I have a Hitec 475 servo on my throttle for my little gas engine. The servo works fine when the engine is not running but once I start the engine the servo surges in the idle position. It works OK from idle to midrange and top end but when I bring it down to the idle position to line up for a landing once it is below half throttle it starts to surge on me and sounds like a WWI fighter landing. This is a analog servo and I was wondering if I should use a digital servo on the throttle?

The ignition is located next to the fuel tank and the receiver is 4" away from the ignition. The servo is 4" away from the receiver and 8" away from the ignition but next to the battery for the ignition and receiver. Any and all help is great full!

My first guess would be the servo has a bad pot although having the RX that close to the ignition would keep me up at night.

Can you swap it out for another servo to see if the problem reoccurs?

Check and make sure that the spark plug connector is fully seated. Leaking RF from the ignition could be the culprit. Servos can be directly affected by RF interference, so it can occur even with 2.4Ghz radios.
Pete
R

Thanks for the input guy's. I will try another servo and maybe move my ignition battery. I do not have a lot of room to move the battery any further back go it will have to me moved forward some. The ignition cap is seated all the way down but it has never lock down tight enough so it won't move some. I will try another igniton on it and see it if will lock down better.

Does your ignition use the small 1/4 - 32 plugs?

Yes the engine does using the 1/4 -32 plug. According to Agape Racing & Hobby Jim says the cap should seat all the way down on the plug and then twist 1/4 turn to lock in place. I have tried this and I cannot get the cap to twist 1/4 turn to lock in. I think this is my trouble. Can the cap be replaced I have never tried to replace a spark plug cap.

I think I found the problem. There was a piece of the spring that was broken off and preventing my from getting the cap all the way down over the spark plug. This was preventing me from getting the cap locked down. I removed the small piece of spring and now I can get the cap to lock in place. Thank for all your help.

Some possible problems .... if that tiny piece of spring was way up inside the cap, you just removed the center electrode contact. People seem to have problems pushing these caps on the plugs far enough before they try to make the 1/8 turn rotation. They will go on the plug but it is a hard push the first time or two. A very tiny bit of dielectric grease on the plug top electrode can aid pushing it on but it should go on w/o the grease. If it is not locked on the plug, some interference is almost guaranteed. At this point, you may have to replace the cap if that spring has been removed. Note is is an 1/8 turn and not 1/4. When locked in place you will not be able to pull the cap off unless your reverse the order and turn it the 1/8 turn to remove it.

Thanks for this info. The spring was just on the bottom of the cap preventing me from pushing the cap all the way down over the plug. I will try and start the engine now that I have the cap locked down on the plug. If it does not start or gives me trouble I will replace the cap or even the ignition.

Jim, at this point I would look into the inside of that cap and see if there's something metal visible to contact the plug. They will run for a while without one, with the spark jumping all the way right from the end of the wire. Problem being the pickup, under conditions like this, will often act as a path of least resistance, offering an option for where the spark can find a ground/return path. As they're not designed to handle anywhere near that kind of voltage, they'll cook very quickly. Same thing can happen turning the engine over without a plug in the spark plug boot, with ign. power on.

Note also that when removing, the cap MUST be pushed down again prior to the 1/8 tun unlocking. Failure to do that will lead to an early demise of that cap. It will wear quickly and eventually be difficult to keep in place.

Thankf again for this info. I'll check out the cap today. It looks like I my have to replace it

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Lifer,

Thanks for the PDF file. This will help me a lot if I have to repace the 1/4-32 plug cap.

That 1/4" cap / plugs are not my favorite and I don't currently own any engines that use it. A year or so ago, a friend was at my house and I helped him install a new Evolution 10 in his airplane. I had a heck of a time teaching him how to install and remove that cap! He was afraid to push hard enough to allow it to rotate so it would lock. I thought he was good to go until some time later we met at the field. On the first start, the cap fell off the plug and you guessed it ..... it wasn't on far enough and locked.

According to Jim at Agape Racing & Hobby the little spring I removed was just to give some extra tension to the cap so it would not come lose once loecked down. The only problem is that with that little spring at the end of the cap I could never get the cap to lock down. Now that I have removed it I can lock it down without any trouble. I do still see a small spring way up at the top of the cap and from others that have posted on this that should be there. I hope to try and start the engine this afternoon and see if it runs any better. Thanks to all

What engine are we talking about here? From your descriptions of the cap, it sounds more like you are talking about the 10 mm, CM6 plug cap. The CM6 cap does have a spring towards the bottom where I have never seen such a spring on the 1/4 - 32 cap. Also the CM6 cap does not lock on with the 1/8 turn, it just pushes on but quite hard like the smaller cap. Just wanted to be sure which cap you have. The cap illustrated in the PDF a few posts back was the CM6 cap. Either cap being a bit loose can cause problems.

On the CM6 cap some people remove the "C" spring from around the base of the cap and substitute a screw type hose clamp to retain the cap. This actually works quite well and is a very positive means of cap retention though this method would not work on the 1/4" cap.

I spray a little shot of silicone spray in cap end....it goes on more easy then. Good to keep out moisture also. Capt,n

My engine is the NGH GT17 and it has the 1/4-32 spark -plug cap. NGH provides and small spring that goes on the very end of the cap to provide some extra tension when you lock it down. The problem was with this little spring in place I could not rotate the cap 1/8 of a turn to lock it down. I contacted Jim at Agape Racing and Hobby and he is sending me a couple more of these springs. He says they do help keeping the cap locked down when while the engine is running. This is the only engine I have that uses this type of cap so I don't know it the EVO 10 has this spring in place.

Well I just started my NGH GT17 engine with the cap locked down this time and that fixed the issue. I have a good 2000rpm idle that is very steady and a 9400RPM top end. I flew my GP Escapade for a 10 minute flight and not one issue. On approch the idle stayed down with a good RPM and a very nice landing indeed. I want to say a big thank you to all od you who help with this.

Good deal the problem is resolved. Thanks for the update on the NGH spring in the cap. I'll file that away in my "not as good as it use to was" memory.

Never had a problem with either of my VVRC's with the 1/4-32 plugs

I have a question about your VVRC's gas engines. Do they have a small spring at the lower end of the spark plug cap? My NGH GT17 came with one and I just ran the engine without it and it runs a lot better with out it.

Nothing like that on the Evo 10's.

I think I will leave that spring off then!