plug fouling

[sonerainut]

Hello All!

I just finished a series of flights with increasing ballast to work up to gross weight. It'll do it but it is not as nimble when solo...

Anyway my last flight was cut short with a rather rough running engine - I made way back to the airport right away. So I'm at full gross, I think the engine is going to quit and I'm high and fast. I can report all came out OK.

So I vowed not to fly the thing until I figure out what is going on. I pulled the cowl and looked around, nothing out of the ordinary. So I went to check the compression and plugs. Compression is OK and all plugs are sooty messes! Problem found - no wonder why it was getting so hard to start and no wonder why it was running so rough - all of the plugs were probably taking their respective turns fouling/missing....

Ok now what?!? I have a Great Plains 2180 cc with mag and secondary ignition. (All are timed correctly) I have a Zenith industrial carb with air filter and airbox that has a fresh air inlet and can be selected for carb heat - it works well too. The plugs were new (less than ten hours of operation). I also have a mechanical mixture control that adjusts the main needle. I run auto gas and have a 4 to 5 PSI facet pump between the tank and the carb (I actually have two in case one gives up the ghost). I do not have an EGT, but oil and CHT's are OK.

As far as the soot problem goes all four exhaust stubs have a little soot at the tips and all eight plugs are equally sooty. Since day one with the engine, I have had problems with fouling plugs and thought I had it straightened out... With my current mixture settings, the idle is almost smooth, it lopes a little. Any leaner (more air on the air bleed) and it quits. At max static RPM (tied down, of course) I can lean slightly, the RPM goes up and a little more leaner, the RPM starts to drop. It all seems OK to me. I really dont think it should be necessary to clean the plugs every 5 hours or so.

Sooooo, what dou guys think? Any input would be helpful.

Regards,

Lee (AKA sonerainut sooty plugs)

 

[privatepilot03]

It's gotta be running rich in flight.. No other possibility really.

Check your intake system for leaks. Grab your propane torch or a can of brake clean and aim the gas at every junction point in your intake system with the engine running at 1500-2000 rpm. If the tone of the engine changes or you get a change in RPM, you have a leak.

From there, I would suspect the float level in the carb maybe being too high or maybe the needle is hanging open. Main jet too large perhaps?

Just some ideas..

Chad

 

[splischke]

It's your primer leaking and causing the mixture to be too rich in flight. Ooopps, that was my plane.

My plugs are sooty too when I remove them after a long taxi or period of running at idle. However, if I take the plugs out after a high speed run-up, they appear to be the correct brownish color. I know this isn't much help but I don't want you thinking [sooty plugs] == [engine rebuild].

I wish I had an answer for your problem. Hunting down these quirks can be frustrating, challenging, time consuming, lonely and immensely rewarding.

Since you run auto-gas, have you given thought to temporarily switching to avgas. You may have received some auto gas with an ethanol blend.

Did you do a compression test or a differential test (leak-down)?

-Scott

 

[garymedic]

You might want to try a different heat range of plug if everything else checks out. Be very careful with this and consult someone you trust with selection as detonation can be the end result of a too hot plug range. Ideally, you want a plug that will transfer heat to the head slow enough to keep the deposits burned off without holding so much heat in the tip as to cause detonation.
I believe two things factor with the soot problem in our aero VW's. We're really low on compression and the magneto ignition mandates such a narrow gap on the plugs. I've read much on "zero decking" VW's to improve the geometry of combustion and think I will use the copper cylinder-to-head gasket and thinner cylinder-to-case shims to attain the necessary compression ratio. We'll see....

Gary

 

[Chuck in Indiana]

Obviously running rich. Just a thought. Could the fuel pump pressure be overpowering the needle and seat? As mentioned before, correct float height? No rocket science involved with these tractor carbs.. ;D
Oh, and congrats on maintaining your composure..
Chuck