Large Model Association Forum

please can anybody throw some light on problem.
the throttle and choke servos on my decathlon 'glitch' a lot when the engine is running, the choke sometimes to the point of making the engine cough and the throttle making it rev and back off momenterally.
The engine is a ZDZ90rvj
The servos are HS625mg on the throttle and HS635hb on the choke, both only done an hour at most.
The servos are fairly close to the ignition unit, mounted with the rubber grommets that come with them, reciver is a JR rd1222 powersafe with two 6volt batts.
After a run of possably fifteen minutes the servo cases are not hot.
The question is where to look first, is it radio, servos, vibration, heat or ignition ?

Don't know if it's applicable to your model but we had the same problem with the throttle servos on the Dak. The engines were "hunting", didn't know why until we saw the throttle links moving, then saw the landing lights were flashing on and off. Traced the fault to a brand new SM Services kill switch which operated the landing lights. Binned that and replaced it with a servo and micro switch....problem solved. Jonesey.

I'd start with the ignition. Is the plug cap pushed really firmly on & making good contact with the hex of the plug, earthing the ignition?

Dave & Rob, thanks for your idears so far, unfortunatly theres no kill switch on this one, just useing the choke for that, plug cap is on tight, so thats eliminated, any more suggestions would be very welcome. Just a pity that we dont have a smilie banging head on brick wall.

Would ferite rings on the servo wires do any good ?

I had the same problem with a throttle servo, which was traced to a spark plug fitted without a resistor. Changed the plug to the correct type and all was fine.

Thanks Barrie, will do some research in the morning.

Like Barry I had glitching all over the place, if it has the cm6 plug then you may have a faulty resistor in the cap or it has broken off its wire which I have had happen.

good luck, these problems are not always easy to find.
Dave

Thanks David, will check it out.

Just a thought, had this problem with a receiver a while ago. The receiver was fine when engine not running but servo jitter occurred when running, tried opto isolators, ferrite rings etc but no joy. Changed the RX and all was fine. That 'dodgy' receiver is in a glider now and, without engine interference, works fine.

Is that a "Rexel" type ignition unit I see in the picture ? If so the problem could be the ignition unit itself. Over the years I have seen similar problems with the "Rexel" type ignitions on our hard working tugs. It appears that something breaks down in the ignition unit itself and causes the problem. Only solution is to change the unit.

John

Hi Paul.

The ignition unit mounting, is it backed by a foam pad then just held on with hard plastic tie wraps? It could be vibration that has damaged the unit as the vibration can transfer along the hard plastic into the unit. I always try to remove the chances of this with another foam pad and a thin ply plate with the wraps round that so the ignition unit is truly isolated from the airframe. Hope you get it sorted mate since the model is worthy of a nice reliable set up!!

Ian.

Thanks everybody, havn't found anything yet, hopefully will get a good sesson on the decathlon at the weekend, will post the results of what I find.

Are the servos digital? I was always told never to use digits on throttles or chokes. Not sure why but always used analogue and never had an issue.

Thanks Mike, the throttle and choke servos are analog type, have got a pair of new 3003s to try, see if that makes any difference.

Depending on what Rx you use, the FrSky type have a setting for high power servos. If you use this setting for ordinary analogue servos then the analogues go very jittery.

Thanks Peter, thats another good point to look at as I'm useing two 6volt packs for the radio, am in the building room at the mo going through all these suggestions, Thanks everyone for your comments so far.

is this the problem ? looks like the plug has failed and is burnt around the base of the ceramic part, although the engine ran ok but not quite as easy to start as it used to be. Can not see any cracks in the white ceramic.

The plug cap may be arching due to a poor connection?

I refer the honourable gentleman to my earlier post on this thread. What does the inside of the plug cap look like, is that a bit brown too?

The brown stains could well be arcing as the plug makes an intermittent earth, giving less spark for good starting.