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New carb runs rich, no idle cutoff.

7

71459

Well Known Member
New carb runs too rich.

Hello VAF brain trust. I have an O-320 powered RV-4 that has what appears to be a fuel-related problem, but I cannot for the life of me understand what's going on.

My engine started running rough a few months ago. In my attempts to find the source of the problem I've changed out the carburetor, the mechanical fuel pump, the plugs, and the harness, with brand-new or 0-time overhauled parts. It still wont run right!

The engine starts up OK, but as the throttle is advanced it loads up with excess fuel, running rough and very rich, pumping black smoke out the exhaust. The engine will not even shut down with the mixture at idle cutoff - it simply continues to run, idling away.

After shutdown (via the mags), fuel drips out of the carburetor throat (note this fuel is not coming out of the carburetor overflow vent, but the carburetor throat itself!). I've checked the float level with the carburetor on the aircraft many times - it always settles exactly where it should.

Reading VAF forums, I though it might be related to the mechanical pump, possibly over-pressurizing the carburetor. So I by-passed the mechanical pump and ran the engine on the electric boost pump. No change.

I'm at my wits end trying figure this out. The only thing I haven't changed out are the mags. Mags only have about 300 hours. Could faulty mags somehow cause this? Should I purchase ANOTHER new carburetor? What else can I look at?

Myself, and everyone else at my airport are stumped.

Thanks in advance for any suggestions..
 
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So are you saying that you have Fuel dripping from the new carburetor,,??
Or was the drip from the old,,
 
N184DA said:
So are you saying that you have Fuel dripping from the new carburetor,,??
Or was the drip from the old,,
Click to expand...

It began running badly on the old carb, although at this point it's not certain that the carburetor has ever been the source of the problem.

On the new carb, it runs worse.. yes with fuel running out of the new carb after shutdown, which has to be performed via the mags because at idle cut off, the engine keeps running. Crazy..
 
snopercod said:
Have you verified that your mixture linkage is properly set?
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I've verified that the mixture is free to swing from stop-to-stop at the carburetor end. But I haven't actually verified that action by using the mixture control in the cockpit. I'll check that, but even if it has come out of adjustment, I can't explain the super-rich, rough running.
 
rocketbob said:
What dash # is the carb? Sounds like you have the wrong jet, and the carb you replaced had the wrong jet.
Click to expand...

The carb is a 10-5009. Thing is this is exactly the same carb model that I had before, which worked perfectly for 15 years! It's also the "factory" carburetor for my engine, an O-320-E2G.
 
Any chance the plug wires were run to the wrong cylinders, have you checked your mag timing? At the time the issue appeared was any work done just prior?

Bill
 
71459 said:
The carb is a 10-5009. Thing is this is exactly the same carb model that I had before, which worked perfectly for 15 years! It's also the "factory" carburetor for my engine, an O-320-E2G.
Click to expand...

10-5217 or 10-3678-32. Works way better than the 10-5009.
 
William said:
Any chance the plug wires were run to the wrong cylinders, have you checked your mag timing? At the time the issue appeared was any work done just prior?

Bill
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I installed a brand-new harness. I carefully re-timed the mags when I did that installation. I checked the harness labels as I went - I believe it is all installed and set up correctly. The engine starts and idles OK - it's just when the throttle is advanced that it "loads up". The mags are still suspect, but I don't know if faulty mags would cause this.
 
rocketbob said:
10-5217 or 10-3678-32. Works way better than the 10-5009.
Click to expand...

Maybe my old one had a different jet - I can't say for sure.

But what's strange is, even with the "wrong" jet, why would the engine keep running at idle cut-off?

It's almost as though the mixture control inside the carb is not right. I'm just guessing at this point.
 
Was the plug wires installed prior to the issue? When you retimed the mags did you make sure to rotate the prop past the timing degree to get the impulse cupling to snap before setting timing? On the magnetos if the internal points setup changes do to cam ware or the screw loosening weird things can happen as well but not likely for both to go out at the same time. It sounds to me like your timing is way off. You?ve confirmed you got the fuel and the air so the last thing is the spark.
 
71459 said:
Maybe my old one had a different jet - I can't say for sure.

But what's strange is, even with the "wrong" jet, why would the engine keep running at idle cut-off?

It's almost as though the mixture control inside the carb is not right. I'm just guessing at this point.
Click to expand...

I had a similar problem with my Warrior, in which the engine would keep running at idle cut off (quite roughly). The problem there was that the two halves of the carburetor had become loose from each other, and allowed excess air to be drafted in through the gap. How this caused the engine to keep running is unclear, but the increased mass flow of air may have induced more fuel flow than expected at the idle cutoff setting.

If you have replaced the carb with a known good unit, perhaps that is not the problem, but an induction air leak downstream of the carb could possibly be causing a similar problem to what I had. I am not familiar with your engine type but could there be a damaged intake tube or gasket?

Are you seeing any anomalies in manifold pressure when this occurs?
 
William said:
Was the plug wires installed prior to the issue? When you retimed the mags did you make sure to rotate the prop past the timing degree to get the impulse cupling to snap before setting timing? On the magnetos if the internal points setup changes do to cam ware or the screw loosening weird things can happen as well but not likely for both to go out at the same time. It sounds to me like your timing is way off. You?ve confirmed you got the fuel and the air so the last thing is the spark.
Click to expand...

Well the mags were both completely off, so I did a "fresh" re-install using the "L" pin hole on the back of the mag, ie, without the harness on and timed them that way. The engine starts right up, but now you have me wondering if the impulse coupling is not disengaging properly. Hmmmm

The problem was the same using the old harness and without touching the timing. I think the timing is good, but I wonder if the mags are damaged internally. On the other hand, what's making the engine continue to run with the mixture at idle cut off? Confusing..
 
