Just bought a second hand Nexstar plane kit and am having trouble getting the engine to run right. I have a hunch the engine is toast, but here goes...

The plane and engine look new and reported to have been ran a couple of times but never flown. It does look this way. Got it running and got the mixtures to where idle and throttle response is good. Anytime I run it wide open for more than 20 or so seconds, it comes to a stop. I watched the fuel line and it was full while the engine died. Filled tank, changed plug. Ricthened mixture. Will not hot restart by hand. Compression seems weak to me, especially when hot. When I hold my finger over venturi, open throttle and turn prop over by hand, cannot get fuel to pull into engine. Can barely get it to happen when cold. That seems very strange to me.

I'm going to pull the head tonight to see what the cylinder looks like.

Any ideas?

NO!!!!!!

That you have it started at all means that the engine should be ok...

It is likely that the engine is not set up/tuned properly.

Find someone who is experienced at setting up engines and take it to them to get working.

You'll learn a lot in the process.

Your engine is a version of this model
http://www.osengines.com/manuals/50s...1fx-manual.pdf
the 46 size obviously.
Your engine may have been damaged by the previous owners but I agree that you should have an experience person look at the engine before tearing it down. It actually sounds like it has some extremely lean runs on it and may have burned a hole in the piston top. Try looking into the exhaust port or glow plug hole to see if there is visable damage. whichever hole you look into, shine a light into the other for illumination.

I'm not a complete beginner. I used to fly quite a bit of control line years ago and had at least two stints in RC trainers. I'm reasonably familiar with tuning a glow motor. As I mentioned in the first post, the things that stand out to me are the relative lack of compression when manually turning over the prop, and the fact that this motor will barely pull fuel into the carb with a finger over it. Don't those two things point to something?

Anyway, emailed back and forth several times with the original owner today and combined with the fact that all of this looks new, believe that indeed he did only run it a couple of times (not in the air). He suggested that I call Hobbico and tell them how it was operating. I did and also told of the history and the lack of a receipt and they said to compose a letter describing the symptoms and they would repair it under warranty. So don't worry, I won't tear the engine apart!

Thanks for the comments!

Nope this is perfectly normal for this engine.

While you may not be a complete beginner, it sounds like you have no experience with this particular engine.

The engine does not appear to be properly set to run, and your questions hint that this is the first time dealing with this type of engine.

Given the assumptions you've made, a little help from someone with a bit more experience will save you a boatload of time trying to figure it out yourself.

The engine sounds like any untuned and unbroken in engine I've run accross, from your description.

The instructions point you in the right direction, but I'll bet the engine is far too rich as is since it starts up for a few seconds and dies out over time.



It's doubtful that there is anything wrong with it. The original owner probably didn't know how to tune it up.

Well I spent some more time with it this evening. I was running some K&B idle bar plug in it. I went back and found an OS 3A plug that the prior owner had in some stuff. Put it in and it ran much better. Started to lean out the high end mixture and things got better. Have leaned out the low end just a tad and now it runs continuously at WOT and idles. Seems as though the plug could not stay lit through my rich settings (and wrong plug?). It still does not have much compression and is hard to get started when hot. Does not pull fuel in at all just hand flipping the prop.

So what now? Send it in anyway since the offer is on the table, or run it like it is?

-thanks

a good place to start is the default settings listed in the OS manual, secondly there are the other needles on the carb to make sure they are set properly the manual will have the proper settings

Thanks for the reply. You are right, I do not have any experience with this engine. The last trainer I fooled with had a new tower 60 in it. It was also a tappered cylinder motor and had MUCH more compression. It seemed much more tolerate of wrong settings. Maybe my memory serves me wrong!

From reading the literature, it seems I still do not have the right plug.

I read throught the manual thoroughly and set the needle at the starting point. The manual didn't say a thing about the low speed setting. Maybe it is because this is an FXi motor and comes preset? Serching the web I found FX46 instructions that told how to initially set the low speed mixture. Needed about a half turn leaner on high speed to get to run continuously at WOT without battery. I don't ever recall a motor having such an issue with kepping the plug lit under rich conditions.

-thanks again

The feeling of lack of compression is normal, especially if your prior experience was with a TH engine.

By comparison the TH engines are difficult to turn over when new and only loosen up when they have been fully broken in.

The O.S. on the other hand feel "light" initially and easy to turn over, almost as if something is wrong. Don't worry about this.

The low speed come somewhat set usually (most of my new O.S. engines turn right over on the first few tries after they get fuel.).

It is likely that the previous owner couldn't get the engine started, so he started playing with the carb settings and promptly screwed things up for you.

The O.S. is actually MORE tolerant than the Tower engines, but the engine you have is probably WAAAY out of tune given it's history.

You have to get the high end within reason, then adjust the low end (pinch test time) then re-adjust the high end again and double check.

Also remember to turn the cylinder head down once you have it tune to check for changes in attitude causing the engine to stall or lean out.

