Kitfox 4 rebuild

246 posts in this topic

Posted

Thanks for the kind words guys, and Paul, I'll look at the Earth-X battery.  What model do you have, and where do you have it located?  JImChuk

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Posted

Jim it is a ETX680C, they cost a lot more but worth it I believe.  I put mine in the original position under the baggage compartment Avid MK IV.  ONE nice thing about the Lithium iron phosphate is how they work in the cold weather, hit the starter for a couple of seconds then wait five and the battery comes up to full power.  Also they hold their charge for months with very little loss. A downside is they take a special charger.

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Posted

Thanks Paul,  I moved the battery 24" forward so it's now right behind the seat.  Lost a few lbs in the move as well.  I was starting to go to far rearward with certain CG setups, but moving the battery took care of that.  Guess I'll stay with what I have for now.  JImChuk

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Posted


5 4 3 2 1 lift off?

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Posted

It's all ready to go except I can't get it to run smooth.  Synced the carbs a bunch of times, my floats are not to heavy, tried different props,  getting tired of spinning my wheels.  JImChuk

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Posted (edited)


Doesn't run smooth at all RPM?

Sounded pretty good in the video maybe check for intake leaks? May have got knocked loose during taxing. Or bad gas.

Edited by TJay

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Posted

Carb boots cracked? That's what happened to my 582. Would start and idle ok. But wouldn't take throttle.

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Posted

Every thing seems hooked up right, fuel from different cans at different times.  New carb boots with no change....  Biggest problem is it wont idle smooth, and then the carbs start to overflow, and of course at that point it's running pretty rich so it runs worse yet.  Funny thing is last week, I was fiddling with the air metering screws, and I had it running like a clock, really purring at about 1800 or even a bit less.  I thought,  wow, great, I finally got it.  About 3 hrs later without having changed anything I fired it up, and it was just like before.... JImChuk

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Posted (edited)

The were several groups of bad floats for the 912/914.  Same sorts of symptoms, I fought mine for months finally found the info.  The pisser for me was I had rebuilt the carbs including new floats so I didn't suspect them to be bad but they were.  There is a SB out on the whole thing.  If you can't find it let know and I will look it up.  It gives you a couple of ways to check them, including weighing them which is the best way. 

https://www.rotax-owner.com/en/rotax-blog/item/34-912-914-float-inspection

Edited by wypaul
added link

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Posted

I weighed the floats with my little electronic scale, and it said they were good, also tried another set that weighed out as being good.  Maximum is 3.5 grams, some of mine were at 2.8.  Just to double check my scale, I used my little balance bar type reloading scale that measures in grains.  It agreed with the electronic scale once I did the conversion from grains to grams.  Like I said, spinning my wheels.  JImChuk

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Posted

I've pretty much got the same carbs on the HKS. Trying to work with Mikuni to switch to their carbs. I know Bing's are simple or at least the 54's are, but I am not fond of the 64 carbs. I used a Mikuni 42SLR on my custom Subaru on the model 4 and it had a mixture control among other neat little features. In the process of researching different carbs now.

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Posted


That stinks Jim all I can say is start with the simple stuff first. Because that is usually what it is.  Maybe fuel flow test.  thread sealant plugging up fuel lines? Another thing I have run into alot with bad running engines was incorrect float height and partially plugged pilot jets.

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Posted

Allen,  while the type 64 isn't the simplest carb ever built, usually they work pretty good and for quite a while at that.  My type 94 on the Jabiru which is just an enlarged type 64 has been doing good for about 600 hrs.  But like anything else, if it's messed up, not set up right or plugged somewhere it will cause problems.  Probably easier to stick with what you have, and just get it set up right.  TJay,  I've been thinking the same thing about the float bowl level.  Really should have dug into that area sooner.  I've never messed with the float level in these carbs, when I got the engine, it had an electrical issue which I resolved.  Will maybe do some checking in that direction tomorrow.  Thanks again for your help guys.  One way or the other that Kitfox is going to fly.  JImChuk

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Posted


Float level is simple to adjust less than a 10 minute job so that is always nice. Usually if fuel is splashing from carbs it means float level to high or the floats them self have sunk to the bottom. I know you weighted the floats and that's good next let them float around in a bucket of gas and make sure they stay on the surface. Really hope you figure it out jim.  Good Luck

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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

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Posted

Well I kind of went in another direction.  I have another 912 on the other Kitfox 4 project I have.  It's been sitting for quite a while, but was fairly close to being able to run.  Jury rigged up a radiator and fuel tank and stuff, and fired it up last evening.  It actually runs smoother then the other 912 I've been fighting with, and all I did to sync the carbs was by eye on the throttle linkages.  It does have a larger mag drop on one ignition and runs kind of ruff on that ignition so that needs to be addressed, but it sure was nice to see that thing running.  Now if need be, I can swap the carbs or other parts one at a time to the other engine to try to track down it's problems.  JImChuk

other 912 runs.jpg

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Posted

The saga continues.  Yesterday I checked the level of the floats in the carbs, and fuel level is down 1/2" with floats in the bowl, so that is right.  I took the carbs off the other engine and ran the shaky engine with the other carbs on it, and it still shakes and pretty soon gas is coming out the carbs.  I'll try other things tomorrow.  I did run the other 912 for a while last evening, before I pulled the carbs off it.  Have quite a rpm drop on one of the ignitions, so swapped the wire plugs for the ignition modules, and then after running it that way, I swapped the wires for the trigger coils.  The miss stayed on the same side, so the issue must be somewhere between the coil and the spark plug.  That is way better then the module or trigger coil.  Funny thing, the engine was running smoother even with the miss in the ignition then the engine in the Kitfox 4 I'm rebuilding.  So it's not the carbs.... Here is a short video of the back burner 912 running last evening.  Like I said, cobbled together enough so I could start it and run it for a little bit.  JImChuk

https://www.youtube.com/watch?v=YfCREMYY3JA

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Posted

So how exactly are the crankcases vented on these engines? If you aren't vented properly or freely or have a pvc valve type  issue that is rpm dependent or related and build up crankcase pressure that could make for a crap idle situation maybe... this is a long shot but you have been chasing carb issues and everything seems to be checking out...

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Posted

Crankcase pressure would go back to the oil tank, and there is a vent line going off the oil tank.   That seems fine.  Thanks for the suggestion though.  JImChuk

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Posted

Hi Jim 

I chased that exact same problem on the 912 in our Zenair 601 for a year. Changed fuel lines, fuel pump, had the carbs rebuilt by rotax, balanced them multiple times.new throttle cables, checked the ignition system multiple times. Nothing seemed to help until I dropped the idle jets down one size. It has run like a top with a smooth idle for 40 hours now.

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Posted

Capture.thumb.PNG.325a063b4b73659740c974

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Posted

Well I'm at a loss, so I figured why not take a shot at it.  Ordered a couple of #30 idle jets.  Thanks for the tip.  If it works.....  :unsure:JImChuk

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Posted (edited)


Might be a dumb question but how is the compression on this engine? Also your exhaust is not plugged up is it?

Edited by TJay

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Posted

Well I'm at a loss, so I figured why not take a shot at it.  Ordered a couple of #30 idle jets.  Thanks for the tip.  If it works.....  :unsure:JImChuk

hmmm, didn't do better with carbs from the other engine so is it fair to assume that it is not likely the carbs? I take it that you have checked for leaks in the manifold and crossover.  Have you Checked the temperatures of all of the exhaust stacks to see if one is lower than the rest? Man I feel for you been there done that and did not get a shirt to boot

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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

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