What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Spark plug fouling

MacCool

Well Known Member
Normally I lean for taxi, but I'm not terribly aggressive about it. The other day however, as I was headed to the runup area, I had some rough running that turned out to be on the right mag. I spent a little time leaning and it did clear.

The engine is an IO-320 D1A with two P-Mags on an RV-9A. The spark plugs are NGK BR9ES. I'm wondering if maybe I should be running a little hotter plug, a BR8ES, or just be more aggressive in leaning for taxi. Thoughts...?
 
I’ve run BR8ES with dual pMags for years. I suspect leaning would make more of a difference than changing to them from BR9ES, but shoot fire - they are cheap. Give it a try.

I lean on the ground until just before cutout. You know you are leaned enough when you add throttle and the engine bogs down - at which point you add a little more gas, then lean again when you pull back the throttle.

Carl
 
Use the 8ES plugs and lean the **** out of it on the ground, including as soon as you exit the runway after landing. When the engine is cooling after landing, the lead is being scavenged and ends up in the plugs! If you are very lean on the ground, I guarantee you that you will not forget to go to full rich for take off, as any advance of throttle will result in a very rough engine!

Regards,

Gary
 
OK thanks. I'll plan on being much more aggressive with leaning for taxi. As to the plugs...I'm going to change the oil on Saturday. I'll look at the BR9ES's that are in there and if they show signs of fouling, I'll replace them with BR8ES's.

The A&P that did the condition inspection in September before I bought it just cleaned those plugs and re-gapped them. At $2.75 per plug, I don't really get that...
 
As everyone mentioned, lean extremely aggressively on the ground ALWAYS. Then run up to 1200 RPM's for about 30 seconds prior to shutdown.

Decalin in the fuel will help with fouling as well.

Vic
 
Also, check to make sure that your idle mixture is set correctly on you injector body. For a bendix style injector, you should get a rise of 10-40 rpm as you slowly bring the mixture back to shutoff.
 
Also, check to make sure that your idle mixture is set correctly on you injector body. For a bendix style injector, you should get a rise of 10-40 rpm as you slowly bring the mixture back to shutoff.

Eh...that's beyond skill level. The kind of thing that I leave to my A&P.
 
Leaning

Funny, I learned to fly in the early 1960s and the instructors always said "keep it rich until you are above 3,000', then lean until the engine starts to run rough, then richen the mixture until it smooths out". On the ground it was always full rich. Now, I lean on the ground until the it's rough, and apply the same procedure when flying no matter the altitude. The only full rich is at takeoff or other WOT below 7,000'. Still learning after all these years.
 
Funny, I learned to fly in the early 1960s and the instructors always said "keep it rich until you are above 3,000', then lean until the engine starts to run rough, then richen the mixture until it smooths out". On the ground it was always full rich. Now, I lean on the ground until the it's rough, and apply the same procedure when flying no matter the altitude. The only full rich is at takeoff or other WOT below 7,000'. Still learning after all these years.
Exactly what I was taught, also in the 60's. Leaning for taxi is a foreign concept.

As I was changing my oil today, I took the opportunity to open the plenum and change all the plugs over. The previous plugs were the NGK's with the removable tip and all 8 of those tips were loose. I replaced them with the solid tip version. Inspecting the old plugs showed them to be a nice light tan color, so I conclude that I need to lean for taxi more aggressively.
 
Eh...that's beyond skill level. The kind of thing that I leave to my A&P.

