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Cylinder and spark plug depth

rvdave

Well Known Member
I?m wondering if this is a problem or not. Having difficulty with engine smoothness running lop at higher altitudes above 8k. Injectors have been balanced but have this question about the plugs maybe not into the cylinder enough for smooth or timely or complete combustion, could this be a factor? As you can see from photos the plug does sit recessed from flush with the top of inside cylinder by about 1/4-3/8 inch. Maybe need longer reach plugs but would helicoil need to be changed or just plug adapters?



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Yes this is a new engine broken in with 50+ hours on it. 8.5:1 io540, airflow performance, dual plasma iii.
 
plugs look right. The parallel valve engines call for at 1/2" reach plug and this is what you have here. DO NOT put in a longer plug.

Do you have mags? Above 8-10K' mags struggle to produce enough energy to fire the plug and the spark can be weak. LOP mixtures are more difficult to light (gets worse the further LOP you go) than rich mixtures. This leads to a potential situation where you can get mis-fires. Mags that are running well with good wires and plugs don't usually have issues here, but if the system is sub-optimal, this is where problems will often present themselves. You want to pull the mag apart and check the egap as well as for carbon tracking and or wear on the cap and rotor.

If you have an EI setup, the EI and coil have no issues with altitude or LOP, but could be a sign of a problem with wires or a bad plug reducing spark energy.

Misfiring only when LOP and only at high altitudes points to marginally low energy being produced by the ignition source or delivery system.

EDIT: Sorry, now see that you have EI. I would be looking at each plug wire. Pull each wire off the coil and check resistance to ground. They should all be within about 2K ohms of each other. Longer wires should have greater resistance. If you find an outlier, pull the wire off the plug and check all of the plugs and wires separately to identify cause.

Larry
 
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Thanks for the feedback, what do you reference for 1/2" plug info? I will check resistances to see where they fall and may try new plugs to eliminate that.
 
length

The cylinder head may call for a 1/2 inch long plug, but there is a 1/4 inch long shoulder on the adapter. I think you need a 3/4 inch reach plug to end up the same place as the large standard plug.
 
Thanks for the feedback, what do you reference for 1/2" plug info? I will check resistances to see where they fall and may try new plugs to eliminate that.

the spec from lycoming says 1/2" reach plug. You can confirm this my measuring the reach of a typical massive electrode avaiation plug spec'ed for your engine. these plugs have a very thick washer and plug will have a reach greater than 1/2" to accomodate the washer. Depending on how your adapter was made, you could end up slightly less than 1/2" if they didn't account for the large washer. However, having a plug short by a 1/16" Will not impact performance in an appreciable way.

Larry
 
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I see in the OP's picture that the Helicoil insert end is short of the end of the spark plug hole into the combustion chamber.
I see the same in some of my cylinders.
Although I would like to have the 'spark kernel' into the combustion chamber as far as possible, I would not like to have any extended threads of the spark plug stick out past the threads of the head. If this occurred and deposits were to form on them, the insert or the Helicoil insert (if not using an insert for 14mm plugs) could be damaged or thread out of the head with the spark plug.
I like to search spark plugs for my Lycoming and Vedeneyev engines, looking for the approved heat range, 5K ohm internal resistor and an extended nose. I have a drawer full of different plugs I have collected but many have not yet been tried, so I won't comment on that part.
I don't get any plugs with excessive long reach (threads).
Look in the spark plug hole and turn the propeller until the piston is at Top Dead Center. you can now measure the clearance to assure your selected spark plug tip will not be hit. Maintain a safety margin.
The longest extended nose plug I have is the Tempest UREM37BY, it's an approved spark plug for O-320 and O-360 engines.
 
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I am thinking about trying this plug with existing adapter. It?s a direct cross for recommended lightspeed plugs.

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It?s probably not the length of your plugs. A lot of people have used that setup over the years. I had a misfire that got progressively worse. Ended up being a bad LSE Hall Effect sensor.
 
+1 on what Jerry said. You may have an other problem, just starting to call for your attention.

That is an interesting plug.
http://www.briskusa.com/brisk_multi_spark_plug_multi_spark_spark_plugs
the link has a short video of the unusual spark.
I would not put a full set of those in my engine all at once.
Ernest K Gann in his book 'Fate is the Hunter' described being an unknowing participant in new spark plug testing. All the plugs in 3 of the 4 engines! :eek:
http://inspire.eaa.org/2018/02/14/five-aviation-lessons-from-fate-is-the-hunter/
 
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I have the dual independent sensors, dual modules, symptom is with both modules isolated, so looking to eliminate plugs.
 
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