The other thread was getting pretty long, but here it is for those that didn't see it. http://www.rcuniverse.com/forum/m_24..._1/key_/tm.htm

Yesterday I ran the engine the first time since making some changes. First I installed a new tank, 10x6 prop instead of a 10x7, new needle valve to replace the pitted one, new head gasket and new backplate o-ring. I also bought a tachometer to get some actual numbers.

Started it up and ran it and adjusted the needle valve to a rich sound and turning 12,500 rpm. First thing I noticed was how much more responsive the engine is with the 10x6 compared to the 10x7. After a while running, the engine picked up 2000 rpm and was running at 14,500 rpm towards the end of the tank. Changing nothing, I stopped the engine and refueled. It then tached at 12,500 again. So, it appears that if I tach the engine on the ground at 12.5K I can compensate for the leaning out towards the end of the tank.

Is it normal for an engine to gain 2000 rpm?

I then ran the engine again and tached it at 12000 rpm, then the max it ever hit during that run was 14000 which keeps it from over leaning. Max rpm I saw was 14,600 with the 10x6 Master Airscrew running on Cool Power 15%.

I also played with the low speed mix. sure enough it DOES affect the high speed needle setting as several mentioned in the old post. I richened the low speed needle until the engine would slowly load up and die. I then leaned it until it would idle for an extended period of time yet still throttle up relatively clean. I think I'm ready for a test flight.

MIke

Good job Mike. I'm still puzzled by the differential rpm between a full and a almost empty tank? Is you tank mounted low relative to the needle valve?

Enjoy,

Jim

It seems like it's definitely running leaner as the tank level drops. Just for curiousity, run it at the 12,500 with a full tank but then try closing the throttle slowly from full open and see how far it closes before the revs drop a little. Leave the throttle at this setting for the rest of the tank and see what rev increase you get. If there's not much increase then I'd say the carb is too big for the revs you're running. Closing the barrel slightly simulates having a smaller carb that has better fuel draw.

The tank centerline is about 3/8 lower than the needle valve. On test stand it didn't seem to matter where the tank was. Even high it would eventually lean out.

With the latest test it's not near as bad. Before I'd have the thing so rich it'd barely have enough thrust to take off our grass runway, yet toward the end of the flight it'd be wailing. Now the difference in rpm is less, I think... I didn't have a tach when I did my early tests with the 10x7.

Mike

Mike.

Are you using the Super Tiger carb?

In the 40 through 90 size Super Tigers, There is only one carb body. They will all fit in either engine.

The difference is in the spray bar. The 40 - 51 engines have a round sided spray bar. The 61 though 90 have a spray bar with flat sides on it.

Even the original stock carb for the 40 is rather large. If you have a ST carb with the flat sided spray bar in it, The carb will be too large and cause the problems you are experiencing.

I forgot to notice which muffler you are using. The economy muffler will produce more muffler pressure than the swing muffler.

Enjoy,

Jim

It's the stock ST carb with the round spraybar. The muffler is the larger volume "silent" muffler. It does act like fuel draw is low due to the large carb throat. might try running it with it less than wide open to see if it leans.

MIke

There is a baffle in the silent muffler. It can be removed by calapsing it with a rod through the outlet hole. Then it can be pulled out through the inlet. If the baffle has been removed, the muffler pressure to the tank will be less.

The muffler pressure nipple can be drilled out a size larger to increase pressure to the tank.

The nipple going into the carb can also be drilled out to increase fuel flow to the carb.

Enjoy,

Jim

Is any of this necessary. I mean there are 1000's of these engines out there and I've never heard of this problem. I don't want to start drilling things out unless absolutely necessary.

Mike

There could easily be a burr partially blocking the pressure fitting on the muffler. You would not be drilling it out so much as checking it out.

Ernie

just to review this.....

do you see a rapid (within 30-60 seconds or so) elevation to the extra 2000 rpm, or does it slowly gain rpm through the run ??

If you throttle back to low/mid throttle..... hold it there for a minute, and then go back to full throttle..... does the engine jump to where it was, or does it drop in rpm and then starts the RPM climb again?

Super Tiger does have quality issues at times. During the last days of the Italian production, I bought a 90 and a 61ABC. They both had 40 type carbs in them. Also I bought a new needle valve along that time to fit in a engine tah was just a year old and the new needle valve was .0005" too large to go into the hole. I polished it down to where it would fit.

Enjoy,

Jim

The one run it did ramp up 2000 rpm in about 30 seconds. Muffler nipple is clear and you can see bubbles flowing back into the tank so I know it's at least getting some pressure. You can blow through the pressure line into the muffler no problem. I'll have to do more testing. This engine has always performed like this. I'm wondering if it is an issue with the entire carb, but then why does it only show it's ugly face after about a half tank of fuel? By the way, this engine is an Italian one purchased in 1996. It had low hours due to this frustration with it so it sat. Recently it was freshened up with a new ring by Frank Bowman.

Mike

Idunno.

All other possible sources for airleaks isolated, and confirmed fuel system integrity...... almost sounds like a crack or pinhole in the case, head, carb or something. Thermal related... lets air in after the engine runs and warms up.

I am a bit confused though. Perhaps I was not descriptive enough in my question. Just trying to isolate cause/effect.

Does the RPM rise happen within 30-60 seconds from when you first take it to stable full throttle (rapid rate) ?
In other words, it runs at 12K rpm for a minute, then over the next minute begins to lean to 14K ?

Does it quickly jump the 2000 rpm at a specific tank level?

Or does it creep up there in rpm as the tank level drops ?

Perhaps best answered following a subsequent test run.

Bob

Let me try to explain it better.

I started it and tuned it to run full throttle at 12,500 rpm. It ran like this for about 3 minutes, then like I mentioned in about 30 seconds, started gaining rpms up about 14.5K rpm. I'll run it again and look at when it gains rpm.

MIke

Does this happen with the tank about half empty (half full ) and are you suddenly seeing air bubbles in the line? Maybe a pin hole leak in the fuel feed line that gets uncovered.

Yeah, thats what I was getting at. This seems more like a cracked pipe, cracked clunk or tube pinhole issue somewhere within the fuel tank.

But I seem to recall it being mentioned that another fuel tank had been used for bench runs. If not, that would be the first thing to do. Different tank, good test stand.

Yes, everything has been replaced....everything. Including copper tubes going to into the tank.

Mike

Remember how the glow engines used to "lean out and scream" near the end of the tank in the old days before muffler pressure?

I'm still wondering if it is a fuel draw and or pressure problem....

Ernie

Yesterday it was unseasonably warm here and PA and I got to fly the plane. The temp was in the low 40's so I headed to the field. First thing I can say is this engine really loves a 10x6 prop. I tuned it just lean of the two-stroke break in sound on the ground. I flew it and the plane was a rocketship compared to the 10x7 on it. Anyway, I flew for maybe 5 minutes and the mixture stayed the same. It did stall right after landing when I tried to throttle up to taxi back. the cold must have affected my idle mixture. I didn't feel like flying again since there were several helicopters up and I hate flying when they're flying since they stay close and are too noisy, drowning out the sound of my plane's engine. After counting the turns of the fuel pump, I found out that I still had about 3.5 ounces in the tank. So I now know that I can get at least 5 minutes of flying with the same settings. One day when there's less people in the air, I'll fly longer and listen for any change in mixture.

Mike