Trouble with a Big Mig

Other than replacing sections of line, filter and bladder, this is the same plumbing I've been flying this one with for a year and a half. It's the only system that's ever been on this plane, and identical to that on my other Tee Dee Streak, which runs fine. The Satan also has the same plumbing and it runs fine too, all on the same Sig Champion 25/22.

I hesitated to bother asking before something as rudimentary as replacing thew glow plug... and I might have, just can't remember, but I figured someone may have an idea before I hit the field again. Next Tuesday is when I'll get my next chance. The platinum glow element is still shiny. There's no internal evidence of leaning and seizing. No discoloration or scuffs. It feels normal when the prop is flipped over. When I run over and pinch off the fuel after it lands, the engine feels normal to flip over hot too. The muffler is an empty can, so nowhere for a blockage to lodge.

Rusty

Hmmm, I wonder if when cool no air leaks are present, but as it heats and expands something appears.

Does it run through a tank on the ground?

Cribbs74 wrote:Hmmm, I wonder if when cool no air leaks are present, but as it heats and expands something appears.

Does it run through a tank on the ground?

Or the opposite.  It runs through fuel on the ground but chokes off in the air where there is significantly added cooling.

What was the air temperature?  If it was below 40F I am sure it was over cooling of the glow plug, maybe to 50F.  Some yarn wrapped in a couple head cooling fins will solve that...at least down to the teens F.  I think this will work fine with an insert type head too.  The risk of blowing a glow plug is higher, but still pretty low.  I am confident there is no other risk to the engine.

All the same I would remove the needle and flush the spray bar again, even though it sounds like the glow plug getting cooled out.

Phil

It was 77 degrees today. If a new glow plug doesn't fix it I'll swap the head too. And change fuel just to be sure. Next, I'll tear it apart again and have a closer look. I'll If all else fails, I have another Big Mig with a Cox fine thread NV on it.

Rusty

OK, not your head over cooling, but it doesn't rule out the glow plug either.

I wonder how G-forces affect your fuel system...

Phil

Check that the cylinder head is tight and the gaskets are good. If anything is too loose the problem may become much greater when the engine gets hot. I had a problem much like this on the Lycoming TIO-540 on a Piper Turbo Lance. There was a very fine crack between the upper spark plug and one of the valves. We could not detect it when the engine was cool but when brought up to operating temp the engine coughed and sputtered terribly. I missed the crack but a friend of mine found it. It was a great find. Good luck and keep us posted.

pkrankow wrote:OK, not your head over cooling, but it doesn't rule out the glow plug either.  

I wonder how G-forces affect your fuel system...

Phil

One of my early suspects was G-forces causing the fuel line to collapse and pinch off at the nipple or the bladder fitting. I secured all the possible flex points and am reasonably confident I ruled that out. Besides, once it slows and I fly flat hoping it'll wake up, but it never does, just shuts off.

Bob, this is the Big Mig you gave me a couple of years ago, as is the NV assembly too. It's been a sweet combo up until lately. One thing that occurred to me as I sit here and type, is that this problem started after my neighbor crashed it and cracked the nose. I didn't fly it again until I fixed it. I couldn't tell anything was cracked except the inboard doubler had separated. I forced CA behind it and clamped it and it seems stiff, but maybe that's another clue. The next flight after fixing it is when this problem started.

Rusty

Like I mentioned... Can it run a tank out on the ground?

Cribbs74 wrote:Like I mentioned... Can it run a tank out on the ground?

Yes, paragraph #3 in the first post. I did it in the stooge today. Ran the remainder of the fuel out at the same needle setting as when it shut off in the air.

Shoot, 0-2 tonight Rusty!  sorry I should have read closer.

Well then it's gotta either be fuel or load related or both then. If the plug was bad it would show itself on the ground.

Heck if I know. I really don't need another project. I might just swap engines and figure it out later.

Maybe it is due for a tear down and clean. Put the other engine on and go fly!

Phil

Rusty, it certainly sounds like the plug is cooling off to me. More vitamin N. Ken

Ken Cook wrote:        Rusty, it certainly sounds like the plug is cooling off to me. More vitamin N. Ken

Maybe I let it get out. I've been storing it in an old pint acetone bottle. I figured if it could contain acetone, it would hold nitromethane and methanol. One or both could be low. Or maybe it has too much water in it. I have an unopened bottle of Sig 15 I'll try while I wait for some new 25. I bet the Tee Dees run hotter so aren't affected as much.
Thanks for the food for thought.

Rusty

Any time I make a repair to an aluminum case I use J B Weld. First I clean the daylights out of the engine with acetone. If a front part of the case cracked J B Weld should be a good adhesive to repair and/or fill in any crack. Try to get the J B Weld in the front and back of the crack. It can be thinned with denatured alcohol if you want to try and get it deep into a crack. You may want to also look at the crank pin/con rod connection and make sure the crankshaft is not hitting the inside of the back plate. If any of the parts were even slightly bent in the crash they may not show signs of damage until they are heated and/or put under a load. Sounds like you have a tough one with this. Keep us posted and good luck.

fit90 wrote:Any time I make a repair to an aluminum case I use J B Weld.  First I clean the daylights out of the engine with acetone.  If a front part of the case cracked J B Weld should be a good adhesive to repair and/or fill in any crack.  Try to get the J B Weld in the front and back of the crack.  It can be thinned with denatured alcohol if you want to try and get it deep into a crack.  You may want to also look at the crank  pin/con rod connection and make sure the crankshaft is not hitting the inside of the back plate.  If any of the parts were even slightly bent in the crash they may not show signs of damage until they are heated and/or put under a load.  Sounds like you have a tough one with this.  Keep us posted and good luck.

