Burned out two plugs today

Surfer_kris wrote:What do you use to start the engines with? It might be due an overcurrent, I can't remember when I last burnt out a glow plug.

I'm using 2 d cells wired in parallel.

Any sign of metal in your exhaust residue, is it darker gray than usual, any sparkles in it? Three shims is plenty safe if your nitro is in the 20% range, even with 30%, 3 should be okay. Any new vibration? I'm guessing the weather in Arizona was well over freezing; frigid air will raise the compression. I would disassemble the engine and look for any sign of metal in the bearings, and scorching and scoring from lean running on the piston and cylinder, scuffing on the backplate or circlip.

Put some shiny new shims in it just to be safe. Those plugs are too expensive to burn out. The only ones I've ever popped were on a Black Widow with 25% nitro but only one shim. That was before I knew about adding shims with more nitro.

I'm not sure how pressure feed would raise the compression.

What was different from the last time you flew?
New fuel?
New Battery's?
I'd lengthen the glow clip wire which will drop the voltage
At the coil slightly.
I have a flight panel with adjustable voltage on the glow
Clip, I think I only blow plugs when I forget to turn the
Voltage down.
What are the D's you're using, dry cell or Nicad/nimh?
Don't forget even though rated at 1.2 v a newly charged
Rechargeable will sometimes be over 2 volts.

The insert plugs as you mention I believe are Merlin plugs. Although I've found them to work pretty well, I did have some concerns. The owner of Merlin is Al Kelly whom I've tried to contact several times in regard to the heat range of these plugs. He recently did return my phone call but I wasn't home to receive it. The plugs I had were hot plugs and the filament was really glowing excessively. Any time I've used a hot plug, the filament is much thinner which I feel could be slightly problematic in the case of these engines and the way were running them. Combined with head design, I feel this plug was causing my engines to be over compressed which needed a minimum of 4 head shims in order to prevent the engine from kicking back during start up. I would prime and the instant I would turn the prop, it would bump immediately. Head shim thickness is all over and I have some here ranging from .002" - .008". I can't comment on who's selling what, I never purchased any from Cox International. I like the filament to be centered in the head of the plug and as evenly spaced in the coils as possible. Using a pin I carefully align the filament as best as possible. The Merlin plugs usually look like they're pushed to the side and the coils are certainly susceptible to getting out of alignment.

In the end, I just switched to Nelson plugs as they just seem to have better durability. I've been making head button designs for most of my engines so that I can just use the same plugs. Even though AL makes a plug that fits the taper and thread of a Nelson plug, It isn't a true Nelson plug. Anytime I have plugs launching, I look for certain things such as prop balance. Too much oil can cause overcompression as well. I don't need to get flamed for my oil choices. I would also take a peak at the side to side crankshaft clearance at 12 o'clock -6 o,clock, 3 -o'clock and also 9. Sometimes play like this can result in vibrations that can just shake the guts out of the plug. I would continue to keep adding head shims until the plugs stop blowing. Anytime the plane hits the ground, this can also scramble a plug. I would also try and prime using a vapor prime rather than dripping fuel into the cylinder. I just use the syringe with nothing in it and squeeze fast to blow the residue in. This is something I do with larger combat engines to avoid floods. Ken

The ones I get from Bernie are described as medium hot. I believe they used to be written up as medium. I don't think I've bought any since that change in description. But 3 is my absolute minimum number of shims for those, and 5 is common. So far I haven't popped one. They fit the Norvel head clamp too.

If part of a filament vibrated over and touched the grounded side(the cathode) all the current would flow through only between the point of contact and the anode; the resistance would quickly drop, causing the current to rise, and it would suddenly get hot enough to burn out in a flash. I always use a straight sharp dental pic to align the coil any time I inspect a plug.

I was actually surprised at how clean my exhaust was yesterday. Both times i burned plugs it was after i stuck it in the ground pretty good, maybe that was it. I am using duracell dry cells. I am going to fly with regular compression cox plugs today and see what happens. Ordered some new inserts and will add shims when those come in.

Don't forget what the heat range denotes Lads.
Hot plug = low nitro
Cold plug = high nitro

Clean exhaust could be too lean which means the engine is running hotter than it should.

Phil

Hi, with a bladder tank it's very easy to run an engine on the lean side (and on the rich side too). With a standard tank the mixture is sucked by the carb, if you are on the lean side, the engine can't draw suffisant fuel, but with a bladder tank the engine don't suck, you adjust the flow but the engine will not have any trouble with the succion.
I hope you understand becaus my english is not very good...
Enjoy your engine,
Rémy