Bad Glow Plug ; Erratic RPM Ups & Downs on the Ground?

[Kafin Noe’man]

Hi, I have an OS LA-S 25 that’s been running great. However there’s something that also makes me confused. After the engine starts, the RPM keeps going up & down erratically on the ground (it can go from 11,800 to 13,500 to 12,300, to 12,800 without changing the needle position).

Normally after 3 to 5 laps up in the air, the engine runs very stable and gives me a very good, solid, and reliable 2-2-2 break.

I can predict/remember the ‘right’ needle position adjustment when the weather gets colder or warmer, but I can’t never tell the launch RPM number. I think it would be nicer if I can ‘see’ the RPM number so the adjustment would be more precise.

But lately, it’s getting worse, it takes more than 20 laps to get the engine settled in solid 2-2-2.

My first thought was leaking problem. So, I had checked everything from the fuel tank to the engine parts and I found one of the backplate screws was missing. I replaced the screw and put the engine back together and went flying. Unfortunately it didn’t stop the erratic RPM ups & downs, even after the plane was up in the air.

The significant difference happened when I replaced the glow plug. The old glow plug was OS #8 and it looked ‘very dark and burnt’. The new glow plug that I tried was an ENYA #3 (although it wasn’t freshly opened from the packaging but it still looked in much better condition).  After this replacement, now the engine RPM is much more consistent both on the ground (now it’s much easier to determine the ground RPM) and up in the air (it runs very stable and gives a solid 2-2-2 break from the very beginning).

Any thoughts on this? Could the bad glow plug be the reason for it?
Looking forward to hearing more from you!


Best,
Kafin

[Dennis Toth]

Kafin,
Sounds like you are making the right moves. You will get a lot of things here but I think first you try a brand new out of the package plug. The Enya #3 is pretty good. You didn't mention what fuel you are using but nitro around 10% seems to work good in the LA's. Also, fuel with at least 23% total oil.

The LA's do like to get up there in rpm so don't overload it something like a 9 1/2 x 4 is a good starting point.

Best,   DennisT

[Colin McRae]

Kafin, it sounds like you already found one potential problem. The glow plug you were using. The OS8 plug is a medium-hot plug. If you are running 5 or 10% nitro, I would recommend a hot plug. The Enya 3 is a hot plug which is good. If you prefer OS, the OS6 (A3) is a hot plug. I run the OS6 plug on all of my OS LA engines.

But the rpm issue on the ground sounds like a fuel delivery issue, maybe a leak. I have also seen this on an OS 25LA, but when it has happened, I could see bubbles in the fuel line. But when the model got in the air the engine ran fine.

Not sure what fuel tank you are using but if a Brodak uniflow tank, the Brodak design places the UF vent pretty close to the fuel pickup location. So, air bubbles from the UF vent can short circuit to the pickup when on the ground. But once centrifugal force happens when the model is in the air, the problem goes away.

I also run muffler pressure on all of my OS LA engines. My opinion is that the LA engines run best with muffler pressure.

[Kafin Noe’man]

If you are running 5 or 10% nitro, I would recommend a hot plug. The Enya 3 is a hot plug which is good.

Hi Colin, thanks for your feedback.
This pretty much explains why I've never experienced this with my Enya engines since I always put the Enya #3 glow plug on them and have the 5% nitro fuel at most of the time.

[John Carrodus]

Kafin - sounds like your tank is too low. Several little problems can sometimes give a passable but not perfect run. Sooner or later two or three little probs will add up to one large one  and your motor just throws in the towel.
Cheers
John C

[jerry v]

In line fuel filter helps too.

Jerry

[Kafin Noe’man]

Kafin - sounds like your tank is too low. Several little problems can sometimes give a passable but not perfect run. Sooner or later two or three little probs will add up to one large one  and your motor just throws in the towel.
Cheers
John C

I’ve experimented with various tank height as well, but it ended up with me had to lower the tank 1/8”

[Kafin Noe’man]

In line fuel filter helps too.

Jerry

I have it installed all the time, I probably should try to clean it.

[Leonard Bourel]

Yes Kafine you should clean it regularly as this will for sure cause an inconsistant engine run if there is any dirt in the filter

[Colin McRae]

Hi Colin, thanks for your feedback.
This pretty much explains why I've never experienced this with my Enya engines since I always put the Enya #3 glow plug on them and have the 5% nitro fuel at most of the time.

Kafin, one simple experiment you can also do to see if the glow plug you are using has the correct temperature range is to see if the engine rpm sags once glow power is removed.

With engine running and needle adjusted with power still on the plug, when the power is removed, if the rpm instantly sags or drops then the plug is not hot enough.

[bob whitney]

went flying yesterday with one of the local sport flyers, he had a combat streak with an older S/T 35 with removable front end, anyway we could not get a steady run out of it .pulled the plug and it looked brand new .not sure what brand ,will follow up on it later. anyway put a new K&B shielded plug in it and it ran great ,as i said the other plug looked new .will try and get it to see what brand and heat range it was.
  RAD
  did not notice an rpm drop when we removed the battery

[Kafin Noe’man]

Hi, I'd like to give you some update report.

I've finally solved the erratic RPM ups & downs on the ground. Beside the bad glow plug, one of the factors was the fuel mounting was kinda loose thus it created some tank's 'movement' that led into having a fuel foaming. I had like 5 or 6 flights last Saturday, and the engine runs on all those flights were pretty consistent both on the ground and up in the air.