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fuel problem

Azjulian

Well Known Member
All - I'm hoping that the collective experience of the group can give me some help on this one. (sorry this is so long but wanted full info in the post)

Plane Info - RV12 ELSA, 5 hours engine time, its in Phase I, located at KDVT (arizona), no modifications, have all my logs in savvyanalysis. KDVT is a very busy airport you often have to wait 15 mins to get up. I am using autogas premium purchased in the last month from Circle K.

Problem - Twice now on take-off have experienced loss of power and engine roughness. This occurred at no other stages of flight.

My investigation so far -

1st Flight The first time this occurred was on the maiden flight, it was quick and we (I'm using the FAA co-pilot rule for phase I) continued with all the test of flight I in the Vans test procedures (flight time 1.5), outside temp 76F, we did perfom taxi tests and waited about 10mins to get up. At that point CHT was 205, stumble happened at about 300' AGL, fuel pres. 4.8psi and on take off had started at 3.6 when full throttle applied and rose up to 5 as we climbed. CHTs dropped back down to around 185-190 the rest of the flight. No other issues.

After the flight I called Roger Lee (Rotax expert in Tuscon), and based on his feedback I pulled the fuel bowls and checked visually the main jets, I also reran the fuel volume test. I also called Vans, they asked me to check that the restriction for the fuel return was in the correct line, I verified that the restriction was on the return line. I also pulled the gascolator apart and checked for debris, nothing. With the cowl off I checked all the plumbing, looked for any issues etc, nothing.

2nd Flight
- OAT 62.6F, CHT on takeoff 172F, entire flight the CHT remained below 180F. No issues, flew for 1.5. performed Flight 2 tests, including full RPM speed box and 10,000' climb out.

3rd Flight - OAT 92.5F, CHT on takeoff 205, but we had waited 15mins and the peak CHT we saw was 220F during that wait. We got a stumble at 600' this one was longer lasting 5 secs. In the air the RPM drop from the logs only seems to be around 200rpm. We decided to call it a day and had no other issues in the pattern or landing.

Things I noticed at this stage - 1. I can see that LEGT gets 150F hotter than REGT just before the stumble shows in RPM (1430F to 1280F) otherwise in flight EGTs are right around 1300. This seems to be temperature related, hot days and hot sun. I also noticed that my LEGT seemed to be rougher than my relatively smooth REGT, I re-did the connections and tested on the ground and got them both smooth. They track each other within 15F at all times.

Ground test - (no top cowl) with the plane on the ground and equivalent conditions. 100F OAT, I let the plane come to temperature and then did a WOT test for 5 mins (suggested by Roger Lee). 1min30s into this my CHTs are at 203F at 2min30 i start to get a stumble it lasts 15 secs, RPM goes down to 3610 my CHT is 210 looking at my logs I can see my EGTs starting to seperate about 50secs before the stumble by 100, fuel pressure is 4.9.

After that I took the carb off completely, stripped the bottom end down (main jet out etc) I can see no signs of any foreign objects.

I also drained my fuel and replaced with 100LL - for two reasons, I want to ensure I don't just have some bad fuel, and I'm wondering if I'm getting some heat problems with the fuel.

Does anyone have any ideas what else I should check ?
 
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Sounds like a classic example of winter blended auto fuel being used when the temps get warmer and causing a little vaporlock.

Would suggest you temporally blend some 100 LL with the auto fuel (have heard 30 % does the trick) until such time you are sure the auto fuel in your area switches to the summer blend which does not vaporize as easily.

Making a couple of flights on warm days using the 100% 100LL you switched to would likely tell the tale as well if there are no RPM drops. (Of course, this assumes the floats are good and adjusted correctly).

Happy flying,
 
I have 100LL in the plane now (16g), I'll report back in that.

Additionally I have noticed that my ground static WOT is almost identical to what I can achieve in cruise. Basically it right at or just under 5200rpm. I think I need to flatten the pitch on that to get me better in range so I can get a max if around 5600 - 5800 rpm. Anyone else seen their static and in flight to be the same ? My density alt is 4400 here with an elevation of 1500
 
I have 100LL in the plane now (16g), I'll report back in that.

