Carburetor questions:

[JOhnH]

I don't know the answers to these questions so I am going to swallow my pride and ask:

What is the mechanism called that adds "extra richness" when full throttle is applied. If I asked the question wrong, feel free to correct me?

Do all carbs employ this in all configurations (engines, aircraft models)?
How is it done?
How would I know if my carb is doing this?

Does that "extra richness" get over ridden if you manually lean it a little bit?

Where is the best place to read about things like this?

 

[Skylane81E]

Economizer or power enrichment valve
Not sure if everything has it
There is more than one way to do it
What carb do you have?
Yes, the mixture control is the boss

 

[JOhnH]

Economizer or power enrichment valve
Not sure if everything has it
There is more than one way to do it
What carb do you have?
Yes, the mixture control is the boss

I had a 10-3878 but I found out it was too lean for my engine (O360-A4M) in a Cessna 172n. (It is the conversion from the O-320).

I have always had issues with high CHTs, especially in climb. It would go to 430 even in a shallow climb. Then in cruise, it was almost impossible to keep the CHT below 390/400 in Cylinder #4. If we leaned much at all it shot up over 400. #1 was always good ~380. #2 is always about 20 degrees cooler than that and 3 and 4 always ran high (>390)

I was told that the 10-5193 was more suited for me because it would allow me to run richer. My A&P had one available and installed it as a loaner to see if it helped. It did help some. My climb was pretty aggressive (800-1000 fpm) and #4 "only" went up to about 410. When we settled into cruise at 4500 ft the CHT came down to under 370 in #4 and even lower in other cylinders. We leaned it and it went up again, but we could keep it at under 390 while still getting ~8 gph.

So I am just trying to increase my understanding of how carbs work.

 

[Capt. Geoffrey Thorpe]

Typically, the air flow through the venturi creates a suction that makes the whole thing work. For the main fuel path, fuel in the float bowl gets sucked through a metering orifice called a jet (main jet). Bigger holes allow more fuel so you run richer. But, of course, you want to control where you are rich and where you are lean... So, the fuel that flows through the jet is mixed with air that comes through a metering orifice called a bleed. Using a larger bleed gives you a leaner mixture - particularly at higher air flows. So you juggle the bleed and the jet to shape the curve of mixture as a function of flow. (There is also a mixing tube the gets in the mix here...). Carburetors will also have additional fuel paths such as the idle fuel circuit with their own sets of orifices. And, you can have valves that open up as a function of the throttle position, "pull over" circuits that add fuel at high air flows, accelerator pumps, etc. etc. etc. Then you get into constant vacuum carburetors, and...

There several ways to control the mixture - from a tapered needle that sticks into a jet to back suction on the float bowl and others that don't pop into my head at the moment.

Every carburetor design is different. And for any given overall design, you can juggle jets and other components to shape the air / fuel ratio quite significantly.

 

[1600vw]

The enrichment valve is called an accelerator pump. Some carbs use them but most aviation carbs don't.
How this works is like this. When you open the throttle it takes a little richer fuel mixture to get things going without a stumble. To do this they put a accelerator pump in the system. Without it you need to run a little richer to compensate for this little extra mixture you need when opening up the throttle.
Without this accelerator pump you will get a bog or stumble when you step into the throttle hard from a low setting or low rpm.
Using an Accelerator pump allows the mixture to be set leaner and not produce the stumble or bog I speak of. Without the circuit the carb MUST run richer or be set up richer.

 

[flyingron]

The enrichment valve is called an accelerator pump. Some carbs use them but most aviation carbs don't.

Umm....not a complete answer. Some aviation carbs do have accelerator pump, but as mentioned early carbs such as the MA-4 on the O-320 do enrichen the mixture when the knob throttle is moved to full open, this is distinct than the transitive squirt of fuel the accellerator gives as the throttle is announced.

 

[1600vw]

Umm....not a complete answer. Some aviation carbs do have accelerator pump, but as mentioned early carbs such as the MA-4 on the O-320 do enrichen the mixture when the knob throttle is moved to full open, this is distinct than the transitive squirt of fuel the accellerator gives as the throttle is announced.

You make me laugh.....

