standard cht... This is what is making hard to pinpiont:( after mis fire no prob and wating for silence all good.. fly home.then all good for 3- 4 hours.. Cant replicate on ground . as there is two ignition circuits cant see both going bad at same time.... 1
ricky m Posted August 2, 2015 Author Posted August 2, 2015 Hi RickyCould be a sticking valve. Try a compression test Will be doing compression test now and check plugs for cylinder prob...im lucky enough to work where i keep aircraft so turn prop twice week to ensure no sticking valves or corrosion... Im at a loss right now :(
old man emu Posted August 2, 2015 Posted August 2, 2015 Before you get thinking about a complete engine rebuild, remember that most intermittent problems can be traced to a simple cause. The first thing I would think about is lack of spark, so trace all your wiring from battery to spark plug, looking for loose wires, or chaffing allowing a short. I'd pay attention to any physical interaction between the throttle cable and electrical wiring as the movement of the throttle cable or the cable outer could be causing chaffing. Also check that the HT leads coming from the distributor are pushed home tight, as is the HT lead from the spark maker. OME
jetjr Posted August 2, 2015 Posted August 2, 2015 Could be many things Dont rule out valve sticking, the guides can close up under heat or just gunk on the stem. Finding it is hard. Some talk that turning engine over by hand cleans off oil layer. Other little one, esasily checked is oil leak onto distributors, remove lids and have a look, sometimes small weep down back of engine. When bad enough shows on mag check.
facthunter Posted August 2, 2015 Posted August 2, 2015 Bear in mind you have dual plugs. The likelihood of two being suss at the same time is very low unless you have oil fouling from something like a worn inlet guide. It could be a hydraulic valve lifter. It your oil pressure a bit high? Other wise check for sticking valves. That's not easy as it's not a constant situation. If on full power. it could be insufficient fuel flow. Kinked hose or filter or a piece of rubber from cutting the end of a hose, anywhere in the system. Check rate of fuel flow to carburetter right to the float needle. Nev 1
ricky m Posted August 2, 2015 Author Posted August 2, 2015 Bear in mind you have dual plugs. The likelihood of two being suss at the same time is very low unless you have oil fouling from something like a worn inlet guide.It could be a hydraulic valve lifter. It your oil pressure a bit high? Other wise check for sticking valves. That's not easy as it's not a constant situation. If on full power. it could be insufficient fuel flow. Kinked hose or filter or a piece of rubber from cutting the end of a hose, anywhere in the system. Check rate of fuel flow to carburetter right to the float needle. Nev[/ .. Will look more mechanical as well as continuing with fuel checks :) was really hoping someone said ahh yes i had this fualt it was xyz ....
aj_richo Posted August 3, 2015 Posted August 3, 2015 Another easy one to eliminate, check the rotor button carbon post up in each of the distributor caps are not sticking or have plastic welding them over. I had an intermittent miss due to this problem, more prevalent in cruise but did happen at full power.
facthunter Posted August 3, 2015 Posted August 3, 2015 Intermittent problems are the most difficult to trace. I don't think you are likely to find another person with exactly the same symptoms, and similar ones may have different causes, entirely. There are a fewer people who can troubleshoot these days, so it's a process of elimination. Do the ones that should be part of normal "build" or post install checks first. I've seen people pull a motor to pieces when it was just a plug lead shorting on a hot part of the motor. THAT shouldn't happen if you go about it methodically. Are you getting FULL power before the symptoms showed themselves.? The choke is only working when the throttle is fully closed or close to it The slide in the carb could be sticky. Check needle too. Nev
JEM Posted August 3, 2015 Posted August 3, 2015 Suggest you get an L2 to do a simple compression test first.