Additionally I have noticed that my ground static WOT is almost identical to what I can achieve in cruise. Basically it right at or just under 5200rpm. I think I need to flatten the pitch on that to get me better in range so I can get a max if around 5600 - 5800 rpm. Anyone else seen their static and in flight to be the same ? My density alt is 4400 here with an elevation of 1500

This makes no sense, other than to say that you set the prop for any static RPM you wanted and then pull the throttle back to run at that in cruise flight, and say they are the same.

The only way to get a valid reading on the prop pitch is to use full throttle in level flight. A full throttle static RPM on the ground of about 5000 -5050 would probably be about right for your typical flight conditions.

I agree with the comment of winter blend fuel causing your problem. Particularly since it sounds like it has only occurred during warm temps. This is the time of year that people have the problem. I predict that with 100LL you will not see a occurrence regardless of the temp.
 
Scott and others are right on...winter auto gas with low vapor pressure (to enhance winter auto starting) being used in higher temps in springtime.

As recommended, add avgas, or run avgas until auto fuel summer gas is available.

We have several similar RV12 situations locally...
 
Scott and others are right on...winter auto gas with low vapor pressure (to enhance winter auto starting) being used in higher temps in springtime.

As recommended, add avgas, or run avgas until auto fuel summer gas is available.

We have several similar RV12 situations locally...

It is always wise to mix 30% 100LL with Winter auto gas...also, I would use name branded auto fuel (Chevron/Shell or the likes). The cut rate station gas is not the same.
 
"I am using autogas premium purchased in the last month from Circle K."

It would really help if folks, when talking about using "autogas premium", would say if they were using non-ethanol premium or premium with ethanol.

Jim
 
I will confirm with the gas station but my understanding is that it is with ethanol. There are very few gas stations in aronzna that I know if that sell pure gas with no ethanol.
 
No Winter Blend Here

First, I am in a small community in Western KY, and I use ethanol free 93 octane premium auto fuel that I purchase from and independent in Hopkinsville, KY. I asked the owner of this small petroleum products chain when the change back from winter blend would occur. He assured me that they never get winter blend in their locations. I fill my 55 gal barrel there, as needed, and I must admit that I have never had a problem using his fuel. Anyone else find that to be true, that they do not receive winter blend auto fuel in their area?
Tom
 
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The OP is located in Scottsdale, AZ. Pretty hard to believe they have winter blend fuel to make starting internal combustion engine easier to start in cold temps. Kinda like when people in Florida put up Christmas lights...

This sounds like vapor lock - fuel boiling in carb bowls due to high temps under the cowling. My guess is 100LL should fix problem because it is less volatile.

Also... don't add switch to shutoff electric fuel pump. It is there for a reason. Cool fuel is circulated thru the fuel distribution system to help prevent vapor lock. Electric fuel pump should always be running. If starting a hot engine that has heat-socked the cowling it is best practice to allow electric fuel pump to recirculate fuel for ~ 5 minutes prior to engine start so cool fuel is available at the carbs.
 
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This sounds like vapor lock - fuel boiling in carb bowls due to high temps under the cowling. My guess is 100LL should fix problem because it is less volatile.

Also... don't add switch to shutoff electric fuel pump. It is there for a reason. Cool fuel is circulated thru the fuel distribution system to help prevent vapor lock. Electric fuel pump should always be running. If starting a hot engine that has heat-socked the cowling it is best practice to allow electric fuel pump to recirculate fuel for ~ 5 minutes prior to engine start so cool fuel is available at the carbs.

I've always wondered about this. IIRC, the fuel return line taps off the spider block at the top of the engine and the cooler fuel doesn't get down to the carbs. Seems like any fuel that was boiled off would go into the intake manifold and result in a very rich mixture when the engine is started. I suppose some cooler fuel would refill the carb bowl after the boil-off.
 
I got a chance to fly again this morning (6am and it was already 80F OAT)

The 100LL did NOT fix this issue, I didn't experience the issue on initial climbout but I did when transitioning to a WOT climbout from a slow flight test.

