Carb MA 4SPA - Need advise please

AJ85WA · #1 09-16-2015, 06:30 PM

Hi Guys

I have an o-320 with ma 4spa carb on it. (2x slick mags 25 before TDC)

In the last couple of months I have been troubleshooting my RV6 having issues starting first thing, temperature is not cold 10-20 degrees C

I have narrowed it down to my carb fuel supply, possibly accelerator pump.
My reason is the following:

When I have my aux fuel pump on and pump the throttle I do not get any fuel in my air box like i usually did.

If I take a tiny bit of AVGAS drained from wings during preflight and throw it into my air intake she will start straight away on first rotation

After the first start of the day, she will have no problems starting again until left overnight again.

I am certain the issue is not related to ignition, and will be opening up the carb accelerator pump and check valves on the weekend. Have any of you guys got any tips on doing this, any parts that I need on hand that need replacing when taking it apart.

Does it sound like i am on the correct track?

Thanks in advance

aerhed · #2 09-16-2015, 07:41 PM

Check the discharge valve and also run a wire down the discharge nozzle.

AJ85WA · #3 09-17-2015, 12:04 AM

Thanks Aerhed

The only thing that confuses me is that the engine is very smooth from idle to full power (throttle pumped in fast stroke). I was under the impression that if the accelerator pump is not working that a pump of throttle to full power will cause the engine to hesitate before making full power

But definitely I am not getting any fuel in the air box with fuel pump on, pressure 5psi, and pumping throttle.

WA85 · #4 09-17-2015, 06:32 AM

The Lycoming engine equipped with a carb was designed to be started cold by using a primer system where fuel is primed directly into the cylinder via primer lines. The intent was for the fuel to be directly sent into the cylinders. Using the carb's accelerator pump to "prime" the engine can have potential issues, namely an intake / carb airbox fire. If you use the carb's accelerator pump to create a pool of fuel in the airbox, a potential backfire through the intake can ignite the fuel and if its not sucked into the engine by continued cranking, you can set the whole mess on fire. It might be recommended to check the primer system first and make sure its working. Also, suggest not running a piece of wire (most likely steel safety wire) through the accel pump discharge circuit since it can scratch the interior surface. Use a plastic tube or use a can of carb cleaner solvent with a plastic tube to blow out the circuit.

aerhed · #5 09-17-2015, 10:18 AM

The discharge nozzle has an orfice in the neighborhood of .020 (I never measured). You won't get any kind of plastic tube in there. It's a copper tube and no you don't want to gouge the inside, but I always use a guitar string for stuff like that. It's the best way to ensure the tube is clear, you don't have to be an alarmist here.

AJ85WA · #6 09-17-2015, 05:21 PM

Thanks guys, good info so far.

Lucky I actually play the guitar

got plenty of strings

I have never needed a primer in the past, she has always started 2nd blade over, so rather than installing one and masking the issue id rather resolve it.

BTW: I dont pump the throttle without turning the engine for a normal procedure

jetjok · #7 09-17-2015, 08:39 PM

Quote:

Originally Posted by WA85

The Lycoming engine equipped with a carb was designed to be started cold by using a primer system where fuel is primed directly into the cylinder via primer lines. The intent was for the fuel to be directly sent into the cylinders. Using the carb's accelerator pump to "prime" the engine can have potential issues, namely an intake / carb airbox fire. If you use the carb's accelerator pump to create a pool of fuel in the airbox, a potential backfire through the intake can ignite the fuel and if its not sucked into the engine by continued cranking, you can set the whole mess on fire. It might be recommended to check the primer system first and make sure its working. Also, suggest not running a piece of wire (most likely steel safety wire) through the accel pump discharge circuit since it can scratch the interior surface. Use a plastic tube or use a can of carb cleaner solvent with a plastic tube to blow out the circuit.

Just for clarification...The primer system on any Lycoming engine does not input the fuel into the cylinder. The nozzle is installed into the intake port, where the atomized fuel is drawn into the cylinder. If you have nozzles in more than one cylinder, it increases the chances that the fuel will find an open intake valve, which could have merits. However, even with a primer system if one is not cranking the engine, the raw fuel can gravitate down the intake tube into the sump, through the carburetor, and eventually into the air box. So, it really matters not whether one has a primer system or uses the accelerator pump. It (IMHO) is all the technique of the operator that will preclude an airbox fire. On my Pacer and RV, I will many times not use the primer, preferring to simply crank the engine, while simultaneously adding fuel using the accelerator pump. YMMV

PCHunt · #8 09-18-2015, 12:50 AM

From reading your initial post, I would agree, it sounds like it might be an accelerator pump issue.

One of the failure modes of the accelerator pump system, is that the long tube that sticks up into the throat of the carburetor can come loose, and fall out. The tube has a 90-degree bend in it, and is fastened with some sort of adhesive.

Make sure your accelerator pump tube is intact, and is actually spraying avgas upward into the carb venturi (throat) with each stroke of the throttle. If all that is good, then you likely don't have an accelerator pump issue. If you aren't getting a spray of fuel:

Rarely, and usually only after a long period of time, the seals in the actual accelerator pump can dry out and go bad.

Also, the carb overhaul instructions call out a test for the accelerator pump check valve. If it has failed, or has dirt in it, the pump won't force the fuel into the tube. Pretty rare.

Hope this helps.

AJ85WA · #9 09-19-2015, 06:35 AM

OK.... So today I took the air filter off to see what is happening.

And this is what I see

The pump discharge nozzle is missing! I then looked back into my building log and it was never there since day one.....sigh of relief!PCHunt you were spot on with your thinking!

I pumped the throttle and fuel was definitely blasting out of the little hole but going straight down. Does anybody know if I can get a replacement for this and if this is the correct part. TUBE, PUMP DISCHARGE - 229-164--F

PCHunt · #10 09-19-2015, 11:04 PM

The tube doesn't just screw in, it is bonded with some sort of special adhesive/epoxy magic stuff.

I'm not sure that this would be an appropriate field repair.

Having said that, and looking back at your posts, you stated that you don't get fuel in the air box when pumping the throttle. If true, the missing tube wouldn't explain that. The fuel should just dribble out of the hole where the tube should be and drain down into the air box.

Have you observed the bottom of the carb while someone pumps the throttle to see if fuel is coming out?

If not, "further investigation is warranted," as they say....