Bruce Tuncks Posted August 3, 2015 Posted August 3, 2015 Is the misfiring on the first climb of the day, then it clears and you can't get it to misfire again? Mine did that, now fixed. The cause was taking off with slightly fouled plugs. 1
cavalon48 Posted August 3, 2015 Posted August 3, 2015 engine missing, check and clean micro filter inside fuel pump if there is one, also do a fuel pressure test, filters may be blocked, it is fuel starvation will happen on full throttle with a backfire. I had similar with rotax 914 and every one i spoke too thought there was fuel contamination. Checked pump one and found the microfilter blocked could not see daylight after 100hrs from new, after cleaning problem solved. 1
ricky m Posted August 3, 2015 Author Posted August 3, 2015 Intermittent problems are the most difficult to trace. I don't think you are likely to find another person with exactly the same symptoms, and similar ones may have different causes, entirely. There are a fewer people who can troubleshoot these days, so it's a process of elimination. Do the ones that should be part of normal "build" or post install checks first.I've seen people pull a motor to pieces when it was just a plug lead shorting on a hot part of the motor. THAT shouldn't happen if you go about it methodically. Are you getting FULL power before the symptoms showed themselves.? The choke is only working when the throttle is fully closed or close to it The slide in the carb could be sticky. Check needle too. Nev Hi Nev luckily i can do the checks myself as licensed mechanic and l2.. Was hoping for someone who had similar prob... I now got too look deeper as first time thought carb ice second found choke not shutting off... Getting full power and still climbs when coughing...it is coincidance i think! That it missis on second full power climb out... As after I readjusted choke checked carb ect went and done 3 circuits all good flew about 4 hoours fualt free then after 1 hour tweenty flight 2. Hours on ground missed on climb at about 500ft and continued till about 1000 Then cleard,couple of laps in the over head cleared and all good :( will do leak down on weekend check lifters and then oil presure if that checks out... Thanks for your thoughts and sugestions :)
planesmaker Posted August 3, 2015 Posted August 3, 2015 Could simply be the fuel, go back to avgas and see if it still does it
Camel Posted August 3, 2015 Posted August 3, 2015 Bear in mind you have dual plugs. The likelihood of two being suss at the same time is very low unless you have oil fouling from something like a worn inlet guide.It could be a hydraulic valve lifter. It your oil pressure a bit high? Other wise check for sticking valves. That's not easy as it's not a constant situation. If on full power. it could be insufficient fuel flow. Kinked hose or filter or a piece of rubber from cutting the end of a hose, anywhere in the system. Check rate of fuel flow to carburetter right to the float needle. Nev I have had one plug foul and able to feel the miss and that was in an o 320, just because there is two doesn't mean you won't feel it, just saying.
old man emu Posted August 3, 2015 Posted August 3, 2015 I now got too look deeper as first time thought carb ice second found choke not shutting off... Told you it would be something simple. People always think the worst. Always trust the quality of your equipment, and mistrust the human who fitted it. OME 1
facthunter Posted August 3, 2015 Posted August 3, 2015 Camel, it doesn't actually miss but has the effect of retarding that particular cylinder, more so when the plugs are located wide apart as on most aero engines of conventional design. There is a slight power loss but nothing like having a cylinder not firing. Nev
deadstick Posted August 3, 2015 Posted August 3, 2015 Hi Ricky m, check that you have the alloy tube directing air from the cooling ducts to the coils. They will behave this way if they get hot! Also check the gap between your coils and the flywheel, I use a piece of paper to set them up. Don't think it would still make good power if a valve was sticking, though I would recommend fitting CHT to each cylinder if you can afford it. 1
facthunter Posted August 3, 2015 Posted August 3, 2015 Sticking valves can be very damaging to the engine, not just hitting the piston crown. It can cause severe detonation in another cylinder, by forcing a larger amount of charge in there. This engine is quite suspect and should be sorted out as soon as can be done. A compression test which is done with engine stopped is not a tell all of engine problems. You could have a weak valve spring , faulty hydraulic lifter . quite a few things. Engines don't often fix themselves. It's in an aeroplane remember. Nev
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