I did test my WOT cruise, it took some time but the max I could get was 5500RPM.

My static WOT is 5200, on my climbout i'm seeing my RPM dropping slightly as I climb, by 3000' AGL i'm at 5060RPM
 
Just throwing this out there. Check the screen on the fuel pump. Had an issue like this once and that was the cause. To this day can't figure out where the trash came from, new tanks lines etc. But cleaned it and all was well.
 
Something else to try - -

Take fuel line off at carb one at a time and put in a container and turn on elec pump. Note volume of fuel, then put hose back on and do other side for the SAME amount of time. Use a stop watch or whatever. I am suggesting this will eliminate a slight restriction in one of the lines. Not likely, but a maybe.
 
John - I had the same idea as you and I didn't do anything scientific with measuring the volume over time, but I did run fuel through both lines when off the carbs and I got good flow...

I'm wondering if this is overload of the engine and that I need to flatten the pitch of the prop, which I will try next. Especially with the high temps here the inlet air to the carbs has to be very high
 
Just as a point of interest

I noticed that my oil temps were going up rather quickly last summer, just after I re pitched my prop to a more coarse setting. After checking out a list of possible causes, I repitched the prop back to a finer pitch and the temps went right back down. Im a few mph slower, but the plane seems a lot smoother and definitely runs cooler. I never had issues like what you describe, though.
I'm of the opinion you should leave the fuel pump on all the time in the RV12 just because we mostly run Mogas and it is more susceptible to vapor lock.
 
I got a chance to fly again this morning (6am and it was already 80F OAT)

The 100LL did NOT fix this issue, I didn't experience the issue on initial climbout but I did when transitioning to a WOT climbout from a slow flight test.

I did test my WOT cruise, it took some time but the max I could get was 5500RPM.

My static WOT is 5200, on my climbout i'm seeing my RPM dropping slightly as I climb, by 3000' AGL i'm at 5060RPM

As I wrote in my other reply, if your static full throttle RPM is higher than your climb RPM then something is wrong. The flight RPM (even in a climb... should always be higher by at least a little bit but the actual difference depends on your climb speed)

As for your roughness issue, if 100LL did not solve it then the next thing I would recommend is inspect for debris in the carbs. It is somewhat common with a brand new build. Stuff can get flushed from the new hoses and lines into the carbs and often causes problems intermittently. It may even be in some way causing a slight power loss only in flight that is effecting your climb RPM.

Your max level RPM of only 5500 when the static is 5200 is another indicator that something is out of wack. A static of 5200 should cause a level full throttle to probably be above red line (5800)
 
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John - I had the same idea as you and I didn't do anything scientific with measuring the volume over time, but I did run fuel through both lines when off the carbs and I got good flow...

I'm wondering if this is overload of the engine and that I need to flatten the pitch of the prop, which I will try next. Especially with the high temps here the inlet air to the carbs has to be very high

I would check the gascolator filter (clean it - I usually order spares from Vans - pretty cheap), and check the fuel tank screen. Then unhook the gas line into the engine driven fuel pump and check the fuel flow per the PAP. (1 gal min in 180 sec) If you are meeting the flow requirements, then you can rule out everything behind that engine driven pump. If not, look at the lines or a possible partial blockage in the electric driven pump.

Next I would weigh my floats, double check my fuel manifold block, and consider the engine fuel pump. Finally, I might consider sending my carbs in to Lockwood for a look over. Keep in mind that some of those checks require a supply of those copper washers possibly.
 
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Hi John - Good thoughts, out of that list here is what I have done so far.

I checked the gascolator and cleaned post first flight I had the issue. I did repeat the volume test by using the input line to the mechanical pump and was well within range. I'm going to repeat the test at the carb fuel fitting.

I talked to Lockwood and they suggested "borrowing" someones carbs from a known working setup to determine if this was a carb issue.

Julian
 
Hi John - Good thoughts, out of that list here is what I have done so far.

I checked the gascolator and cleaned post first flight I had the issue. I did repeat the volume test by using the input line to the mechanical pump and was well within range. I'm going to repeat the test at the carb fuel fitting.

