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Brief roughness in flight

YellowJacket RV9

YellowJacket RV9

Well Known Member
Patron
Hi all,
Today I had my first minor scare after about 90 trouble free hours. I am hoping to get some advice on how to track down the issue, so here is the background. Lycoming O-320-E3D with apr 750 total and 90 since new cylinders. Short story: I thought my high FP was flooding carb resulting in 5 seconds of roughness, but engine data indicates a 100 degree EGT drop on #1 only, along with 100 RPM. No other symptoms and all returned to normal after event. Long story:

Quick flight after installing nose gear fairing and wheel pant. Normal taxi and run-up. Very small, (<1 sec) hiccup on application of takeoff power, but was in the air and climbing well immediately after, so I continued the climbout and thought little of it, but stayed close and high.

Flight was uneventful for apr. 20 minutes, although I noticed fuel pressure a bit higher than usual. It was in the 5.5-6.8 range, when usually it hovers between 5-6. As I was descending to land, at about 55-60% power, there was a sudden and brief engine roughness and loss of power. I immediately traded airspeed for extra altitude, added carb heat, hit the boost pump, switched fuel tanks and confirmed mixture full rich. By the time I was done doing this the engine seemed to be making decent power again. I was babying it and due to nerves I really can't judge whether it was running rougher than normal from there on. If it was, it wasn't extremely rough. Made a straight-in high approach and landed without incident.

My initial thought was that the high fuel pressure flooded the carb, and possibly also did so during the takeoff run. I also considered carb icing (80 degrees OAT, 93% humidity). I usually apply carb heat below 55% power. I didn't notice any increased roughness after adding carb heat so I don't think that is a strong possibility.

I came home resolved to install a new fuel pump (I have heard that they can indeed fail high), as well as pull and inspect carb and float. But when I uploaded my engine logs to Savvy, it is not what I expected. At the time of roughness, there is about 100 degree EGT drop on cylinder #1 and 100RPM drop. No other changes. The fuel pressure immediately preceding the event was 6.0, so not extremely high. After the event, all parameters return to normal, although fuel pressure creeps as high as 6.9 again.

I also noticed when reviewing the logs for the pre-takeoff period that right after runup my fuel pressure was as high as 7.8, when began to drop to around 7 on application of takeoff power. This is definitely too high.

So I am thinking I have a fuel pressure problem for sure, which I will fix with a new pump hopefully. But I'm also thinking that didn't cause this specific issue as it was only an EGT drop on cylinder 1. I guess I will pull plugs next and inspect them.

I apologize for the long post but hopefully I provided enough information for the experts to weigh in. I do not intend to fly again until I am confident I have addressed however many issues need to be addressed.

This is the data log if it helps: https://www.savvyanalysis.com/flight/1487707/8c337abe-41ca-418f-a2be-c63e6e033094

Thanks!

Chris
 
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Engine is about 750 SMO, but only 90 since new cylinders. Hangared and never in the rain. I will check gascolator and fuel tanks just in case however. Good thought.

Chris
 
I am starting to seriously consider the sticking exhaust valve theory. Even though I have less than 100 hours on the new cylinders, I have been running quite rich most of the time for break-in and low-level cruising around, and I hear the low-compression engines are even more susceptible to getting gummed up with 100LL.

I have still ordered a new fuel pump in an attempt to lower fuel pressure, but am going to check the exhaust valves as well.

Chris
 
YellowJacket RV9 said:
I am starting to seriously consider the sticking exhaust valve theory. Even though I have less than 100 hours on the new cylinders...
Click to expand...
It's not unheard of for an engine with less than 100 hours on it (from new, even) to have a sticking valve.
 
YellowJacket RV9 said:
As I was descending to land, at about 55-60% power, there was a sudden and brief engine roughness and loss of power. I immediately traded airspeed for extra altitude, added carb heat, hit the boost pump, switched fuel tanks and confirmed mixture full rich.
Click to expand...

It's a curious plot. Mind a few questions?

How much altitude did you gain?

Confirmed mixture was already full rich, or did you move it to full rich?

Is your red cube located before or after the engine driven pump?

What are you logging on Aux3?

CS or fixed pitch?

I came home resolved to install a new fuel pump (I have heard that they can indeed fail high), as well as pull and inspect carb and float
Click to expand...

