Problem with FS91SII
 

A I reported in another thread before, I am having an odd issue with a FS91SII. It races at idle with the booster on, but as soon as it is taken off, the RPM drops quite a bit and then it runs for 10-15 seconds, misfiring increasingly on the way and then dies. I tried both richnening and leaning the LSN but doesnt help. The characteristics changed with a different plug but not much. A few months ago, I went through three plugs trying to sort this out. Those are the only plugs I have right now. Putting the plug on the booster, all coils except the topmost light up on all three. I am using Klotz fuel with no nitro. What could be the problem?

Ameyam

RE: Problem with FS91SII
 

Booster? Glow driver perhaps?

RE: Problem with FS91SII
 

Have you tried putting the LSN back to its factory original position? Sometimes they can get far enough out of whack that you can adjust all day long and not find the "sweet" spot. It kind of sounds like you might be super rich. I just looked in the manual for the instructions to reset it: Turn the needle all the way in (clockwise) until it stops. For the 91, turn the needle out 2 1/2 turns if it doesn't have a pump, 2 turns if it does. I hope this helps.

RE: Problem with FS91SII
 

I have been having this problem for some time as I havent run the engine in a month or so. Last time I run it, I replaced the plug twice. Today, I put in another new plug. Problem is the same

I have also reset the LSN to 2.5turns out from closed as per the manual

Another thing, I hear a sucking sound when wiggling the prop... you know... a kisk-kisk. I don recollect this from before. Think its coming from the carb but not sure.

Any ideas what the problem is? I havent run at full throttle, running in the house so I cant check the top end

Ameyam

RE: Problem with FS91SII
 

Ok.. I won't ask anymore.

RE: Problem with FS91SII
 

Hey, I didnt mean any offence. Running the engine at idle is only to check the setup on this new airplane I just finished setting up. If it runs for half a minute, I wont run it any more. I want to resolve the issue before I go to field, and then lose a day off flying because I didnt setup something correctly, thats all

Ameyam

RE: Problem with FS91SII
 

The sucking sound could be a problem.... or it could be perfectly normal. It really depends on where it's coming from. If it is, in fact coming from the carb, I would bet that it's normal. You're hearing air being drawn through the carb during the intake stroke. If it's coming from the backplate, it could be the soure of an air leak, which could be the cause of your poor running condition. Try tightening the screws that hold the backplate on and see if it makes a difference. If this engine once ran well and was only stored in the interim, it should still run.

RE: Problem with FS91SII
 

Just one other note while I'm thinking of it. It is really difficult to set the mixture without first setting the HSN. I'm afraid that you're going to have to take this one to the field to try it. I'd put it on a test stand so it won't take up much space. Mess with it between flights on whatever else you plan on flying that day.

RE: Problem with FS91SII
 

The sucking/sloppy fuel sound is very normal for a 4 stroke.. they want to be sloppy rich for a start-up.

My first knee-jerk reaction to your problem is the LSN setting... once you take the glow heat away, the engine slowly dies... Old and/or condensated fuel is another cause.

Forgive me if you already know this, but the LSN is soooooo sensative to adjustment, just 1/16th to 1/8th will make a very big difference in it's reliability on idling.

When you have the engine running, what are the symptoms when you go from idle to full power quickly???

**Edit** A loose backplate is not a 'problem' on 4 strokes, a 2 stroke engine needs a sealed crankcase... a leak there IS a problem.

RE: Problem with FS91SII
 

When you decided to go to FAI fuel you effectively made your engine a lot harder to tune. When I was being taught engine tuning years ago and I asked about people running 30% nitro in there YS engines I was told high nitro is for those that can't twist a needle and get an engine in tune. Did you ever read the article I sent you so you had some understanding of what the nitro was really doing?? With the FAI fuel you are using, that low end needle is way more touchy then if you were running 10% or 15% nitro. You may never hit the sweet spot. Here is a big question for you. What type of Alcohol are you using??? Where did you buy the Alcohol. What type of container did the Alcohol come in? The answer to these questions may hold the key to your trouble. Are the O-ring for the carb/manafold in good condition?? A bit more info may point us in a different direction, your having way too many engine problems to be normal.

