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-   -   fuel problem (http://www.vansairforce.com/community/showthread.php?t=149960)

Azjulian 05-29-2017 11:56 AM

fuel problem
 
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 ?

snopercod 05-29-2017 12:00 PM

Does your exhaust system pass anywhere near the fuel lines or carburetor?

Azjulian 05-29-2017 12:54 PM

I did check that and there is good separation between the fuel line and the exhaust.

John-G 05-29-2017 12:55 PM

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,

Azjulian 05-29-2017 01:45 PM

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

rvbuilder2002 05-29-2017 03:29 PM

Quote:

Originally Posted by Azjulian (Post 1176417)
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.

deene 05-29-2017 04:52 PM

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...

Phantom30 05-29-2017 09:26 PM

Quote:

Originally Posted by deene (Post 1176461)
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.

Jim T 05-29-2017 09:53 PM

"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

Azjulian 05-30-2017 12:43 AM

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.


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