Calibrate the fuel pressure gauge first. I too have heard an engine driven pump can fail high, but based on principle of operation, I cannot determine how. On the other hand, pressure senders are notorious.

Not sayin' it has anything to do with this problem, but it sounds like a used engine from another application, so it certainly can't hurt to make sure your carb is up to date:
http://msacarbs.com/technical-data/service-bulletins.html
 
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I had a similar engine roughness then smoothing. Mine occured running lean of peak and was "cured" going rich of peak. When I landed I did a mag check and had problems on BOTH mags not firing #1 and #2 cylinders. Decowled and noticed two spark plug had vibrated and settled to where wires touching. I have PMags and auto harnesses, so very high voltage and wires touching each other or other metal is a big no no. I looked carefully and saw where an arc had burned a pinhole in one of the wires.
I separated them and did another run up and now only one mag had a problem, still didn't fire #1 and 2 cylinders. A call to Emagair verified that one of the two coils in the left Pmag was probably ruined by the plug wires touching and arcing.

If you have electronic ignition check that it is working correctly.
 
DanH said:
It's a curious plot. Mind a few questions?

How much altitude did you gain?

Confirmed mixture was already full rich, or did you move it to full rich?

Is your red cube located before or after the engine driven pump?

What are you logging on Aux3?

CS or fixed pitch?



Calibrate the fuel pressure gauge first. I too have heard an engine driven pump can fail high, but based on principle of operation, I cannot determine how. On the other hand, pressure senders are notorious.

Not sayin' it has anything to do with this problem, but it sounds like a used engine from another application, so it certainly can't hurt to make sure your carb is up to date:
http://msacarbs.com/technical-data/service-bulletins.html
Click to expand...

I only gained about 800' before I knew I had a landing assured.

The mixture was indeed full rich and had been left there the whole flight.

Aux4 is fuel pressure X 10 psi
Aux3 is manifold pressure x10

It is fixed pitch

Fuel flow meter is in the cockpit before both pumps.


I am at the shop now where compressions are all good and valve appears to function normally when viewed while pulling the prop through. mechanics say it is fine; I am trying to get them to do a wobble test.

Chris
 
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A&P says the rem37by plugs are too cold and meant for an engine with more blow-by. They were transferred over after installing new cylinders so this could be. He assures me that 8 new urem40e plugs will solve the problem. Cylinder bore scope and valves look like new. Thoughts?

Chris
 
Champion says the REM37BY plug is approved on the O-320E2D in Cessna 172I-M models and on all Piper Cherokee O-320's (many -E3D's). I'm no expert but it seems unlikely that's the source of your issue.
 
krw5927 said:
Champion says the REM37BY plug is approved on the O-320E2D in Cessna 172I-M models and on all Piper Cherokee O-320's (many -E3D's). I'm no expert but it seems unlikely that's the source of your issue.
Click to expand...

I agree. I'd love it to be that simple but find it unlikely....

Chris
 
YellowJacket RV9 said:
A&P says the rem37by plugs are too cold and meant for an engine with more blow-by. They were transferred over after installing new cylinders so this could be. He assures me that 8 new urem40e plugs will solve the problem. Cylinder bore scope and valves look like new. Thoughts?

Chris
Click to expand...

I run those plugs with good success. Never had an issue, though I rarely run ROP beyond t/o and climb. While the 37BY is a colder plug, it was specifically designed to fire with lead deposit build ups, common on the Lycoming plugs. It is generally recommend for all 320/360's that have problems with lead deposits. It is unlikely the plug selection is your problem.

Colder plugs are generally better if they are hot enough to burn off deposits. I have over 100 hours on my 37BY's and they are very clean and have never seen deposits on them.

Larry
 
Double checked timing on a whim. Left is a full 30*btdc, and right is off a few degrees as well. Going to correct this issue before throwing new plugs at it.

Chris
 
krw5927 said:
Champion says the REM37BY plug is approved on the O-320E2D in Cessna 172I-M models and on all Piper Cherokee O-320's (many -E3D's). I'm no expert but it seems unlikely that's the source of your issue.
Click to expand...

Some engines are sensitive to plug choice.

A friend with a Grumman Cheetah with a O-320-E2G (similar to the OP) bought new REM37BY plugs and had a not-so-smooth engine. Going back to REM40 plugs solved it.

Meanwhile my same airframe Grumman Tiger (with a O-360 carb) seems to be smoother and fouls plus less with the extended tip REM37BY plugs.

