tmac48

Thanks BobH, I think I may leave the original pulse pump connected as well, can't see how it would interfere with addition of the electric pump, anyone else done it this way???
tmac46.

Captainbob

My Feedback: (4)

the diaphragm you are all talking about is in the metering section of the Walbro carb. The metering section will not allow any fuel to flow through the carb unless and until you close the choke and flip the prop. Thi s causes a partial vacuum in the carb which sucks the diaphragm down thus opening the metering needle. also, when the engine is running, a partial vacuum exists in the venture which keeps the diaphragm in and the fuel flowing.

BobH

My Feedback: (2)

The vacuum is actually in the crank case. The pulse (change in pressure) is transfered to the carb and that is what moves the inlet needle.. Both diaphragms move in concert. One moving in the opposite one moving out.. Then they change direction.during each pulse.
Works pretty well.

RichardGee

My Feedback: (156)

Guys, the Walbro carburetor pump on a Moki is not activated by crankcase pressure. Because a four stroke crankcase is vented to the atmosphere, there are no positive/negative pressure pulses... whereas a two stroke case is sealed and does generate positive and negative pressure that can be used to move the carburetor pump diaphragm.

The Moki uses a small air pump driven by the camshaft. Because the cam follower that drives the pump must remain lubricated, Moki packs the area with molybdenum grease. As the grease gets hot it liquefies and gets pushed into the air tube that connects the air pump at the front of the engine to the carburetor. When the tube becomes blocked by grease, it no longer provides reliable pressure pulses to the carburetor and the engine can be starved for fuel. This is why so many no longer use the air pump and have switched to an external electric fuel pump.

tmac48

Yes but this does not explain why when I first turn my pump on it takes for ever to push the fuel the last 40mm into the carby. Some one suggested pumping the prop to promote fuel flow but with no crank case pulse or in this instance air pump I fail to see how this would help in the flow of fuel.

tmac48

RichardGee

My Feedback: (156)

Is your pump close to the tank or the carburetor? These APS pumps are much better at "pushing fuel" than sucking it.
Have you tried applying the choke, cracked throttle, and flipping the prop to see if this then enables the fuel to travel into the carburetor?

tmac48

Tanks are high in the fuselage pump is below tanks.the question is, with no crank case pump connected to carby what is the thinking behind pumping or flipping the prop? Another question to those that have electric pumps that are having success are you disconnecting the Moki standard pump and do find that fuel gets to the carby without having to flip the prop?
tmac48.

RichardGee

My Feedback: (156)

If this is the APS pump recommended by Vogelsang, high or low isn't as important as how CLOSE it is to the fuel supply?
There is no reason to flip the prop if the goal is to make the carb "pump" - once the air line has been disconnected from the engine air pump, the carburetor no long pumps fuel.
CLOSE the choke and flip the prop. This creates a vacuum that will pull fuel in the fuel line into the carburetor.

tmac48

Thank you Richard.
tmac48.

Captainbob

My Feedback: (4)

tmac48, you still don't get it. The metering section of the Walbro has cut off all flow of fuel. It will allow fuel to flow only when the choke is closed and the prop is flipped. When the choke is closed and prop flipped , (or when the engine is running under its own power), there exists a low pressure area within the carb, the metering diaphragm is deflected by this low pressure (downward/inward) opening the metering needle and allowing fuel to flow. You can leave the pump on until the cows come home but no fuel will flow until a low pressure in the carb deflects the diaphragm which opens the needle allowing fuel to flow.
Remember, this type of carburetor was designed primarily for yard implements, weed eaters, chainsaws, leaf blowers, etc. The primary function of the metering section is to prevent fuel from leaking from the implement regardless of the position the implement is stored in. Hung on a wall or thrown in the back of a pickup or tossed on the floor, no fuel can flow to the carb until a low pressure exists in the throat of the carburetor.

tmac48

Captainbob, thanks for the clarification, I did get it but I wrongly assumed that the electric pump would push the fuel into the first side of the carby , that is what was stopping me from seeing reality---all good and thanks to all for your patience. Fuel now dripping from carby after turning prop with choke on and throttle open!!!!!!!!
tmac48

Captainbob

My Feedback: (4)

tmac48, good for you!
I,ve just-about given up on the Walbro and am in the process of adapting a two-stroke nitro carb to the engine. 13 flights on my comparf Corsair with a half-dozen engine shutdowns, one in-flight, has left me with little confidence in the factory set-up. In my opinion, the Walbro carb is way oversized for this motor (the moki 250). The engine's demand for air is much smaller than the carb was designed for thus creating a marginally small pressure differential in the carb throat. I have been running my engine with the choke 60% closed with no power loss.