 

[charlie]

The enrichment valve is called an accelerator pump. Some carbs use them but most aviation carbs don't.
How this works is like this. When you open the throttle it takes a little richer fuel mixture to get things going without a stumble. To do this they put a accelerator pump in the system. Without it you need to run a little richer to compensate for this little extra mixture you need when opening up the throttle.
Without this accelerator pump you will get a bog or stumble when you step into the throttle hard from a low setting or low rpm.
Using an Accelerator pump allows the mixture to be set leaner and not produce the stumble or bog I speak of. Without the circuit the carb MUST run richer or be set up richer.[/QUOT

The enrichment valve AKA economizer is NOT the accelerator pump. Separate items and many aviation carbs have them. Those that don't are some O-235's and most lower power engines. Charlie Melot Zephyr Engines

 

[JOhnH]

The enrichment valve AKA economizer is NOT the accelerator pump. Separate items and many aviation carbs have them. Those that don't are some O-235's and most lower power engines. Charlie Melot Zephyr Engines

By the way Charlie, thanks for pointing me in the direction of the carb.

I have been asking this question and spending a lot of money over the past several years and aside from Brian also mentioning that he had engine problems until he found out he had the wrong carb, nobody was able to help. The 10-5193 hasn't completely fixed it, but it is much better. As you and many others have told me, some of these things just run hot, and some of them do it for a loooooong time.

John

 

[Skylane81E]

The enrichment valve is called an accelerator pump. Some carbs use them but most aviation carbs don't.
How this works is like this. When you open the throttle it takes a little richer fuel mixture to get things going without a stumble. To do this they put a accelerator pump in the system. Without it you need to run a little richer to compensate for this little extra mixture you need when opening up the throttle.
Without this accelerator pump you will get a bog or stumble when you step into the throttle hard from a low setting or low rpm.
Using an Accelerator pump allows the mixture to be set leaner and not produce the stumble or bog I speak of. Without the circuit the carb MUST run richer or be set up richer.

No,

Beyond that many aviation carbs have them, the MS carbs with a "P" in the model number all have them

 

[Skylane81E]

I wanted to be sure of the meathod before opening my yap,

The standard MS aircraft carb enriches the mixture at full power by restricting the air bleed.

 

[Skylane81E]

You make me laugh.....

 

[flyingron]

You make me laugh.....

That last word should be advanced. Darned auto corrector.

 

[flyingron]

And by the way, Duncan was right. Lycoming does call it the economizer.

 

[Capt. Geoffrey Thorpe]

Accelerator pumps pump a shot of fuel into the intake as the throttle is moving to compensate for fuel that ends up in a larger intake "puddle" as the manifold pressure increases. Once the throttle stops moving, no more extra fuel is provided by the accelerator pump.

When you close the throttle and the manifold pressure drops, extra fuel evaporates from the puddle and the engine runs extra rich for a moment, but it usually tolerates it well.

Just to be clear, yes. There is liquid fuel on the walls of the intake manifold. If you look at automotive intake manifolds, some even have ridges molded into the plenum to direct the flow of the liquid fuel along the manifold and overcome some of the effects of the throttle plate throwing fuel toward one end of the manifold.

Carburation is very much a black art. And, each practitioner will have his/her own personal preferences from their book of devious methods.

 

[Silvaire]

...So I am just trying to increase my understanding of how carbs work...

Basically they started with a tin full of gasoline that dripped into a manifold and over the ensuing 150 years or so continually designed and developed various improvements, fixes and work-arounds to get the thing to work better. For each and every one of these improvements the old adage "there's more than one way to skin a cat" rings true but in your case you just want to know about Marvel Schebler carbs and specifically the MA-4-5.

Here is a short article that skims some of the basics: AMT Article

For any specific type certificated engine there are only specific carburetor part numbers allowed as listed in the Type Certificate Data Sheet which, for the Lycoming O-360-A4M would be E-286 (see Note 9) In your case however, you are operating under an STC so it would be in that paperwork that you might find the allowable part numbers.

It seems like you are working with some people who are familiar with your specific configuration so I'd listen to them as well as the STC owner. The carburetor may be simple in concept but it is probably one of the most over-engineered devices on the planet just because people have had so long to fiddle with it and reading a few articles and books isn't going to be enough to give you knowledge that could trump that, which in the case of your modification, probably involved a lot of hit or miss, trial and error that went on to get to the point they are at now.

 

[SteveR]

sacskyranch was such an incredible website for stuff like this. It had absolutely invaluable information. I'd gladly pitch in $50 or $100 to get that information from John Schwaner.

 

[Capt. Geoffrey Thorpe]

Basically they started with a tin full of gasoline that dripped into a manifold

A lot of the early vehicles actually just bubbled the air through the gasoline tank - that resulted in an extra rich mixture (particularly since they heated the tank) so there was an extra air inlet to let air in to control the mixture.

