Morning Bax!

I've been having some troubles with my 2 year old engine regarding transitioning from idle to midrange. Background follows"

Received NIB 2002
Broken in according to manual
Started using Cool Power 15% fuel, switched to 10% Fuel for 1 season since it was given as a gift. Has had 3 gallons of fuel run through it.
Remote needle backplate has been replaced 3 times due to crashes
Turning 11X6 power point and Master Airscrew propeller since installation in Avistar (in 2002)
OS #8 plugs only

The engine ran great in the beginning, but in the last two summers, when I have started it and tried to go from idle to 3/4 throttle for takeoff, it dies at just under 1/2 throttle. My Instructor is puzzled why this is happening. We have both changed all fuel lines, verified there are no air leaks comming into the carb (no bubbles in the lines) and have changed the glow plug numerous times trying to fix this problem. It will even do this while the ignitor is still attached.

He has also made adjustments to the carb needle, but to no avail. We were flying at an exhibition yesterday and out of 8 flights, 3 were deadstick after getting up to altitude. This was after numerous missed takeoffs due to not being able to get past 1/2 throttle.

I took it out of the A/C last evening and took it apart to verify there weren't any issues concerning the piston/lining and bearings, all looked good with no scoring or pitting. I removed the carb and verified that the "O" ring was still serviceable and put a bead of red RTV sealant around the carb to crankcase joint.

Can you offer any further insight or advice with what may be causing this engine to run this way?

Thanks alot in advance!

Don

1) Richen the idle mixture a LOT! Open high-speed needle about 2 turns out from closed.

2) Start the engine at 1/3-1/2 throttle.

3) Leave the glow plug battery connected and advance throttle to full.

4) Lean the high-speed mixture to just rich of peak RPM.

5) If the engine will sustain running there, then you can retard the throttle and adjust the idle mixture. You will likely have to start leaning it at about 1/2-3/4 throttle as it richens when it enters the midrange.

If your engine won't take a set, then it's likely that something's wrong, such as a bad piston/liner fit that causes compression loss when the engine warms up.

Okay, I'll try all the above. I take it that I should have a new piston/liner available for replacement? Just in case? Thanks!

Don

Only if the engine has a bad fit. The only way to determine this is to remove the piston/cylinder assembly from the engine and see if the piston will come up to the top of the liner. If it can be pushed up to the top or beyond, then the fit is gone and you need a new assembly. It should tend to stop about 3/32" or so below the lip of the liner. You should see a burned ring at about where the piston stops on its regular upward journey. Hand pressure should find it quite tight at that point.

Gotcha.

I went ahead and took the engine apart last night anticipating the arrival of my new assembly that I ordered and after taking out the liner, I discovered an area around the bottom of the left side inlet hole where it looks like the liner had "chipped". There are noticeable spots that are rough to the touch although they are on the outside of the liner, between the walls of the liner and crankcase. Also, when I took the piston out, I was able to get it up to about 1/16th below the top of the liner lip. I'll go ahead and change out the assembly and see what happens.

Don

Hi Bax,

Just an update. I received the new piston/sleeve assembly yesterday and compared it to my original one. There is some peeling on the original sleeve on the outside of the ports. Also, the piston is damaged as such as it goes all the way to the upper lip without alot of resistance. I can't imagine how this happened, since both myself and my Instructor are very meticulous when we set the needles for flight. Anyhow, I hope this takes care of the problem. I'll keep you informed.

Don

Interesting... I have an FX that's about the same age, with approximately the same run time&conditions(without the accidents) using Byron 15/20, and I'm also having a problem keeping it running at mid-range. It's been running great, with no problems until about 6 months ago (cleaned it up, and shot a bit of oil in while I was gone for four months, engine was all plugged up and empty for storage).

I took a good look at the piston/liner, and that looks good (but I'll look again). I've re-set the needles( as suggested above ) a couple times, changed glo plugs(I did have a bad one - still not working after changing the plug). O-ring in the carb neck looks ok, but I haven't replaced it yet, and all the parts seem to be in good shape, and tight. Idle is great, full power is good, but advancing the throttle(even slowly) kills it maybe about half the time. Transitions ok on the ground usually, but several minutes into flight will die when throttling up. I don't think it's the tank, or position of the tank as inverted flight and manuevers don't seem to affect it. I seriously doubt it's overheating, it's mounted in the open, no cowling or obstructions.

I'm kind of lost on this one, any suggestions on what else to check?

If the engine's compression is shot, not only will transition be bad, but idle will be, too. Many times, the idle starts to go and the bottom-end mixture is adjusted to get some of it back. This throws the midrange out of whack, and sometimes affects the full-throttle running.

Best thing to do is richen the idle mixture, richen the top end, and then start the engine at part throttle. Leave the battery on and open to full. Adjust the high-speed mixture. Once the engine's running well at full throttle just lean of peak RPM, you can retard the throttle and work on the idle mixture and RPM. Many times, people try to idle soo slowly. You can likely get a very slow idle, but this will also be a cause of a problematic midrange. Any idle RPM below 2,500-2,700 RPM is considered good. A number of years ago, anything below 3,000 RPM was considered a good idle on these small engines.

Still having problems with this one, reset both needles as described-again. The compression is fine, it's got a great idle (maybe a little on the slow side around 2K plus or minus), still intermittent mid-range, and high speed is good with lots of power. I have both set slightly rich (faint smoke trail at high throttle) but every second or third attempt at throttling up results in the motor dying(on the ground and in the air).

I'm sure any warranty has expired, but I might have to send this one in - it's got me baffled. Probably something really small that I'm overlooking(and everyone else at the field too). Wierd thing is, I have another that is also about the same age(bought within a month or so) has been run under identical conditions, broken in and tuned the same(per directions), and with about the same amount of flight time, without any problems. I know they vary from engine to engine, but one that's perfect, and one that just randomly stops? I don't get it. [&o][sm=confused.gif]

Hello All,

Just an update on my engine woes. I did get a chance to run the engine with the replaced liner/piston and after an initial time of it wanting to run backwards, it finally settled down. I didn't have a tach handy, so I am unable to give RPM information. Suffice it to say that the engine runs just like it did when NIB. Lots of power and a smooth transition from idle to WOT.

Now then, right after I got this straightened out, I started to get the same issue from my second 46FX. This time, I ordered a replacement part from another Company which makes ABC liner/pistons for OS engines. After disassembling the engine, I noticed a spot on the back internal wall of the liner that had brass showing( 5mm x 2mm). I wanted to see if there will be any differances in the way the two will run and see how the new parts hold up. I'll update this thread when I fly the engine in the Spring. I have already been told that it will take a lot of break in since the tolerances are VERY tight on this particular setup.

Anyhow, wish me luck!
Don

Without trying to step on Bax's toes, what company is making pistons and liners for OS engines. I am trying to find one for an old OS 61 FSR (no longer supported by OS or Hobbico)

John