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First flight with new (to me) engine - possible detonation event?

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00Dan

Well Known Member
I finally got everything squared away with my new O-320 install (or so I thought) and took my first flight with it today. The ground runs earlier in the day had been uneventful, with function and leak checks going fine. Shortly after initial takeoff my engine monitor alarm started going off for CHTs exceeding 400 dF. This was accompanied by a couple pops a few seconds apart. I reduced power and continued the climb out without any detectable loss of thrust, and CHTs came back down with the reduced throttle setting.

After I got it back on the ground I pulled all the plugs and found the bottom plugs fouled quite badly, to the point that I suspect they were grounding out. I also scoped the cylinders and all looked normal save for one spot on the edge of the piston face of #4. Pictures of the plugs and the piston are attached, as well as the savvy link for the engine monitor data.

https://apps.savvyaviation.com/flights/6630447/55228b29-cb24-4fee-98c7-f00b726832bc

The engine is a carburteted O-320 with 10:1 pistons, but otherwise stock. Two slick magnetos timed to 25* BTDC and fine wire plugs. Fuel flow on takeoff peaked at 12 GPH at approximately 2200 RPM before I pulled the throttle back to control the temperatures.

I suspect the popping at full throttle was a result of the plugs fouling shortly after takeoff and unburnt mixture getting into the exhaust. Would this have also contributed to the high CHT, or is that going to be more due to 25* timing being a bit hot for the compression I'm running?

My planned actions for tomorrow are to clean and reinstall the plugs, and retime to 20* BTDC. Anything else I should be considering?
 

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Mike S said:
This what I was going to suggest.

Brand new engine and 10:1 C/R ------- talk to the engine shop for timing guidance???
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New to me, I bought the engine as a 200 hour takeoff from a Pitts. Penn Yann did the overhaul in 2019 and dataplated it experimental but still with 25* timing.
 
We run 23 degrees timing using 9.5 pistons. 25 seems aggressive for 10 to 1. Your fuel flow seems a bit low for a high performance 320.
 
sailvi767 said:
We run 23 degrees timing using 9.5 pistons. 25 seems aggressive for 10 to 1. Your fuel flow seems a bit low for a high performance 320.
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I’m running an MA4-SPA 10-3678-32 with a 47-862 nozzle. I pulled the throttle back today before the prop had a chance to wind up to climb RPM, but what number do you think would look more realistic?

Mike S said:
Same ignition as when it was on the Pitts? Same timing etc--------any changes from the Pitts installation?
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Same mags, different plugs (engine was using massive, I switched to equivalent fine wires I had). The previous owner claims he ran it at the data plate value of 25*. I don’t know if he had the engine instrumented or not.
 
00Dan said:
Same mags, different plugs (engine was using massive, I switched to equivalent fine wires I had). The previous owner claims he ran it at the data plate value of 25*. I don’t know if he had the engine instrumented or not.
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Other than getting hot (and hot is a relative term here, Lyc limit is 500 IIRC) how did the engine act?? Run smooth, try to backfire or other strange behavior ?

Have you asked the previous owner about CHT's he saw ?

All CHT's high or only part of them? Plug wiring verified?
 
Mike S said:
Other than getting hot (and hot is a relative term here, Lyc limit is 500 IIRC) how did the engine act?? Run smooth, try to backfire or other strange behavior ?

Have you asked the previous owner about CHT's he saw ?

All CHT's high or only part of them? Plug wiring verified?
Click to expand...

It ran smooth other than the full throttle backfiring right after TO, which I suspect is the fault of the fouled plugs. During an earlier static RPM ground run it didn’t exhibit any strange behaviors, although obviously I only left it full throttle long enough for the RPM to stabilize. Taxi and run up were normal.

I fixed the Savvy link in the OP. All but #4 exceeded 400 before I pulled the throttle back. The #4 trace is a bit odd in that it shows depressed CHT relative to the others, but EGT tracks normally with the group. It caught up to the pack as the flight progressed.
 
Looking at the Savvy the #3 and #4 higher EGT could definitely be the retarded timing resulting from the two non firing plugs. I have read that one plug/mag out is equal to 3 deg retard, but have reasons to think it might more than that. I have never seen a timing sweep on these engines.