Thanks, I'll go over it with a fine toothed comb. I wish I hadn't given my black light away last week, it might have helped me spot a crack. It nosed in, but at least it was soft grass. I'm hoping it's impotent fuel.

Rusty

Hey Buddy, where you don't mention the .061 going "lean" before quitting... it could be a stale fuel/cold plug thing, which would probably take a "few laps" as you said... before the engine cools and quits.

I realize your flight weather was warm... but Phil's suggestion with a few turns of yarn in the head's cooling-fins might be something to try on Tuesday (before a fresh-fuel flight) It would be a simple/quick test you can perform at the field, and may be revealing.

Roddie

Rusty it is time for you to get Torco on e-bay to send you a 1 gal jug of 100% nitro so you and adjust your winter and summer blends and accommodate the TDs and NVs needs.... You got me into the NVs and Ken got me sorted on the fine thread NVA for the little screaming buggers.... you will be astounded at upping your current 25% to 30 or 40% will do for that Big Mig

The little bit of nitro added will not dilute your current oil content enough to fuss with full blending...

In other words upping your Sig 25 to 30 or 40% in a half pint or smaller to test with does not need any castor adjustment ( IIRC you always are over 20% oil with the blends you are using)

Rainy Saturday, wife out of town for the day, what's a boy to do?????????? SHOP TIME!  

I started out with ambitious plans. Prep for tomorrows flying excursion, take the offset out of the Skyray rudder, balance a spare prop, work on the Yak, fix my mildly broken Clownlikething, replace the frayed leadouts and bladder on the Li'l Satan, fix the Big Mig on the Baby Streak, clean up the shop.......

So I started with the Big Mig. Set up the wing stooge and got 3 different bottles of fuel. For the baseline I used Sig 25 and lit it up. Previously I had stated that it would run fine in the stooge, but I used more fuel this time and that proved to not be the case. After a half ounce burned off at full speed, she started slowing.

I knew the outcome so I played with the needle. The longer it ran, the more I had to richen it. Bit by bit, up to a half turn by the time it used all the fuel. So next was some Sig 35. Same results but had to set the needle back to the original starting point to get it cranked. Then bit by bit, richen it up to the same half turn to keep it going. The thing is, the 35% fuel is older than the 25%, and stored in the same type empty acetone bottle, so this isn't really much of a useful test. I filled it up with a bladder full of Powermaster 10/22, and it seemed to do a little better, though it sounded like lower RPM(my tach batts crapped out), and it still dirfted off peak the same way. Soooo, I put a new glow plug in it, ran all three fuels again and the results were the same. It did seem faster with the new plug, but I can't swear to it.

I want to take this plane to fly tomorrow, so I get to finally field rest the Big Mig I put the old Cox fine thread NV in. I bolted it up and ran some of each fuel through it and it ran great. Now I'm about out of the 25% and 35%, so I have a brand new bottle of Sig 15% that I've had since switching to 10% for the stunters. I'll burn it up in the Norvel so it doesn't go to waste. I'm going to order some stuff from Sig tonight so I'll go ahead and get some fresh 35%, since I'm going to be needing it for the Mustang racer.

And I didn't get a damn 'nother thing done in the shop today. I'll organize my flying equipment when I go to turn off the heater and lights later tonight.

At least you figured it out!

Everything always takes 3 times the amount of time you think it will.

Yeah well, I just re-read my post and I was a little unclear. I actually didn't figure it out, I just bolted another Big Mig on it and threw the bratty one in the drawer. The one I bolted on is the one I put the Cox FT spraybar in and only bench ran it once.

The fleet I have lined up for tomorrow is the Skyray, the BFS II with the swapped engine, the Li'l Satan and the ol' Shoestring. It hasn't flown since H'ville and I was afraid it was getting lonely. And I want to see if I can fly it any better now than I could then. I've flown more patterns now than I ever had before the contest. Wayne, Will and Watt from the H'ville gang are coming. Will is practicing for the King Orange contest in Jacksonville and wants some practice judging from Wayne. I'll be on hand to supply the comic relief.

Rusty

I got to the field before my companions showed up this morning. I took the opportunity to fly the Baby Streak before the wind picked up, and it was the first field test of the converted Cox fine thread NVA I featured in a thread last Spring. It ran beautifully. This was already a faster Big Mig on the test bench than my other one. This one will tach 28,800 rpm with an APC prop, and it was considerably faster on the lines too. I flew it with a Master Airscrew 5-3/4" prop and Sig 15% fuel. That's the lowest nitro I've ever used in a 1/2A engine, but it sang just fine. It was one badass little plane. I'm glad I didn't have a 5-1/4" prop on it or it might have wound me up in the lines. I put 3/4oz in the bladder, and it's amazing how efficient these Norvel engines are. I didn't think it was ever going to run out. My buddies were there when I landed, so I flew it once more and wound the lines in and set up the Stunter. Five hours later we had flown a lot of flights and had a generally great time. I flew about five patterns without aborting a single maneuver. Some were ugly, but they were complete. No unplanned landings. A good day for all.

Rusty

Glad it was a good one, it helps offset the numerous bad ones   

28.8K! Wow