Kitfox 4 rebuild

246 posts in this topic

Posted

Other things looked at, compression was 80/80 on 2, 78/80 on 1, and 79/80 on the other.  Checked valves today, didn't put a dial indicator on them, but all looked to move down the same amount.  Tried a different prop today as well.  Mag checks are pretty much equal drop.  Tiny tack is currently just on one side, so can't give any exact rpm drop, but seems same to my ear.  Not a lot rougher running on either mag.  Much alike with both sides, but slower and not quite as smooth.  Store only had 6 plugs in stock, so changed that many, but the other two looked fine.  JImChuk

Hey Jim, what was you mag drop rpm? A normal mag drop should be between 80-120 rpm, anything above hints to ignition system malfunction (leads/caps/coils/pickup etc)

I also had a hard time for 6 months figuring a bad mag drop..

Also I found it not always clear if it's an ignition issue or carb issue, hence the 80-120 rpm drop test. I had 200-300rpm drop...was bad , even though book says up to 300 is ok

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Posted

I can't give exact numbers right now, but I would say 80-100 seems likely for the mag drop.  I did also install all new spark plug wires this winter, I move the ignition modules to the firewall side of the motor mount so it would be subject to less vibration and the broken wires that can cause.  But if I had one bad plug or wire, it should show up in the mag dropping much more on one mag then the other.  What did you find the problem to be on yours, flywise?  JImChuk  

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Posted


Good luck today Jim.

A few ideas run it with out the muffler and check all wiring harnesses maybe a small pin broke off in one of them.

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Posted (edited)

Thanks TJay,  as far as wires go, anything is possible, but I would expect to see a broken wire show up in the mag check???  I even was wondering if something was amiss in the gearbox, bad bearing or something.  Pulled the magnetic plug out, and it did have some black paste like stuff on it, rubbing it in my fingers it felt like graphite paste or something.  No pieces of metal that I could detect.  Wish I had the # 30 idle jets right now, be interesting to see what difference they will make.   I do go back to the one time a week or so back, I was fiddling with the air metering screws, and I had it running as smooth as could be, and I said, I've got it finally, but a couple hrs later when I started it, it was back to rough running with not changes done other then it sitting there for a while...JImChuk

PS,  and before anyone says: do you have the air cleaners loaded up with to much oil, it runs the same with or with out air cleaners....:dunno:

Edited by 1avidflyer

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Posted (edited)

I can't give exact numbers right now, but I would say 80-100 seems likely for the mag drop.  I did also install all new spark plug wires this winter, I move the ignition modules to the firewall side of the motor mount so it would be subject to less vibration and the broken wires that can cause.  But if I had one bad plug or wire, it should show up in the mag dropping much more on one mag then the other.  What did you find the problem to be on yours, flywise?  JImChuk  

Hey Jim, I found three things : I had different length slider springs in my carbs ,  my choke housing seal was not totally airtight. The other thing was that the mechanical rotax fuel pump is delivering way too much fuel pressure (5-7psi) at idle. I called the bing manufacturer and they confirmed that the bing carbs are built for a maximum of 2.9psi pressure...So I installed a fuel pressure regulator/damper and all is great.   took me ages to find these problems...

Edited by flywise

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Posted

Thanks flywise  (got a first name? :-)   I do have a fuel return line installed in my system with a .040" orifice in it, and I put a temporary fuel pressure gauge in the fuel line just before the carbs, and it was showing 3 lbs.  I suppose I could rig up a temporary gravity feed system and see if it makes any difference, but I'm guessing it wont.  Do you have the fuel return line?  Swapping carbs should have covered the other two issues, so again I'm at a loss.  :dunno:  JImChuk

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Posted


Frustrating but don't let it get you down. I remember Larry chased a similar issue a number of years back and it ended up being carb sockets. IIRC, you said you 'swapped' carb sockets. Swapped between the two 912 engines you have, or actually replaced with a brand new set? If new, JBI or Rotax?

