what happens

Tom-D

Tom-D

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Tom-D
when you stroke the throttle on a MA3-SPA carburetor.


I installed this new carb today, after the conversation we had on priming Thought you might like to see how long it requires to have the fuel fall down to the air/box.

And remember the primer nozzle on a 0-200 is about 2" above the carb.
 
Ummmm, purple? :dunno:

I've never fully understood rhetorical question day.
 
Tom-D said:
when you stroke the throttle on a MA3-SPA carburetor.


I installed this new carb today, after the conversation we had on priming Thought you might like to see how long it requires to have the fuel fall down to the air/box.

And remember the primer nozzle on a 0-200 is about 2" above the carb.
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Yeah but, a backfire and burned hand would have really driven the message home.
 
GlennAB1 said:
Yeah but, a backfire and burned hand would have really driven the message home.
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Not likely when the engine isn't cranking and the mags off.
first step in any maintenance, is make the aircraft safe for maintenance.
 
Timbeck2 said:
Ummmm, purple? :dunno:

I've never fully understood rhetorical question day.
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Rhetorical?? no just allowing those who disbelieve what it really looks like. real time.
 
Thanks, Tom.
 
Really need to know your airplane as some do really well using this method and others its a great way to get a fire started.

I've had a carb fire on Cessna 150, thank god it was minimal damage, dad though I burned the plane to the ground when I told him.
 
Greg Bockelman said:
Wow. That is a LOT of fuel per stroke. Is that normal?
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exact same engine and carb that you have on 34V. and yes that is what the accelerator pump delivers each stroke, on all models of the MA3-SPA and remember the primer pump is twice as large (CC wise) as the accelerator pump. and it delivers fuel 6" up stream of the carb in the left "Y" pipe of the 0-300-D in your aircraft.
 
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Here is a look at the position of the accelerator pump nozzle, note it is below the throttle plate.It is the smaller of the two, simply a #80 drilled orifice tube
 

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Tom-D said:
exact same engine and carb that you have on 34V. and yes that is what the accelerator pump delivers each stroke, on all models of the MA3-SPA and remember the primer pump is twice as large (CC wise) as the accelerator pump. and it delivers fuel 6" up stream of the carb in the left "Y" pipe of the 0-300-D in your aircraft.
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The accelerator pump is a function of the throttle is it not? Am I watching the effect of the primer as it's pulled and pushed out of the panel, or is this from strokes of the throttle?
 
iflyvfr said:
The accelerator pump is a function of the throttle is it not? Am I watching the effect of the primer as it's pulled and pushed out of the panel, or is this from strokes of the throttle?
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If you are speaking of the video, I was moving the throttle arm. not the primer.
 
I have had just this happen in an arrow that HATED to start cold. Someone ended up coming running across the ramp making a cut signal because there was fuel coming out of the cowl. And before the peanut gallery chimes in, I was following the checklist.
 
(I was following the checklist.)
That's your problem. Most starting procedures I have seen call for a massive amount of prime and too much throttle. It is more important WHEN to prime (just as or after you hit the starter). That way the prime shot goes up into the intake manifold and flashes, rather than run out the bottom soaking the air intake etc. It takes three hands but really worked for me using just a nearly-closed throttle.
 
nrpetersen said:
(I was following the checklist.)
That's your problem. Most starting procedures I have seen call for a massive amount of prime and too much throttle. It is more important WHEN to prime (just as or after you hit the starter). That way the prime shot goes up into the intake manifold and flashes, rather than run out the bottom soaking the air intake etc. It takes three hands but really worked for me using just a nearly-closed throttle.
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OMG ... some one advocates the check list might be wrong. Ya think??
 
I always thought that the accel pump sprays fuel as mist into the TB, especially hopefully above the butterfly. I guess I was wrong. :eek:
So why does the accel pump nozzle not have a fine jet to spray the fuel instead of streaming it in? It would help atomize the fuel, especially with the engine not running.

For the record, I never liked priming carbs by pumping throttle for this reason. But I had an actual primer pump so I did not have to use the throttle. (lucky me)

P.S.: Thank you, Tom, for the nice video. MANY pilots will hopefully get to see it.
 
If my engine is warm at all, 2 pumps of the throttle and she'll start right up. If she's cold, 2-3 hits with the primer followed by ignition works every time. I tried messing with that procedure and I had trouble starting. I did take away from POA that I need to hit the starter immediately after prime. I love videos like Tom's but I'm not very mechanical so any/lots of context is appreciated.
 