I talked to Lockwood and they suggested "borrowing" someones carbs from a known working setup to determine if this was a carb issue.

Julian

I would offer mine - but the plane is up north and I'm hoping to finish my annual this weekend.
 
Update 6/3/2017 - I completely re-setup my carbs after cleaning, both mechanically and them using a carb sync tool. It was very hot yesterday (over 100F) with a density altitude of 4500'. With the engine hot (CHTs over 200) and the hot OAT I was able to replicate the stumbling I have heard in flight. It's a little random being able to replicate, it can take some time holding WOT and then the issue can just go away, have to say this is extremely frustrating.

https://youtu.be/mS7NNFnSRrg
 
Have you adjusted the prop pitch yet? Lugging the engine is bad for it and makes it run hotter. If the engine still stumbles after fixing the prop pitch, consider installing a blast tube from the cowl duct to the left carburetor bowl.
 
Hi Joe - yes I should have added that I flattened the pitch by 0.4 degrees which has increased my static rpm to 5350
 
I got a chance to fly again this morning (6am and it was already 80F OAT)

The 100LL did NOT fix this issue, I didn't experience the issue on initial climbout but I did when transitioning to a WOT climbout from a slow flight test.

I did test my WOT cruise, it took some time but the max I could get was 5500RPM.

My static WOT is 5200, on my climbout i'm seeing my RPM dropping slightly as I climb, by 3000' AGL i'm at 5060RPM

It is going to be hard to help you when you ignore some information being provided.....

The static RPM you originally mentioned was fine or actually just a bit too high, but now you have reduced the pitch even more. You are now way under pitched.
I previously mentioned a serious discrepancy in the information you were providing..... that your static RPM on the ground was higher than what it is climbing after takeoff. This should never happen, so it is probably a major clue in solving your problem.

Like I said before.... you very likely have a carb. problem caused by contamination resulting from it being a newly completed airplane (very common). Lockwood even tried to head you that direction by suggesting you try a different set of carburetors. What process did you use to clean the carbs? It will require more than just removing and cleaning the float bowl.
 
Hi Scott - I apologize if you feel that I am ignoring advice. I'm actually trying to attack this in a very methodical way. On my last flight I checked exactly what you asked, with my 5100-5200 static WOT RPM that does change with temperature, I was seeing a slight increase in climb (about 50 or so rpm). On cruise WOT max RPM I could see on that day was 5500 with a density altidtude of around 6500'. I talked to both Lockwood and Roger Lee, who both suggested flattening the pitch by about 0.4 degrees, which I did. My mistake on interpretation of the static WOT was not do a same day direct comparison as I am seeing a slight difference in WOT static RPM from day to day depending on temperature.

To clean the left carb I removed the top and the diaphragm, the float needle and assembly. then used carb cleaner and compressed air and blew out every single orifice and jet. I took out the needle and check position and cleaned the assembly.

I did not clean the right carb to that extent yet as due to the EGT reading Lockwood and Roger Lee both felt that it was on the left side that I had the problem.


I also ran the fuel lines with the carbs off to ensure no contamination.

I then reassembled and started at the beginning of the carb chapter in line maintenance I setup the carbs from scratch, idle mixture, mechanical setup, the. I synced the carbs and did that sync from idle all the way up to WOT. They are now very balanced.

When I changed my prop I triple checked that my pitch on both blades to ensure balance.

Additionally I did squirt carb cleaner at the manifold and at the carb socket to see if I had a leak anywhere.
 
Scott - can I confirm your concern, from your message I'm reading that you would expect the static WOT RPM to be lower than the climb WOT RPM and then that cruise WoT RPM ?

We are just not seeing that here, I had a friend who has an SLSA RV12 check and his ground static WOT is 5500RPM, at 75knt climb out he is seeing between 53000 and 5350. Then in level flight WOT he gets 5600
 
FWIW, here are my approximate RV-12 specs (with wheel pants): Ground WOT @ 4,950 rpm, Climb-Out WOT (@ 75 kts) @ 5,250 rpm, and Cruise WOT (@ 6,500' msl) @ 5,650 rpm. My Idle is set 1,650 rpm static on the ground and this results in @ 2,000 rpm on short final. I typically cruise at @ 5,250 rpm and see @ 108+ kts IAS.

