After a couple of failed attempts last week, repairs and rebalancing this week I finally got my Lahaie 63" Mirage in the air today. It's appr. 13.5 pounds wet, Dynamax and OS .91 with a JMP system 2 pipe. My field is not the best being somewhat long grass - 1 - 1.5" and pretty thick. To get the plane in the air I fitted 3.5" wheels and it took about a 500 foot run before it finally rotated. The first circuit was pretty wide and slugish then it got up to speed, not wildly fast since those huge wheels were still hanging out. Now that I know it flies I'll see about enlarging the wells some.

Anyway, the motor was running sweetly at full song. I had it leaned out as much as I dared for the takeoff then richened it up with the in-flight needle after the first circuit. Last week I played with the idle mixture, I leaned it a 1/4 turn to get a halfway decent transition from idle. Before the leaning it would seriously cough and stutter on accelleration if allowed to idle for more than a few seconds. When it came time to land I was leary of partial throttle running so I only backed off a little until I got it 1/2 way through the turn onto final then I dropped to 1/4 throttle. It died quite soon after pulling the stick back. To me it sounded like it died lean rather than rich. I have zero experience with DF motors, I mostly fly EDF. I did fly glow (prop) for years so I know my way around a motor. Is this normal? How to fix? Does the idle mix need to be quite rich on the ground to get it to keep running in the air or is this just a tiny adjustment needed to get it to keep running.

Thanks!

I have a feeler gauge set that I have modified 3 of the blades (19,20,21) to be able to fit into the throat of the OS.91 carb. The three blades add up to .060 or 60 thousanths. Put the gauge into the throat with the throttle opened to full and adjust the low speed needle to this clearance. Once set, I have never had to go back and change the setting.

Chris:

I may not be an authority in thie matter, but how low of an idle are you trying to achieve?

It is really not necessary to go down on RPM's too much at idle, since there will be pratically no thrust at less then 1/4 throttle. It soudns to me like you leaned the idle too much. Remeber, just lke any other 2 stroke engine, if you lean the idle mixture you may have to richen the high speed needle a little to maintain constant fuel flow.

David

Are you using pipe pressure? It sounds like it is lean on the low end and the cough is do to lack of fuel, are you using a hopper tank? I'm running an OS 77 right now with a hopper tank above the engine and there is enough fuel flowing down to the engine, How about your fuel? The adjustment of the low end needle should be done like any other engine, you did it right by moving it 1/4 but it may be to lean now, which glow plug are you using.

Make sure you are checking the head screws every 3 flights or 4 they do get loose do this when the engine has cooled down, with the screws loose the engine will run lean as you flight it, to prevent it from burning the engine turn the inf. mix. to full rich and land ASAP this has happened to me 2 times in the last month, I have been flying fans for about a month.
Any way let me know if you have more questions.

CARS II

Thanks (both of you!)

My idle speed is reasonably high I think - and besides I wasn't trying to drop to idle but around a 1/4 - 1/3 throttle when it cut. I will try richening it up some since it sounded like a lean cut. If that doesn't work I'll go find some feeler gauges - first time I've heard that idea but it sounds like a consistant way to set things up...

I'm using fan pressure.

Fuel is Wildcat Jet-A 5% DF fuel with 23% oil.

Yes there is a 2 ounce hopper tank although it may be too far forward - I have it under the spine just infront of the engine hatch front, say 6 or 8 inches from the engine. It runs perfectly at WOT, no complaints - like a little sewing machine. The space directly over the fan area is pretty limited, I'd have to glue the hopper in place. Now that the fan and thrust tube doesn't need to come out I suppose I could do that...

Plug is a McCoy #9. I haven't checked it today but it only had maybe 3 runs on it.

I will check the head bolts, I swapped the stock parts for the JMP high-strength bolts which I think is supposed to alleviate that problem?

Another way to set the low end is to close the idle screw all the way in ( with the throttle closed all the way and the speed needle a few turns out ), now put a piece of fuel tubing in in fuel port and blow air through it then turn the screw out 'till you can feel and hear air coming through, this should put you very closed to the low end setting, now go back run and adjust the speed needle.


CARS II

looks like you are doing everything right, I had a JHH Mirage, yes not enough room for the hopper tank, mine was of oval shape and it went just behind the fan between the trust tub and the fuse, about 4" away from the carb.

CARS II
Former E-120 Driver

I heard the .060" trick came from Tom Cook. At the time we heard about it, there were about 6 of us running OS.91s and Dynamaxes around here. We all started using this method and we never had to readjust the low side. We all used JMP pitot pressure.

