My cessna 185 runs rough and drops power when operating at high power settings. what could be the cause of this
There are many things to check. Start with the simple things.
First, how good are the compressions? Check, gap and test the spark plugs in a proper tester. Check your Fuel Filter and check your fuel flow. To do that remove the fuel pipe downstream of the filter from the mechanical (engine driven) fuel pump, (Fuel Selector OFF), then position a one gallon fuel container at the end of the pipe, turn the fuel on and make a note of the time it takes to drain off one gallon. If it takes for example 2 minutes, then clearly it is delivering 30 gallons per hour. The aircraft maintenance manual will tell you how many GPH it should deliver. For your aircraft it should be at least 20 GPH. if not you might have a fuel starvation problem. Just because it delivers more than it consumes does not necessarily follow that it is delivering enough fuel to create a good 'Head of pressure'.
After that have a good look at the magnetos. When checking the mags, it is best to remove them. Look closely at the impulse coupling, especially the spring. It is not always obvious that the impulse spring has broken, as when the spring is wound up it will bind on itself and the impulse will work, but when it is loose in its housing a broken spring will allow the coupling and timing to drift and the engine will run rough and loose power as you describe. Both Slick and Bendix manuals are readily downloadable from the internet. Slick magnetos require frequent checking, especially the internal timing and points gap. If you don't have the kit to set the timing, let your local aircraft engineer do it for you, You'll need to be qualified to re-install the mags anyway. Both types of mags have problems with rotator bearings, Slick more than Bendix. If the bearing at the points cam end is worn the points opening will be effected badly and will cause the engine to misfire badly, particularly at speed, but will purr like a good'n at low RPM. This happens because the rotor will try to 'climb' the bearing and will often result in premature points opening or not opening at all. Moving the rotor from side to side against the points will show how much the points gap is effected. The mag also needs to be checked for internal tracking, (where the spark has decided to go elsewhere). Both Slick and Bendix have had problems with the coils in the mags. The faulty coils act just fine when cold but as soon as things warm up a bit, they 'break down' in the windings and fail to deliver a good solid consistent spark. Your Mag should be check that you have the correct coils installed (there is and AD on this).
Having done all that, the injected continentals (I'm assuming you have an IO-470), do sometime have problems with the engine driven fuel pump and metering unit. These units are notoriously difficult to set up in the field and should be bench set. It will be expensive. You can most often tell when there is a problem here as the engine will only run nicely at one particular RPM and you have to mess about with the mixture to get it to do so. Don't go there? if you suspect the fuel unit, send it away for repair!!
Best of luck, Rgs. Bob.
9/9/2018 11:17:07 AM •
Cessna 185...
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Answered
on Sep 09, 2018
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