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Posted (edited)

I had a set of the JBI sockets sitting for a number of years in an unopened bag.  Not out in the sunlight either.  I would say that is not the problem.  Thanks though Doug.  It's got to be something.  After I change out the idle jets, I will swap fuel pumps, this engine has the upgraded new higher pressure pump, the other engine has the old style one yet.  Or else rig up a gravity flow fuel tank. Maybe both if the first try doesn't work.  JImChuk

One thing I should mention, one of the ways I synced the carbs was with a large vacuum gauge I have, ran a line from each intake manifold to a T  that had one leg connected to the vacuum gauge.  Both lines from the manifolds had valves in them, so I could open one and shut off the other to get a reading on either side.  On the left side, the vacuum gauge bounces back and forth about 3 lbs  ( 1 1/2 lb each way)  on the right side it only bounces about 1 lb.  (1/2 lb each way)  not sure if that is telling me anything or not.  Other way I synced the carbs was with two clear bottles filled half full of prestone, interconnecting tube between the bottles, and lines going from each bottle to each manifold.  If level in bottles stays the same, you have same vacuum on each side.  Seems to be pretty accurate.  Here is a video of that procedure  https://www.youtube.com/watch?v=Y_WDF6glD5k

Edited by 1avidflyer

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Posted (edited)


Kind of curious re: the needle bounce on the gauges. Did you swap vacuum tubes to see if it was just a twitchy gauge or if it followed the same carb? [edit: oops, never mind, I should have read a bit closer]

If the roughness persists after the jet change, this is the only other thing I can think of... and I know you don't want to go there. At least you have another gearbox to swap.

https://pointsforpilots.blogspot.com/2018/11/bellville-washers-in-gearbox-of-rotax.html

Edited by dholly

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Posted

I was just using the one vacuum gauge, so something was different on the one side to give the bouncy reading.  I just watched a video about reading a vacuum gauge, and he says a bouncing needle is an indication of warn valve guides.  Engine has 750 hrs on it.  Leak down compression is excellent, so makes me wonder.  Maybe it can have good compression with worn guides, but is that likely?  I don't know.  How common is worn valve guides on the 912?  Thanks for the article on the bellville washers.  How likely at 750 hrs?  JImChuk

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Posted

Still waiting for the idle jets from Bing carbs.  I did run the other 912 this evening, got the miss sorted out, one plug was misfiring.  If the idle jets don't do the trick, I'm swapping engines.  JImChuk

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Posted


Jim did you cover the wings on this recently. Trying to figure up a chemical materials list?

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Posted

I did the wings about 3 or 4 years ago.  I would figure 4 gallons of poly brush and 4 or 5 gallons of poly spray for the whole plane,  Finish paint depends on what you do with it.   JImChuk

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Posted

Well they say don't look a gift horse in the mouth, but still I wonder....   I got the # 30 idle jets for 912 today that I couldn't get to run smooth.  Put the new jets in and never saw a drop of gas come out the overflow tubes.  Seems to have been the cure.  At least for now......   Which brings up my saying that I started out with.  While I'm happy that it's running better now then before, I'm still puzzled with why.  Why do almost all other 912s run fine with the # 35 jets, including my other one, and this one doesn't?  If it needed a richer jet, I would think air leak, but having to lean it out even though a second set of carbs didn't do any better has me wondering.  Any ideas?  JImChuk

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Posted

Hi Jim

I’m glad to hear that helped, as far as why it helps I have no idea. I have wondered the same thing and would by interested to know why. A interesting side note is that I have the same bouncy vacuum gauge on the left carb on our engine, not nearly as much on the right. As far as the worn valve guide theory goes our engine only had 40 hours on it when we bought it and had the rough idle issue so I kind of doubt it but I guess anything is possible. We have just under 90 hours on it now and it’s running great.

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Posted (edited)


If an engine is running rich it either cant get air or cant get rid or air.

Either way I'm looking forward to your first flight.

Edited by TJay

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Posted

Well it's not over, back to spitting gas again.  Today I changed the new style fuel pump for the old style one that makes less pressure.  No difference. Mag drop is about 150 on one side, little less on the the other. Last thing I'm changing is the ignition modules and coils from the one engine to the other.  I'm pulling the other engine off and will have a look inside, cause it had sat quite a long time.  If there is no corrosion, and the ignition switch doesn't fix my problem, I'll switch motors.  I did do a compression check on the second 912 a couple of days ago, and three cylinders were 80/80, and one was 79/80.  No corrosion on valve stems or lifters and such, so hopefully the internals are still ok.  I do know the guy at LEAF told me that these engines usually hold up pretty well, nicasil cylinders so no corrosion on them and such.   Right about now, If I was to run across a project with an unused 912, I would maybe see about snapping it up.  The idea of dumping another $17,000 or so after sales taxes and such kind of puts me off from a brand new one.   I have been thinking about the differences between the Jabiru and the Rotax.  The ignition on the Jabiru is permanent magnets in the flywheel, running past two Honda coils.  The sparks go through two small distributors that run off gears from the cam.  Carb is just one type 94 Bing.  Sure would have been nice if Rotax had done something simple with some aspects of their design.  I'm not saying Jabiru is better, overall, I think the 912 is a better engine, but they sure could have made some things simpler.  JImChuk

