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Thread: Rough O-200

  1. #1

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    Rough O-200

    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 ?

  2. #2

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    Carb mounting washers.

    Quote Originally Posted by Eric Brown View Post
    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.

    these symptoms are typical of a carb/intake spider that is mounted solid.

  3. #3
    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

  4. #4

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    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.

  5. #5

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    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.

  6. #6

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    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.

  7. #7
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    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!

  8. #8

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    Eric, what brand of spark plugs do you have? The issue of resistance is emphsized by Tempest. Seems to have come to the fore in regards of Cirrus and lean of peak running where is it claimed a problem with Champion. I havent heard much about this in the last year, and Champion has a new design of part of their plug, I have used Champions for decades, never had much trouble other than a cracked one. I do know of cases where the pilot thought engine was running fine, no complaints and then tested resistance and decided the plugs were defective on that basis. I rely on mag test while runnng or on pressure tester when plugs are serviced. I have checked resistance but only changed one plug on that basis. A C150 engine should be easy on plugs, not demand much.
    Let us know what you find asap?
    Last edited by Bill Greenwood; 05-18-2017 at 01:48 PM.

  9. #9

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    Gentlemen.
    Here's what I have done so far. Visual check of induction system. Different spark plugs. Tried another O-200 carburetor. Removed both ignition leads from one cylinder at a time and ran engine below 1000 RPM to see if miss would disappear with the dead cylinder. Tried setting the idle mix very lean (1/4 turn). Tried very rich mix (3 turns). The miss is still present below 1200 RPM.

    Riddle me this. If the crank and cam gears were off by 1 tooth, would this create this problem ???? There were no index marks on my gears. I set them by feel of the #1 cylinder valve overlap per the OH manual.
    Last edited by Eric Brown; 05-18-2017 at 07:44 PM.

  10. #10

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    I dont know about setting gears. But youve ruled out plugs and carb it seems. So, sounds silly but Check each ignition wire with a lead tester as well as visual and that they are connected correctly, I have seen this switched.
    My next gu ess is a valve problem, and a compression test may find any valve leak, wont be surprised if you dont find one. A sticking valve is most likley on a engine started cold. That sort of leaves mags, but cant see how they would clear up at 1200 rpm, but may be worth a look inside for a crack or crack in cap and the timiing setting.
    It would be worth a phone call to try tech support (770-358-4430)and just might be able to help.
    Last edited by Bill Greenwood; 05-19-2017 at 10:30 AM.

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