One of the first (but not quite successful) aircraft had full inverted fuel - Prof. Langley used a box full of wooden balls that had been soaked in gasoline.

But, that's not really relevant to fixing the OPs CHT issue.

 

[brcase]

Basically they started with a tin full of gasoline<snip>

Reminds me of book I read about a pilot flying in Peru quite a few years ago. He was visiting a small village when a truck came driving through town. There was no hood on the truck, there was a boy sitting over the engine with a rubber hose that was siphoning fuel out of a gas can sitting on the top of the truck. The boy was pinching the fuel line to control the fuel flow into the intake manifold of the engine since there was no carburetor on the engine. They were en route the the next village which was something like 15 miles away.

Anyway it gave the pilot the idea to run a second outlet from the fuel tank in his PA14 (I think) down the door post, parallel to the primary fuel line with a needle valve in the line. It then went directly into the intake manifold of his engine. A bit of testing, pulling the mixture to idle cut off and opening the needle valve he found he could pretty easily keep the engine running pretty well. Made a very simple alternate fuel system when flying over the jungle.

Brian

 

[Dan Thomas]

You make me laugh.....

Note the two cam-driven devices in this picture, both operated by the throttle. One is the accelerator pump; the other is the enrichment valve that opens at high power settings and allows extra fuel to bypass the main jet (which is just below the diffuser) and add to the overall fuel flow.

Some auto carbs use a vacuum-operated power valve that senses manifold pressure and opens at high power settings. I don't know if any aircraft carbs have that. My old '81 Pontiac Parisienne had a mechanical enrichment valve controlled by throttle position, so I know both systems were common in automobiles. Most basic light airplanes won't have the enrichment valve and some don't have the accelerator pump either.

Dan

 

[Henning]

The enrichment valve is called an accelerator pump. Some carbs use them but most aviation carbs don't.
How this works is like this. When you open the throttle it takes a little richer fuel mixture to get things going without a stumble. To do this they put a accelerator pump in the system. Without it you need to run a little richer to compensate for this little extra mixture you need when opening up the throttle.
Without this accelerator pump you will get a bog or stumble when you step into the throttle hard from a low setting or low rpm.
Using an Accelerator pump allows the mixture to be set leaner and not produce the stumble or bog I speak of. Without the circuit the carb MUST run richer or be set up richer.

No, that enriches the fuel momentarily while the engine accelerates to a new RPM while the throttle is being moved. Some carbs also have a spring and diaphragm 'power valve' that opens a small jet when the vacuum drops below a specified level so when you are at full throttle with low vacuum it will be open putting a bit more fuel in it. Some carburetors also do this with a mechanical linkage that lifts a metering pin out of a separate jet, and some by lifting a metering pin out of the primary jet. These are not dependent on continuous movement of the throttle linkage as is an accelerator pump.

 

[1600vw]

No, that enriches the fuel momentarily while the engine accelerates to a new RPM while the throttle is being moved. Some carbs also have a spring and diaphragm 'power valve' that opens a small jet when the vacuum drops below a specified level so when you are at full throttle with low vacuum it will be open putting a bit more fuel in it. Some carburetors also do this with a mechanical linkage that lifts a metering pin out of a separate jet, and some by lifting a metering pin out of the primary jet. These are not dependent on continuous movement of the throttle linkage as is an accelerator pump.

This is correct and they all do the same thing just do it different is all.

 

[1600vw]

What I was trying to say was without this enriching circuit a carb does not perform or work correctly and the engine will have a stumble or hesitation. One needs to understand this in order to perform correct maint. on an engine.
I have seen some really weird things done to engines because this enriching circuit was either not working correctly or out of adjustment.

 

[1600vw]

Note the two cam-driven devices in this picture, both operated by the throttle. One is the accelerator pump; the other is the enrichment valve that opens at high power settings and allows extra fuel to bypass the main jet (which is just below the diffuser) and add to the overall fuel flow.

Some auto carbs use a vacuum-operated power valve that senses manifold pressure and opens at high power settings. I don't know if any aircraft carbs have that. My old '81 Pontiac Parisienne had a mechanical enrichment valve controlled by throttle position, so I know both systems were common in automobiles. Most basic light airplanes won't have the enrichment valve and some don't have the accelerator pump either.

