M20 C Engine Start problems

[Urs_Wildermuth]

Hello all,

 

I'd like to pick your brains on a problem which has been driving me slightly miffed in recent weeks.

I own my C model since 2009 and since then, usually start up was never a problem at all. Our procedure as set was always the same:

- Electrical fuel pump on, till pressure shows, then off.

- Priming 6-8 times with the throttle (cold engine) or 2-4 times (warm engine)

- Engage starter and the engine would run.

 

Since this year, start up has become a real pain. I've had to leave the airplane two times unable to start it, had to request mechanic assistance (ext power) twice more and I keep getting feedbacks from the other pilots.

The engine turns, most of the time fires shortly but when you disengage the starter,it stops. In subsequent tries,it either never ever fires again or again just once or twice.

What we found is that during the start up one has to pump vigorously with the throttle, which sometimes leads to a start.

2nd and 3rd start after the engine has run is unproblematic, even though even then pumping the throttle is required.

In two unsucessfull attempts, the engine definitly was drowned, with fuel leaking on the front tyre.

Two of the pilots report they have NO problems at all and think we are too stupid simply. Well, it's possible,but we follow the exactly same procedure (one of them filmed it) and have no success.

As I said, the previous years we never had any problem whatsoever. Maintenance has looked at it and found nothing wrong, even though they also were unable to start the engine at least once. They report, plugs and magnetoes are fine, carburettor as well.

My feel is the engine does not get enough fuel the first start, then finding a good mix for starting in following attempts is difficult.

Any idea on where to start looking would be appreciated. We have a major problem with this, as our airport has departure slots and we can't afford dicking around with the engine for hours every time we want to go flying. I also feel this is not how a standard O360 A1D should behave.

Thanks a lot.

[smwash02]

I've gone through a few rounds of this. Since the plane seems to run fine what it's going, that helps narrow it.

The C has the shower of sparks, unless it's converted. Your switch could be bad or the shower of sparks could be bad.

While they might say they mags are fine, I'd still get them a 500 hour inspection. What equipment did they have to test? If the shower of sparks is good getting it started, but when you release and it dies, the spark from the mags could be weak. Every set I've sent in at 500-800 hours needed something.

How did they test the carb? Did it receive a flow test? Does it idle properly and cut off at the right RPM rise?

And finally, induction leak. I was missing some piece of the intake gasket on a cylinder causing me to have VERY hard starts, but once it did it was smooth sailing (though I'm sure my cylinders weren't too thrilled).

[Urs_Wildermuth]

Just now, smwash02 said:

The C has the shower of sparks, unless it's converted. Your switch could be bad or the shower of sparks could be bad.

It does have the shower of sparks. I wonder if the switch is bad indeed or intermittent. Would explain why some people have this problem and others don't. Could also mean that when maintenance checks it, it won't do the evil deed.... nothing as frustrating than an intermittent fault.

5 minutes ago, smwash02 said:

If the shower of sparks is good getting it started, but when you release and it dies, the spark from the mags could be weak. Every set I've sent in at 500-800 hours needed something.

The mags have 300 hrs since a full overhaul. Once the engine is running, they work perfectly, mag check is good (it was not before we got them overhauled). From my experience as well as what the other pilots who have the problem said, it was always once that the engine had properly fired, it runs very well. It's to get it to fire the first time, which is the problem.

It also feels like there is not enough fuel to get it to fire or it runs out of fuel after that burst of firing I've been describing. The question is, what do you do next? You give 6-8 pushes on the throttle, it doesn't fire. So how many do you do now? Why does it start if you pump vigorously? Is that an indication of too few fuel available to start?

 

8 minutes ago, smwash02 said:

Does it idle properly and cut off at the right RPM rise?

Yes. No idea how they tested it but it had the annual and they knew about this and checked all they know to check. Same maintenance organisation has had this plane for over 20 years and know it well. They are as puzzled as we are. One of their people had it happen to him pre-annual, after the annual it started fine, so they assumed that whatever they had done had fixed it, but it re-occurred.

 

10 minutes ago, smwash02 said:

And finally, induction leak. I was missing some piece of the intake gasket on a cylinder causing me to have VERY hard starts, but once it did it was smooth sailing

Right, thanks for the tip. I'll have that looked at. Engine has around 300 hrs since overhaul (2011), with around 50 hrs flown p.a. since.

[smwash02]

With my plane subsequent starts were harder than initial when I had my induction leak.

It sounds like you've got a good handle on the plane's state, which is good. Given your responses I lean 70% carb, 30% switch.

