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2200 to 2230 Max RPM

Recently I modified my Osborne tip tanks to feed into my main tanks as opposed to feeding directly to the engine. This change necessitated removing the 5 way fuel selector valve and installing a rebuilt, standard 3 way fuel selector valve (Part #36-380026-1). At the same time I changed out the air filter. I have a turbo-normalized IO-520 that has been on the aircraft for 150 hours of Hobbs time.

The engine starts fine, idles fines, and accelerates normally until approximately 2200 RPM at which point adding fuel results in no additional RPM but reduced RPM because. Turning on the boost pump at this point reduces the RPM even more.

I’ve checked all connections for fuel leaks and found none. I checked the mixture at idle and found 25 RPM rise as the mixture is slowly pulled out. All cylinder temperatures are similar and normal. The character is the same on both the left and right tanks. I am starting to suspect an air leak that gets worse as RPM increases, but see no evidence of a fuel leak when I pull the mixture off and run the boost pump to circulate the fuel through the system.

Model: Bonanza F33A
Aircraft Serial Number: CE-560
Posted 11/24/2023 - 3 months ago
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I should add that we have also hooked up a gauge to the unmetered fuel and have 10 PSI at idle. The spec for this engine / turbo set up at idle is 9 - 11 PSI. At 2200 RPM the unmetered fuel pressure is 17 to 18 PSI. That pressure is reached at about 1800 RPM and doesn’t really increase.

Posted 11/24/2023 - 3 months ago
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I also need to say that as a part of my troubleshooting I suspected the prop governor. So I the governor removed and bench tested and it tested good. After I put the governor back on the aircraft I ran the aircraft up and got to 2200 rpm. I then leaned the mixture and could get to 2250 rpm. In my mind that ruled out the governor as the issue.

Posted 11/24/2023 - 3 months ago
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Tom;

Suggest checking the fuel return system.

It sounds like there is either a restriction or blockage and that is forcing the engine to 'try' to consume the fuel, but is bogging down from being over-rich. Assuming the fuel levels are at least 5-gal low in each main, with engine not running, the throttle open and mixture at ICO, boost pump on high: the fuel return stream can be seen through the fuel cap opening looking outboard with a mirror and flashlight. Check both tanks. With left selected, the stream will be seen in the left opening only, and same with right tank; through right opening only. If return is only a dribble or non-existent, have the return system plumbing at the fuel valve inspected for conformity. If plumbing is good, I suspect the 'new' valve itself as the issue.

Mike
ABS Technical Advisor

Posted 11/24/2023 - 3 months ago
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Checked the return fuel stream to both left and right tanks. The flow was return was a steady stream - with left selected left tank had return stream, with right selected right tank has return stream. Any checks I can do on the valve? Also, are there any pressure checks I should do to see if there is an air leak?

Posted 11/24/2023 - 3 months ago
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That test also proved the return flow path was unimpeded and correct in the valve. If you had an air leak in the fuel lines large enough to do this RPM reduction, you would have fuel leaking out at a pretty good rate also. A couple of questions:

How long was the aircraft out of service (sitting without running) during the plumbing change?

If the two valves (feed and return) inside the selector assembly were not clocked correctly (there are two possibilities in this clocking), it would reduce fuel flow for sure. This can be checked by directing the -6 fuel line from the firewall to engine pump inlet into a bucket and turning on the fuel selector and boost pump to gauge flow rate; it should come out with some exuberance. Was the valve overhauled by a quality source?

Mike
ABS Technical Advisor

Posted 11/24/2023 - 3 months ago
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Valve was overhauled by The Aviation Group in Peachtree City GA. I will check for exuberance later today.

Posted 11/25/2023 - 3 months ago
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Aircraft was sitting for about 4 months during the mod. I took the fuel filter off and am cleaning that now. Don’t know what new looks like but it looked like it could be cleaned. Reinstalled fuel filter. Flowed the fuel at the fire wall using the boost pump. The flow was very strong. Ran again today and still have 2200 RPM max.

Posted 11/25/2023 - 3 months ago
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What happens when you are idling with the aux fuel pump off?

On my Bonanza (with a 550), the only time I turn on the aux fuel pump is to prime the engine. In this action, I operate the electric pump for about 2 or 3 seconds and then turn it off.

If the RPM is still limited to about 2200 RPM with the fuel pump off and the mixture full rich, then I would want to see with someone had done with the controls. IE, have the messed up the throttle and the RPM controls?

Tom

Posted 11/25/2023 - 3 months ago
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1. Is your modification supported by and IAW the Osborne STC?

2. Could this be a tank venting issue?

Posted 11/26/2023 - 3 months ago
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Found the issue. After going through the fuel supply issue in some detail we determined the fuel system was ok and started suspecting the turbo induction system. During the troubleshooting we took a look at the turbo and found it seized. So not good news but at least I know what needs to be done.

Posted 11/26/2023 - 3 months ago
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2200 to 2230 Max RPM