RV-4 runs rich at cruise.
 

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..

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.

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.

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.

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?

Air filter?
 

Could the air filter be collapsing when it gets hot?

Assuming you have one.

Finn
(Not a Lycoming guy)

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.

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

Why not try pulling the mixture well back when you get to cruise. Pull it back until the engine is running rough and then push richer until is smooths out. That is what we did years ago before EGT gauges were common. Check your plugs later if you have no roughness problems.

Could it be a leaking primer system?
 

Recently I experienced a badly leaking primer system that caused my newly-overhauled carburetor to seem to run overly-rich. Fuel was continuously being leaked into the cylinders (bypassing the carburetor).

Perhaps this is your root cause but why it would take a half hour to manifest itself is beyond me. Mine was continuous and I could troubleshoot it on the ground.

--
Joe

Yes true, but I was trained (40+ years ago) that no leaning is necessary below 3000'.

Maybe things vary depending by how a carb was last overhauled. I'll clean up the plugs and try flying it using aggressive leaning, though that has never been needed in the last 1100+ hours.:confused:

I don't have a primer on the airplane.

Another old wives' tale that regularly shows up as false when put to the test.

If I ran full rich at 2000' i'd be burning 30% more gas than I need to and not going much (if any) faster.

On my other aurplane I installed a fuel flow gauge and sensor. I discovered that I could reduce the fuel burn by 25% with no loss in power at low altitude by leaning. That system cost me $500 but paid for itself in no time and probably reduced the amount of crud in my engine. So leaning can always be done for cruise regardless of altitude.

Yes but in my case it's so rich the plugs foul up in less than 1 hour and the engine is running so rough it's not safe to fly. This is something completely new, thus my concern. At only 2000' the engine should run perfectly fine at full rich without fouling.

Yes can do this, but this has never been necessary in the past, especially at a low altitude, thus my concern.

Thanks for the suggestion. I don't know about collapsing, but the filter is very old (but kept cleaned and oiled). Maybe it's worn in such a way as to restrict air flow enough to cause a too-rich mixture. I'll put a brand new one on and see what happens.

Possible Cause?
 

?. You said you recently changed fuel pumps! Check your fuel pressure is not
over 5 or 6 psi, This is definitely a possibility, as those carbs are very sensitive
to fuel pressure. Thanks, Allan..:D

Right, it's such a small amount of fuel leaking by it takes an hour to foul the plugs up. The last overhauled carb I bought did exactly this and it was the mixture valve.

I have been "taught" MANY things in my life that I later "learned" were incorrect. Just because someone is your teacher doesn't mean everything they teach you is correct. Often not their fault, just bad info passed across generations.

Larry

Your new carb likely has a larger jet than stock or someone otherwise monkeyed with the bleeds, making it a bit richer than normal. Either way, try leaning it and you'll know. If they adjusted the float more than about 1/16" or so high and you get the same over richness.

Well after reading a lot of old posts, it seems this is not an uncommon issue. Though for most the problem is the other way around - mixture too lean, temps too high, and a need to enlarge the main jet. It seems the jet is too large for my setup and the rich mixture ends up fowling the plugs.

It's also interesting that the same carb part number can show quite a range of flows and different performance results. Crude technology indeed.

For me I guess the best thing to do is just be more aggressive with leaning the mixture and fly on. Only other options seem to be exchange the carburetor (again!) or buy and install a smaller jet (probably not worth the trouble and expense).

Thanks to everyone who has posted and helped with this - appreciated!

Wanted to follow-up on this and say thanks to Finn for pointing me in the right direction.

Turns out the problem was with the air filter element. Not collapsed, but otherwise not working as it should. After installing a new one the engine has returned to running normally.

Backstory: For a long time, my engine had a small oil leak. Nothing serious - just a "nuisance leak". But the leak was on the top of the engine and over many hours of operation the oil would slowly work its way around the cylinders, to the bottom of the crankcase, and yes eventually deposit itself right on top of the air box. I now think all that motor oil, over many years, clogged and perhaps damaged the filter element until it would no longer pass enough air. Yes I did service the filter often per the K&N service kit instructions, but I think that motor oil eventually got the better of it.

The K&N filters are good quality (expensive too!) but I don't think they were designed to work in an often oily, dirty, hot place like the bottom of our engines. These are actually "automotive" filters that would normally be found on top of or away from a car engine, exposed to outside air and nothing else.

So, clean them regularity, and be sure not to over-oil them!

I hope my report might help others avoid this problem.

PS The "nuisance leak" has also been fixed.