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Classic Wings Magazine WWII Naval Aviation Research Pacific Warbird Digest
Final Cut-The Post War B-17 Flying Fortress and Survivors - 5th Edition


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PostPosted: Sat Dec 31, 2016 1:11 pm 
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Been troubleshooting an odd problem with my left engine (R-985) on the Twin Beech and running out of ideas, so I figure I'll throw this out here and see if anyone has any thoughts, or experienced this.

Both engines are perfectly matched below about 6,500' MSL. As I climb above 6500' I have to start pushing the left throttle up past the right to keep MP matched. By 8000' I'm WOT on the left with MP dropping in the climb. For reference the right engine at WOT at the same altitude will give about 2" MP more than the left.

Now, the interesting thing I recently noted is that when I transition from climb to cruise up high, I lose MP on the left engine when I pull the prop back to cruise RPM. I've never seen that before in a constant speed prop. For example, if I level off at 11,000', I'll be a WOT on the left and 26.5" MP. As I pull the prop back from 2100 to 1800 RPM, the left MP will drop to 25" MP (right engine doesn't change with the prop at that altitude, so I end up pulling the right engine back to match.

I don't believe it is a gauge issue: The engines are perfectly synced and fuel burn between sides is within a gallon or two after a 3+ hour flight (for reference, cruise burn is about 22-23 GPH per engine).

We have been talking extensively with Covington and have not been able to find any cause. We soap checked the engine at the annual (last month) and couldn't find any induction leaks. Per Covington's recommendations, we inspected the blower and it is in fine shape. No evidence of FOD damage. Compressions are near perfect.

Any other ideas to check?


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PostPosted: Sat Dec 31, 2016 2:22 pm 
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Did you try switching the manifold pressure hoses between the left and right indicators to be absolutely certain that it's not an indication problem? One other thing might be to try disconnecting the manifold pressure line at the gauge and blowing back toward the crankcase with low pressure shop air. That cleared up a similar problem in a DC-7 once. The line had a small amount of oil or carbon restricting it slightly and blowing back cleared it.


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PostPosted: Sat Dec 31, 2016 2:34 pm 
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Larry Kraus wrote:
Did you try switching the manifold pressure hoses between the left and right indicators to be absolutely certain that it's not an indication problem? One other thing might be to try disconnecting the manifold pressure line at the gauge and blowing back toward the crankcase with low pressure shop air. That cleared up a similar problem in a DC-7 once. The line had a small amount of oil or carbon restricting it slightly and blowing back cleared it.

I might try that. But, if it is the a gauge or gauge line issue, wouldn't I have uneven fuel burn or props out of sync when the needles are matched on the gauge?


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PostPosted: Sun Jan 01, 2017 12:58 pm 
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If the problem is with an incorrect indication and the manifold pressures are actually equal, there wouldn't be any difference in fuel flow and prop sync has to do with constant RPM rather than manifold pressure. It's probably worth switching the lines just to eliminate a gauge or partially blocked line as a possibility. It will make things interesting while trying to set up power on the engines in flight, though, with the readings switched from left to right and vice versa.


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PostPosted: Sat Jul 07, 2018 12:24 am 
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You have some kind of blower or induction leak IMHO. Please let us know what you find. I work on R2800's for a large cargo outfit in Alaska and when this happens usually the engine gets pulled for rebuild. I would start by carefully checking all your induction components you can get to first. Be careful don't overtorque anything....Good luck. -Robert

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