RV-4 runs rich at cruise.

[71459]

Hi, was wondering if anyone here could help with an issue re a rich-running RV-4.

Airplane starts right off, idles fine, run-up is perfect. T/O proceeds alright, maybe power isn't 100% there, but good.

After about 1/2 hour, engine starts to run rough. Gets rougher over time. Able to smooth it out by leaning, but shouldn't have to at 2000'. Airplane has run perfectly smooth for years, but now something is not right. Head back to the field pronto. Back on the ground, plugs are completely blackened with soot - definitely showing over-rich mixture.

Recently changed out to new plugs, new harness, new mechanical fuel pump, and overhauled carb. I did open the carb and change the float to the blue epoxy model. Float level checked 10 times - seems to be perfect (on the ground).

Did I screw up the float installation? Is the overhauled carb the wrong model for an RV-4? But it's the same model I've used for years without any trouble (10-5009).

Out of ideas, getting nervous to fly it.

Thanks for any insight..

[AlexPeterson]

What power settings are you using at 2000'?

[71459]

Originally Posted by AlexPeterson

What power settings are you using at 2000'?

Usually run it at about 2600. Nothing new here - no changes in operation after 20 years flying the same airplane. But something in the airplane has changed and it has me baffled.

[joeboisselle]

Originally Posted by 71459

Recently changed out to new plugs, new harness, new mechanical fuel pump, and overhauled carb.

That's what changed.


Plugs, harness and fuel pumps don't regulate mixture ratio so that narrows it down to the carburetor. If you're running an O-320-A1B, D1F, A2B, B2B, D2C, D2F, E1A, E1C, E1F, E1J, E2A, E2C, E2D, E2F, E3D, E2G, E3H, H1AD, or H2AD you have the correct carb.

I'd pull the carb, split it in half (take the top off) and inspect the mixture valve. Verify the "slit" isn't worn out and that it doesn't move around loosely when installed with the top still off.

Wish I had a picture of the last one I had apart.

[71459]

Originally Posted by joeboisselle

I'd pull the carb, split it in half (take the top off) and inspect the mixture valve. Verify the "slit" isn't worn out and that it doesn't move around loosely when installed with the top still off.

Yes it may be running rich all the time, but takes an hour or so for the plugs to foul up enough to cause it to run rough. Just guessing.

It is a freshly overhauled carb with less than 20 hours on it. Maybe it's a poor overhaul and I need to exchange it for another carb.

Another thought - clogged crankcase breather? I wonder if, after a few "zero-G" rolls and loops, some oil is getting up into the top of the engine and clogging the breather. I understand this can cause increased fuel pressure and an overflowing carb.

[Toobuilder]

I'm guessing your EGT's are way off from normal (last 20 years). Do they change in flight, tracking with the other signs of rich running? If so, can the EGTs be brought back to normal with mixture?

How about manifold pressure - is that the same as before? Do you have a new blockage upstream of the airbox?

[FinnFlyer]

Could the air filter be collapsing when it gets hot?

Assuming you have one.

Finn
(Not a Lycoming guy)

[71459]

Thanks for the suggestions. Sorry, it's a very basic airplane. No MP or EGT gauges.

I may just order a rebuilt (not overhauled) carb. The one I have installed is freshly overhauled, but it has been troublesome ever since it showed up.

But first I'll take a close look at the airbox and air filter. And disassemble the intake components and see if I find anything blocking.

[AlexPeterson]

I'm surprised that you can (before) fly along at 2000' with full rich. Can you not lean it any there? Sorry I don't have a sense for approx % power at 2600rpm at 2000'.

[lr172]

Originally Posted by AlexPeterson

I'm surprised that you can (before) fly along at 2000' with full rich. Can you not lean it any there? Sorry I don't have a sense for approx % power at 2600rpm at 2000'.

+1

I would never expect a carb to be anywhere near optimal / best power mixture if left at full rich, especially at something higher than sea level. Next time try leaning until it stops giving upward improvements in RPM(you mentioned no EGT instruments and I am assuming FP prop). That will put you in the neighborhood of 100* ROP / best power. Then see if you are still getting the same problem.

. IMO, cruise flight at any altitude should involve proper leaning and it can be done without EGT; Not as effectively, but can be done. This is true even for those desiring to run ROP.

The mixture knob is there because variable altitude makes it impossible ot cost prohibitive for a mechanical device to be built that is "turn it on and forget it."

Larry