Uh, no, no it's not. I'm fighting this dragon right now, and leaning adjustments are super-easy to do. Turn the linkage turnbuckle in the opposite direction of the "R->" marking one detent at a time and check the rpm rise at idle as you slowly pull the mixture lean from full rich. 40 rpm rise is within spec, but apparently still too rich for my engine, so I'm looking for 10 rpm this afternoon
 
Uh, no, no it's not. I'm fighting this dragon right now, and leaning adjustments are super-easy to do. Turn the linkage turnbuckle in the opposite direction of the "R->" marking one detent at a time and check the rpm rise at idle as you slowly pull the mixture lean from full rich. 40 rpm rise is within spec, but apparently still too rich for my engine, so I'm looking for 10 rpm this afternoon

You might consider the lowest MAP approach. If you choose to run at the mixture that your engine likes, this is the best approach. You set the RPM to 800 and tweak the mixture until you get the lowest MAP possible at 800. Be sure to readjust the RPM back to 800 after each adjustment. The lowest MAP is a direct reflection of what you engine "likes" for idle mixture. Best to do this at a median temperature, as your engine will like different mixtures based upon ambient air temps. Colder temps mean more fuel.

Your engine will run leaner than best MAP, but this is considered optimal and provide the smoothest running.

Larry
 
Last edited:
Uh, no, no it's not. I'm fighting this dragon right now, and leaning adjustments are super-easy to do. Turn the linkage turnbuckle in the opposite direction of the "R->" marking one detent at a time and check the rpm rise at idle as you slowly pull the mixture lean from full rich. 40 rpm rise is within spec, but apparently still too rich for my engine, so I'm looking for 10 rpm this afternoon

I do appreciate the input.

General Aviation in the US is great...there's something for everyone. I know, especially on this website, that there are some very knowledgeable and skilled airplane tinkerers and mechanics, and that that's an aspect of flying that is a huge attraction to them. Me...? I've been a pilot a long time, but I'm a flyer. I enjoy the flying part. As I learn more about Experimental Aviation in general and this airplane in particular, maybe the beyond-routine maintenance aspect will become a bigger part of this hobby for me. But for now...my A&P/IA works about 20 yards from my hangar. He's a great and readily willing resource, and he works cheap. I don't mind tinkering with my tractor, boat, or car because if I get it wrong, it's simply an inconvenience. With this airplane...well, the stakes are higher. For now at least, I want to make sure that the tinkering is done by someone who knows what they're doing.
 
You might consider the lowest MAP approach. If you choose to run at the mixture that your engine likes, this is the best approach. You set the RPM to 800 and tweak the mixture until you get the lowest MAP possible at 800. Be sure to readjust the RPM back to 800 after each adjustment. The lowest MAP is a direct reflection of what you engine "likes" for idle mixture. Best to do this at a median temperature, as your engine will like different mixtures based upon ambient air temps. Colder temps mean more fuel.

Your engine will run leaner than best MAP, but this is considered optimal and provide the smoothest running.

Larry

Interesting, Larry. Boy, sure sounds like a lot of iterations and that means a lot of climbing in and out around a moving prop or a lot of start cycles on the engine (I generally have to work by myself out here). Also I'm at 1060' MSL and wouldn't want to get so lean my engine isn't happy at sea level on a high pressure day.
 
Also, check to make sure that your idle mixture is set correctly on you injector body. For a bendix style injector, you should get a rise of 10-40 rpm as you slowly bring the mixture back to shutoff.

+1

I believe this is the larger issue. I don’t lean on the ground. 700 hours on the 6 and 120 hours on the 10 without ever having a fouled plug. I do ensure that my idle mixture is correctly set though and i do run up to 1400 rpm for a vit at shutdown.

Larry
 
Interesting, Larry. Boy, sure sounds like a lot of iterations and that means a lot of climbing in and out around a moving prop or a lot of start cycles on the engine (I generally have to work by myself out here). Also I'm at 1060' MSL and wouldn't want to get so lean my engine isn't happy at sea level on a high pressure day.

I have someone in the pilot seat while I adjust. I akes about 2-3 minutes. I can’t imagine it is any more iterations than hitting a target egt rise. Best map is not really that lean and should have no problem dealing with a 1000’ lower altitude

Lowesr map is a best power mixture. It is rop by just the right amount for that barometric pressure and temperature
 
Last edited:
I've been adding TCP to the gas for about a year now and it seems to help. At the last annual, I only had a few lead balls on one plug. I also switched to the UREM37BY plugs on the bottom.
 
Back
Top