When it is properly set up the O.S. engines seem to BARELY change when they go nose up. Far less than the Tower engines, maybe due to more muffler pressure(?)...

It sounds like you are getting there, but the engine is probably still not in tune. Run a few tanks of fuel through it too to get it broken in.

As you do this change and check the settings at different attitudes.

Once you have the low end dialed in right, everything else becomes easy IMHO.

Hey, boat,
Fellow NexSTAR flyer, here. A couple hints...


Sounds to me like the engine has not been broken in, yet.

1. Take the limiter off of the high end needle. Start at the recommended settings and take your time. Small adjustments after a short pause. Let the engine catch up to the adjustment.
2. Use a tach. It's better than trying to listen to the changes.

As mentioned before, you can turn this motor over the compression. But, it will feel a little tighter after break-in.

Good luck,

Have a safe flight day,
E J

Limiter?

I don't think his O.S. Engine has a limiter, does it?

I think he is talking about the little plastic needle limiter that comes with the nexstar. It was off when I got the model.

Well, I have to say thanks a lot guys.

The limiters are on the Evolution motors.

There are limiters on the OS 46 FXi also,if it has not already been removed.The i in the FXi signafies that it is a remote needle for the high end adjustment.Be sure and remove the piece of fuel tubing running from the remote needle to carb and inspect for pin holes by stretching hose in your fingers.They are bad about getting a pin hole in that area even if it hadn't ran much.

take the limiter off. this engine is broken in at the factory. all the other comments are correct as well. this is a great engine. good luck russm

sounds like your low end mixture screw is improperly adjusted. If you don't get this close to the proper setting the high speed will never work properly over the operational range and you will just be chasing the settings all around 1 click to rich one click too lean. how many turns is the low end out from all the way in?

Dunno, I started wayy to far out and then visually adjusted it to where I could see the outlet just going over the tip when the intake was just closing. Kept leaning it out until throttle response just got a hair sluggish, then went to leaning out the high end mixture. It was at that point that the engine stopped dying after 30 seconds at full throttle with glow starter removed. That was the original issue. The engine seems much more sensitive to rich running that what I was used to.

When trying to prime the engine, do you have the throttle fully open? If not, it won't pull fuel when you block the carb or exhaust outlet & hand flip the prop.

Other than that it sounds ok to me -- you just need to get used to tuning it.

Yes, I do. It still does not prime when hot, but will now start. Thanks for all the help.

how many turns out is your low end screw that is in the throttle barrel?

My OS 46AX doesn't prime when hot. It wasn't like this when I first bought it, it's been two months now and I've tried my best to preserve it from any damage or lean run.
Probably this is how they are, no?

Also when hot, I doesn't show any sign of compression, and I can't start it with a chicken stick. I need the electric starter to spin it.
Usually, when I land, I just wait 5min before refueling and taking off again. That will cool the engine, and I will be able to start with a chicken stick from the first flick!!

Works all the time...

Don't know how many turns it is, just adjusted it by behavior. It's a pain to get to in this plane. Have to take the throttle crank off to get to it. Do you have a suggested setting? I'll try it.

Fadi, it's good to hear that I'm not the only one that notices this. Sure seems strang to me.

Also noticed that the engine is not mounted straight in the fuselodge. It is definitely cranked over to one side. In fact the firewall is not perpendicular to the fuselodge. Normal? Not?

From looking over all the posts, I suspect the glow plug "that the previous owner had in some stuff" is possibly a marginal plug at best. I would purchase a new OS A3 plug and put that in the engine.

Second, you never mention which prop you have on the engine. It could be either under or over propped, which will also have a dramatic effect on the ability to get the engine properly tuned. Too large a prop will tend to make you want to run the engine too lean, leading to overheating, and exactly the problem you described with short runs at WOT. Further, the lean runs will greatly shorten the life of the glow plug. Too small a prop and you'll peak the rpm too high, again possibly leading to overheating and shortening of the plug life. For the OS-46 AX, try an 11X5 or 11X6 APC prop on the Nexstar.

As for the engine not being mounted straight, this is normal. The Nexstar has a fair amount of right thrust built into the firewall.

There have been many posts on engine tuning. Getting the low end properly set is an art that may require a bit of trial and error. Make small adjustments, and reset the high speed needle after every low end adjustment. Then use the throttle response to tell you if its correct. Its often very difficult to tell the difference between a rich or lean low end setting. Slightly rich hesitates and then advances to WOT. Too rich and the engine simply sputters and dies. Too lean, and the engine will sputter and die when the throttle is quickly advanced. Use both the throttle reponse and the pinch test to help you figure it out.

Good Luck!

Brad

This plane has the stock nexstar prop on it (11x5 or 11x6, can't remember). The plug I dumbly put in was a new K&B RC plug. I TOOK OUT an OS3A plug. [&:] When I put back the old plug, things got better. I'll pick up a couple of news ones here soon. BTW this is an FXI engine, if it makes any difference.

Thanks for the help.