The mechanic could be correct even though Lycoming approves the -37BY for most engines....:)
 
Well timing is fixed and test ground run sounded good. But, after shut down there was a steady dribble of fuel from the air box, and I noticed it didn't want to stop when I pulled the mixture out. Fuel pressure was over 7 during the ground run. Going to replace fuel pump. But also pulling carb and taking it in for a throrough check.

Chris
 
I see nothing in the plots to suggest a plug problem. Power loss in one cylinder would require two bad plugs on different mags. Failure of one plug would show an EGT rise on that cylinder.

I also discount fuel pressure. Plot fuel pressure (Aux4) against fuel flow (FF) and add RPM (Tach) to the other graph. Their relationship is entire stable for the 3 minutes prior to the roughness event. Now examine the three indicators after the power reduction around 6:12. If the float was being overpowered, flow would not track RPM downward, nor would pressure rise.

Carb ice does not appear to be supported. I would expect some indication in all cylinder traces.

Now plot manifold pressure (Aux3) against EGT. Manifold pressure rises in lockstep with the loss of cylinder#1 EGT. That suggests a stuck exhaust valve. With the exhaust open, there is no working intake stroke in #1. The throttle plate provides a fixed orifice; manifold pressure rises because now there are only three cylinders to drawn pressure down.

When power is restored (about 5:19), EGT 1, 3, and 4 rise roughly 45 degrees, likely in response to Chris unloading the prop a bit while trading airspeed for altitude. We could know with more certainty if we had altitude, airspeed, etc. However, #2 rises, then falls, which suggests it is leaner than the others, and loading pushed it over peak. I'd look for a bad intake gasket on that one.
 
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Thanks Dan,

Your points make sense, and I am still leaning towards a valve sticking. I got tired of working through a language barrier with the two A&Ps on the field today and couldn't explain that I wanted a wobble test. They were convinced after pulling the valve cover and looking inside with the scope that things were clean as a whistle and guaranteed me there was no valve issue. I am going to borrow or buy the tool and do the test on my own.

It is also apparent that either way, there are a few other issues. The timing was well off, left was set at 30* BTDC and right about 22*. Surprising because it has run pretty smooth, but in any case it is dead on now. Both mags were overhauled prior to me hanging the engine.

After my ground run, the fuel spilling out of the air box means I definitely have a carb issue, as well as the fact that it didn't want to shut down. I took the carb off and will be going through it with a local expert. It does have the blue epoxy float.

I also went ahead and pulled the fuel pump, because I have a new one on order anyways. Might as well see if that lowers my FP a bit.

Anyways, plane is grounded till everything is back together and I do the wobble test, but I am enjoying the learning experience and am confident I can get to the bottom of this and be running better than ever....

Chris

DanH said:
I see nothing in the plots to suggest a plug problem. Power loss in one cylinder would require two bad plugs on different mags. Failure of one plug would show an EGT rise on that cylinder.

I also discount fuel pressure. Plot fuel pressure (Aux4) against fuel flow (FF) and add RPM (Tach) to the other graph. Their relationship is entire stable for the 3 minutes prior to the roughness event. Now examine the three indicators after the power reduction around 6:12. If the float was being overpowered, flow would not track RPM downward, nor would pressure rise.

Carb ice does not appear to be supported. I would expect some indication in all cylinder traces.

Now plot manifold pressure (Aux3) against EGT. Manifold pressure rises in lockstep with the loss of cylinder#1 EGT. That suggests a stuck exhaust valve. With the exhaust open, there is no working intake stroke in #1. The throttle plate provides a fixed orifice; manifold pressure rises because now there are only three cylinders to drawn pressure down.

When power is restored (about 5:19), EGT 1, 3, and 4 rise roughly 45 degrees, likely in response to Chris unloading the prop a bit while trading airspeed for altitude. We could know with more certainty if we had altitude, airspeed, etc. However, #2 rises, then falls, which suggests it is leaner than the others, and loading pushed it over peak. I'd look for a bad intake gasket on that one.
Click to expand...
 
bullojm1 said:
Chris,

Read through my write-up on checking for sticking exhaust valves here first:

www.vansairforce.com/community/showthread.php?t=128673

You shouldn't need a wobble testing tool yet - your fingers will let you know if you have a stuck valve by how smooth the valve runs in the guide. If there is any friction at all, buy the reamer and clean out the guide.
Click to expand...

