Engine running rough after service

[glenn reynolds]

thanks in advance for your assistance.

I flew two weeks ago and found the mag drop in the > 150 range when I did my run up to return home.  I ran the engine up and cycled the mixture multiple times and got the mag drop to less than 75 rpm.  Flew home but the engine was missing more than I care for.  Called mobile mechanic and explained issues.  the engine is an io550G with 500 hours since new, with massive plugs.  Mechanic told me to change plugs and mags, which I did.  on the post service run up, the engine started like a top, but ran rough.  the vibration or roughness was much worse at 750 rpm than it was at 1700 rpm.  The mag drop is 50 rpm on both mags.  No cylinder dropped out cold during mag check.  At this point I don't think it is the new plugs.  I don't suspect the mags which are zero hours after an overhaul.  the mobile mechanic removed two intake pipes to better access the mags and he re-used the gaskets.  I'm thinking the number one cylinder has a bad intake gasket based on the engine monitor data dump.  I'm not very knowledgeable about reading the data so would some assistance.  

Flight_ N9624M 2024-07-31 22_49, 0h 8m 24s.pdf

https://drive.google.com/file/d/1N4AH8vbgNz7HzE6ocsMg9X1B9STUZ2wn/view?usp=drive_link

 

[Fritz1]

Sounds like intake leak, take shop vac, connect to intake with throttle open, blow into intake, spray soap water onto intake system, bubbles indicate leak

[glenn reynolds]

That is a great way to troubleshoot that. Thank you. I replaced both the two gaskets cyl 2&4 which we disturbed during the mag replacement and then due to what the engine monitor data showed I also replaced the gasket on number one. Frankly I think I may have put pressure on the hose (never unbolted the 1 intake during mag swap) but I cleaned all three faces, installed new gaskets and then used sealant when I reinstalled the three hoses. Test run was perfect. No roughness. Test flight was perfect engine super smooth. Thanks for reading. The plan is tomorrow fly coast to coast.  So it’s nice to have the machine in good form. 

[Will.iam]

11 hours ago, glenn reynolds said:

That is a great way to troubleshoot that. Thank you. I replaced both the two gaskets cyl 2&4 which we disturbed during the mag replacement and then due to what the engine monitor data showed I also replaced the gasket on number one. Frankly I think I may have put pressure on the hose (never unbolted the 1 intake during mag swap) but I cleaned all three faces, installed new gaskets and then used sealant when I reinstalled the three hoses. Test run was perfect. No roughness. Test flight was perfect engine super smooth. Thanks for reading. The plan is tomorrow fly coast to coast.  So it’s nice to have the machine in good form. 

In the future to test which cylinder has an  induction leaks you can simply go fly at WOT note your egt’s. Then pull back MP 10” from where it was at WOT and look at EGT’s the cylinder that has the induction leak will have a leaner EGT than the rest of the cylinders that do not have a leak. 

[GeeBee]

On 7/31/2024 at 10:55 PM, glenn reynolds said:

the mobile mechanic removed two intake pipes to better access the mags and he re-used the gaskets.

You need a new mechanic. (I always keep intake and exhaust gaskets on hand by the way)

[201Steve]

10 hours ago, Will.iam said:

In the future to test which cylinder has an  induction leaks you can simply go fly at WOT note your egt’s. Then pull back MP 10” from where it was at WOT and look at EGT’s the cylinder that has the induction leak will have a leaner EGT than the rest of the cylinders that do not have a leak. 

I’ve not heard this one. Seems feasible if all other things remain equal. However, for other reasons, maybe spark plug gap or maybe fuel injector flow rate, you could do as prescribed and have a cylinder with “leaner EGTs” (which should result in higher EGT temps than the rest if your mixture is on the rich side of peak) that’s the result of some other factor and not have an intake leak. Maybe the leanest cylinder in that test has a slightly smaller injector orifice than the others, showing it to be leaner for that reason. Or if carbureted, the mixture could be slightly different at different pressures due to the routing of the intake tubes. 
 

I guess doing a gami spread test could tell you what the fuel is doing per cylinder, and then you could compare against what the air is doing in the above. 
 

maybe I’m overthinking it. Is this procedure written up anywhere?

[Will.iam]

30 minutes ago, 201Steve said:

I’ve not heard this one. Seems feasible if all other things remain equal. However, for other reasons, maybe spark plug gap or maybe fuel injector flow rate, you could do as prescribed and have a cylinder with “leaner EGTs” (which should result in higher EGT temps than the rest if your mixture is on the rich side of peak) that’s the result of some other factor and not have an intake leak. Maybe the leanest cylinder in that test has a slightly smaller injector orifice than the others, showing it to be leaner for that reason. Or if carbureted, the mixture could be slightly different at different pressures due to the routing of the intake tubes. 
 

I guess doing a gami spread test could tell you what the fuel is doing per cylinder, and then you could compare against what the air is doing in the above. 
 

maybe I’m overthinking it. Is this procedure written up anywhere?

Mike bush explains it in detail. That was the boiler plate version.