Anyone with extensive carb experience

[moetzmoet]

I have a Marvel MA-4SPA carb on my 0320 powered 6. Recently, I have experienced a problem shutting down the engine when pulling the idle mixture to cut off. The engine continues to run at very low rpm and very rough. The only way I can kill it is to shut the fuel selector to off for a couple of minutes then pull the mixture to off...and it shuts down as expected. I replaced the float/needle and seat thinking this was the problem but it continues. I have played with the idle mixture setting too to no avail.

Does anyone with extensive carb experience know what my problem may be. Also, what should the idle mixture setting be.

 

[fixnflyguy]

You are not likely getting full travel on the mixture cutoff..perhaps your cable moved, or has interference somewhere, allowing a small amount of fuel to get through. The Mixture setting can be a moving target depending on several factors, but the Marvel Schebler manual has an initial setting (I cant recall exactly) as I dont have the manual on me..I think around 1.5 turns out from closed.I highly recomend saftey wiring the knurled idle mixture even know it has a spring movement restrictor...they can work their way out and even fall out completely. Setting the throttle stop and idle mixture for smooth idle may take some adjusting, Heavy metal props will allow slightly lower idle speeds do to inertia in most cases.

 

[bhester]

I have a Marvel MA-4SPA carb on my 0320 powered 6. Recently, I have experienced a problem shutting down the engine when pulling the idle mixture to cut off. The engine continues to run at very low rpm and very rough. The only way I can kill it is to shut the fuel selector to off for a couple of minutes then pull the mixture to off...and it shuts down as expected. I replaced the float/needle and seat thinking this was the problem but it continues. I have played with the idle mixture setting too to no avail.

Does anyone with extensive carb experience know what my problem may be. Also, what should the idle mixture setting be.

I had that problem a few years back and had to replace this part, worked just as you are explaining and worked great after replacing it with a new one (top):

 

[Roadjunkie1]

I had that problem a few years back and had to replace this part, worked just as you are explaining and worked great after replacing it with a new one (top):

I had the same problem with a carburetor that had sat around for too long. The mixture control looked like it was working from the outside but it wasn't doing anything inside the carburetor. The mixture mechanism at the end of that "cable"-appearing part was stuck in its little hole and the cable was just twisting. Got everything cleaned up and a new mixture control 'thingie' and it worked fine!

 

[jselvatici]

I had that problem a few years back and had to replace this part, worked just as you are explaining and worked great after replacing it with a new one (top):

View attachment 56546 View attachment 56547

Thank you for sharing that.
Did you rebuilt your MA-4SPA? if you did, do you remember the Part Number of the kits you have used?
I saw MS has a regular kit and a "Deluxe" kit. And I think even on the Deluxe kit, the new blue float is not included, not sure.
Appreciate if you can share more information. I'm thinking to rebuild my carb, just seems there's not a lot information out there about MA-4SPA rebuild (like a step by step video).

Thanks!

 

[OKAV8r]

Are you applying the Lycoming protocol for engine shutdown? It says you operate the engine at 1000 RPM for 30 seconds before pulling the mixture to ICO.
Truth is, the mixture control is a gate valve that, even when new, will allow a small fuel flow into the main metering well. a little bit of RPM will cause an increased demand for fuel that will exceed the amount that can leak past the mixture gate valve, resulting in a clean shutdown.

 

[Freemasm]

I have a Marvel MA-4SPA carb on my 0320 powered 6. Recently, I have experienced a problem shutting down the engine when pulling the idle mixture to cut off. The engine continues to run at very low rpm and very rough. The only way I can kill it is to shut the fuel selector to off for a couple of minutes then pull the mixture to off...and it shuts down as expected. I replaced the float/needle and seat thinking this was the problem but it continues. I have played with the idle mixture setting too to no avail.

Does anyone with extensive carb experience know what my problem may be. Also, what should the idle mixture setting be.

If this is a new condition, check the tightness of the bowl/related fastener torque. A resulting leaky gasket can be just enough to let some fuel to atomize as the engine turns and result in what you describe. A similar thing happened to me many years back. IIRC, a few years later there was an AD in certified world to check for such. Won't cost anything to check. If I can find the AD, I'll post a link.

Edit = They don't mention the run-on but I'm pretty sure this is the related SB. Good luck. Hoping for simple and cheap for you.

https://msacarbs.com/pdf/SB-17_O 081210.pdf

 

[Tailwind/RVGuy]

Are you applying the Lycoming protocol for engine shutdown? It says you operate the engine at 1000 RPM for 30 seconds before pulling the mixture to ICO.
Truth is, the mixture control is a gate valve that, even when new, will allow a small fuel flow into the main metering well. a little bit of RPM will cause an increased demand for fuel that will exceed the amount that can leak past the mixture gate valve, resulting in a clean shutdown.