When you're on the ground, put it in a low idle for a minute or so then quickly push it forward (go around simulation). Goal here is to test the accelerator pump. If it dies or struggles you could be right it's not getting enough prime, but once it's started subsequent starts don't need as much. However, it doesn't fully explain how it will run with the starter going, but dies once you let go unless it's not really started but rather just popping here and there.

Hope you figure it out. I will be curious to know what it ends up being.

[Urs_Wildermuth]

7 minutes ago, smwash02 said:

However, it doesn't fully explain how it will run with the starter going, but dies once you let go unless it's not really started but rather just popping here and there.

popping here and there is what a good description is of what it's doing.

Thanks smwash02. I'll be glad to hear more opinions and will take what I can learn here to my maintenance.

[Greg Ellis]

2 hours ago, Urs_Wildermuth said:

Hello all,

 

What we found is that during the start up one has to pump vigorously with the throttle, which sometimes leads to a start.

2nd and 3rd start after the engine has run is unproblematic, even though even then pumping the throttle is required.

 

This may be unrelated to the actual starting issue but the need to aggressively pump the throttle may be a problem with the carburetor, in particular, the carb on these engines had no other prime other than pumping the throttle a couple of times to fill the carburetor bowl.  This is an accelerator pump I believe, anyone please correct me if I am wrong.  There is a diaphragm that if it is old and or cracked, you have to do as you said and aggressively pump the throttle.  This happened to me soon after I bought my airplane about 11 years ago.  They overhauled the carb and replaced this diaphragm.  After they did that, all it takes to start is 2-3 pumps and did not matter what the temps outside were.  Now your other issues could be the starter, shower of sparks, like folks have said but I just wanted to put this out there to you since this symptom you describe that I have quoted is exactly what happened to me.

[rbridges]

maybe I'm doing it wrong, but I only prime mine twice on a cold start unless it's <40F outside.  I don't think it's the issue since it's worked fine in the previous years.

[Hank]

@Urs_Wildermuth, I had this exact problem a few years ago. Like Rob above, I generally pump the throttle 1-2 times during warm weather, and no more than 4-5 in the middle of Ohio winters. in cold weather, after building fuel pressure and pumping the throttle, I wind and set the clock and put on my headset to give the fuel time to evaporate. Then turn and push.

When she was showing her hind end, I did everything--plugs, checked timing, checked mags, sent out carb for OH . . . Turned out to be worn points in the Shower of Sparks. It's easy to access, simple to test and inexpensive to replace. Check them before messing with your ignition switch.

Let us know how it pans out for you . . . .

[yvesg]

My start procedure is different: I leave the electrical pump on while pumping manually the throttle then turn it off then start.

Yves

[Raptor05121]

1 hour ago, Greg Ellis said:

in particular, the carb on these engines had no other prime other than pumping the throttle a couple of times to fill the carburetor bowl.

 

If my understanding of the carb system is correct, the fuel pump fills the bowl, and the throttle simply engages the accelerator pump.

4 minutes ago, yvesg said:

My start procedure is different: I leave the electrical pump on while pumping manually the throttle then turn it off then start.

Yves

I doubt you'll see much difference one way or another. You won't drain the bowl with a few pumps, and the mechanical pump will fill it as soon as it fires. I turn my elec pump off just to conserve battery power for the starter.

[neilpilot]

17 minutes ago, yvesg said:

My start procedure is different: I leave the electrical pump on while pumping manually the throttle then turn it off then start.

Yves

My start procedure has me turning the electric pump off only after the engine starts.  That's the way I read the procedure on page 16 in the 1965 M20C Owner Manual.

[cliffy]

 

Shower of Sparks- there is a procedure to test the system. Find the manual on line and test as described first. 

Double check the retard points to make sure they are clean, gapped properly and timed correctly. They are timed when the regular points are timed properly. They are only on one mag. 

Has anyone ever checked the condition of the P leads and their insulation coverings? Sometimes, after years of use, the inner insulation cracks from being brittle allowing the shielding on the wire to come in contact with the inner lead wiring and ground the points. This can cause intermittent problems. 

One other item that might be a player is if the mags have been "adjusted" for timing at each annual for a few years. If they have been moved to "adjust" the timing and if they move the same way every time you might be changing the "E" gap internally in the mag lowering the spark available at low speeds of the engine. Thus if not turning fast after firing, when you release the mag switch, there is no spark available to run after the Shower of Sparks shuts down. 