Dan

These two systems work independent of each other. If you notice there is a small passage between the two. This is so when one opens the throttle a little the Accelerator pump will shoot a extra shot, but when one pushes harder or opens the throttle more the enriching circuit will open also.
One needs to understand this when trying to solve and engine running problem.
Some of the enriching circuits work off vacuum and if there is a problem with the vacuum they will not work either. People will mess with the carb and its a vacuum problem.
But the most messed with part when a carb is out of adjustment...the timing. Everyone wants to blame it on timing. Engine stumbles it must be timing.

 

[Dan Thomas]

But the most messed with part when a carb is out of adjustment...the timing. Everyone wants to blame it on timing. Engine stumbles it must be timing.

In reality, 90% of engine performance problems are electrical, meaning ignition. Most people will fool with the carb when there's nothing wrong with it. Carburetors make very little trouble, and in aircraft the carbs are usually much simpler than automobile carbs and are even more reliable. As long as they get clean fuel they're fine.

The typical aircraft carb has very few moving parts. No choke, no enrichment valve, many have no accelerator pump. Just a throttle shaft and a mixture valve, and some don't even have the mixture valve. There's almost nothing to adjust.

Dan

 

[Henning]

In reality, 90% of engine performance problems are electrical, meaning ignition. Most people will fool with the carb when there's nothing wrong with it. Carburetors make very little trouble, and in aircraft the carbs are usually much simpler than automobile carbs and are even more reliable. As long as they get clean fuel they're fine.

The typical aircraft carb has very few moving parts. No choke, no enrichment valve, many have no accelerator pump. Just a throttle shaft and a mixture valve, and some don't even have the mixture valve. There's almost nothing to adjust.

Dan

My favorite was always "You need a new carburetor" No, you need an $18 Jiffy Kit and maybe a $10 float. I've rebuilt more carbs on the side of the road than I can remember, takes about 20 minutes.

 

[Tom-D]

My favorite was always "You need a new carburetor" No, you need an $18 Jiffy Kit and maybe a $10 float. I've rebuilt more carbs on the side of the road than I can remember, takes about 20 minutes.

I guess you have not priced aviation carb parts lately.

Accelerator pump plunger for a MA3SPA is over $140.00

I hope your aviation carb never places you on the side of the road.

 

[Tom-D]

Dan

This is not the circuit of the MA3SPA, which is the carb on most of the light single aircraft in service today.

All the 0-200, 0-235, 0-300, 0-320, and 0-470 use the MA3 or MA4 that does not operate as this diagram shows.

 

[Henning]

I guess you have not priced aviation carb parts lately.

Accelerator pump plunger for a MA3SPA is over $140.00

I hope your aviation carb never places you on the side of the road.

I have none, if you think parts for an aviation carb are expensive, check out the price list on FI parts.... BTW, IIRC that plunger typically needs a piece of leather that can be had for very cheap and used legally as an 'Owner Produced Part'.

 

[Dan Thomas]

This is not the circuit of the MA3SPA, which is the carb on most of the light single aircraft in service today.

All the 0-200, 0-235, 0-300, 0-320, and 0-470 use the MA3 or MA4 that does not operate as this diagram shows.

I know that. Those MA carbs don't have the enrichment circuit. I think the HA-6 might but have never had one opened up. Many of the MA carbs for smaller engines don't have the accelerator pump, either, and I don't know if they even use an accelerator "well" like the Stromberg did. My old Stromberg uses a well and it doesn't miss the pump much at all.

The MA's mixture control valve controls ALL the fuel flow out of the bowl; that diragram I posted shows only idle flow being controlled, with high power flow being controlled with back suction on the bowl.

There is very little info on the 'net, particularly diagrams, of aircraft carbs. Lots of stuff on auto carbs.

Dan

 

[Silvaire]

"Roadside" PS-5C carburetor tester

 

[Tom-D]

There is very little info on the 'net, particularly diagrams, of aircraft carbs. Lots of stuff on auto carbs.

Dan

I have pictures of the MA3SPA and all the parts, as does AS&S.

1st pic shows the float bowl, and the accelerator pump bore.
2nd shows the throttle shaft and pump operating mechanism.

 

[Tom-D]

I have none, if you think parts for an aviation carb are expensive, check out the price list on FI parts.... BTW, IIRC that plunger typically needs a piece of leather that can be had for very cheap and used legally as an 'Owner Produced Part'.

Sorry no,, that plunger can not be made at home. (I've tried) to get the old leather off you destroy the shaft.

 

[Silvaire]

Checking the needle drop on a Stromberg