The basis for this comment is the Savvy time 17:00 to 18:00 where elevated EGT match the lower CHT for #3 & #4.

25 static timing definitely is not ideal, and 23 (or less) would be more in order. Some with 10:1 experience should say where precisely to begin.

I don't believe it was detonation based on the data. The plug photos are too fuzzy to add clarity to a conclusion.
 
sure looks like 3&4 were running on one plug, probably due to fouling. In my experience, going from one plug to two will raise EGTs between 150-200*. Two plugs definately look shorted. What is that shiny build up on the plugs? Is it lead or aluminum? Looks like Al. Very important to determine. Former is a non issue and latter is a confirmation of detonation or pre-ignition (the detonation melts the Al on the piston or chamber and the molten melt attaches to the plug).

Piston shows evidence of detonation. Notice how some force completely removed the deposits from that area of the piston? That is not normal. Usually the entire crown is coated with deposits even if the color and thickness varies across the crown.

I would be very carefully examining the ignition system. IMHO, 25* should not cause detonation at CHTs below 400, even with 10:1 pistons. I do agree with others that 12 GPH is not that low for 2300 RPM. What was it flowing as the RPMs rose during the take off? I would be expecting at least 13 or more during the latter stages of the roll and this could be part of the issue. It is tough to use EGT comparison across planes, BUT it looks like you were right around 1300 during the take off phase and that doesn't seem to me that you were too lean. However, without having an idea what peak EGTs are, this really doesn't help to eliminate lean mixture as the source of the detonation. I would be focused on positively confirming no ignition anomaly is occurring and also a more detailed borescope on the cyl associated with the piston in the pic looking for a small metal protrusion that could be a source of pre-ignition..
 
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BillL said:
I have read that one plug/mag out is equal to 3 deg retard, but have reasons to think it might more than that. I have never seen a timing sweep on these engines.

.
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Way more than that. To reach peak pressure with 2 ign events, the two flame fronts travel less than half the D of the Cyl. With one ignition event (remember that plug is near the edge), it has to travel almost the full D.

I have my timing advance adjustable with a Pot (-4/+5 from the table), so I regularly sweep 3-4* of advance while looking for best speed at a given FF and set of conditions. The EGT changes are around 20-30* with that level of change. If you drop one ignition source, you will likely see a minimum of a 150* EGT rise. I am confident that the effective change in advance is way more than 3*.
 
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00Dan said:
I’m running an MA4-SPA 10-3678-32 with a 47-862 nozzle. I pulled the throttle back today before the prop had a chance to wind up to climb RPM, but what number do you think would look more realistic?
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I would be thinking 15 to 16 GPH minimum.
 
sailvi767 said:
I would be thinking 15 to 16 GPH minimum.
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That is beyond excessive for a 320 running 2400 RPM. According to Lyc, Minimum fuel flow for a 160HP 320 (8.5:1) is 13.5 GPH at SL and 2700 / 29". It should be CONSIDERABLY less at 2400/25" where the OP data said he had the problem. Lyc says at 85% power, min fuel flow is 12. 2400/25 is right around 75% power, so OP was right in the neighborhood, though I agree that a bit more fuel due to the high CR is appropriate. High compression does not require more fuel per se, it just detonates more easily due to the extra work it is trying to extract from the fuel/air mixture and therefore excess fuel can keep detonation under control, but that can be more effectively controlled with less advance.
 
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00Dan said:
I’m running an MA4-SPA 10-3678-32 with a 47-862 nozzle. I pulled the throttle back today before the prop had a chance to wind up to climb RPM, but what number do you think would look more realistic?
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Dan,

I would be careful about pulling the throttle back on takeoff. It might contribute to the damage issue instead of helping. Carbs have a mixture enrichment feature when at full throttle. Part throttle operation bypasses it. I would listen to what the others are saying about the engine timing. Is your field near sea level?

Also, regarding oil fouled plugs, can you post pictures of the cross hatching on the cylinders? Did the original owner have oil consumption issues? And what does a compression check indicate?