 

[carusoam]

On my C... I would pressurize the fuel system with the electric pump. Pump off. Then watch the FP decline with each pump of the throttle...

The accelerator pump (listen up, Alex ) dumps heavy fuel droplets around inside the carb.  Waiting some time allows for evaporation as Hank pointed out.  The excess fuel heads for the drain and front tire.

I used to turn on the fuel pump after it fired... I also didn't want to wait for the fuel to fully evaporate, knowing the the fuel I wanted in there was trying to leave the carb...

using the fuel pressure to limit the amount of fuel is being delivered seems to be a good way to add some consistency to the procedure.

 +1 Shower of Spark maintenance make sure it is working properly. Spark timing for start is critical.

+1 Magneto OH at 500hrs. Spark strength is critical.

+1 Fine wire plugs.  Getting the mixture to wear the spark occurs is critical.

+1 cleaning out the lead balls that form in the lower plugs...

Warm weather starting of the O360 should be pretty consistent.  Once you define the right procedure that works... number of pumps of the throttle, throttle setting, time to wait

Cold weather starting for me was a graph of OAT vs. number of throttle pumps.

Old fuzzy PP memories of my M20C...

Best regards,

-a-

For Alex.... Trivia: How many pumps does an M20C have...   Answer: 8

• Electric fuel pump
• mechanical fuel pump
• oil gear pump
• Prop governor's gear pump
• flap pump
• Left brake pump (cylinder)
• Right brake pump (cylinder)
• Accelerator pump (actuated by moving the throttle in)
• Option 1: de-icer fluid pump
• Option 2: icebox AC cooler (bilge pump)

PP knowledge only, not a mechanic...

[jamesm]

May I asks few more questions ?
Do you hear buzzing from the shower of spark  when you push to start the ignition switch ?

Do you original equipment manufacture circuit breakers ? (I think they look like  red button)

The reason I ask many years ago I had my Ignition/cigar lighter circuit breaker go intermittent on me. Being that cigar lighter  had problem in the past It was tricky one for me to catch. Since this probably easy one to look at , I might start here. Ensure that you with Voltmeter  make sure that circuit breaker hasn't gone intermittent you.

What kind of starter is installed ?  Delco-Remy, Prestolite  or ???? SkyTec

have you tried Skytec starter troubleshooting  guide it may not be 100 % applicable to the starter you have installed but could lead you right direction.

http://www.skytecair.com/Troubleshooting.htm

When was the last time Magnetos timing to the engine checked ? 

When was last Magnetos overhauled ?  Is it possible that the Magneto internal  timing needs service or has slip ?

here some info on the magneto troubleshooting ...

http://www.aircraftmagnetoservice.net/magneto-troubleshooting-guide

how do the spark plugs Look ? How does the spark  plug gap look ok?   what spark plugs are you using ? Tempest  Champion ? or ??? massive or fine wire plugs?

You may want to check your spark plug resistance

http://www.qaa.com/resource-center/tempest-tech-tips/tempest-tech-tips-the-right-way-to-check-spark-plug-resistors

I don't know much about this product but it may give you good suggestions and overview about spark plug resistance(s).

Those usually first thing that come to mind.

Hope this helps,

James '67C

Edited July 13, 2017 by jamesm

[Urs_Wildermuth]

1 hour ago, jamesm said:

Do you hear buzzing from the shower of spark  when you push to start the ignition switch ?

Yes I do.

1 hour ago, jamesm said:

Do you original equipment manufacture circuit breakers ? (I think they look like  red button)

I think so.

It could well be that we have an intermittent fault as not everybody has had it. I'll definitly investigate in this direction.

1 hour ago, jamesm said:

What kind of starter is installed ?  Delco-Remy, Prestolite  or ???? SkyTec

I believe Skytec. Need to check. A new starter was installed at overhaul 300 hrs ago. Just read the troubleshooting guide, none of the problems. The starter turns the prop just fine.

1 hour ago, jamesm said:

When was the last time Magnetos timing to the engine checked ? 

When was last Magnetos overhauled ?  Is it possible that the Magneto internal  timing needs service or has slip ?

Both at engine overhaul in 2011, 300 hrs ago.

I don't know if it's possible that the magneto timing needs service, but I'll ask that question.

1 hour ago, jamesm said:

how do the spark plugs Look ? How does the spark  plug gap look ok? 

I will check if the plugs got replaced at last annual or cleaned. Normally they do that. The last time they found the spark plugs contaminated or faulty, I was told. This annual went through without any issues at all, just very basic annual (the first of it's kind in 8 years of ownership where there was no issues found).