RE: Problem with FS91SII
 

As others have said, you're not going to be able to properly adjust the low speed on this engine until the top end (high speed) has been set. If you're just trying to start it at an idle and then adjust it you'll never get it properly set. Take the engine outside and adjust the high speed setting, and then tackle the problem with your low speed.

Ken

RE: Problem with FS91SII
 

Ok, I dropped the idea of tuning by running at idle for the time being. Am setting to factory and leaving it there.

I had the HSN at 3 turns out. I usually set it there when I am starting after a break for this specific engine (and always have this problem, thats a good point). I adjust before the flight

Ok, I see your point. By the way, I read the email you sent me but there was no attachment. I had replied to you but you may have missed the email or something. Can you resend?

Alcohol is industrial type. All of us are using it. In my country, you need a licence to buy the stuff, so my mentor gave me some. It was bought in bulk and stored in a sealed container. And this is the same fuel that resolved my problems on the 2c 55ax and 75ax. Same can, so no nitro. I have some of the 10% nitro fuel though, will use that at the field.

That sound is from the carb. The backplate is secured. However, there are bubbles in the tube going from the crankcase nipple to the carb if that indicates anything.

Basically, I spent so much time trying to get the setup right in the last few days and trying to get the engine running today, I am out of patience.

Guess you are right regarding both the HSN-LSN issue and the fuel. Now that I think of it, when I had this problem last time and took to the field the next day and tuned the HSN properly, I didnt deadstick. So, will take it to the field Saturday and see

Thanks for the suggestions though, its really appreciated

Ameyam

RE: Problem with FS91SII
 

Normal. It's a crankcase breather vent just dumping into the carb.

Older OS 4 strokes just dumped the crankcase pressures out, without going back into the carb.

RE: Problem with FS91SII
 

This should be the file.

RE: Problem with FS91SII
 

Thanks, maybe I neednt have spent so much time setting the idle today[>:] Anyway, live and learn

Ameyam

RE: Problem with FS91SII
 

I just wanted to know what you mean by a "booster", that's all. If you mean glow driver, then fine. I just have never heard it called that before.

CGr

RE: Problem with FS91SII
 

Booster, glow stick, glow driver, hot shoe, hot shot, ignitor and many more names that leave my head spinning.
Ameyam, there is a saying that is close to 100% true. The high end doesn't effect the low end but the low end does effect the high end. Three turns out on the high end is way too rich. You adjust the high end first then the low end then after you are finished you can tweak the high end a little to get it set, it can be off a little. Nitro is your friend. I have a couple of the .91 FS engines I use all the time, they also like a heavy prop to smooth out the idle. I'm guessing your just out of tune and your engine may want some nitro to help you find that sweet spot. I started using only one blend of fuel about 15 years ago, Power Master 15% nitro and it has an 18% blended oil. I run everything on that one fuel. Other then a very high compressed make of engine that required the FAI fuel the PM has worked in everything I have ever used. I'm not brand loyal but I have always gotten the best prices on the Power Master. Not all engines like FAI fuel, it works best on high compression engines, your .91 isn't one of them. FAI fuel is also Methanol Alcohol so I'm not sure what your bulk alcohol is. It may or may not be something your engine likes. A real good engine tuner can make FAI work in almost any engine but I question if it is going to work out for you. Try changing over to some 10% or 15% fuel, an APCprop, maybe a 15X6 and see if things work out better for you. Make sure you have a good type F plugh in the engine too. It is surprising how the glow plug can effect the way an engine runs.

RE: Problem with FS91SII
 

Ameyam

Are you using a 4 stroke glow plug? such the O.S F plug?
Your engine may not work properly if you use a two stroke plug, specially without nitro. Two strokes and 4 strokes are different beast as you may know...and they need a different glow plug.