IMHO, if your Ground WOT rpm is higher than your Climb-Out WOT rpm, then that sounds like an engine issue, not a prop pitch issue.
 
I'd clean out the right carb too. It is not unusual to have the instrumentation switched between the right and left side. You may be troubleshooting the wrong carb. Easy to mistake if wiring from the front and using your right instead of plane right.
 
I unplugged my EGT lead prior to ensure that I could be a 100% on which lead was giving the higher reading.

I have to say I'm completely out of ideas now and seriously concerned that I am going to have a mechanical issue with this engine. I will strip the right carb as well. But this issue remains, has been present since flight 1.

I don't know anyone whose carbs I can swap out.

Personally I'm starting to think this is to do with the internals of the engine, a sticky valve sleeve at higher rpm for instance.

I built a plane with a brand new engine for a reason I didn't want to have to chase these issues. Has anyone gone the extra step in a situation like this and pursued a warranty claim ? How do I go about that ?
 
As I wrote in my other reply, if your static full throttle RPM is higher than your climb RPM then something is wrong. The flight RPM (even in a climb... should always be higher by at least a little bit but the actual difference depends on your climb speed)

As for your roughness issue, if 100LL did not solve it then the next thing I would recommend is inspect for debris in the carbs. It is somewhat common with a brand new build. Stuff can get flushed from the new hoses and lines into the carbs and often causes problems intermittently. It may even be in some way causing a slight power loss only in flight that is effecting your climb RPM.

Your max level RPM of only 5500 when the static is 5200 is another indicator that something is out of wack. A static of 5200 should cause a level full throttle to probably be above red line (5800)

Scott - can I confirm your concern, from your message I'm reading that you would expect the static WOT RPM to be lower than the climb WOT RPM and then that cruise WoT RPM ?

We are just not seeing that here, I had a friend who has an SLSA RV12 check and his ground static WOT is 5500RPM, at 75knt climb out he is seeing between 53000 and 5350. Then in level flight WOT he gets 5600

Yes as indicated in my earlier post

More bad data I would guess because an RV-12 would be a horrible performer with a WOT static RPM of 5500. It is impossible to have a WOT static RPM of 5500 and level flight WOT of only 100 RPM more.

Doing a static RPM check with a wind blowing will give a false reading but to get a static of 5500 with a properly adjusted prop pitch would probable require the nose to be pointed into a 100 MPH wind so that is not likely the explanation.

Every RV-12 I have flown or worked on (at least a dozen I would guess) and every fixed pitch propeller RV I have ever flown or worked on has had a WOT static RPM that is lower than the WOT climb RPM. Basic physics makes that happen.
You are correct that the static RPM will be different from one day to another (or even time of day) because of the influences of air density, but the variation isn't typically very large unless you compare coldest day middle of the winter to warmest day, middle of the summer.
 
Personally I'm starting to think this is to do with the internals of the engine, a sticky valve sleeve at higher rpm for instance.

You are using Aero Shell Sport 4 Oil?

Did you do correct oil purge prior to first start so hydraulic lifters pump up?

At risk of getting yelled at here... Try adding Marvel Mystery Oil 2oz/5gallons to your fuel to add top-end lubricant for valve stems. MMO is synthetic and will pass through the engine without burning.
 
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HinJim - yes that oil came with my engine, and I bought a new oil filter to replace the one that was on the engine when it shipped. I performed the oil purge procedure and tested each of the lifters as part of that procedure and they were all good.

It's interesting you mention that I was talking to a Rotax certified mechanic last night about coming to take a look at the engine next week. When I walked him through my symptoms he said that he had seen a sticky valve stem issue only surface at high RPM causing very similar symptoms....
 