 

The very irritating thing is that it apparently does not happen to all of us, just to me and one more pilot. This starts to massively impact my confidence in the airplane and in myself. Why can two other people start this plane on first try every time and the other guy and me have problems every time? What the hell are we doing wrong if we do everything exactly as they do? One of them even filmed his start up and he does everything the same way we've done it for years and the darn thing starts right up.

 

 

[Urs_Wildermuth]

11 hours ago, Hank said:

I had this exact problem a few years ago. Like Rob above, I generally pump the throttle 1-2 times during warm weather, and no more than 4-5 in the middle of Ohio winters. in cold weather, after building fuel pressure and pumping the throttle, I wind and set the clock and put on my headset to give the fuel time to evaporate. Then turn and push.

1-2 strokes won't do anything and never did. We always needed 6-7 cold and 3-4 warm. So I'd say there is a pre-existing problem somewhere.

11 hours ago, Hank said:

Turned out to be worn points in the Shower of Sparks. It's easy to access, simple to test and inexpensive to replace. Check them before messing with your ignition switch.

Will do, thanks.

[cliffy]

Let's talk float carbs for a moment- Back to school

On a float carb the fuel (at what ever pressure it arrives at the carb at (gravity pressure, think Cessna or Mooney, think low pressure, 6 psi, fuel pump) enters through the float controlled neddle valve and seat into the float chamber.    Once the float chamber reaches its full limit depth the needle closes off the flow of fuel (at whatever pressure it is coming in at) and the fuel in the chamber stays at atmospheric pressure from then on until enough is used in the bowl to lower the level and the needle once again opens on the seat and lets in more fuel. At no time does the float bowl  ever exceed atmospheric pressure. What pushes the fuel through the main metering jet into the throat of the carb is the difference in pressure between the float bowl at atmospheric pressure and the lowered pressure in the throat of the carb due to airflow through the narrowed throat (Bernoulli' Priciple).

Pressure from the fuel pump, in our case, will have no effect on the fuel flow through either the main metering jet or the idle circuit of the carb or the accelerator pump.  Fuel injected engines have a totally different system of fuel delivery to the cylinders. 

If the engine sits for a while the level of fuel in the bowl can evaporate and lower. By using the fuel pump before start we make sure the bowl is full and that the best conditions exist to draw fuel through the idle and main metering circuits when we start the engine. 

[jamesm]

15 hours ago, Urs_Wildermuth said:

 

Both at engine overhaul in 2011, 300 hrs ago.

I don't know if it's possible that the magneto timing needs service, but I'll ask that question.

I will check if the plugs got replaced at last annual or cleaned. Normally they do that. The last time they found the spark plugs contaminated or faulty, I was told. This annual went through without any issues at all, just very basic annual (the first of it's kind in 8 years of ownership where there was no issues found).

 

The very irritating thing is that it apparently does not happen to all of us, just to me and one more pilot. This starts to massively impact my confidence in the airplane and in myself. Why can two other people start this plane on first try every time and the other guy and me have problems every time? What the hell are we doing wrong if we do everything exactly as they do? One of them even filmed his start up and he does everything the same way we've done it for years and the darn thing starts right up.

 

Don't beat up  yourself too bad....  some times this is how the intermittent problems show themselves.  I Recall many  years ago,   my  Dad's first Mooney ('62 C).   He and his airplane partner kept it at a busy controlled  airport. When my  Dad would use the airplane  the com radio on  transmit  was intermittent.  When his partner used the airplane always worked fine until one day .... I can't  remember if  they both flying the palne together or the intermittent com transmit  problem manifested  itself.  The ground plane for com antenna was corroded.  The problem wasn't so evident at first until it  became a   frequently occurring problem.

The skytec starter ( i believe mine is the NL -149 model)   has given me  confidence  to fly other airports  without  fearing  of getting stranded at some remote airport someplace.  with  the old delco-remy starter  and generator you would never knew when it was going to fail. It was usually a matter of time. I have moved to Plane Power Alternator as well. (not to knock on generators) since I had prop off I converted to the alternator many years ago.

To clarify ... magneto timing  (which probably isn't your problem)  the Magneto have two type of timing....  they have an internal timing  within themselves and the timing  that gets set when the magneto is overhauled and there  is a timing  for the magneto gets mounted  to the engine. They are not the same timing.

