Can one completely dead spark plug cause hard starting on an O -235-C1?

[Lndwarrior]

Trouble shooting a new hard starting problem on my 4 cylinder O-235-C1.

Electrical, battery, starter all seem good.

Found one completely dead spark plug on the bottom #4 cylinder. It did not register at all on the ohm meter. All other plugs were within specs. (Tempest UREM37BY)

Can this, alone, cause hard starting?

It does start but after many attempts.

This is new problem not experienced in the 8 years since I installed her.

Before I put everything back together I'd like to know the answer to this question in case I should keep looking for additional issues.

TIA
Gary

 

[Gary Ward]

I think you need to keep looking…
How many hours are on the magneto?
How many hours are on the plugs?

 

[Dan Thomas]

We see, from the TCDS, that only one impulse mag is on an O-235-C1. If that engine has Slick mags, it might have been upgraded to two impulse mags like a lot of O-235s, which can be notoriously hard to start.

Anyway, weak spark is the usual cause of hard starting. When were those magnetos last off for internal inspection, cleaning, and replacement of anything that needed it (usually the points)? Every400 hours for Bendix/TCM, 500 for Slick/Champion.

In the mags, eroded points will shift the E-gap, affecting the mag's internal timing and drastically weaken the spark. Fixing this requires some knowledge as to how to do it, and without that knowledge, things get worse.

Are you pressure-testing those sparkplugs? The ohm check only checks the internal resistor. It will not identify cracked ceramic that can allow shorting of the spark to ground. And if this the same airplane that had those UREM-37BY plugs with the huge gap, there's a big part of your problem, if the rest of those plugs are gapped like that. These aren't plugs for a car's high-energy ignition system, with .050" gaps or more. You want .016 to .018" for the plugs for an O-235. Too much gap will affect starting.

Another factor can be the plug leads. They do age and start to break down and leak the spark current. You need a lead tester for that. It applies spark voltage to the lead, and you need to know how to use that, too.

One more factor: what ignition switch are you using? If it's an ACS, you might have problems there. You have to get a look at the switch to see if it's a Bendix or ACS (or Gerdes, which I think is the ancestor of the ACS). Those ACS switches have a 2000-hour AD on them that also requires a diode across the starter contactor coil to prevent contact burning in the switch that can affect magneto operation. The AD demands an internal inspection and relubrication of the contacts. This applies only to those switches with a Start position. The diode across the starter contactor coil is to shunt the voltage spike generated by that coil when the switch is released, avoiding arcing inside the switch.

 

[Dana]

Assuming only one impulse mag, does it normally feed the top or bottom plugs? Because if the ignition switch is wired to only fire one mag on starting, and that one has the bad plug, that would certainly make starting difficult.

 

[Dan Thomas]

Assuming only one impulse mag, does it normally feed the top or bottom plugs? Because if the ignition switch is wired to only fire one mag on starting, and that one has the bad plug, that would certainly make starting difficult.

There's a good point. And if someone has switched the P-leads, only the non-impulse mag is firing on start. That will take a long time to fire up.

On the Lyc, the left mag fires the top left and bottom right plugs. The right mag gets the rest.