Actually you need to hit the starter before you prime with the throttle. This way the cranking engine sucks the fuel up into the cylinders. If you stroke the throttle then crank you already have fuel running into the air box.
 
Vaflier said:
Actually you need to hit the starter before you prime with the throttle. This way the cranking engine sucks the fuel up into the cylinders. If you stroke the throttle then crank you already have fuel running into the air box.
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Your Primmer pump dumps raw fuel into the same system.
 
BigBadLou said:
I always thought that the accel pump sprays fuel as mist into the TB, especially hopefully above the butterfly. I guess I was wrong. :eek:
So why does the accel pump nozzle not have a fine jet to spray the fuel instead of streaming it in? It would help atomize the fuel, especially with the engine not running.

For the record, I never liked priming carbs by pumping throttle for this reason. But I had an actual primer pump so I did not have to use the throttle. (lucky me)

P.S.: Thank you, Tom, for the nice video. MANY pilots will hopefully get to see it.
Click to expand...


Mainly because the function of the accelerator pump is not to prime the engine. It is only there to provide a burst of fuel to prevent an overly lean mixture when you quickly increase the throttle. Therefore, you don't really want that fine of a mist, you want the fuel there NOW. Think about what happens in some C-150's when you hit the gas for a go-around and the Carb is a vanilla MA-3. You get an initial stumble before the the fuel flow catches up to the air flow. In addition, a lot of carbs also run richer by use of an auto-enriching, (or economizer valve) for extra cooling at high MP/Throttle settings
 
zodiacflyguy said:
Mainly because the function of the accelerator pump is not to prime the engine. It is only there to provide a burst of fuel to prevent an overly lean mixture when you quickly increase the throttle. Therefore, you don't really want that fine of a mist, you want the fuel there NOW. Think about what happens in some C-150's when you hit the gas for a go-around and the Carb is a vanilla MA-3. You get an initial stumble before the the fuel flow catches up to the air flow. In addition, a lot of carbs also run richer by use of an auto-enriching, (or economizer valve) for extra cooling at high MP/Throttle settings
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When you get a stumble when advancing the throttle on a C-150- you need to change the accelerator pump setting. there are three holes in the arm that will change the effective stroke of the pump. most run in the middle hole some need to be in the hole that give the longer stroke.
the Primer nozzle on the intake spider is about 1 inch above the throttle plate. IOWs the primer places fuel in above the throttle plate and the accelerator pump puts it in below the throttle plate.
Bottom line here... no air flow, the fuel will drop thru the carb and into the airbox from either input.
 
Tom-D said:
When you get a stumble when advancing the throttle on a C-150- you need to change the accelerator pump setting. there are three holes in the arm that will change the effective stroke of the pump. most run in the middle hole some need to be in the hole that give the longer stroke.
the Primer nozzle on the intake spider is about 1 inch above the throttle plate. IOWs the primer places fuel in above the throttle plate and the accelerator pump puts it in below the throttle plate.
Bottom line here... no air flow, the fuel will drop thru the carb and into the airbox from either input.
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Yes, and I have done that, but when I looked at this carb, it was as I remember an MA-3, or MA-3A with no accel pump. (Old straight tail C-150)
 
zodiacflyguy said:
Yes, and I have done that, but when I looked at this carb, it was as I remember an MA-3, or MA-3A with no accel pump. (Old straight tail C-150)
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All the MA3-'s had accelerator pumps. I've never seen 0-200 that had a tillotson carburetor, with out a pump.
A-65 thru C-90s did . the C-140 came from Cessna with a tillotson carburetor, but most have been upgraded to a MA3-SPA
 
Tom-D said:
When you get a stumble when advancing the throttle on a C-150- you need to change the accelerator pump setting. there are three holes in the arm that will change the effective stroke of the pump. most run in the middle hole some need to be in the hole that give the longer stroke.
the Primer nozzle on the intake spider is about 1 inch above the throttle plate. IOWs the primer places fuel in above the throttle plate and the accelerator pump puts it in below the throttle plate.
Bottom line here... no air flow, the fuel will drop thru the carb and into the airbox from either input.
Click to expand...

My 150M has the stumble, nobody (and I've tried 4 or 5) has been able to fix it. If you think you can, I'll fly over there and pay you what you want (let me know where you are and how much you want for it). When would you have time to do it?
 
Sounds like the common problem that occurs after the two piece venturi is replaced with the one piece. The fix is to install a different nozzle that atomizes the fuel better during the transition from the idle to the main fuel delivery circuit.