Another thing to look at  is the  grounding  and electrical connections going to the magneto and master s/w  especially  if the wiring is the original.   the connections may look fine  from distance but closeup sometime you can find some of them hanging by few wire strands  creating a high resistance  connection. If wiring is 52 years old, the   newer Tefzel   wiring is much much better , durable and lighter.

I learned the hard way when I figured out the Circuit breakers can go   intermittent .   If  you had ask me  can a circuit breaker could go intermittent ?  I would had said no. they either work or they don't.   It still irritates me  even today  that it took me so long to find  my starting problem many  years ago. I had large helping of humble pie that day.

curious when you say "cold engine" what would you  consider  to outside temp  for cold engine?

(I know it is not always possible or convenient ) but have  you  tried applying heat gun / hair dryer to the cold engine ? maybe 30 min or so.

I'll divide the time on the cylinders and also sometimes  I 'll  run the heat gun up the tail pipe to heat the exhaust side of the cylinders.
I typically pre-heat my engine anytime the outside air temp gets below 40 degrees F (4 degrees c) Seems to help mine starting.

Sounds like our engines were overhauled about same time.

I know many of us  have different starting techniques.  I 'll be first to admit I am no expert in this area  just maybe fortunate ....
 

James '67 M20C

 

 

Edited July 13, 2017 by jamesm

[DanM20C]

This is a great thread,

I would be suspect of the accelerator pump.  Two things to look for to verify its functionality.

Before starting run the fuel pump until the pressure stabilizes. Turn the pump off, then watch for fuel pressure to drop as you pump the throttle.  If I recall correctly, mine would drop 1/2-1psi each pump. 

The second thing, does you engine stumble with a fast application of power?  Most people (at least I did) advance the throttle slowly at take off.  A bad accelerator pump may not be noticed with a slow advancement of the throttle, but will be evident with a quick advance.

If the pressure drops when pumping and the engine stumbles with rapid advance of the throttle, I would bet (I'm not a betting man) that the accelerator pump is the problem.

A properly working accelerate pump is similar to a squirt gun.  When I had my o-200's Marvel schebler on the bench it would squirt fuel 8-10 ft when moving the throttle. It was impressive.

Good luck,

Dan

Edited July 14, 2017 by DanM20C

[Urs_Wildermuth]

On ‎13‎.‎07‎.‎2017 at 10:56 PM, jamesm said:

curious when you say "cold engine" what would you  consider  to outside temp  for cold engine?

Hello James,

 

with that I merely meant that the engine has not run that day. The massive problems usually only are present at the first start up, consequent start ups during the day work better, but still not good.

As an example: Last week one of my pilots did our standard procedure, pumping 7 times, waiting 30 seconds and then engage the starter. The engine turns fine, then fired once or twice, when he disengaged the starter it stopped. He then had to try 4 more times until it did start and he had to pump the throttle vigorously in order to get it to fire.

He then flew to a nearby field, where the engine started on the first try, but still with a lot of pumping of the throttle.

 

On ‎13‎.‎07‎.‎2017 at 11:29 PM, DanM20C said:

Before starting run the fuel pump until the pressure stabilizes. Turn the pump off, then watch for fuel pressure to drop as you pump the throttle.  If I recall correctly, mine would drop 1/2-1psi each pump. 

Thanks for this procedure. Will try it asap. As far as I remember, I have never ever seen a drop in pressure when pumping the throttle.

So you are saying the fuel pressure should drop or not? I am confused now: If the fuel pressure drops with pumping, is the accelerator pump broken? Or the other way around?

On ‎13‎.‎07‎.‎2017 at 11:29 PM, DanM20C said:

The second thing, does you engine stumble with a fast application of power?  Most people (at least I did) advance the throttle slowly at take off.  A bad accelerator pump may not be noticed with a slow advancement of the throttle, but will be evident with a quick advance.

Well, ok, if I can get the thing to start next week, I'll look at that too. i am not counting my chickens anymore before the engine runs. So far, I've had to go home 3 times as it simply refused to start.

From what I read here, it definitly looks like there is something wrong with the accelerator pump, the engine appears not to get enough fuel. I'll have that and the shower of sparks checked next week. I would think, from what you guys posted, this is a high probability that it's one of the two.

[smwash02]

Just now, Urs_Wildermuth said:

So you are saying the fuel pressure should drop or not? I am confused now: If the fuel pressure drops with pumping, is the accelerator pump broken? Or the other way around?

Once you run the boost pump to get your initial fuel pressure in the green, pumping the throttle and turning over the engine will cause this pressure to drop as it feeds that fuel into the engine.

