About three weeks ago, my friend buried my VMAR Hotrod into a cornfield. The engine was caked with mud and mud got into the carb and crank. I spent several hours carefully cleaning and flushing all compontents of the engine. Here is what happened when I went to run it.

I normally use an OS A5 plug, but I was told to try a Fox plug. I put a Fox Standard plug in it. The engine would not stay running once the driver was unplugged. I finally gave up and put an OS A5 back in it, she fired up just fine.

1st Question. What is the recommended plug for this engine? The manual lists several. I have been told to run a 3 or an 8... what is the difference between the 3, A5, and 8 plugs offered by OS???

OK, so now the engine is running, I power it up, it immediately sputters and dies. This engine had never died after break in and I have not had to adjust either needle since.
I assumed it was running rich so I leaned the needle a notch. Still dies. So I richen the mixture (dialing it counterclockwise). After three to four clicks to the rich side, it was making power now. I make several runs from idle to full power with no problem, but then its starts bogging down when throttling up. I'd have to back the throttle off to get the engine to recover. Once in a while it would hit power, at that time, I would dial the mixture for best RPM. The sweet spot ended up being about 4 clicks CCW from my pre-crash setting. (Note that the outside temps were about 30 degrees during this run. The last time I ran the engine before the crash, the temps were 50+.

2nd question. How much affect does the cold have on the engine and does the mixture need to be changed when its cold out???

I got the aircraft up for a couple flights and it seemed ok, but on the last flight, the engine lost power on takeoff, I brought the throttle back and it recovered and I quickly landed the aircraft. I took it back to the pits and once again the engine is losing rpm when accelerating from idle.

Could the low end be too rich? I had to lean it 1/4 turn when breaking the engine in for this problem. Could the crash have thrown off this setting???

Please help! This engine ran perfectly before the crash. The engine doesn't seem to be damaged in any way. Could I possibly have another bad glow plug?

And here is something else I noticed. I run Byron 16% nitro 17% oil in this engine. It used to spit out nice golden brown oil while running. Now, its coming out pure black.. what is the deal???

Dan,

The O.S. Max #8 plug is just about the best general purpose glow plug on the market. The #A3 is suggested by O.S. to be used in engines .40 and smaller, and is a slightly hotter plug than the #8. The #A5 is suggested for .61 and larger engines, and is a cooler plug than the #8. The #A5 also is better for higher nitro applications.

Afater a crash, any number of things can happen. It's possible that something's bent, which cold be the problem source.

For the carb, try richening the idle a lot, starting the engine with a rich high-speed needle. Leave the battery on and advance to full throttle. Set the high-speed needle to peak RPM, and then work with getting the idle mxiture set.

You may have to disassemble the carb and clean all the passages. "Blowing it out" with air may not do the job. You may have to run a fine wire through the spraybar to eliminate any blockaged.

Check the fuel system to make sure it's clean and that the clunk line goes to the back of the tank. Many times, after a hard landing with a noseover, the clunk winds up in the front of the tank. It unports when the nose goes up, and when the fuel level lowers a bit.

Finally, it's possible that the rod got slightly bent in the crash. This would cause the engine to gradually overheat and quit partway through the tank. A sudden stop of the engine can easily cause the rod to bend. Even the tiniest amount of misalignment can cause enough friction to cause the engine to overheat and quit.

Thanks for your info. I'll try a number 8 plug. One of the things I was wondering was if the outside temps were a factor. At full take off power, I could hold the cylinder head and it almost felt cold to the touch, the engine wasn't very hot after shutdown either.

I forgot to mention that the I had to replace the HotRod with a new one, it was a total loss. So the fuel set up on the new plane is good. (The clunk did go to the front of the tank on the crashed plane though!)

When I disassembled the engine, I checked very carefully for damage, especially in the carb. Everything seems to be in good order and the engine turns over well. I ran two tanks through it in the air on about half throttle, I didn't tempt taking it to full throttle in the air.

I'll try retuning the low end. The power is dropping off during the transition.

Thanks again.

Dan,
colder outside temps=denser air=richer mixture is a general rule, especially for the high end. Try starting with the low needle 1 full turn CCW. Set the high end as normal, a few hundred rpm shy of peak rpm. This should get you in the "ballpark" barring other problems.

"Almost cold to the touch" is a mystery.

Thanks for the info, I'm going to do a full tune on it to see if the problem goes away. The cold thing is weird, but at full power, my hand was getting numb setting the need from the freezing cold propwash!

Dan,

Air temps below 40 degrees F can certainly cause engine running problems! Glow fuel won't atomize as well, and the cylinder can easily get over-cooled. With normal running, we've seen cylinder head temps of around 350 degrees on aircraft engines. If you can hold your hand on it, it's definitely being over-cooled.

You should not have to mess with the idle mixture much at all, but your high-speed needle will need richening to accomodate the denser air at low temps.

It's also a good idea to go to a fuel with all-synthetic oil because castor can easily freeze in cold temps. It also becomes MUCH harder to clean off.

When flying in winter, getting the engine started is the main problem. Once it's running, it should be OK. If you continue to have problems, you can try small strips of aluminum foil to block part or all of the cylinder fins to see if that helps. That's just what full-size vehicles to in winter. They're 'tuning' the cooling system for cold weather operation.

Are you saying the Byron 16/17 fuel I'm using isn't the right type? All the guys in my club swear by it and they run it year-round.

I did have to richen the fuel mixture on the high end. It seems like my problem is from idle to take off again.

I haven't gone out to retune yet, though. Hopefully it will be warmer this week.

Also, any idea why the oil is now coming out of the silencer pure black instead of golden brown like it was before? Is this an indicator of how the engine is running???

Dan,

If the fuel's been successfully used year-round, there's no reason to change. I mentioned the fact because a fuel with a high castor content gets hard to clean off the model in cold weather. I've even seen it freeze when it's really cold.

With the reduced atomization cold weather causes, low-end and acceleration can get problematic.

If the engine's not too lean, you can still get oil in the muffler burning dark. If you need to check it, capture some of the exhaust stream on a piece of white card stock. The oil will get absorbed into the card and you can see if the engine's "making metal.

Usually, though, it's just the oil from the fuel getting "cooked" in the muffler.

Mystery solved, I guess? Went to the field today, it was about 15 or so degress warmer than my last outing. Engine fired up fine and needed slight leaning at power, did 10 flights today, she did not so much as sputter!

I guess this engine really needs some TLC when it gets in the 30's and lower!