Originally Posted by
cub builder
Let's take a methodical approach to this. You have to have compression, fuel, spark, and the spark has to be at about the right time. One of these 4 items isn't right. So let's do a quick walk through each of these requirements to diagnose your problem.
1. Do you have compression when you pull the engine through?
a. no - fix it.
b. yes - go to step 2.
YES - I CAN FEEL IT WITH MY THUMB OVER THE PLUG HOLE. COMPRESSION STROKE ALIGNS WITH THE TIMING MARKS ON THE PROP FLANGE
2. Does it spark? Pull all the top spark plugs to ensure that there is no compression available, and the engine can't possibly start. Connect the leads to each spark plug and lay them on top of the cylinder head. Turn the magnetos on and pull the engine through in the hangar with the doors closed and lights off. Does the impulse coupler snap? (if not, fix your magnetos) If you are in a relatively dark hangar, you are going to see the arc on a spark plug if it sparks when the impulse snaps.
Is there a spark?
a. No - Possible causes. Bad magneto, incorrect mag switch wiring. (Note: when the switch is in the on position, it opens the P-lead connected to it. In the off position, the switch is actually closed to ground the P-lead.)
b. Yes - Continue to step 3.
COULD NOT SEE THE SPARK IN DAYLIGHT; THE GARAGE IS TIGHT AND THE WEATHER HAS BEEN COLD; GETTING WARMER NOW. CAN'T ROTATE PROP WITH DOOR DOWN, SO WILL RETRY IN THE NEXT EVENING OR TWO JUST TO SEE IF i CAN SEE SPARKS.
I DO NOT HAVE P LEADS CONNECTED AS NOTED IN MY POST SINCE I WANTED TO ELIMINATE WIRING/SWITCH ISSUES. IMPULSE COUPLERS (2) SNAP.
3. Is the timing correct? With the spark plugs still out and you have turned the engine to where you just get a spark on a spark plug as the impulse snaps, turn the magneto switch off, and back the propeller up 1/4 turn. Put your thumb over the spark plug hole and pull the engine back forward 1/4 turn. There should be compression there. If not, your timing is off, likely 180° if you have checked it with a timing tweeter.
Is the spark timing correct?
a. No - correct he magneto timing, possibly removing and re-installing the magneto.
b. Yes - Continue to step 4.
NOW I'M CURIOUS. I PREVIOUSLY (COUPLA DAYS AGO) PULLED THE PLUGS, ENSURED I HAD TDC COMPRESSION, AND IT MATCHED THE PROP FLANGE TIMING MARKS. IMPULSE COUPLER SNAPS AT THIS POINT. WHEN I INSTALLED THE MAGS, I MADE SURE I INSERTED THE TIMING PIN IN THE LEFT HOLE IN THE MAG (MAGS SAY LEFT ROTATION ON THEIR ID PLATE), AND INSERTED THE MAG, THEN CHECKED WITH THE TIMING TWEETER. YES THE MAGS WERE INSTALLED WITH THE CAM/CRANK TIMING MARKS ON TDC.
SINCE THE ENGINE POPS WITH STARTING FLUID BUT DOESN'T SOUND LIKE ITS TRYING TO START, AND THE PLUG HARNESS ONLY INSTALLS IN ONE ORIENTATION, IS THERE SOMETHING ELSE THAT MIGHT HAVE RESULTED IN MAGS BEING 90 OR 180 OFF?
4. Are you getting fuel into the engine? You said the accelerator pump on the carb is working. Slowly open and close the throttle plate through it's full stroke several times. If the accelerator pump is spraying fuel, you will have fuel dripping out the bottom of the carb. If you don't have any fuel running out, your accelerator pump is not pumping fuel. There are two reasons why that may happen. Either your accelerator pump is not working, or more likely, you aren't getting fuel to the carb. Let's check to see if the carb is getting fuel. At the back of the MS carb, there is a pipe plug. It could be an allen socket, but is more likely a square or hex shaped pipe plug that is safety wired into place. Cut the safety wire and remove the plug. If there is fuel in the carb, you will have about a cup of fuel drain out through the plug hole when you remove the plug. If not, then you aren't getting any fuel to the carb. Check your fuel system, check the inlet filter on the carb, and check to ensure the float/needle is correct and opening.
Assuming your engine is assembled correctly, you are going to find your issue if you spend about 30 minutes and go through these steps. Your problem will be found somewhere along the way.
I DRAINED THE FUEL BOWL YESTERDAY JUST TO MAKE SURE IT HAD FUEL IN IT. UNFORTUNATELY I DIDN'T COLLECT THE FUEL. THE CARB HEAT BOX / AIR FILTER IS ON THE BOTTOM OF THE CARB SO I DIDN'T SEE FUEL DRIPPING ON THE FLOOR. I'LL REMOVE THE SCAT TUBE AND SEE IF ANYTHING IS IN THERE.
-Cub Builder