If you pump the throttle and the accelerator pump is not working it will not move fuel into the engine and the fuel pressure won't change (This is a simplification, even a broken pump will probably move SOME and slightly lower the pressure). If the pump is working you will begin losing fuel pressure and this is what you want to see.

 

[carusoam]

Sounds like the accelerator pump needs some fixing...

7 pumps for my M20C was for very cool OAT.

10 pumps was for 0°C.

3 pumps should move the fuel pressure needle significantly.  

IIRC warm summer days, 3 pumps would have been a lot...

You are saying it takes a lot of pumps, and the Fuel pressure isn't going down...

 

quick test....  pressurize the fuel system.  Mixture in. Pump off.  Pump throttle until the fuel pressure declines.  Set throttle.  Start.

 

sounds like you aren't getting any fuel through the accel pump.  Makes you want to get hand pump for fuel like a C152...

or just get the accel pump checked.

PP thoughts only, not a mechanic...

Best regards,

-a-

[Urs_Wildermuth]

2 hours ago, carusoam said:

Makes you want to get hand pump for fuel like a C152...

Yea! Never had such problems with the C150 I used to own....

[Guest]

On July 12, 2017 at 10:08 AM, Urs_Wildermuth said:

Hello all,

 

I'd like to pick your brains on a problem which has been driving me slightly miffed in recent weeks.

I own my C model since 2009 and since then, usually start up was never a problem at all. Our procedure as set was always the same:

- Electrical fuel pump on, till pressure shows, then off.

- Priming 6-8 times with the throttle (cold engine) or 2-4 times (warm engine)

- Engage starter and the engine would run.

 

Since this year, start up has become a real pain. I've had to leave the airplane two times unable to start it, had to request mechanic assistance (ext power) twice more and I keep getting feedbacks from the other pilots.

The engine turns, most of the time fires shortly but when you disengage the starter,it stops. In subsequent tries,it either never ever fires again or again just once or twice.

What we found is that during the start up one has to pump vigorously with the throttle, which sometimes leads to a start.

2nd and 3rd start after the engine has run is unproblematic, even though even then pumping the throttle is required.

In two unsucessfull attempts, the engine definitly was drowned, with fuel leaking on the front tyre.

Two of the pilots report they have NO problems at all and think we are too stupid simply. Well, it's possible,but we follow the exactly same procedure (one of them filmed it) and have no success.

As I said, the previous years we never had any problem whatsoever. Maintenance has looked at it and found nothing wrong, even though they also were unable to start the engine at least once. They report, plugs and magnetoes are fine, carburettor as well.

My feel is the engine does not get enough fuel the first start, then finding a good mix for starting in following attempts is difficult.

Any idea on where to start looking would be appreciated. We have a major problem with this, as our airport has departure slots and we can't afford dicking around with the engine for hours every time we want to go flying. I also feel this is not how a standard O360 A1D should behave.

Thanks a lot.

It takes fuel, air and sparks to make it run, I think your trouble is sparks.

If you go to the Continenetal engine web site (they make the shower of sparks) down load manual X43003-2 for the shower of sparks.  Everything you need is in it.  While there you can get a copy of X43002 ignition switch manual.

You can test your SOS system this way:

Remove all spark plug wires and the power wire from the starter relay(solenoid) to prevent the engine from turning,

Remove the top spark plug from cylinder number 1, turn the engine to top dead centre on compression stroke for cylinder 1.

Hold the spark plug wire with an insulated plier,

Have someone turn on the master switch then turn the starter switch to the start position,

Hold the spark plug wires 1/2" from the engine one wire at a time, 

From one of the spark plug wires on cylinder 1 a shower of spark should arch to the engine, you'll have to check both, only the wire from the left magneto should spark.  Sometimes turning the prop by hand around top dead centre is required to find the exact spot.

Turn the prop 180 degrees should move the sparking to cylinder 3, another 180 should move it to cylinder 2, another 180 should move it to cylinder 4.

Clarence

 

[flyhigh603]

I had this problem in my C right before annual. Very hard to start last 4 flights , come to find out at annual I had sloshing compound coating my tanks which is totally illegally.well the compound disintegrated and was clogging up my carb.Clogged up to the point where my primer pump filter screen was clogged solid. My Mech can't believe I even got it started on my last flight and said I was incredibly lucky the motor didn't quit in mid flight.He couldn't start it at all to bring her across the tarmac to get her in the hangar to start my first annual.Im in the process of getting bladders installed but my point being have your Mech check your primer pump and primer nozzle. Just could be debris in the primer pump nozzle or primer screen .