Joe
 
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Caveman said:
Dan,

I would be careful about pulling the throttle back on takeoff. It might contribute to the damage issue instead of helping. Carbs have a mixture enrichment feature when at full throttle. Part throttle operation bypasses it. I would listen to what the others are saying about the engine timing. Is your field near sea level?

Joe
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Pretty sure the 3678 series does not have a WOT enrichment circuit, but it has been quite a while since I studied the diagrams. I used to have one on my 320 when I overhauled it. Long since replaced with FI. The ultimate test is FF. As long as a calibrated FF instrument shows a safe and appropriate flow for the RPM/MAP configuration, you are good. Only issue is additional flow for increased margin due to CR.
 
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lr172 said:
Two plugs definately look shorted. What is that shiny build up on the plugs? Is it lead or aluminum? Looks like Al. Very important to determine. Former is a non issue and latter is a confirmation of detonation or pre-ignition (the detonation melts the Al on the piston or chamber and the molten melt attaches to the plug).

Piston shows evidence of detonation. Notice how some force completely removed the deposits from that area of the piston? That is not normal. Usually the entire crown is coated with deposits even if the color and thickness varies across the crown.
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The two plugs that look shorted are the bottom plugs for #1 and #2. I assume the deposits are lead, how would you tell?

The cross hatching on all four cylinders looked normal to me, and that one spot on #4 was the only one that caught my attention. I’ll grab some additional photos today. Looking at the traces for my flight, I’m not sure what I found on #4 occurred at that time; maybe it’s older?
 
00Dan said:
The two plugs that look shorted are the bottom plugs for #1 and #2. I assume the deposits are lead, how would you tell?
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It would be quite uncommon for lead to form like that and have serious doubt it is lead. Lead tends to form roundish balls. It has VERY low to zero adhesion strength (it is technically a lubricant) so usually only collects in the gap between the shell and insulator or other cavity where it is basically stuck in place and not adhered to the plug. Your deposit appears to be adhered to the steel and lead cannot do that. My money is on Aluminum that melted off something else and solidified on the plug (it could have come from the piston OR the cyl head). Break it off and put it in a cup with muriatic acid. If it starts to fizz like alka seltzer, it is most likely Alum and definitely not lead, as lead doesn't react to acid (a battery is nothing more than lead plates sitting in an acid solution). Alum is about the only metal with a low enough melting point to do this in a cylinder environment. What I see on the plug is a classic pre-ignition signature, though I don't do this for a living and offer no guarantee on my diagnosis. I absolutely would not assume this is lead without some form of confirmation of that hypothesis.
 
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Here's pictures of all four piston faces. Additionally, I got up close to that one spot on #4 and as best I can tell its just a spot with less deposits; there does not appear to be any material missing from the piston itself.
 

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00Dan said:
Here's pictures of all four piston faces. Additionally, I got up close to that one spot on #4 and as best I can tell its just a spot with less deposits; there does not appear to be any material missing from the piston itself.
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how many hours on those? Looks like deposits have been worked off. Usually a much more even pattern. Did someone clean them but not completely. Very unusual looking
 
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00Dan said:
Here's pictures of all four piston faces. Additionally, I got up close to that one spot on #4 and as best I can tell its just a spot with less deposits; there does not appear to be any material missing from the piston itself.
Click to expand...

how many hours on those? Looks like deposits have been worked off. Usually a much more even pattern. Should look at th cyl head as well.
 
I retimed the mags to 20* BTDC and did a ground run just now. It ran fine at idle all the way up to full throttle. Idle RPM was either side of 500 RPM so I could stand to bump that up but it still ran. Both mags checked good. Static RPM was 2200 with fuel flow stabilizing at 11 GPH full throttle.
 
IMHO, an idle setting of 500 RPM is just perfect...... If you want a higher RPM, then just push the throttle fwd a bit.
 
Timing

0 320 with 10-1 piston, no other mods, on a VERY tightly cowled Wittman Tailwind. I ran 22 degrees timing for break in and 25 degrees since. Your problem is not the timing. Your engine is broken in and short excursions to 425 CHT will not cause damage.
Verify no intake leaks. REPLACDE the fouled plugs and if the problem persists test the plug leads. Then try a different mag.
 