Excellent write up thanks! I will work on making one of those tools...

Chris
 
YellowJacket RV9 said:
Your points make sense, and I am still leaning towards a valve sticking.
Click to expand...

I do the data analysis for fun, like my mom does crossword puzzles. You have a Savvy Pro subscription; what did they have to say?

I got tired of working through a language barrier with the two A&Ps on the field today and couldn't explain that I wanted a wobble test. They were convinced after pulling the valve cover and looking inside with the scope that things were clean as a whistle and guaranteed me there was no valve issue.
Click to expand...

Did they pull the spring and play with the valve? Scoping the head of the valve from inside the combustion chamber is good practice, but won't tell much about the stem and guide.

It is also apparent that either way, there are a few other issues.
Click to expand...

Yep, more than one issue is common, perhaps even guaranteed if you look enough. Some issues will contribute to the gross symptoms. Others are just there, perhaps causing some deterioration of performance without being very apparent. Last, some issues are the result of maintenance or investigation.

Point is, never expect just one problem.

After my ground run, the fuel spilling out of the air box means I definitely have a carb issue..
Click to expand...

No doubt. Do remember that a flooding carb should affect all cylinders, not one.

I also went ahead and pulled the fuel pump, because I have a new one on order anyways. Might as well see if that lowers my FP a bit.
Click to expand...

It might. The nameplate range is 4 to 6 psi for a low pressure engine driven pump. Actual operating pressure (as seen on an accurate gauge) varies with flow rate. Maximum pressure is a function of the spring that pushes on the diaphragm. That maximum cannot increase in service, unless you know some way to make a spring more powerful with more use.

The replacement pump may have a slightly weaker spring, or a slightly leaky check valve, thus a slightly lower (but within range) maximum pressure.

Tempest has a very good fuel pump video on YouTube. It illustrates with a high pressure engine driven pump for constant flow injection, but the operating principles are identical for a low pressure pump. The difference is the size of the diaphragm spring.

https://youtu.be/PYKoLUsTJU4

We've also had nice postings here on VAF in which an owner dismantled a pump or two so we could all see inside. Try the search function.

In any case, the TCDS for your engine lists maximum fuel inlet pressure as 8 psi, and I'd bet a carb in good working order will handle more.
 
So the guys at Savvy emailed me this morning suspecting a fuel/carb issue due to EGT and CHT drop and they also noticed smaller EGT drops on 3 and 4 that I hadn't looked closely for. I still plan on checking the exhaust valve, however, just in case, before flying again.

I took the carb to Performance Carburetor here in Clearwater this morning where we took it apart and immediately noticed that the discharge tube was gone. I didn't look closely in my air box yesterday but I bet I'll find it there when I go back and look (I hope, has anybody ever had one get sucked through an engine?). Also, there was a lot of play and significant wear on the main shaft. Ron, the owner, took over an hour to explain things to me, show me examples of other carb issues, and take mine apart. He's got 40 years experience with airplane carbs so I am going to let him rebuild it and test it in shop. He did also say that this carb will take 6psi, "and not a lot more".

I am pretty confident that with the timing fixed and a fresh carb and new fuel pump I will be running better than ever. I also think that fixing the advanced timing well help with CHT's that were fine in cruise but ran a little hot in climb.

I will report back next week when everything is back together. Wife says I have to stay home and do yard work today.

Chris

DanH said:
I do the data analysis for fun, like my mom does crossword puzzles. You have a Savvy Pro subscription; what did they have to say?



Did they pull the spring and play with the valve? Scoping the head of the valve from inside the combustion chamber is good practice, but won't tell much about the stem and guide.



Yep, more than one issue is common, perhaps even guaranteed if you look enough. Some issues will contribute to the gross symptoms. Others are just there, perhaps causing some deterioration of performance without being very apparent. Last, some issues are the result of maintenance or investigation.

Point is, never expect just one problem.



No doubt. Do remember that a flooding carb should affect all cylinders, not one.



It might. The nameplate range is 4 to 6 psi for a low pressure engine driven pump. Actual operating pressure (as seen on an accurate gauge) varies with flow rate. Maximum pressure is a function of the spring that pushes on the diaphragm. That maximum cannot increase in service, unless you know some way to make a spring more powerful with more use.

The replacement pump may have a slightly weaker spring, or a slightly leaky check valve, thus a slightly lower (but within range) maximum pressure.