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Posted

Well that really sucks! I was hoping to log on and read your first flight report. I feel for you that can be really frustrating. Will it idle long enough that you can turn the fuel valve off and let it run out of fuel? If it smooths out for a few seconds before it dies that tells you conclusively that your dealing with a rich condition vs a electrical one. Just a thought that’s easy to try.

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Posted

Wings are usually folded, so the header tank runs out of fuel occasionally.  Didn't notice it getting better before it would die.  Rougher actually.   I'm frustrated.  JImChuk

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Posted


Maybe you need to walk away for the day go fly the avid around. When you come back the issue will be right there in front of you.

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Posted

Thanks TJay,  You're right, the float level is easy to adjust... if the carb is off the plane, and you can turn it over when you are putting it back together.  I know I have fought to get the needle valve and float arms back on my Jabiru carb with it mounted in place.  Last times I have done it, I ended up putting grease up into the jet, and push the needle up in there.  Then try to hook the little wire holder over the tab on the float arms and get it in place so you can put the pivot pin in.  All while laying under the plane looking up through bifocal glasses that are never in focus it seems.   And in a 4" space, with 5" hands.    After I got the pin back in place, I let enough gas flow through to wash out the grease.  The way the carbs sit on the 912, especially the left one, with it close to the firewall, and my water recovery bottle also in the way, I think I would take a hammer to things before I got it back together out of flustration.  I should have pulled the carbs off completely last week when I changed out the carb boots, the carbs were half way off then, just cables and fuel line still holding.  Wish I had now, maybe would have solved the problem, or at least been able to say:  well it's not that either.....  :dunno:  JImChuk

Thanks TJay,  You're right, the float level is easy to adjust... if the carb is off the plane, and you can turn it over when you are putting it back together.  I know I have fought to get the needle valve and float arms back on my Jabiru carb with it mounted in place.  Last times I have done it, I ended up putting grease up into the jet, and push the needle up in there.  Then try to hook the little wire holder over the tab on the float arms and get it in place so you can put the pivot pin in.  All while laying under the plane looking up through bifocal glasses that are never in focus it seems.   And in a 4" space, with 5" hands.    After I got the pin back in place, I let enough gas flow through to wash out the grease.  The way the carbs sit on the 912, especially the left one, with it close to the firewall, and my water recovery bottle also in the way, I think I would take a hammer to things before I got it back together out of flustration.  I should have pulled the carbs off completely last week when I changed out the carb boots, the carbs were half way off then, just cables and fuel line still holding.  Wish I had now, maybe would have solved the problem, or at least been able to say:  well it's not that either.....  :dunno:  J

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Posted

Situations like these keep me from wanting a 912. I think I'll save my pennies for a bird with a Cont or Lyco on the nose. They may be old, outdated, heavy and came here on the Mayflower, but they are pretty simple and reliable. I know from all reports that the 912 is a great engine, or at least till something starts going south.

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Posted


Well if it runs good then doesn't then does then doesn't that means its a problem that is being over looked. Probably just a loose wire or bad ground somewhere Keep looking Jim I wish you the best. Look for kinked wires with white ish marks on the outside that usually means a cracked wire inside the insulation.  Poor spade connectors. Look everywhere Jim or give me a good deal on another 912 Ha

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Posted

Hey Jim,

 

talking about the grounds...forgot to tell you about the importance of the ignition modules ground wire being 100% grounded. I had a ground wire fitted to a tab on my engine mount and found that the engine mount to airframe ground had some resistance (my engine mount/frame is powder coated), Try and pull a ground wire directly from your batt. neg. to see if things improve. Also the earlier 912 engines had wires shorting in the braided sleeve going to the coils which was an intermittent fault sometimes not powering one module or the other. Also make sure your stator coils are good (measure impedance through the ign module connector)

I am sure you already know, but all these items have a service letter or service bulletin you can find @ https://www.rotax-owner.com/en/support-topmenu/service-bulletins . your solution to your problem might be somewhere else in this long list....

Laurent

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