Preventing Spark Plug Fouling

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
Experienced my first excessive mag drop since I've owned my plane on the runup today.

I've flown it about 15 hrs since the annual was completed in October. The drop on the left mag was about 200 rpm so suspecting plug fouling, I leaned and ran it at a high rpm for a bit. That seemed to correct the problem fine and I went flying.

So, immediate problem solved, but since I am the only pilot of this plane, I'm trying to figure out what I am doing wrong - as a longtime renter, I've been under the impression that plug fouling is usually the result of 'some other guy' running the plane too rich and/or not leaning on the ground. I always lean on the ground and in flight. I have been doing alot of pattern work in the plane in the last 15 hrs, but the 2 flights immediately preceeding today's experience were both x-countries slightly over an hour each and I leaned as much as possible at 6000' and around 2450 rpm (airplane is a 170 with O-300).

So, is this common with O-300s, or is there something I need to check? I can't imagine running the engine any leaner than I was doing on the trip this weekend.
 
If you're absolutely certain it's plug fouling, you can add some of this into the fuel: Alcor TCP

Otherwise, Occam's razor says suspect a bad plug, then ignition lead, and continue from there...
 
I lean aggressively on the ground and in the air, however, I do a leaned out run-up prior to shutdown, or before I do the mag check to clear any fouling from running at low RPM's in the pattern, and landing.
 
dmccormack said:
If you're absolutely certain it's plug fouling, you can add some of this into the fuel: Alcor TCP

Otherwise, Occam's razor says suspect a bad plug, then ignition lead, and continue from there...
Click to expand...

That will help for lead balls (admittedly, the most likely culprit) but not all sources of plug fouling.
 
The O-200/O-300 series are prone to oil fouling of the plugs when the engine is warming up. The bottom plugs are really low in the head and any oil getting past the rings during warmup runs into them. If it misfires when checking the mags (big mag drop), go to Both and run it at full throttle for ten seconds or so to burn the oil out. Try the mag check again.

I use REM37BY plugs to get around the problem. Their electrodes stick out farther and don't foul easily.

Dan
 
Fearless Tower said:
Experienced my first excessive mag drop since I've owned my plane on the runup today.

I've flown it about 15 hrs since the annual was completed in October. The drop on the left mag was about 200 rpm so suspecting plug fouling, I leaned and ran it at a high rpm for a bit. That seemed to correct the problem fine and I went flying.

So, immediate problem solved, but since I am the only pilot of this plane, I'm trying to figure out what I am doing wrong - as a longtime renter, I've been under the impression that plug fouling is usually the result of 'some other guy' running the plane too rich and/or not leaning on the ground. I always lean on the ground and in flight. I have been doing alot of pattern work in the plane in the last 15 hrs, but the 2 flights immediately preceeding today's experience were both x-countries slightly over an hour each and I leaned as much as possible at 6000' and around 2450 rpm (airplane is a 170 with O-300).

So, is this common with O-300s, or is there something I need to check? I can't imagine running the engine any leaner than I was doing on the trip this weekend.
Click to expand...

It's a current problem with all Kelly carbs for the 0-200/0-300. What is your fuel burn rate at cruise leaned ?

try running a 3 auto fuel to 2 100ll mix or find some 100VLL fuel see if it helps.
 
Last edited:
Tom-D said:
It's a current problem with all Kelly carbs for the 0-200/0-300. What is your fuel burn rate at cruise leaned ?
Click to expand...
Haven't done enough x-countries in it to say for sure, but so far I'm estimating around 9 gph at ~2450 RPM, but that includes the climb which makes up around 15 minutes of a 1 hr flight.
 
How lean do you run on the ground? I set my mixture so lean that I know I will never take off without richening as the engine quits as soon as the throttle is advanced.
 
Fearless Tower said:
Haven't done enough x-countries in it to say for sure, but so far I'm estimating around 9 gph at ~2450 RPM, but that includes the climb which makes up around 15 minutes of a 1 hr flight.
Click to expand...

the MA3-SPA will run rich in the WOT mode. When doing pattern work you should plan on cleaning plugs every 25 hours or more frequently.

9 GPH is middle of the road for pattern work.
 
Last edited:
Skylane81E said:
How lean do you run on the ground? I set my mixture so lean that I know I will never take off without richening as the engine quits as soon as the throttle is advanced.
Click to expand...

With the old fashioned mixture pull knob (not the modern vernier twist type) I am not able to lean quite as much as in the 172s, but I am able to get it lean enough that I can't get more than 15-1600 RPM before it dies.
 
Tom-D said:
It's a current problem with all Kelly carbs for the 0-200/0-300. What is your fuel burn rate at cruise leaned ?

try running a 3 auto fuel to 2 100ll mix or find some 100VLL fuel see if it helps.
Click to expand...
Isn't 100VLL pretty much what we run now? How could you tell? They are both colored the same blue with the same dye.

http://www.aopa.org/advocacy/articles/2011/110915faa-oks-use-100vll-fuel-for-ga-aircraft.html

The new fuel specification, called 100VLL, has a maximum lead content nearly 20 percent less than the existing 100LL specification and better represents the actual amount of the lead additive used in aviation fuels today. The SAIB goes onto state, “The FAA determined that grade 100VLL meets all of the performance requirements of grades 80, 91, 100 and 100LL and will perform identically in existing aircraft and engines.”
Click to expand...
 
Dan Thomas said:
The O-200/O-300 series are prone to oil fouling of the plugs when the engine is warming up. The bottom plugs are really low in the head and any oil getting past the rings during warmup runs into them. If it misfires when checking the mags (big mag drop), go to Both and run it at full throttle for ten seconds or so to burn the oil out. Try the mag check again.

I use REM37BY plugs to get around the problem. Their electrodes stick out farther and don't foul easily.

Dan
Click to expand...

I was going to mention. I had chronic fouling problems and high oil consumption until I got a top overhaul.

When I flew the test flight after the overhaul the plugs fouled. They even pulled a plug and popped out a lead ball out when I complained.

TCP only helped a little. I'm convinced it was mostly oil fouling.

The fouling stopped once the rings seated and the oil stopped going to waste.
 
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