XJet

I was flying a plane with a TT61GP in it this afternoon when the engine began to make funny noises.

It sounded very much like it was detonating -- but only when the engine unloaded in a dive -- pull the nose up and it ran strong and smooth.

I landed and checked that everything was still tight and the prop hadn't been nicked on take-off or anything, but no problems could be seen.

Just in case, I richened the needle a quarter turn and flew again -- no change, except that now it began mis-firing like a gasser with an intermittent ignition system. The misfires were so bad in fact, that I wondered if it wasn't something wrong with the throttle servo causing it to go full-open then closed again.

After landing again I found that I could richen the needle a full turn without much effect -- so i figured it might be dirt in the fuel system but, just on a hunch, I thought I'd try a new plug.

I've seen bad plug cause all sorts of problems in the past and this one had seen quite a bit of service in an engine that gets hammered like hell -- so I pulled it out and popped a new one in.

I was also pretty sure it wasn't dirt in the system because my fuel is triple-flitered and I've *never* even once had any problems in this regard. I also knew that, since I'm running just 5% nitro and only turning a 12x6 prop, it would be pretty hard to get this engine detonating no matter how lean it was running.

With the new plug in it, the engine was immediately back to normal -- albeit running *way* rich now because I'd wound the needle out so far. I leaned it up again and flew the snot out of it. It was back to its good old self, really winding out in those vertical dives without a hint of detonation or excessive vibration.

Also gone was the really bad intermittent misfire that had started.

Just to make sure, I flew another hour or so with no further problems.

Looking at the old plug (an OS#8) it was easy to see that it had begun to crystalize and was looking rather dull and, to make absolutely sure, I put it back in the engine -- whereapon the symptoms immediately returned.

So, sometimes, before you strip down your fuel-system or start chasing demons, just try a new plug and see what happens.

I was also surprised to find that an the 61GP tollerates a Fox Long plug -- most of my TTs really prefer an OS#8 but I can see no difference between the two in this engine.

Hobbsy

My Feedback: (102)

Fox makes a #8 but it does not say so on the plug.

w8ye

My Feedback: (16)

You can throw that old plug in your starter box for a couple months. It may run OK when you put it back in the engine?

bob27s

My Feedback: (19)

A good rule of thumb....

If you have a good running engine, which very suddently starts running strange - first thing to change is the glow plug.

Plugs are consumables, and do fail just of age. Sometimes they fail from contamination (carbon chunk?). Most often, they fail due to a lean run (but those are most obvious)

What you decribed may be either contimination of the element, or quite possibly a leak in the plug post seal. Rare, but it happens.

Usually a leaking seal will cause the engine to run lean or very hot. Visible portion of the plug becomes discolored.

Contaimination in the coil can cause a short - or a hot spot. This will cause mis-firing.

Probably nothing you did wrong. Just change the plug.

Ive seen the posts/seals come loose a few times on installations where a remote glow driver connection was used, and the vibration from the mass attached to the post eventually caused a failure. Although, that shouldnt really happen either.

Anyway...... math is easy. Plane: $500 Glowplug $4. It is a consumable.

Harry Lagman

Nice report XJet - thanks for sharing.

Are you sure it's not plug damage caused by that new "secret weapon" oil you guys are running?