X2 on this. Not saying that you don't have an issue with the part described above but a good place to start and might help even if you do have a problem with that part. You should check linkage first to make sure that the idle arm goes to it's stop. I have flown airplanes that if you try to pull the mixture at idle RPM (650 or so) it will not have a clean shut off. My idle shut down method is to run up to 1400 RPM for 10-15 seconds to help clear any lead deposits and then rpm back and hold at 1000 rpm - AND THEN pull the mixture to ICO. It helps with a clean shut down.

Good luck.
Keith

 

[Tailwind/RVGuy]

X2 on this. Not saying that you don't have an issue with the part described above but a good place to start and might help even if you do have a problem with that part. You should check linkage first to make sure that the idle arm goes to it's stop. I have flown airplanes that if you try to pull the mixture at idle RPM (650 or so) it will not have a clean shut off. My idle shut down method is to run up to 1400 RPM for 10-15 seconds to help clear any lead deposits and then rpm back and hold at 1000 rpm - AND THEN pull the mixture to ICO. It helps with a clean shut down.

Good luck.
Keith

I meant make sure the mixture arm (not idle arm) goes to it's stop

 

[Ralph Inkster]

Hopefully adjusting the Lean control cable fixes your problem - but - in addition to the flexible shaft part mentioned in post #3, there is another SB addressing a leaning issue that might be affecting your carb. This one involves the slotted sleeve that is pressed into the bottom of the fuel bowl coming loose & rotating, essentially allowing the leaning valve to rotate, maybe enriching the circuit, or worse over lean or shut the circuit off!

This condition happened to me resulting on a power loss & forced landing. I had an Avstar carb installed (Marvel MA-4SPA clone) but the fault was exactly the same as described in:

https://msacarbs.com/pdf/SB-18_A%20031511.pdf

 

[petehowell]

Is the run on heat related? I run a lot of Mogas, and if it is a hot day, the fuel can boil in the carb, letting some fuel get by after mixture cut off. It does not happen on a cold day and rarely happens with 100LL. Winter Mogas on a warm spring day is the most likely situation. The engine will diesel on for a bit.

This sounds like a different situation, but I thought I would mention it.

 

[bhester]

Thank you for sharing that.
Did you rebuilt your MA-4SPA? if you did, do you remember the Part Number of the kits you have used?
I saw MS has a regular kit and a "Deluxe" kit. And I think even on the Deluxe kit, the new blue float is not included, not sure.
Appreciate if you can share more information. I'm thinking to rebuild my carb, just seems there's not a lot information out there about MA-4SPA rebuild (like a step by step video).

Thanks!

No, I did not rebuild my carb but I did replace the floats with the smurf blue floats and I replace that mixture shaft. There is a maintenance manual floating around the web for the carb with part numbers, I don't have that readily available. Remember you'll need new gaskets and locking washers.
Here is a link to my pictures in case they might help:

Carb Work

36 new items · Album by Bobby Hester

photos.app.goo.gl

 

[jselvatici]

No, I did not rebuild my carb but I did replace the floats with the smurf blue floats and I replace that mixture shaft. There is a maintenance manual floating around the web for the carb with part numbers, I don't have that readily available. Remember you'll need new gaskets and locking washers.
Here is a link to my pictures in case they might help:

Carb Work

36 new items · Album by Bobby Hester

photos.app.goo.gl

Thank you Bobby for sharing your knowledge.
I will start getting all the parts needed for a future float upgrade. Much appreciated.

 

[Ethanol1]

https://www.insightavionics.com/pdf%20files/MA-4%20Carb%20Manual.pdf

 

[bhester]

Thank you Bobby for sharing your knowledge.
I will start getting all the parts needed for a future float upgrade. Much appreciated.

If I remember correctly when I ordered the Float upgrade kit, it came with all the parts necessary to do the upgrade. I just went and looked at those pictures in the link that I sent, when you click on a picture text will pop up with a lot of information that should help you.

 

[Taltruda]

I would echo the tip to check that the mixture goes all the way to the ”ICO” stop on the carb body. As for correct idle mixture setting, go fly it to warm up everything. Then set the idle to 650 to 680, or as low as you can go and still have it smooth. Then SLOWLY pull back on the mixture knob, you should get a 50 RPM rise. If you get more than a 50 rpm rise before it drops off and dies, it’s idling too rich, turn the idle mixture screw clockwise (in) a little bit (1/4 turn) and try it again. If you don’t get a full 50 rpm rise, them turn the mixture screw out a little until you can get it. You may have to reset the idle speed as they both effect each other.