RichardGee

My Feedback: (156)

So, this may ruffle some feathers but here goes... I flew my CARF P-47 with Moki 250 many times. Used Red Line 50:1 and no air baffles. Stock "top hat" ignition and stock air / fuel pump (NO auxiliary electric pump). In other words, by today's conventional wisdom, I was doing almost everything WRONG... Even though at the time, I was following Moki's instructions (with the exception of NO baffles) and using the factory engine accessories. The engine ran perfectly with only a couple exceptions, both of which can be attributed to flying in exceedingly hot weather (engine getting too hot), and the air tube to the carb pump clogging with grease and needing to be flushed out.
It never once quit, but would run slightly rough.
The engine always started using exactly the same choking and starting procedure as every other gasser I have ever run.
The Solo 4-blade prop was tuned to produce 3900 rpm on the ground and absolutely HOWLED in the air.
Was I lucky? Don't know.
But I do believe some tend to over-complicate things by trying to solve simple problems with complex solutions.

• The most common problem is that too many do not understand how to properly tune the carburetor
• Some are using inferior wiring and connectors (high resistance) which drop voltage to the ignition
• Some are getting the engine too hot and not using the proper oil or oil mix
• Some have overheated the engine so frequently that engine components no longer have the proper fit and the engine will never run evenly

I am using a fuel pump on my new 180 ONLY as an insurance policy and to eliminate the maintenance headache of cleaning the air/fuel pump tube.
Switched to Amsoil Dominator 40:1 at the behest of the Moki retailer who will honor the warranty
Added the baffle to keep the engine running cool

These engines are superbly designed and thoroughly tested to run properly right out of the box.

Captainbob

My Feedback: (4)

RichardG, I am happy you are having good luck with your motors. And I agree with you that usually simple fixes are best. However, due to my bad experience with the motor I have read every post on this forum since 2009 and have found that many have had similar problems. The Walbro type fuel pump couldn't be simpler and millions of them are in operation round the world, yet "the experts" recommend and the factory offers a substitute electric pump, why?. I have been in modeling for 60yrs. and have never heard of an application requiring an electric fuel pump.
I agree with you, these engines are superbly designed. But thoroughly tested, I don't think so. Remember the brass cam gears?, the grease in the cam compt. getting into the pump pulse line? The pushrods that fell into three pieces when they were removed? The fact that the cam lobes differ in height so that one must adjust each valve's lash on every lobe, or risk picking the lowest lobe and possibly suffering a burnt exhaust valve.

Blancr

+1 for me. My Moki 250 in Carf P47 is set up as per manufactures instructions. I follow the starting instructions and it always fires 3rd or 4th flick. I do have baffles but not convinced they are essential. Mine is tuned to 3400 rpm on the ground which unloads to about 3800 in the air - more than enough to perform all of the scale maneuvers and absolutely no risk of throwing a rod. It still sounds great. All I do is oil the rockers and every other outing clear the carb breather tube.
Bob

009

My Feedback: (1)

Craig B.

RichardGee

My Feedback: (156)

Craig, Not sure what you mean "timing the carb" ?

Jim Henley

009, has the engine been sitting for a long time? I had to rebuild the carburetor before the engine would run correctly. You also may want to richen the high side and see if that helps.

009

My Feedback: (1)

BobH

My Feedback: (2)

Sounds like you may have an air leak? If the engine runs with the choke engaged its getting O2 from some place. If you advance the throttle you lose RPM because you don't have enough air for it to run properly.
This is just a guess but some where to start looking.

Jim Henley

009 in addition to Bob H suggestion, are you sure that the throttle blade is closing all the way? You may want to check to see that the idle adjustment screw is not holding the throttle blade open.

009

My Feedback: (1)

Captainbob

My Feedback: (4)

Two years ago I bought a Moki 250 for a comp arf F4-U i was building. Initial test running on a test stand was not satisfactory. it wasn't possible to get the high speed mixture rich enough (turn the needle out three or more turns no drop in rpm). I settled for this, finished the airplane and installed the engine and test flew. However, the Moki quit frequently, once in flight, and was just very temperamental. I started reading others comments in this forum and found out that many were having similar problems. At one point V.A. came out recommending an electric fuel pump. To me, this was an admission from the factory rep. that the engine had issues. Finally, out of total frustration, I discarded the factory Walbro carb. In it's place I used an O.S. Max 8B carb I had in my "junk box" (I believe O.S. used this on their 108 motor). GUESS WHAT! All problems disappeared. I use no pump of any kind, not even muffler pressure. The throttle response is much more linear and CH temps have dropped from somewhere around 250F to less than 200F (Jeti telemetry). I've had many flights on the Corsair since making this change and I no longer have any concerns about the engine stopping in flight or any other time.

Hinckley Bill

My Feedback: (569)

Very enlightening ... quick question, was the switch to the OS carb a 'drop in' or did you have to perform some 'modification(s)' to use it?