CARS II
Put some red loctite on your head bolts and they will stay tight a lot longer. In fact they won't get loose unless you have a real lean run.
Your OS .77 has a different carb on it so this trick won't work for you. Did you know that you can run the backplate, rotor and carb from an OS.91 on your .77? I did it to one of mine and the performance was outstanding. Anyone that has run OS.91s for several years will probably have some parts laying around. If you want to give the mod a try and can't find the parts locally, let me know. I might can scrounge up the parts you need to do the switch. I had to do it because there is a very small screw inside the barrel of the OS.77 carb that has a tendancy to break. When I ran several .77s, I kept a few in my flight box. Now I'm sure they're impossible to find. Once it breaks you will have a heck of a time keeping it running, at least I did.

Really? I happened to have a burned OS 91 that was given to me. What kind of performance are we talking about?

CARS II

I also read here not to use red loctite on the head screws because it may give you a false sense on security.

CARS II

I didn't have another .77 at the time to compare it to, but it appeared to me to run almost as good as the .91 that I had run in the same airframe.
At the very least, it's an option for you if you do have problems with the .77 carb, which you may not have. I had a .77 on a Byron fan in a Regal Eagle that I never had a problem with and I sold it after about 300 flights.

I have also read what you're saying about the loctite. I used it on every DF I ever owned, about 15 of them and some I flew for several years. I never had a problem related to head bolts. I used blue loctite on all the rest of the bolts, including screws in the retracts. I have seen DFs vibrate the screws out of a retract unit.

300 flights, It must be some kind of record. What I have I got it used I'll say about 1/2 way used, I had some problems with the carburetor getting stock in the body but solve that little problem with some sanding paper after that and now the engine is running just fine haven't had any other problems with the carb. so far, I'm burning Power Master's 10% with 1 oz. of turbine oil a gallon.

CARS II

Red loctite, something new to try. Why not I'm here to learn and experiment.

You can see my OS 77 set up mounted on my D/F trainer if go to the Gold Coast Jet Fan thread it is next to Dan's F-100.

CARS II

That Regal Eagle was my first DF. Unfortunately, I was never able to get any where near those kinds of numbers on later airframes. I had several that lived only long enough for 1/2 a flight. Actually one of them was about 9/10s of a flight. Crashed on a go around after a botched approach. To give it some perspective, I keep a spreadsheet with all of the planes I have built and owned. I also keep an approximate count of flights made on each airframe. I just looked at it and it says I have about 550 DF flights over the last 15 years. That means that after the first 300 flights on one airframe there are 250 flights on the other 16 airframes combined.

Just be careful not to overtighten the head bolts. The loctite acts like a lubricant during asy.

I hope when I grow up I get to do that many flights, so far I think I got about 30 with 3 different airframe.

Thank you for the advice on the head screws.

CARS II

How many flights were you able to get out of the OS pipe "O" ring gaskets.
So far I been getting no more than 4 flights per gasket and about 10 flights out the glow plug.

CARS II

I used 5% WildCat or PowerMaster DF fuel. I've had O rings last 10 - 15 flights. And then put a new one in and only get 4 - 5 flights. I had the best luck with the white ones that JMP used to sell. They are getting hard to find. Also, all of my .77s and .91s had the JMP machined exhaust header on them. I think that helped O ring life. We used to replace glow plugs every 4 flights, whether it needed it or not. The last several years I have just been running them till they die and some lasted 20 or more flights.

Thanks again, will chat later ( wife is here now ).


CARS II

i know its a old thread.. u say u add 1oz oil to per gallon.. im using wildcat premium 10%nitro 18% syn/castor oil.. i add 5oz of racing castrol to per gallon.. is this tomuch oil? the engine does smoke alot! or does turbine oil not require as much added?

i sent you a PM, but just in case.. .060 would be about the thickeness of a piece of notebook paper? i just looked at my os91 and it had like 1/8-1/16th of a inch gap at full throttle. and its been cutting out after idling for 30 secs+/- when i full throttle it for take off. it runs fine at full speed.. does hesiate and blows little xtra smoke before cutting out.. and after reading this thread i think i know why...[8D]

right now i just turned the mixture screw in like 1 to 2 turns from its location.. i can still see a gap in the carb but its much much smaller than before.... but i'd say it probly the thickness of 3 pieces of paper.. sound about right? or should i keep tightin it?

thanks for your input?

HOLD ON THERE. 1/16 of an inch is .062". That is, .060 is almost exactly 1/16 of an inch. A lot thicker than a piece of paper. So, if you have a piece of 1/16 ply or a length of 1/16 music wire, they'll probably do as a feeler to set the low speed needle.

ROGER DAT! i meant to say it was like 1/8th-1/4 inch open before i changed it...i just turned the screw in like 2 full turns to get the 1/16th inch... will run it tomorrow.. [sm=punching.gif]