Started my freshly field overhauled O-200 for the first time, today. Engine is in a C150. The engine has a scatter miss below 1200 RPM. Is the same on either mag. Mixture setting makes no change. Both mags were oh'd. Carb was oh'd with latest float and nozzle. Above 1200 RPM it is smooth with 50 RPM mag drops. Almost feels though one cylinder is not firing all the time. Sticky valve ? Have not run above 1500 RPM.
Has new Millenium cylinder assemblies. Whada ya think ?
Do you have the Intake spider mounted to the case with the proper stat-o-seals? Check the IPB to see the part number.Originally Posted by Eric Brown
these symptoms are typical of a carb/intake spider that is mounted solid.
Measure spark plug resistance with Volt-Ohm Meter. If greater than 6000 Ohms, you need new spark plugs. With such high resistance the spark is very weak, which manifests in rough running at low RPM when fuel/air mixture is very lean. Increasing RPMs increases fuel air mixture and engine smooths out.
Joel Marketello
I do have new stat-o-seals at the spider mount. All spark plugs are new, but I could check each one just in case. I'll let you know.
Next step, sniffer test around all of the intake hoses. see if the engine surges when a little WD 40 is applied at each hose. (both ends)
After that, disconnect the primer line and cap it at the spider.
Unlikely that new plugs are the fault. There are advocates of 6000 ohms resistance being significanat and others who say if it fires well on the screen under pressure testing it should be good. A defectivee plug is not likley to clear up at 1200 rpm unless it was fouled, and then more likely to need more rpm to burn off.
The carb may have problems at idle and low speed and then engine runs good when it is on main metering system above 1200.
If you start engine from cold and it fires right up it is likely that ignition system is good. You can start from cold,run 20 sec with it missing, then shut down and check which if any cylinder is cold or if any spark plug is cold, ie not firing.
I'd tend to agree with Bill on this one. I've seen "overhauled" carbs that show the described conditions coming off the idle circuit on to the main circuit usually due to the passages not cleaned out completely or have sat around with old fuel in the bowl.
If God had intended man to fly He would have given us more money!
I would go with possible carburetor problem and ask, was it overhauled by aviation carb overhaul shop or someone local ? I have seen some bad ones done by people who don t do it completely. Also, if it has new one piece venturii, was the "flash" left by manufacturing process cleaned off before installation ? Just listing some of the things I have seen over the years. Also, was new gasket installed between carb and intake mount ?
Bill L
Bill L. you may have missed it but Eric wrote that he had tried an entirely different carb on the engine and no improvement, same as he tried other plugs, other mag , other harness. etc.
Eric, when you tried the new mag was it with an whole new cap and harness? Caps can have fine cracks.
And since we are at the Ouiji board stage, are you sure the engine is missing and not just a vibration. I once in flight lost an entire cylinder on at 4 cylinder 0360 in a Mooney and it was definterly rough no doubt about . Turns outh the OH had not secured the valve guide right and it worked its way out of the head and hammered both plug gaps closed.
I once had what I thought was a miss on cliimb out in my TSIO 520 in Bonanza, just a slight rough, could see it in the magnetice compass rocking. All gauges normal , but with a passenger and rougher terrain ahead I returned to the ariport and took an airline. Didnt get attacked by thugs on the ariline and had the equivalent of a checkup and tuneup on the plane the next 2 days. No problem found. Came back to fly home and still the same, a very slight vibration, worse with more rpm. Decieded it was not a miss but a vibration and took it to the shop. And a simple answer was found, where the deice boots on the prop went into the hub, a small piece of rubber had come off, hard to see, and just enough to slightly unbalance prop. So small that putting one tie wrap on the base of the prop made it smooth again.
The hard part on yours is clearing at 1200? Try that phone number I gave, supposed to be an expert on this engine.
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