O-470 R Continental

O-470 R Continental

<HTML>I had a factory new O-470 R engine installed in my Cessna 180.  It seems to run normally and well most of the time, except at full throttle.  It starts cutting out as if it has carb icing occurring.  Carb heat or a slight power reduction will clear the roughness.  This problem doesn't happen during take-off, when at the full 2,600 RPM, but consistently occurs when trying to set up   a full throttle cruise at altitude.  Any ideas as to what's wrong?</HTML>

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Re: O-470 R Continental

<HTML>Does it do it when at full throttle on the ground with the prop governed back at cruise?  (Say 2400 rpm?)</HTML>

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Re: O-470 R Continental

<HTML>Yes.  If I govern the prop back to about 2450 rpm almost immediately after lift-off (while leaving the throttle "full"), the cutting out starts within a few seconds.  It clears up quickly if power is reduced or if carb heat is activated.  It has done this since the very first test flight, 75 tach. hours ago.  It hasn't been much of a problem, due to the usual reduced throttle low level flying I usually do on floats.  However, it does not seem normal to have to use carb heat continously for full throttle high level cruise at say 6,500 msl or higher.</HTML>

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<HTML>Any ideas out there?  Should I call up the tech. rep. at Continental?</HTML>

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Re: O-470 R Continental

<HTML>It sure sounds as if you have a carburetor/intake malfunction.  The enrichment valve should come into play at full throttle, providing a slightly over-rich mixture during takeoff/climb.  Reducing rpm would normally give a slight increase of MP which would ordinarily cause a slight leaning of the mixture.  This could be the cause of the roughness, which in turn is improved when the mixture is enrichened again by application of carb-heat.
On the next flight, test the engine on one mag to see if it clears up.  (I suspect no difference will occur, or maybe even a slightly more noticeable roughness will appear.)  This will alter combustion timing and possibly help diagnose and/or eliminate a plug breaking down.
A broken or weak valve spring can also give this symptom, but if you've recently overhauled your engine, this is an unlikely cause.  Nonetheless, it won't hurt to pull your valve-covers and look.
Check for leaking intake manifold hoses or loose clamps. 
If none of those items lead to a solution, I suspect you'll need to confirm your carburetor model/status and send it for flow-bench check.</HTML>

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Re: O-470 R Continental

<HTML>P.S. And to answer your question, Yes, it never hurts to notify the TCM tech-rep.  They have usually seen all possible gremlins.</HTML>

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