Tempest has a very good fuel pump video on YouTube. It illustrates with a high pressure engine driven pump for constant flow injection, but the operating principles are identical for a low pressure pump. The difference is the size of the diaphragm spring.

https://youtu.be/PYKoLUsTJU4

We've also had nice postings here on VAF in which an owner dismantled a pump or two so we could all see inside. Try the search function.

In any case, the TCDS for your engine lists maximum fuel inlet pressure as 8 psi, and I'd bet a carb in good working order will handle more.
Click to expand...
 
Wow, you guys are all amazing. Such productive interaction. I read this thread like a good novel. It's interesting how much stuff I can read and put away in my little brain in the "just in case" file. Never know when this will pop up in my own adventures. Glad to be a part of this group!
 
How a Tempest fuel pump works

This is an excellent Youtube video produced by Tempest and repeated here for easy searching of archives in the future. BTW, I found it interesting that the presenter claims a pump pressure can be as low as 10 PSI at the pump outlet of a high pressure pump and still provide enough full for full power of a 200HP engine.

https://youtu.be/PYKoLUsTJU4

Bevan
 
YellowJacket RV9 said:
So the guys at Savvy emailed me this morning suspecting a fuel/carb issue due to EGT and CHT drop and they also noticed smaller EGT drops on 3 and 4 that I hadn't looked closely for.
Click to expand...

Sincerely, I'd like to read their explanation. Can you post it or email it?

In the meantime, I'll be sporting and double down. Take another look at those EGT and CHT traces: https://www.savvyanalysis.com/flight...e-c63e6e033094

The event starts just after 4:47. That's when MP and RPM change, useful as a marker because they respond much more quickly than the EGT probes.

There is no CHT drop on #2, #3, and #4. Only #1 drops, at 5:03. It lags the the #1 cyl EGT drop, as we would expect, by seven seconds.

Now go to EGT. Starting at 4:47,#3 falls by 38 degrees in 10 seconds. #4 wobbles around a bit, max fall being 15 degrees for the whole event. #2 doesn't fall at all, but rather, rises by 14 degrees. Cylinder #1 is the only one with a significant drop, down almost 200 degrees 16 seconds into the event.

So, #3 has a notable EGT drop (38 degrees), but still makes power. Cylinder #1 is dead. The other two are pretty much normal. What do #1 and #3 have in common?

Firing order is 1-3-2-4. Approximately the last 90 degrees of #1's intake valve open period overlaps the #3 intake open period. With the #1 exhaust valve stuck open, #3 pulls its intake charge from the carb, and from the #1 intake tube, cylinder volume, and exhaust pipe. Having both of #1's valves open doesn't bother #2 and #3, as they are on compression and power strokes at that time.

 
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DanH said:
Sincerely, I'd like to read their explanation. Can you post it or email it?
Click to expand...

Absolutely:

I reviewed your flight data and I believe I see the event you are concerned with which occurs at 4:56 into the data. The event shows a significant drop in EGT1 of 200F, with erratic FF and RPM. We also see the EGT1 drop is collaborated by a small drop in CHT1 along with smaller EGT drops in 3&4. Seeing the CHT drop along with EGT tells us this is most likely a fuel or mixture related event so I think we can rule out ignition. There are many different causes to these kinds of events yet often they stem from hot fuel leading to temporary vapor lock. Some times though these can also be carburetor issues. If it happens again, just try turning on the fuel pump to see if that alone clears it up; assuming you have time and altitude to trouble shoot a bit.


I tend to discount the vapor lock theory, as I have had no such trouble under much the same or hotter conditions all summer. I run 100LL exclusively as well. Knowing that my carb was in serious need of overhaul, I could believe that contributed, perhaps in combination with over-advanced timing?

Again, with my limited knowledge, the sticky valve seems to fit all the clues. Only odd part is no "morning sickness" symptoms, in addition to the relatively low time, not that that rules it out. Valve will be inspected next week to conclusively hopefully solve the mystery.

Your analysis as well as explanations are extremely helpful, so thank you for taking the time to walk us through it. I really enjoy learning this stuff.

Chris
 
YellowJacket RV9 said:
I really enjoy learning this stuff.
Click to expand...

We're both learning Chris. Engine monitors have become common, so data analysis is now a useful skill. Like everything else in EAB, the best way to learn is to jump in and do it.