I tried again today with the timing reset to 20* BTDC. As with yesterday, full throttle temperatures quickly became excessive. No popping or such today, however, so I let the prop spin up a bit to see what the fuel flow was. At full throttle (~28” MAP) the prop spin up to 2650 before I pulled the throttle back for CHTs. The fuel flow peaked at 12.5 GPH.

https://apps.savvyaviation.com/flights/6632266/39a8f27d-be6a-4143-961e-942954948358

I let it cool off and climbed to 7500’ and tried a full throttle run there. I saw 2715 RPM at 21.5” MAP and 11.5 GPH full rich. #2 was still running hot here.

On takeoff when #1 and #2 got hot I see the EGT is a bit depressed relative to 3 and 4 however still climbing before I throttled back. Maybe it’s just mixture distribution.

Either way, I’m starting to buy into the theory that my carb is mismatched and I’m running lean based on the fuel flow at that instance.
 
jrs14855 said:
0 320 with 10-1 piston, no other mods, on a VERY tightly cowled Wittman Tailwind. I ran 22 degrees timing for break in and 25 degrees since. Your problem is not the timing. Your engine is broken in and short excursions to 425 CHT will not cause damage.
Verify no intake leaks. REPLACDE the fouled plugs and if the problem persists test the plug leads. Then try a different mag.
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Which model carburetor are you running?
 
Dan, I’m running similar engine you are. 10:1 compression. My carb. is the 3678-32 (.1065 diameter main jet) and in climb I see 13.5 gph. On a pretty warm day #3 will approach 400 degrees in climb but I level off and it will cool quickly. My engine does seem like the air/fuel distribution is not great and would like to figure out a way to improve that.

I run my timing at 23deg after trying 25 & 20. Power felt down at 20 deg.
 
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Zero4Zulu said:
Dan, I’m running similar engine you are. 10:1 compression. My carb. is the 3678-32 (.1065 diameter main jet) and in climb I see 13.5 gph. On a pretty warm day #3 will approach 400 degrees in climb but I level off and it will cool quickly. My engine does seem like the air/fuel distribution is not great and would like to figure out a way to improve that.

I run my timing at 23deg after trying 25 & 20. Power felt down at 20 deg.
Click to expand...


I have the same carb setup with a .1065 nozzle (https://vansairforce.net/community/showpost.php?p=1518984&postcount=34) but I’m pulling down 1 GPH less than you. I don’t have reason to question my red cube, I had previously dialed it in such that the totalizer is within 0.1 of the gas pump when I fill the tanks.

What is your takeoff RPM that you see that fuel flow at?
 
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To get popping - you have to have both plugs in a cylinder fouling... then unburnt stuff goes into the exhaust and ignites either because of the hot exhaust or because of the other cylinder's exhaust igniting it.

Otherwise we'd all get popping every time we do a mag check.

It looks like aluminum to me. Lead is much more gray colored. Also I see 3 plugs with deposits not 2 (one is hiding near the wire).

I don't trust that detonation can't happen with CHT < 500.
It could be well worth paying the Savvy Analysis fee and get their opinion. Not just was it detonation (which they've seen a lot of), but what to do if it was.
 
You should be getting around 13.5 gph at sea level

You should be getting at least 13.5 gph at sea level. Im not sure if at higher altitudes if the fuel flow will decrease or not due to mp decreasing so hopefully someone on here who knows will chime in but 12 gph at full power at sea level is definitely not enough. When you get the carb readjusted for higher fuel flow climb to 2000’ at full throttle and see what the temps peak at unless it goes beyond 450-460 deg F. Going that high for a few minutes is not going to hurt the engine unless you are regularly doing it. Going to 410-415 or so regularly for a few minutes is not going to hurt the engine.

Of course check your baffling for finger size leaks. Too big/many leaks decrease airflow pass cylinders and increases the pressure in the lower cowling resulting in a decrease of differential pressure drop between the upper and lower cowling
 
FireMedic_2009 said:
You should be getting at least 13.5 gph at sea level. Im not sure if at higher altitudes if the fuel flow will decrease or not due to mp decreasing so hopefully someone on here who knows will chime in but 12 gph at full power at sea level is definitely not enough. When you get the carb readjusted for higher fuel flow climb to 2000’ at full throttle and see what the temps peak at unless it goes beyond 450-460 deg F. Going that high for a few minutes is not going to hurt the engine unless you are regularly doing it. Going to 410-415 or so regularly for a few minutes is not going to hurt the engine.

