I have been flying for some years & have just bought my first gas engine, a 26cc with cdi electronic ignition. I fitted it to a CMpro large Mustang. My radio is 2.4 Spectrum.
I was getting severe interference when the engine was running, with all servos randomly jumping about. I tried moving the ignition control unit to various locations with no result. I burned out three servos as a result of the violent reactions. I have found that grounding the wire cased spark plug lead to the engine stops the interference. I have temporarily cable tied it to one of the engine stand off legs.
My question is, is it normal to have to do this & if so, should I make a more permanent ground, such as soldering a wire from the plug wire sheath & bolting it to the engine.

Standard procedure

make it permanent

Thanks. I was just about to give up on solving the problem. I presumed the plug cap grounded the wire sheath to the body of the plug, obviously not & the engine instructions made no mention of the requirement.

In most ignitions the shield IS grounded through the plug. Think DA50 and all the clones out there. It seems, in your case, the braid is not making good contact with the metal cap that goes over the plug so you will have to ground it as you have and just make it permanent.

Just for curiosity what make of engine/ignition is it?

Sounds to me like the metal cap isn't pushed all the way on the plug! These push on hard the first time or two and many newbies fail to push it on far enough. When in place, the hex part of the sparkplug will be covered.

The engine is a 26cc J'en from Just Engines in the UK. I made sure the plug cap was pushed on fully. Looking at the cap, the only contact with the plug appears to be at the top of the plug. At no point does the body of the cap touch the hex of the plug.
What is the best way to ground the plug lead? Is it ok to just run a wire from the case of the ignition box to the engine, or fix a wire to the actual plug wire braid & ground that?

The body of the cap should touch the hex of the plug. I think you don't have it pushed down far enough

There may be the wrong plug in the engine and the plug is too tall

From the picture on the JE website. they show a metal Spark plug cap. The lower part of that cap should snap around the hex of the plug. If you don't have a metal cap I would wonder what ignition you have and if it's just a rubber boot I would send it back. If it is a metal cap you can put a hose clamp around the base of the cap right where it covers the hex and tighten until the cap makes contact with the Hex...there also should be braided wire covering the plug wire, from the spark plug to the ignition box, if that's not there I would surely send it back as the picture on the site does not match what you have.

gene

On the Just Engines site it appears to be a XL type ignition and also appears to show a ground wire from the cap with a ring connector on the end.

yup that looks like a ring with a wire lead soldered to it. the question is even if he has that wire and ring, he's saying it does not make contact with the hex on the plug. so the ring and wire do nothing to help, unless he connects the other end of the wire to the motor case, that would close the loop between the motor, spark plug and ignition. Personally I would do away with the ring and wire and make sure the cap is snug around the Hex on the plug. One less wire and solder joint to worry about... I have a CRRC GF50 and in most threads I've read about interferance with running them the solution was to use a hose clamp and make sure the cap is snug on the Hex....

Gene

It uses an RcXel ignition, the plug cap must be pushed down far enough to cover the hex of the spark plug, if it doesn't it is not on all the way, eat some Wheaties and push hard! Once the cap is all the way on you will not need to ground the shielding to the engine at all and your interference will go away providing you have separation from the rest of the flight controlcomponents. The extra wire you see in the picture is not for grounding, it is an accessory lead with a battery plug on one end for connecting power to the RcXel ignition module, it's included with all RcXel ignition systems but almost never used.</p>

I concur with Jody. The cap likely is not all the way seated. Next is determining if he has the right size and type of plug.

There is no wire attached to my plug cap. The plug is an NGK BPMR6F. The cap is pushed on as hard as possible & clicks on the plug top. There is a rubber insert in the plug cap that centralises the cap on the plug. With the cap in place, there is a gap of about 1/8th" around the hex. It does not look possible to close this gap easily without distorting the cap badly.
I think the best solution is to solder a wire to the plug metal braid, with the added security of a couple of cable ties around the joint & ground that.

They may have sent you an ignition with a cap for a different plug. If the cap goes down far enough to cover the hex of the plug but does not grip it tight you probably just need to use a BPMR6A spark plug, it has a 3/4" hex instead of the 5/8 that is on the BPMR6F.

does the cap cover the hex on the plug....if it does then use a hose clamp.. it will close that gap and make contact....if it doesnt cover the hex the plug is too long...IMO you really don't want high voltage taking weird paths back to the Ignition box...there's a real good reason for the braided wire... it provides a shield and a direct return path for the high voltage going to the Plug...It will work by grounding the engine to the ignition box, but you have that nice braided wire that can take tons of current sitting 1/8th of an inch away from the Hex...Also the cap is designed to flex a little, if you notice there are slits cut up the side of the cap...

in short, connecting the engine to the box should work...so you can choose either way...

g

that's why he's a Jedi...

gene

The cap covers the hex on the plug, it looks as if it was made for a larger size plug. The rubber insert in the cap extends almost to the end of the cap & the only way a hose clip will work is if some of the rubber was removed. Even then I would have to cut slots up the cap side to avoid distorting the cap to much.
I think the BPMR6A plug would fit the cap properly.
Many thanks for your help to a gasser newbie.

Go for that new plug

The plug is the fastest/easiest solution but, not trying to confuse you, some engines will have clearance problems between the larger size plug and cap and cooling fins, if this happens in your case you can replace the plug cap with the correct size, very easy to do and inexpensive.

You are 100% correct.Jody,Seems we are getting more RF complaints with 2.4 then we had on 72 and this should not be. Most of the time the plug cap is not pushed all the way on or is the incorrect cap for the spark plug being used. As you say the snap ring should be over the hex on the spark plug and it is a PITA to get on and off when new.Some times we relax the pressure on the wire ring a little to help to get the cap off. You do not need to use vise grips pry bars or Dynamite to get the cap off. Sounds like this is cap for a BPMR6A plug 3/4 hex ,trying to fit on a F 5/8 hex spark plug. The RCEXL 3/4 hex cap does not fit the Champion RCJ7Y well at all but that is another story. It fits the NGK and Bosch OK.When I install the A caps I use the boot for the F plug .This gives you a little more room for the spark plug. I have all of the cap kits if you don't.I have seen several of the RCEXL ignitions that have
had the braid broken all the way around where it exits the eyelet on the ignition case,this is easy to repair. Have all so seen them lose ground inside the case ,this is almost unfixable.
BCCHI

Thanks Bill, don't get me started on the 2.4 thing.

Jascat, if you needa new plug cap give the good folks at CH Ignitions a call, they will take good care of you. http://www.ch-ignitions.com/

Those engines are supplied by RCGF/GRPro/SV (Same factory for the older crankcase style as shown on that 26cc and a couple of others). There was quite a large Qty of them supplied with mismatched Plugs and Caps. I am guessing that the change of plug will solve it.

Hi, The A plug has a 3/4 inch hex and is washer mounted. The F plug is for a tapered seat and has a 5/8 inch hex. Putting a tapered plug into a cylinder head that is not designed for it has been done but it's not the best thing to do. There's a lot of stess on the head and the seal is not so good. Try some other plugs and see what fits. Hopefully the seat will match the cylinder. Dan.

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