I may be right, or I may be wrong, but I can't lose.
 
Chris, I had a tight valve guide on the no. 2 cylinder exhaust valve which would periodically stick in flight for short periods of time. No morning sickness symptoms either. Reamed it and problem solved.
 
DanH said:
It's a curious plot.



Calibrate the fuel pressure gauge first. I too have heard an engine driven pump can fail high, but based on principle of operation, I cannot determine how. On the other hand, pressure senders are notorious.

Not sayin' it has anything to do with this problem, but it sounds like a used engine from another application, so it certainly can't hurt to make sure your carb is up to date:
http://msacarbs.com/technical-data/service-bulletins.html
Click to expand...



Dan, I'm not proposing this as his problem, but I read here on VAF recently that high crank case pressure from a plugged breather, will act on the back side of the fuel pump diaphram adding to the spring pressure, and that's how the pump can fail high. Never seen it myself, but it makes sense and certainly begs a quick look at the draft tube.

Tim Andres
 
tim2542 said:
Dan, I'm not proposing this as his problem, but I read here on VAF recently that high crank case pressure from a plugged breather, will act on the back side of the fuel pump diaphram adding to the spring pressure, and that's how the pump can fail high. Never seen it myself, but it makes sense and certainly begs a quick look at the draft tube.

Tim Andres
Click to expand...

Good point and that was one thing I checked early on when I noticed my high FP. Breather tube is totally clear.

Chris
 
tim2542 said:
Dan, I'm not proposing this as his problem, but I read here on VAF recently that high crank case pressure from a plugged breather, will act on the back side of the fuel pump diaphram adding to the spring pressure, and that's how the pump can fail high. Never seen it myself, but it makes sense and certainly begs a quick look at the draft tube.
Tim Andres
Click to expand...

Like any theory, it begs an experiment to determine if it is true or not. Recall that I've been running a case evacuator on my IO-390 for quite some time. If the theory is true, crankcase pressure should work both ways; negative crankcase pressure should decrease fuel pressure.

The evacuator pulls about 2.75" Hg negative pressure at cruise, or 1.351 psi. If I have the opportunity, I'll disconnect the evac line and see if my fuel pressure rises by that amount. I think it probably will, unless I'm missing some detail of pump construction.

In the meantime, let's quantify a normal install. IIRC, the Lycoming spec for maximum positive case pressure is something like 5" H2O (check me). That's 0.18 psi. A full 1 psi increase would require almost 28" H2O, but that doesn't seem crazy for a blocked breather.

Of course, it's a failed breather, not a failed pump.
 
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Don't forget to multiply by the sq in. of the diaphram, I'd guess the diaphram at 3" dia so that's about 7sq in?

Tim Andres
 
tim2542 said:
Dan, I'm not proposing this as his problem, but I read here on VAF recently that high crank case pressure from a plugged breather, will act on the back side of the fuel pump diaphram adding to the spring pressure, and that's how the pump can fail high. Never seen it myself, but it makes sense and certainly begs a quick look at the draft tube.

Tim Andres
Click to expand...

Doesn't seem likely. The spring is causing the diaphragm to exert around 6 PSI of pressure on the other side of the diaphragm. While I am not a mechanical engineer, logic tells me I would need over 6 PSI of pressure on the backside of the driaphragm to create an increase beyond the 6 PSI already exerted by the spring. Anything less should have no impact on the fuel pressure. I am guessing that a blocked breather, producing 6 PSI of crankcase pressure would blow out the front seal. However, I have no real idea what pressure level would be required to do that.

An example of the logic. Take your air compressor set at 100 PSI and connect it to a gauge; You get a reading of 100 PSI. Add a tee to the line and add a second compressor, set at 80 PSI to the line. The gauge will still read 100 PSI, not 180 PSI. The gauge is equivalant to the other side of the diaphragm. In fact most use a diaphragm and spring to measure the pressure.

Larry
 
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tim2542 said:
Don't forget to multiply by the sq in. of the diaphragm, I'd guess the diaphragm at 3" dia so that's about 7sq in?
Click to expand...

Diaphragm area would be required to determine the necessary spring force for a desired fluid pressure.

shaft force (lbs) / diaphragm area (sq in) = fluid pressure (lbs per sq in)

so

fluid pressure (psi) x diaphragm area (sq in) = shaft force (lbs)

Here we're merely adding or subtracting psi above the diaphragm from psi below the diaphragm...same units.
 
