Carburetor Question

[rmalone]

I have been running the 0-320 on my PA 22/20 getting ready for the first flight. I am noticing as I get the RPM to around 1200 the engine gets rough and then smooths out as I continue to increase the RPM. It was very pronounced when the engine was cold, even dying once, and became much less so as the engine got to temp. The engine was stored for about two and a half years before I started running it again. I have been running it on and off some for the last year and a half or so. At full power the engine was making about 2150 RPM with a 74 X 60 prop and runs smoothly. Any information would be appreciated.

 

[Troy Hamon]

I'm not the expert, they'll be along shortly...but just to clarify: Is 2150 the static run or in-flight? If that is in-flight I think you have a problem...

Never mind, I see that you are getting ready for first flight, so it must be static...

 

[Smokey]

Have you considered the magneto points? The tungsten in the points develop oxidation on them as they sit. Especially when they are touching. At low RPM the magneto is not creating as much voltage and the ignition system load is the greatest. As the engine warms the inductive reactance decreases enabling the magneto to create more voltage to overcome the corroded point. Bendix recommended pulling a business card through the points to remove the tungsten oxide.

 

[Gilbert Pierce]

A lean mixture caused by the carb as you come off idle can cause this problem.

When a carb comes off the idle jets to the power (main) jets there is sometimes a lag which results in a lean mixture.
As you advance the throttle the vacuum behind the throttle plate that is pulling fuel through the idle jets drops as the throttle plate opens and less fuel is drawn in. At the same the air flowing through the venturi increases resulting in more fuel being drawn through the main jets to the fuel nozzle. If for some reason the float level is not set exactly right (too low)there is lag in fuel flow in main jet and nozzle fuel flow which results in a rough running, usually around 1200 rpm.

The opposite can also happen if the float level is set too high. The fuel starts flowing sooner then it should from the nozzle which results in a rich mixture. Rich mixtures are less prone to rough running then lean. Also a cold engine needs more fuel, richer mixture, and is more susceptible to rough running when lean. That is why old cars have automatic chokes and the spark retards when you pop the throttle quickly.

Another cause could be the accelerator pump. This aguments the flow of fuel from the nozzle into the venturi as you advance the throttle. It squirts fuel into the carb throat. It consists of a spring loaded piston (rubber seal), check ball and orifice. Trash in the orifice or worn piston seal can cause problems with accelerator pump. Pump the throttle a couple of times without the engine running and fuel should drip out of your air box onto the ground if this is working.

When all else fails contact Bobby at http://www.aircraftcarbs.com/carbs.html there in Dallas. His help kept me from having to tear down my carb recently.

 

[longpacer]

That 2150 static rpm is worrisome -- the minimum static for an O-320 with a Sensinich prop is 2250. So if your tach is accurate (many aren't), you are dealing with two anomalies here, not just one. I would try to address the tach question to determine whether you have a static rpm issue as well as a roughness issue. The Tru Tach devices sold by Spruce and others are easy to use and accurate.

Once you know what you are dealing with, you might check stuff that's relatively cheap to investigate. Cylinder compression, mag timing, tightness of the carb to engine bolts, the screws that hold the throttle body to the carb bowl, the slop in the throttle bushings, the clamps on the intake hose connectors, the intake tube-to-engine bolts. Maybe you could get lucky on one of these items.

This is yet another situation where a multiprobe EGT/CHT would be really valuable.

I wouldn't fly it until I got it figured out. Good luck.
Bob

 

[Lownslow]

I think Gilbert presents one of the most likely scenarios.... As the throttle is advanced you are switching the functioning mode of the carb from idle mixture control to the power mode that utilizes the carb venturi and air flow to meter fuel into the manifold. Is there a chance that auto fuel has been allowed to set up in the carb that could have "gummed up" the carb's small metering jets? It would only take 3-4 months of auto fuel sitting in it cause such symptoms, whereas the mixture is becoming too lean at that throttle setting. As he also mentioned, if the accellerator pump "leathers" have dried out and have'nt had time to soak and expand in fuel then your accellerator pump that aids in this transition as the throttle is advanced may not be working as it should.

I suspect that your idle jet has some restriction, not allowing full flow of fuel through the range to where the power phase takes over. I had a similar problem with mine after auto fuel had been run through it by the previous owner. The idle jet is a very small tube and orifice and doesn't need much to plug it up. I disassembled the carb and soaked it in Chemtool to clean it up. Works perfect now.

Lou

 

[rmalone]

Thanks for the suggestions.

 

[zalto]

Have you tried adjusting the idle mixture per the Lycoming instructions? Basically you adjust the mixture screw (with the engine hot) until you get max rpm at idle, then slowly back it out to achieve a 100 rpm drop. This results in a rich idle mixture. If you can achieve this, the idle circuit should be clear. I have read that if the idle mixture isn't adjusted rich in this manner that you can have a "flat spot (lean)" when advancing the throttle from idle.

I would also try and see what difference leaning the engine at WOT and see if it affects static rpm.

Dusty

 

[gliderman]

Complete agreement
just had this very problem: upon closer inspection the accelerator pump was a mass of rust and the "Leathers" were toast. A good cleaning and a new accelerator pump fixed the problem.
I know that i'll get some static for this but: I think that contaminated auto fuel ( water)had a great deal to do with this problem judging by the condition of the pump. this is the lowest part of the carb - great place for water to collect
Shane

I think Gilbert presents one of the most likely scenarios.... As the throttle is advanced you are switching the functioning mode of the carb from idle mixture control to the power mode that utilizes the carb venturi and air flow to meter fuel into the manifold. Is there a chance that auto fuel has been allowed to set up in the carb that could have "gummed up" the carb's small metering jets? It would only take 3-4 months of auto fuel sitting in it cause such symptoms, whereas the mixture is becoming too lean at that throttle setting. As he also mentioned, if the accellerator pump "leathers" have dried out and have'nt had time to soak and expand in fuel then your accellerator pump that aids in this transition as the throttle is advanced may not be working as it should.

I suspect that your idle jet has some restriction, not allowing full flow of fuel through the range to where the power phase takes over. I had a similar problem with mine after auto fuel had been run through it by the previous owner. The idle jet is a very small tube and orifice and doesn't need much to plug it up. I disassembled the carb and soaked it in Chemtool to clean it up. Works perfect now.

Lou

 

[rexandkim]

I told Rex that I wouldn't fly in it until the carb was overhauled - last overhaul was 1995. I'm a worry wart. It's on it's way to get overhauled.

 

[Lownslow]

Good luck, Rex. Would like to know what you findis the culprit.

Lou

 

[P.o.P.]

Couple things:

1. Have you ASSURED the the Throttle is hitting the stop on the carb?

2. How does it accelerate with Carb Heat ON?

Carb Heat will RICHEN the Mixture.

Generally I associate lack of acceleration with a LEAN mixture.

3. Idle Mixture should be set a little rich so the fuel is already there when the Throttle is opened.

You can always manually lean during taxi.

With the engine warmed up you can check as follows:

1. RPM - 800

2. Note RPM & SLOWLY move Mixture Control to LEAN.

There should be a rise of about 25 RPM before it shuts off.

NO RISE + LEAN mixture

More than 25 RPM rise = RICH mixture