Scott - I understand exactly what you are saying and I wasn't there when friend tested so let's exclude that data. But what I can tell you with certainty and I'll post an image of the log (when I work out how to on this forum) that when I start my ground roll at WOT my rpm reading was 5170 (this is prior to adjusting pitch of my prop, I haven't flown since doing that), in my climb out (I did not touch my throttle, its pinned to the firewall) I see the rpm drop slightly and at the top it's 5070 and I can see in savvyanalysis it's been declining gradually. At top of my climb I level out still WOT and manage 5510.
 
It's interesting you mention that I was talking to a Rotax certified mechanic last night about coming to take a look at the engine next week. When I walked him through my symptoms he said that he had seen a sticky valve stem issue only surface at high RPM causing very similar symptoms....

Sticking valve at high speed is a possibility. Aircraft engines typically use low force valve springs to reduce wear on cam lobes/lifter.

I use Marvel Mystery Oil at 2 oz/5 gallons about every tenth fill-up. Now at 230 hours with like-new performance.

Couldn’t hurt to give it a try…
 
Log of climbout

9aUVLh0p3QUNv4fjm8DNTnUXDF1jb6-NWe6RFhCI_Ba3_BMWXG73TejBrurRQbM7aldVq1x3nOUORNZQwJRDKO7HNfI_7c4VJHeIqKe1NptjfkekMynkhzCFuxnjPy3PONI8JZOhR5dfXTRTFhdKaG2KcNYwvpUt5rw32KgcfqEDpz0G1DCshDdJ1z0P762hadSe8me7imzsCtqs8fPDMtw9Fz6HIRZO-labZS6MxT0SmNM6pQs9DrjyMjbgXBkHkKvjmSwBtH-ryqjEQuFalYYi30kjSsHhW_dMFd_iFuge5xm2hCno8WxX00MI9tMA2EtvB641nrckDASiqSwOzRpVWcepOqvrhOjGtrElxF0OTbxCXEqtQL3cCRgI0m2ozSB0hitioAp2hPLBXDk4MTOOXj5S01p-Zlx5ggG1wNnjoVDDpnHUuJOgGHTvKeKX2IS2oEXm7aiD6_8o7w3ns6b8ksusYNJQY47algpELTq_EpeopiYqb8qO7Ot5rS2Iy901wlCIj7mLZBBp58d5WALWIdOGArBKQRnqd9v1YQXmgrcmpQgOdipYy1Q8QBcW0K_MtrB0l6H9S3SjOc9Xl01fhN4L0LcPsBa0mupmBnx4RbBr=w2208-h1242-no
 
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Very odd indeed. Could be valve hanging up. What happens if climb to 2K and level at cruise power instead of longer climb to 4K?
 
Try two tings...

1) Reduce climb rate - I can't tell from scale on your graph but appears to be near max 1000 fpm. So try slower climb with slightly reduced power - pull throttle back ~ 1" after initial climb is established. I Assume you are flying single-place and min fuel to stay lightly loaded.

2) Reduce power when you level off so engine stays 5000 rpm. Ok to let speed build slowly and not push over the top to gain speed.
 
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I don't understand why both EGT's start going low in the climb. Maybe vacuum diaphragms that set fuel mixture based on altitude are working correctly. Maybe you have induced pressure or weird air flow in the cowling causing carbs to calibrate mixture incorrectly.

Do you have newest design where fiberglass plenum is no longer used to direct cooling air to cylinder fins?
 
it does have a solid climb rate, I am using the FAA co-pilot allowance to do my phase I so its weight is 1250 in the tests at takeoff. climb rate is around 700fpm.

