The International Cessna^® 170 Association

For the past couple of days I've leaned the engine during taxi to the point where any additional leaning results in rough running. So far, no fouling no matter how long I taxi or how low the rpm during taxi. I leave it leaned like this until it is time to do the run-up and at that time, I go full rich. Also, using this technique the mag drop when running on the bottom plugs drops less than when I was taxiing out full-rich. This is a surprise to me but it is definately a noticable difference.

I guess I'm learning a couple of things...1 - just because auto fuel doesn't contain lead, the plugs can still foul with too rich a mixture at low RPM. I wonder what they are fouling on? and 2. Proper leaning technique during taxi seems to be eliminating my fouling problem.

The question that I still would like to answer for myself is, are the Unison plugs more susceptable than Champion, as this fouling started immediately after the switch to Unison, both on 100LL and unleaded auto fuel? I've ordered some Champion plugs and am going to switch back just as an experiment. I'll report what happens after I get some experience.

Regards,

Curt

Curt,
I think you thought every thing out well. But I have had problems with Unison plugs! Also mogas will foul plugs (it's just not lead, but carbon instead) It seames to me that the other plugs out there don't spark as hot as champs! (just a personal opinion!!) And that would note for your carbon (if that is the case) problem. Leaning will help carbon and lead clean out! (due to higher cyl. temps.) So that could be the case of your last post.
brad

CBogle wrote:For the past couple of days I've leaned the engine during taxi to the point where any additional leaning results in rough running. So far, no fouling no matter how long I taxi or how low the rpm during taxi.

Regards,

Curt

So maybe your idle mixture screw was adjusted too rich during your annual?? Or it's always been too rich but the champions were less effected? The manual says with a properly adjusted idle mixture screw setting you should see between 25 - 50 RPM increase when pulling the cabin mixture control to Idle Cut Off. Any more than that and you are idling too rich.

Brad: Very interesting that you've had fouling problems with Unison plugs. My plan is to install the champions and go back to my old (maybe bad) habit of on leaning on the ground, and see if I still get a fouling problem. If I don't, we'll have good evidence that the Unison plugs were the difference. Either way, I'll probably go back to leaning on the ground as it is probably better procedure.

Corey: The idle mixture setting was not changed at the annual. I get the recommeded rise in RPM most of the time when shutting down so I believe it is set correctly.

Thank you both for contributing your ideas and thoughts. I'll report back my results when I get some time on the Champion plugs.

Regards,

Curt

When your engine is idleing, the vacuum in the intake manifold is at its maximum. If there is any leakage from your primer, it will manifest itself itself the most at this time. Prolonged operation with this condition could cause fuel fouling of the lower plugs.
Another consideration is the ignition system. Unlike most all other engines, the C-145/O-300 series fire ALL the botton plugs with the LEFT mag. If that mag is weak at idle speed, the bottom plugs may not be firing regularly.

BL:

Bruce also mentioned earlier the possibility of the primer leaking. The annual is coming up in a couple of months so I'm going to change the O-Rings in the primer as suggested. They are so cheap that it isn't worth not doing.

I've also considered that the left mag could be getting a little weak. As I mentioned before, I used to get almost no perceptable mag drop on either mag at run up, but, immediately after changing to the Unison plugs I noticed the left mag dropping a little more than it ever had before, but, still within the limits. However, over the past few days with leaning during taxi, I've noted that the left mag isn't dropping quite as much as it was during run-up as when I didn't lean.

I'm going to try going back to Champion plugs just for the experiment of doing so but I'm going to take yours and Bruce's suggestion to replace the O-rings in the primer just-in-case.

Thanks,

Curt

Curt,

The O rings on the primer have no bearing on leakage to the engine.

BL:

Thanks for the clarification. I understand.

Curt

CBogle wrote:...
I guess I'm learning a couple of things...1 - just because auto fuel doesn't contain lead, the plugs can still foul with too rich a mixture at low RPM. I wonder what they are fouling on? ...

Carbon. (Carbon is conductive.) The most likely cause of sudden fouling is a failure of some sort, either magneto failure (timing, point-gap, etc. should be checked annually), mag-lead, carb (or primer), or induction blockage/issue.

Spark plug brand is not suspect, IMHO.