Sound coming from the carb is because air is leaking through the valves...and may be nothing wrong with that, when the engine is cold...without much oil, or not properly run in..it will seal properly while running.

RE: Problem with FS91SII
 

I have just adjusted the valves- atleast one of them that had a lot more play than the feeler gauge. Basically, aligned the "t" engraving on the hub with the web on the crankcase, then opened the nut, tightened the screw till I could just remove the feeler gauge and then retightened the nut. Hope I did correct. It was the one on the LHS.

One point that I noticed, the last firing of the engine just as it cuts seems to discharge backeard through the carb. I noticed this because I was holding the airplane there and felt the draft on my fingers. Dont know if that is important / of concern.

Like I said, I have some 10% fuel from earlier, will try with that. May be I will try to run it in my backyard just long enough to set the top end and request the neighbours to bear with me for a few minutes

Ameyam

RE: Problem with FS91SII
 

When you rotate the engine to adjust the valves you mentioned the small T or TDC mark on the thrust washer and only one valve { rocker }was opened or had slop in it. Do you know the difference of top dead center and being 180 degrees off?? I have sent the instructions for valve adjusting out to a lot of people, I think I sent them to you at some point. Do you have anyone you can have show you how to adjust valves? It's a lot easier to have someone you are sitting next to showing you these things instead of people trying to tell you things on the net. Tunining and adjusting an engine is easy after someone sits down with you and shows you how to do it. Something just seems wrong when you mentioned one rocker arm being opened a lot more then the other. I will look and see if I have a better how to do it book on hand then the instructions that come with the adjusting kit. If I do I will post it here in a file for you. When at TDC both rockers are up and off the valve stems, you cna move the crank several degrees both directions left and right and the rockers don't move. I have never set the valves or one of them when I was off TDC so I'm not sure what the reaction of the engine would be but the one valve would be opening way more and probably not be closing all the way? Not sure if the engine would run at all but it's worth looking at. I just haven't ever tried it to see what it would do.

RE: Problem with FS91SII
 

These pages are taken from A guide to OS model 4 strokes by Grahm Rice and the book was sold through the RCM anthology library series. If you look up RC Modeler magazine they may still have this little book listed. It is old but for someone new to 4 strokes it is worth it's weight in gold. I bought this one years ago when I started rebuilding my own engines, it's step by step, some thngs have changed but not a lot so it still works today.;) Print it out so you can read it and tack it to your wall!!

RE: Problem with FS91SII
 

I know what you are talking about. Am an engineer and automobile engg was one of our subjects and in any case, I was aware of this since school days, so I am aware of what you are pointing to.

If I am 180 Deg off TDC, the exhaust valve should be starting to open. Also, I should have no resistance to turning over the crank. That was not the case. To start with both valves were closed when I started to check the adjustment. To align the TDC mark with the webbing took quite a bit of effort with the engine off the airplane but with the glow in so I should be at the correct TDC. The adjustment I made was very minimal. There was some slack, slightly greater than 0.14 mm (both feeler gauges inserted together) between one of the rocker arms and the valve stem. I was able to insert both gauges together easily whereas in the other one I was able to insert only one. So I opened the lock nut by quarter turn, closed the adjusting screw about half a turn till I was just able to pull out the 0.04mm feeler and then tightened the locknut again. Turned it over a full 2 turns to check if it was OK. Had I not done this at the correctly, the engine would have never fired. I still get a very good engine note with the glow driver in place, there are no misfires then. It only starts to misfire some seconds after I take off the driver and just before it cuts-off . I am still to take it to full power since then though

Ameyam

RE: Problem with FS91SII
 

An engineer, this is good to know. Hard to know what someones back ground is on the net. No need to be trying to simplify things for you. I thought you were a youngster just having more problems then normal. Sorry if I was trying too hard to dumb things down for you.
Gene

RE: Problem with FS91SII
 

Ameyam...again (sorry to insist) what glow plug are you using?

RE: Problem with FS91SII
 

F Type plug. Thats what Bax asked me too.:D:D

Ameyam