This is what happens when the stumble occours mid flight

O_98QEvhgYl0pH3tZ3wtl1Q6foMYiiLcWOdTGQL--Y6CmHf7sslvRd5AOfztCgUIcRS51hGD-EIb_dw6wY5ce0Est6NBJxFp7TGFyqcKjcW9ExXfvW5-XNMTfx5d2TjCfBDfpZQh0W56Z9h7PIaopGvq6AcXIb4fOX7ogiofTzTIgRTBXAndPLrUaWceyHf6P6JoltaBK9K7B0d-CgBPLh-6c3QQvdUNQsvRoJVHP_hmUbe8gskJrbpFHirRVNhdd6ETanuSym2wJbDv-9O9pFG8HGg2qtnGuyx9aVwWgQdo0S2hPJRK0mhW7TbKkUKxnideVsDORXLfxQGMMqGCZFlxbO9gQZgOeC8LDKNUbmhEl4ZJEonZehrq2GvwSPqqfo11bXaw_AzEzyJeirnPT2E7WkZ-GfTzdYC4-uXs4bq18zft5tPZX8fYki3sVkRQtJNWJhiBYyyrCkCI39r6X8vlqrn_C3OVeqPEWVh9rDgX_Y5B4XBjBlgVgYScJPi8szxJjBI9k_8fxDEw_qHsiFR8QzqXRODVo0WKjzSNTEzkxPJUSiPX0_GA7LfxW9PSWFA6_P0NQ6K6lzJt4R6cALMk8SVFf4EBL7j1ykzxh3hNgRFQKIa_xE3Bun0dH9v8W-7cSFPotbDDQtpYuGMExVpkT0RI5jexJyciUz6mnQ=w2208-h1242-no
 
Could also be intermittent ignition. Next time in cruise flight mode with engine behaving well - try shutting off one ignition at a time and note rpm drop. Should be very little drop at cruise rpm.

Do same as above when engine starts doing its dance and again note rpm drop between both ignitions.

One ignition module could be susceptible to thermal problem.

As an aside... I frequently test both ignitions in flight especially before jumping over large expanse of water.
 
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In the video I put a link to above, the plane is on the ground at WOT the same symptoms occur, stumbling, EGT gap. I test switching off the ignitions alternately and they have no impact to the condition.

If I was significantly down on power I'd expect lower climb rates, but I really am right in the ball park for the density altitude on my climb rate, my cruise speed with 5500 rpm is right at 100knts.

I dont know how stable everyone elses engine is when set to a particular RPM but I do notice a wandering of about 75-100 rpm at all ranges. so I set to 4000 to do a mag check and its moving around in a 75-100rpm range.
 
Tank?

Trying desperately to keep it simple here - a long shot and easy to check. Check your return line fuel flow and tank vent, etc. Hope this helps.
 
Another guess entered - -

Have you carefully looked at both diaphragms ? Screws holding plates above them tight ? Diaphragms in proper clock position ? Balance tube and related all seem good ?
 
on the return flow - that was a suggestion that Vans tech support had. I disconnected line at return (on firewall) and at the fuel pressure sensor. I confirmed that the restrictor was in place on the return line and there was fuel flow. I also can see that i'm getting a constant flow when the engine is off but electric pump is on of 1.5GPH.

I did check both diaphragms and can confirm the rubber key / tab was in the correct position in both the diaphragms were secure and in good condition.

The balance tube etc is all secure and I confirmed the M6 screw in front of the balance nipple connector (to attach a balance nipple) was secure on both manifolds.
 
next time I fly I will check the ignition test in cruise as you suggested

Even with two people aboard, I would still reduce power after initial climb established (VS~500 fpm), perhaps only climb 2000 AGL, and reduce power further as you approach cruise altitude so that cruise becomes 5000 rpm with no engine speed overshoot.

Please report back after next flight. Best of luck.

King Donald would have you powering it with "clean coal"... :D
 
1) I asked in early post - do you have fiberglass shroud that directs cooling air to the cylinder fins? This is a big fiberglass mask on top of the engine that is connected with scat tube to left side air inlet on front of cowling.

2) What type of fuel tank vent do you have - vented cap or new vented tank?

3) Because your situation is so weird... how about looking into fuel tank filler neck and visually determine if return fuel is seen as a "fountain-of-liquid" coming back to the tank? The return line terminates at the base of the filler neck and the discharge is oriented vertical "up". Do this with only electric fuel pump running, and of course, no open flames...
 
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sorry Jim - I meant to say I do have the fiberglass shroud over my engine fins connected to the scat tube on the left.

its the new vented tank

the vent line terminates near the base of the filler neck, but the return line terminates right next to the fuel output on the bottom of the tank
 
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