Of course check your baffling for finger size leaks. Too big/many leaks decrease airflow pass cylinders and increases the pressure in the lower cowling resulting in a decrease of differential pressure drop between the upper and lower cowling
Click to expand...

After some research, on paper my carb model and the main jet I have in there should be flowing 15 GPH. Short of trash in one of the lines what would cause a carb to flow lean of spec?
 
00Dan said:
200 hours. I didn’t see anything on the cylinder heads and the valves looked normal.
Click to expand...

Piston crowns don't normally look like that. You can see where the deposits are being cleaned off in an irregular way like a 10 year old tried to scrape it off. That is not normal. While the deposit patterns can vary quite a bit, they generally don't look like yours do. Detonation is a strong force and will knock the deposits off the metal and guessing what is happening here. Just look at all the shinny metal on the piston crown. Not at all common for a 200 hour engine; Usually fully coated with deposits. Glad you are looking at all this stuff, but you really need a pro to come in and tell you if this is detonation and help you figure out the root cause. If detonation turns to pre-ignition, which it can, it only takes seconds to blow a hole in your piston.

Also look at all of the poc marks on the metal and deposits on the piston crown. Yours looks like the surface of the moon. That is classic detonation signature. Several guys here have posted pics of their borescope work. I strongly encourage you to review those and compare to what you see on yours.
 
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00Dan said:
I tried again today with the timing reset to 20* BTDC. As with yesterday, full throttle temperatures quickly became excessive. No popping or such today, however, so I let the prop spin up a bit to see what the fuel flow was. At full throttle (~28” MAP) the prop spin up to 2650 before I pulled the throttle back for CHTs. The fuel flow peaked at 12.5 GPH.

https://apps.savvyaviation.com/flights/6632266/39a8f27d-be6a-4143-961e-942954948358

I let it cool off and climbed to 7500’ and tried a full throttle run there. I saw 2715 RPM at 21.5” MAP and 11.5 GPH full rich. #2 was still running hot here.

On takeoff when #1 and #2 got hot I see the EGT is a bit depressed relative to 3 and 4 however still climbing before I throttled back. Maybe it’s just mixture distribution.

Either way, I’m starting to buy into the theory that my carb is mismatched and I’m running lean based on the fuel flow at that instance.
Click to expand...

Yes 12.5 is too low for 2700/28 with 10:1 pistons. Minimum of 13.5 required and a bit more wouldn't hurt. Have you cleaned the screen in the carb? Confirmed the mixture arm goes to max travel? 3 and 4 EGT are going to 1400, which is too high for full power down low in most cases. Hard to confirm that in your case due to not knowing peak EGT for your setup. An 8.5:1 engine should be fine at max power mixture (80-100 ROP), but most recommend 200 ROP at 100% and probably mandatory with 10:1 CR, depending upon where your advance is.
 
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blaplante said:
T

I don't trust that detonation can't happen with CHT < 500.
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You absolutely DO NOT need temps anywhere near that high to get detonation. It is true that very high CHTs can CAUSE detonation on it's own, but there are many other causes and in those cases, excessive heat is NOT required. At full power and 10:1 pistons, I have all confidence that I could force detonation at 350 if you let me play with the mixture and timing. Heck, every modern car does this. It keeps advancing the timing until the sensor picks up detonation, then backs off a bit. However, they are smart enough NOT to do this under heavy power/load situations where things can run away from them and cause damage.
 
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I’m going to pull the plugs again today and take another look with a set of more experienced eyes by my side. Checking the finger screen on the carb inlet is next on my list, but unless I find a smoking gun in there I’m not really sure where to go next on getting the carb to flow more. I verified control travel at installation and again the other day when I took those borescope pictures.

