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Miss-firing cylinder

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Dave Gardner
(@mahoganyakgmail-com)
Posts: 8
Active Member
Topic starter
 

My number one cylinder in my Franklin 165 has quit firing below 1450-1500 RPM.   Above 1500 it operates normally.  We have checked compression, valves, plugs, plug wires and looked for air leaks.  No change.  Engine starts easily and idles on five cylinders.  Mixture and mag switch makes no difference.  Anyone with a similar experience?  Thanks for any help.   Dave  

 
Posted : 24/08/2022 12:01 am
Bret
 Bret
Posts: 55
Trusted Member
 

Dave - have you tried changing spark plugs?  I had a spark plug fail which caused problems similar to what you are reporting.

 

 
Posted : 24/08/2022 2:05 am
Dave Gardner
(@mahoganyakgmail-com)
Posts: 8
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Topic starter
 

Bret - thanks.   we clean and tested the plugs, and they were working fine, Regardless I've ordered two new plugs (ouch) and will install them when they arrive     thanks again

 
Posted : 24/08/2022 3:45 am
Bret reacted
Bret
 Bret
Posts: 55
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Dave - Thanks for the reply.  When you tested them, did you test them under pressure?  The reason I ask is because I was not able to test our plugs under pressure, but they were clean.  In fact the one that had actually failed was reasonably new.  It tested was tested simply by checking to see if it conducted spark setting against the cylinder for ground. However it failed when under a certain pressure.  

I understand about the cost of the plugs.  They are way too expensive!   Another option to buying new plugs is to swap plugs with another cylinder.

Do you have a CHT sensor that allows you to determine which cylinder is cool and not firing?

This sounds like a very frustrating problem! I hope others jump in here to help you.

Good luck Dave.

 

Bret

 
Posted : 24/08/2022 10:52 am
Tim
 Tim
Posts: 64
Trusted Member
 

Change the top plugs on #1 & #3. See if the problem follows the plug. If not the swap the bottom plugs and check again. If the problem doesn't follow then it's something else. When was the last time the mags had the 500 hour inspection or were overhauled?

 
Posted : 24/08/2022 11:39 am
Bret reacted
Dave Gardner
(@mahoganyakgmail-com)
Posts: 8
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Topic starter
 

Yes - I have a JPI Engine Analyzer and I'm sure it's number one cylinder.     i'll concentrate efforts on the plugs and will look into the 500 hour mag check.    Thanks again

 
Posted : 24/08/2022 3:30 pm
Robert Picard
(@bob-picard)
Posts: 86
Estimable Member
 

Are you sayng that the cylinder is not firing at all? If so that would mean that your problem is not electrical because both plugs and/or both wires an/or both mags have the identical problems on exactly the same cylinder. What are the odds of that? I would suspect the engine analyzer and it's associated wiring connections and thermocouple, especially if it seems to idle properly as you mentioned.

Bob Picard

Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska

 
Posted : 24/08/2022 6:48 pm
Carsten H. reacted
James
Posts: 7
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We need to know the plug resistance and egt if you want any real help. I assume the first thing your mechanic did was put a multi meter on the plugs and check resistance? Did it have low exhaust Temps or none? A single plug can create a pretty substantial miss. High resistance will not only foul a plug on the bottom end but is the main culprit of burnt up slick mags. I check plugs once a year as I consider it preventative maintenance. Did your mechanic have you order parts without correctly diagnosing the problem first?

 
Posted : 25/08/2022 11:08 am
Dave Gardner
(@mahoganyakgmail-com)
Posts: 8
Active Member
Topic starter
 

For additional clarification:

1.   Yes, the cylinder is not firing at all, below 1500 rpm.  When the engine is cold and I start it, but keep the rpm's below 1500, the number one cylinder remains cold.  The other five are too hot to touch.  So I've discounted the JPI as the problem.

2.   plug wires have been tested and are fine

3.    We swapped the top plug with number 3 cylinder thinking that if both number one plugs were the problem, then swapping one plug would allow the number one cylinder to fire and confirm a plug problem.  However, no change.  So we decided to install two new plugs in number one cylinder, in case the old plugs in the engine (Champion REJ 38) might all be showing similar wear and causing similar problems in the number one cylinder.  Plugs have about 330 hours.  New plugs have not arrived yet.

4.  checked extensively for air leaks and intake/carburetor loose pipes.   No leaks found and all mating surfaces look tight (and all bolts are tight), however we have not yet pulled the intake manifold.    Also, the engine monitor data shows that immediately before this situation started, the EGT for number one cylinder jumped about 30 degrees (and was the hottest cylinder) , so we have not given up looking for a "lean" situation in that cylinder.  

5.    There is a slight, but noticable vibration below 1500 rpm, which goes away when number one starts firing.  There is also a slight change in engine sound, and the engine monitor begins to indicate an egt rise in number one.   

I appreciate enverone's thoughts and suggestions - what a puzzler!

 
Posted : 25/08/2022 2:37 pm
James
Posts: 7
Active Member
 

If the problem is only on #1 cylinder and plugs are ruled out then it has to be internal.  Intake leak would create a high idle/lean mixture on all cylinders and be visible on the monitor so you can rule that out. Of course it did get hot when all this started so might be worth a check.With it running you can spray carb cleaner where the intake meets the head and instantly know if its leaking.Two mags won't go bad at the same time so we can forget those. I would triple check my plug wires and plugs. Maybe look for a place the wire might be shorting out. Checking resistance on the plugs with a multi meter will quickly eliminate them as a issue. If plug resistance is real high it won't fire under high pressure/low speed.

 
Posted : 25/08/2022 4:33 pm
Robert Picard
(@bob-picard)
Posts: 86
Estimable Member
 

Perhaps it's time to pull the valve cover and look for a bent pushrod or a damaged rocker.

Bob

Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska

 
Posted : 25/08/2022 6:39 pm
Carsten H.
(@carl)
Posts: 116
Estimable Member
 

This is a good one! Mounting a pressure transducer in the cylinder and monitoring what happens as the cylinder fails might be of some use.

 
Posted : 25/08/2022 6:57 pm
Bret reacted
Dave Gardner
(@mahoganyakgmail-com)
Posts: 8
Active Member
Topic starter
 

Thanks everyone.    We installed new plugs and checked the valves/pushrods.    No change.    Bad gas has been suggested, so next we are going to get different gas and also check for air leaks using carb cleaner.    I’ll stay in touch!   Dave

 
Posted : 28/08/2022 4:26 pm
Bret reacted
Eddie
(@906co470)
Posts: 36
Eminent Member
 

I am going to go way outside the box here. Pull the valve cover on number 1 and number 3. Using a dial indicator, check to see that the cam is moving the valves the same on both cylinders. If you have a bad lobe on the cam, it "could" give you the indications you are describing. A stuck plunger in a lifter could be the issue also. Hope you do not have to go that deep. 

 

Eddie Stewart
Elmore City, Oklahoma
N906C 108-3 Continental O-470

 
Posted : 10/09/2022 4:00 am
Bret reacted
Dave Gardner
(@mahoganyakgmail-com)
Posts: 8
Active Member
Topic starter
 

After a few days away from the plane, my mechanic returned and we decided it was time to start removing parts for inspection.     We started with the left intake manifold and found a 3/4 gap in the gasket to number one cylinder.    We hope that is the problem (extra air/too lean at low manifold pressure).    When new gaskets arrive we will reassemble and will keep all of you posted.    Thanks again to everyone for your ideas.

 
Posted : 11/09/2022 3:32 pm
Bret reacted
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