Hummel engine runs on only one cylinder

[CHARLES DEBOER]

I need some help.  I have a full case 1/2 VW hummel engine that fires on only one cylinder.  The engine has only 22 hours total.  I have 145 psi pressure in both cylinders.  I have spark at both plugs.  Plugs are gaped to spec.  The valves are set to spec.  I assume the #2 cylinder has gas because it was running for at lease 30 minutes before it quit.  I have gas in the tank.  It will start and run on the number one cylinder alone.

[Dave Stroud]

To be clear, did you ever have it firing on both cylinders ? You say it's got 22 hours on it...was that by you and all of a sudden it quit firing on one cylinder ?  What type of ignition system do you have ?

[CHARLES DEBOER]

To be clear, did you ever have it firing on both cylinders ? You say it's got 22 hours on it...was that by you and all of a sudden it quit firing on one cylinder ?  What type of ignition system do you have ?

The engine has a magneto supplied by Hummel.  It has been running on both cylinders and yes it quit firing on one cylinder all of a sudden.  It also has single ignition and dual mukuni VM28 mm carbs.  I have owned it since it was new from Hummel.

[dalemellen]

I have had that happen to me also, My engine would only develop 1600 rpm on one cylinder. When I checked for spark one cylinder was kind of hit and miss , turned out to be a bad spark plug wire you might want to start there.

[s johnson]

I was thinking it could be a stuck valve  but you mentioned it has good compression on both cyl.  Have you tried to prime the cyl that doesn't fire? Maybe take the air filter off, put you hand over the opening and pull the prop over to suck fuel and then try it. Just shooting in the dark here.

Scott J.

[stevejahr]

No running says go back to the basics:

Do you have suck?  Does it pull air in through carb?
Do you have squeeze?  Sounds like you do with compression reading.
Do you have bang?  Sounds like yes/no in that you have spark.
Do you have blow?  Does it push air out the exhaust?

Those handle the basic mechanical air pump stuff.

Put another way it takes compression, fuel, and spark.  You have checked two and found good so must be the other one.

I might swap carbs and see if the problem moves.  If it does you have a fuel/carb problem.  Could also potentially check this out using the sparkplug wet or dry test.  Could also check if fuel is in float bowl of dead cylinder's carb.  I presume the cylinder is dead at all RPMs and throttle levels?

Long shot but that spark also has to occur at the right time.  Is it possible to have different timing on one cylinder?  I might guess not.

[CHARLES DEBOER]

Back to basics was the key.  
Swapped plug wires but still only ran on the #1 cylinder.
I pulled the float bowl from the #2 cylinder carb furthest down the line from the tank and it was almost dry.
My gas tank vent is not enough to relieve all of the vacuum in the tank.  Barely enough gas was being released from the tank to feed only one carb, #1 cylinder which is closest to the tank.  
Also, the gas level was just low enough in the tank that low head pressure reduced the fuel flow.

[Dave Stroud]

Glad it was something simple, Charles. 

Don't forget that when doing fuel flow tests before first flight, the system should be able to flow 150% of the engines requirement at full throttle when the airplane is at a 15% angle nose up and nose down.