1001001 said:
I had a similar problem with my Warrior, in which the engine would keep running at idle cut off (quite roughly). The problem there was that the two halves of the carburetor had become loose from each other, and allowed excess air to be drafted in through the gap. How this caused the engine to keep running is unclear, but the increased mass flow of air may have induced more fuel flow than expected at the idle cutoff setting.

If you have replaced the carb with a known good unit, perhaps that is not the problem, but an induction air leak downstream of the carb could possibly be causing a similar problem to what I had. I am not familiar with your engine type but could there be a damaged intake tube or gasket?

Are you seeing any anomalies in manifold pressure when this occurs?
Click to expand...

That's interesting - so idle cut off is not necessarily 100% complete fuel cut off.

I don't have a manifold pressure gauge in my airplane so I can't say about that.

I have not disassembled and examined the intake manifold. From the outside, it looks perfect - like new.
 
Improper assembly?

When a carburetor is assembled, and the bottom half is bolted to the top half, it is easy to make the mistake of not ensuring that the mixture control from the top half gets properly inserted into the mixture receptacle in the bottom half, or the bowl.

Your symptoms sound very similar to problems caused by that improper assembly. The only way to check it is to take the bowl off the carb, and see where the mixture control is located.

The mixture control rod that sticks down is flexible, and it's easy to miss getting the rod into the receptacle.
 
Here's a photo of the mixture metering valve (that half-cylindrical thing on the springy shaft, just to the right of the blue float)

4weIWE.jpg
 
PCHunt said:
it's easy to miss getting the rod into the receptacle.
Click to expand...

I did have the carb apart (several times). Could I have botched assembling it and not gotten the mixture value in its seat? I recall it's pretty much a blind operation, guiding the valve down into place. I'll take it apart and double check it.
 
Some progress made..

So, yes indeed I did have the mixture valve off-center (slaps forehead). That explains the erratic run and inability to shut down. Thanks for the suggestion to check that.

However.. it still doesn't run correctly, and I'm now convinced that the main discharge nozzle is wrong for this installation. I had the exact same carb installed for many years and it ran great, but I'm suspicious that the jet in that one had been changed out.

This "stock" 10-5009 carb runs way too rich. It will start fine, but when throttling up it bogs down, overloaded with fuel. If I bring the mixture back, the engine smooths out and runs perfectly.

There are a lot of posts on VAF about drilling the jet up larger. It seems I need a smaller one. Does anyone here know which jet I should use? Is there one that has proven to work well in these engines in the RV installation (mine is the O-320-E2G, 150HP).

Thank You
 
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71459 said:
It will start fine, but when throttling up it bogs down, overloaded with fuel. If I bring the mixture back, the engine smooths out and runs perfectly.


Thank You
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excess fuel delivery (beyond that delivered through the main jet) during throttle application is managed by accelerator pump and is adjustable via the connection between the rod and lever arm. If your over rich condition is only present during throttle advancement, I would start there. There should be 2 or 3 different holes in the arm to adjust the accell pump delivery.

Larry
 
lr172 said:
excess fuel delivery (beyond that delivered through the main jet) during throttle application is managed by accelerator pump and is adjustable via the connection between the rod and lever arm. If your over rich condition is only present during throttle advancement, I would start there. There should be 2 or 3 different holes in the arm to adjust the accell pump delivery.

Larry
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It runs too rich after throttle up (not during throttle movement). Once power is set, for example at 1500 RPM, I can move the mixture control back and it smooths out.

I wonder if having the filtered air box on would make a difference? All these test runs have been with it off the engine.

But I suspect I need a different, smaller jet. But how would one go about choosing which one to install?
 
You may have hit on it!

I had trouble getting my engine to run correctly, and the issue was just what you mentioned: The air filter and box were off the engine. That must somehow disturb the airflow over the carburetor intake, is my guess. As you add power, the prop pushes the airflow past the carb throat, maybe causing turbulence, and not enough air goes into the carb, maybe??

An easy fix if that's the problem! Put the air filter box with filter installed on the carb, and try again.

Hope it works! :)
 
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I have had a similar problem with a C-185, would exhibit all of your symptoms when running with the lower cowlings off, once cowlings installed-back to normal. The airflow over the open intake was disrupting normal airflow to the engine intake.
 
71459 said:
It runs too rich after throttle up (not during throttle movement). Once power is set, for example at 1500 RPM, I can move the mixture control back and it smooths out.

I wonder if having the filtered air box on would make a difference? All these test runs have been with it off the engine.

But I suspect I need a different, smaller jet. But how would one go about choosing which one to install?
Click to expand...

It is unlikely that any stock jet for a properly sized carb is going to be excessively rich at 1500 RPM, at least to the point of rough running. I would look at the float adjustment. Too high of a fuel level in the bowl will provide an overly rich condition across the RPM range. Wouldn't be overly surprising to see a new carb show up with an improperly adjusted float. Turbulence at the carb inlet can cause pressure problems in the main metering circuit and impact fuel flow (flow is metered, in part, by the difference between static pressure at the entrance to the carb throat and pressure inside the venturi. However, the air filter should prevent any significant turbulence.
 
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Resolved

Installing the air box and filter has solved the problem. Engine runs perfectly.

I can't believe how much time I spent trying to solve this. I never suspected what I found as the cause. It was right there in front of me the whole time. This is one for the notebooks, that's for sure.

Thank you to everyone who offered suggestions and insights - all of your contributions are greatly appreciated!
 
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