Obviously I won’t be flying it until I can address the fuel flow at minimum, assuming my testing hasn’t managed to eat a piston crown.
 
lr172 said:
You absolutely DO NOT need temps anywhere near that high to get detonation. It is true that very high CHTs can CAUSE detonation on it's own, but there are many other causes and in those cases, excessive heat is NOT required. At full power and 10:1 pistons, I have all confidence that I could force detonation at 350 if you let me play with the mixture and timing. Heck, every modern car does this. It keeps advancing the timing until the sensor picks up detonation, then backs off a bit. However, they are smart enough NOT to do this under heavy power/load situations where things can run away from them and cause damage.
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I know this... but some seem to believe so long as CHT is under redline, all's well. :)
 
Next steps

Run a fuel flow test on the system.

Disconnect the line at the carb. Put hose or something to aim into a catch jug. Don't make hose so long that it helps siphon the fuel! Elevate nose to a climb attitude. Turn on boost pump and determine how many seconds for 1 gallon of flow. Repeat for the other tank.

Minimum flow is something like 150% of the max flow of the engine per the data sheet. In your odd engine case I'd used something like an o-360 180 hp as your baseline. Lycoming has the official flow numbers in the operator's guide if I recall correctly.

LOTS of other things can restrict flow... tank pickup, trash in the selector valve...

Repeat with wings level.

Best to test with minimum fuel in the tanks.

Side note... had an issue on a Mazda decades ago... low flow. Found the issue - a part of a twig had gotten ingested into the pickup tube, and gotten stuck at a bend in the line.
 
blaplante said:
Follow-up on my earlier message...

http://www.accelerationtech.com/photos/mooney/manuals/Lycoming Engine Operators Manual.pdf

page 3-25, 87 lbs per hour, 2700 rpm, 100% power. -> 14.5 gph

Correction - only gravity systems need 150%. Pressure systems need to demonstrate 125% of required flow. Hence you need to see your fuel system produce 18.13 GPH in the carb disconnected test I recommended above.
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This is good info - I’ll test it, which should at least let me know if it’s the lines or the carb.

I scoped the engine again this morning and there were definitely less deposits on the pistons after yesterdays flight, along with what appeared to be the “textbook” sandblasted look between the valves. I don’t think I saw any pitting or piston crown erosion but I have an A&P coming by later to give it a look.
 
00Dan said:
I scoped the engine again this morning and there were definitely less deposits on the pistons after yesterdays flight,
Click to expand...

Sorry but that is detonation. Pistons don't clean themselves without it. Water injection can do some of that, but will not leave the jagged edges. Fuel flow testing on the ground is not enough. Several issues in the carb can prevent adequate fuel delivery even if the upstream system is delivering 400% of required flow. SOmething as simple as a saturated float can do that or a varnished up jet or air bleed passage. Definitely check the system for delivery capacity, but don't consider success there to be a problem solved.
 
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lr172 said:
Sorry but that is detonation. Pistons don't clean themselves without it.
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Agreed.

SOmething as simple as a saturated float can do that or a varnished up jet or air bleed passage. [/QUOTE]

Methinks a saturated float would cause it to go rich.
 
I’ve been brainstorming how I want to tackle the troubleshooting, and am wondering if there is a way to me to bench test the carburetor flow myself? I haven’t come up with anything in my searches thus far.

Another potentially contributing factor - the other day when I ran it up to 7500’ for the full throttle run I noticed my fuel pressure had dropped to about 1.5 PSI. On the ground with the electric pump off it was running at 3-4 psi. Turning the electric pump on would cause the pressure to jump to 5-6 psi, but there was no change in EGT or fuel flow. My old engine ran pretty steady on fuel pressure, so is this something else to investigate? My gage is mechanical, so obviously nothing electrical to check.
 
Hey Dan, The things I've read as suggestions in this thread are to retard your timing a bit, perhaps down to 23 degrees, and to check for intake leaks.

Another thing to look it is do your mixture and throttle cables move fully to the stops on the carburetor in each direction?

I think technically it is possible to check flow in a carb on the bench, but you probably need something that will move a measured amount of air through it as well, which is probably going to be quite a job to construct.

You might have mentioned this, but are you running 100LL? Is the fuel "fresh"?

And as usual, Kitplanes has some good advice in this article:

https://www.kitplanes.com/carburetor-maintenance/
 
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