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Problem with FS91SII

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Old 05-22-2011, 01:10 AM
  #1
ameyam
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Default 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?

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Old 05-23-2011, 06:06 AM
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Default RE: Problem with FS91SII

The symptoms you describe indicate a bad glow plug.  You should replace it with a brand-new one.  The O.S. Type F would be our recommendation.  Almost always, if the battery connector is removed and you lose RPM, then it's the glow plug.


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Old 05-23-2011, 09:49 AM
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Default RE: Problem with FS91SII

Bax, this is a new plug. In fact these are three new plugs. I am running fuel with no nitro though. Dou you want me to try some with 10% nitro?


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Old 05-24-2011, 06:12 AM
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Default RE: Problem with FS91SII

Depending upon the plug, the fact that it's new doesn't mean it's good.  However, contaminated fuel can cause problems, incorrect idle mixture,  and so on.  You may try starting the engine at part-to-mid throttle.  Keep the battery connected and advance the throttle to full.  Get the  high-speed needle adjusted first, then remove the battery connector from the glow plug.  If the engine still quits, it's definitely a bad plug.  If the engine keeps running well, then retard the throttle slowly and adjust the idle mixture to keep it running well.  Eventually, you should be able to retard the throttle to the point where  you have a good idle.


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Old 05-24-2011, 06:51 PM
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Default RE: Problem with FS91SII

Well, I ran the engine yesterday with the 10% nitro fuel. Ran it behind my appartment, so ran if for less than a minute so as to not irk the neighbours or attract a crowd.

I have no idea where the LSN was, I just took it out and ran it. Initially it did the same thing- it cut at idle before I could get around to stand across to give full power. Second time, it started struggled and then actually ran backward sending exhaust out (I think) through the carb which surprised me, it ran only for a couple of seconds. Third time I kept the driver on till I was able to get around to stand across. Then I went to full power and adjusted the HSN fairly and then came back to idle. This time I idled for 20-25 seconds and it didnt cut, which I think is fairly ok as it wouldnt last more than 10 seconds before.

Thats about all I wanted to and can do without going to the field. Remaining adjustments can be done on-field. I will check the LSN again, I think it is at 3 turns out, left there during the Sunday's trial-and-error. I will set it to 2.5 before I go and will also use the plain castor on field (though I will carry the nitro fuel as well)

One surprising thing is the way it sounded when outside- like a motorcycle engine. In fact, it sounded different than even the very same engine when it was on another airplane. I am quite surprised that it requires nitro. I would like to have one can of fuel for both my 2c & 4c engines, so I need to drop my nitro content. Do you think 4-5% is enough?

By the way, this was with the same same "old worn out" plugs. Guess they were not really worn-out

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