Remove your carb and send it to a proper repair station. Send a good explanation of the problem with it. They should be able to go through it and set it up. If it has been monkeyed with in the field and/or has a lot of time on it since new or last overhaul, I am willing to bet it is due for some expert TLC.
http://www.qaa.com/aircraft-carburetors-and-fuel-servos

Also ... https://www.pilotsofamerica.com/community/threads/rough-running-150.28123/. Sound familiar?
 
mtuomi said:
My 150M has the stumble, nobody (and I've tried 4 or 5) has been able to fix it. If you think you can, I'll fly over there and pay you what you want (let me know where you are and how much you want for it). When would you have time to do it?
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Look up the Zip code 98277,, when you land at OKH call me.

take a picture of the left side of the carb, post it so we can see where the accelerator pump is set.

it requires 2 four cent cotter keys, and about 5 minutes to make the adjustment.

Which Venturi do you have installed?

also take a picture of the data tag on the carb for me.
 
JAWS said:
Sounds like the common problem that occurs after the two piece venturi is replaced with the one piece. The fix is to install a different nozzle that atomizes the fuel better during the transition from the idle to the main fuel delivery circuit.

Remove your carb and send it to a proper repair station. Send a good explanation of the problem with it. They should be able to go through it and set it up. If it has been monkeyed with in the field and/or has a lot of time on it since new or last overhaul, I am willing to bet it is due for some expert TLC.
http://www.qaa.com/aircraft-carburetors-and-fuel-servos

Also ... https://www.pilotsofamerica.com/community/threads/rough-running-150.28123/. Sound familiar?
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Read all the AD written on this carb. Doing what you said never fixes the problem.

Precision air-motive no longer holds the production certificate for the Marvel Schebler carbs, Kelly now does and are the ones that were making the parts for Precision air-motive.
Every carb that I have replaced with a Kelley carb runs 1 gallon per hour richer than the old 2 piece Venturi, and the old fuel nozzle.

And the fowl plugs.
 
Doesn't the venturi AD allow for you to revert to the 2 piece venturi, if the one piece proves to be detrimental to proper operation?
But it then needs inspection periodically. (been a while since I read that AD)
 
bluerooster said:
Doesn't the venturi AD allow for you to revert to the 2 piece venturi, if the one piece proves to be detrimental to proper operation?
But it then needs inspection periodically. (been a while since I read that AD)
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This is true, but finding a two piece is a problem. by the time most figure out they need the two piece the garbage has gone out.
 
I helped a guy with his 150 who had a very similar problem. Carb needed a proper oh. Fixed and flying. YMMV as usual.
 
JAWS said:
I helped a guy with his 150 who had a very similar problem. Carb needed a proper oh. Fixed and flying. YMMV as usual.
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Describe " proper overhaul" the only moving part that wears is the throttle plate and the bushings it rides in.
 
JAWS said:
Sounds like the common problem that occurs after the two piece venturi is replaced with the one piece. The fix is to install a different nozzle that atomizes the fuel better during the transition from the idle to the main fuel delivery circuit.

Remove your carb and send it to a proper repair station. Send a good explanation of the problem with it. They should be able to go through it and set it up. If it has been monkeyed with in the field and/or has a lot of time on it since new or last overhaul, I am willing to bet it is due for some expert TLC.
http://www.qaa.com/aircraft-carburetors-and-fuel-servos

Also ... https://www.pilotsofamerica.com/community/threads/rough-running-150.28123/. Sound familiar?
Click to expand...
The one-piece venturi does not cause stumble on acceleration. It causes roughness at some power settings. Stumble at acceleration is either a bad accelerator pump or, more likely, the idle mixture is set too lean. Adjusting the mixture richer to get a 50 RPM or so rise when the mixture is pulled to idle cutoff at idle speed will often stop stumbling. Weak spark is also a common cause for such behavior, as I keep pointing out: ignition troubles can cause a wide range of symptoms that are most often blamed on the fuel system, and many dollars are spent chasing fuel problems while the ignition gets away with its laziness. After all, why do we have two ignition systems and one fuel system? Because mags and plugs make WAY more trouble than carbs and gasoline and so on.
 
You might be able to "fix" it temporarily by adjusting the "throw," but, if it had worked fine but now has a problem, that adjustment is just masking the problem.
 
GlennAB1 said:
How about the accelerator pump components? The most likely culprit.
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Yeah, OK, it moves and could cause this stumble. it's an easy check..pump the throttle,, see what comes out the bottom.
 
how do we know it work prior?
 
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