I wanted to post here and wrap this up as everything is back together and running well. The #1 exhaust valve tested fine, so it does not appear to be the culprit, as much as the initial clues pointed that way. It looks like in this case there were 3 things in play: Incorrect timing (advanced on left, retarded on right), overpressure from the mechanical fuel pump (sometimes over 7psi), and a poorly performing carb.

The timing was easy enough to fix, but after shutting down during a test run I noticed a good bit of fuel coming out of the drain in the air box. I went ahead and pulled the mechanical fuel pump and carb. I took the carb to Ron at Performance Carburetor in Clearwater right next to KCLW. He was extremely knowledgeable, walked me through disassembly, and ended up doing a complete overhaul to new specs. Price was extremely fair and included an hour's worth of free knowledge. I highly recommend him if you are in the area and need carb or engine help. He did comment that the carb does not like much over 6 psi and I certainly could have been over-pressuring it. Most interestingly (and embarrassing), he found the discharge tube for the accelerator pump missing. I hate to admit that I'm not 100% sure it was there when I initially installed it. It is MIA in any case, and I borescoped the entire intake plenum out of paranoia that it might have just been sucked into it.

Today I installed a new mechanical fuel pump as well as the carb. Made a few small idle speed and mixture adjustments and all was perfect for several ground runs. Fuel pressure stayed between 3-4 psi, much nicer than the 6-8 that I had been seeing. Then flew for 45 minutes with no squawks. Fingers crossed the issue seems to be resolved. I appreciate everybody's suggestions, and was also very pleased with the expert analysis from Savvy. I come away a tiny bit smarter after each issue I resolve! And to whoever posted the tip about using some string to help hold the fuel pump drive pin up while installing the pump - you sir are my hero.

Chris
 
YellowJacket RV9 said:
Wobble test. IIRC .026, within specs.
Click to expand...

Sounds good. Report back in 10 hours please.

In the meantime, I'll think about how a carb problem can be intermittent, and kill one cylinder. And where is the clue in this?

 
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I'm as lost as you. But, in the plot above, I do see the yellow line (forget which cylinder that is) actually starts to drop noticeably before #1 (in red), suggesting more than just #1 was involved. Perhaps a terrible mixture distribution contributed to #1 dropping off much more than others? In any case, I will certainly keep an eye out for any signs of continued problems, including stuck valve symptoms.

Chris
 
I would think if the cylinder was dead for a few strokes and the valves are working properly, the mixture would not be affecting the remaining cylinders so directly. They would keep getting the same stuff. Now a dead cylinder won't have the extraction from blowdown as normal, but being cold, should just allow it to pump a little more volume. Still, the other 3 should not be affected.

Thinking out loud -

Mixture - not for one cylinder- unless there is some vent in the carb that would send massive fuel only to one cylinder if it flooded badly. Not feeling likely.

Fuel slug - maybe if the EGT listings is off in the data call out and it is really #3 or #4 due to some yaw?

Spark - both mags? No, don't think so.

Back to valves, yes, feeling pretty likely. Again, is the EGT data probe assignment accurate? And were all the valves checked? Still feeling like a valve, with the "cross talk" effects on EGT of the remaining jugs. Dan had a good analysis of this. Rereading - what if a chunk of ice caught in an intake port?

Yes, do keep us informed as you get some more hours.
 
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az_gila said:
Did you disassemble and clean the lifter as recommended in the wobble test SB 388C?
Click to expand...

Yep.

Just throwing out ideas here as to causes - I know that carbs have a poor mixture distribution, so some cylinders will be richer than others. What if the mixture became so rich that the richest cylinders were actually too rich to combust, while the leaner ones managed to keep firing? In this case it would appear that #1 was obviously the richest and stopped producing power, but #3 also maybe became too rich and started losing power as well. This could explain the disparities between cylinders. I would have expected that 1 and 2 would have been richest due to the setting of the throttle plate, but there's a lot of swirling air in there so who knows. I do know that with the rebuilt carb and new fuel pump, my full-rich fuel flow is down significantly so it was definitely running too rich previously.

I'm not sure if there is anything else I can do to rule out the stuck valve theory, is there? If so, I will be glad to try. I am certainly keeping an eye on things and will always ground the plane at first sign of trouble, but right now it is running better than ever, with 2 known issues resolved.

Chris
 
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