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#2 Cylinder running cold.

in 912 / 914 Technical Questions 13 years ago
Rotax 912S. Engine runs rough. Found number 2 cylinder running about 550-580 degrees F vs 1,3,4 running at around 890-900 F at 2,000 RPMs. Performed compression check, 1,3 are at 78 and 2,4 are at 76. Pulled #2 cover, wayched valve train move when pulling prop through by hand.Plugs on 1,3 are greyish in color. Plugs on 2, 4 are black. Carbs had just been rebuilt by another company., aircraft owner says the other company installed carbs on another engine and that engine ran fine. This problem started after carb installation. Inspected carbs, needle jet is on position number 3 (from the top). I suspect plug wiring but have never had this problem before. Any suggestions?

  • Re: #2 Cylinder running cold.

    by » 13 years ago

    It’s not the ignition. The 2/4 plugs should not be black and they are fed by the same carb: check for slide alignment, choke assembly (should have an "R" stamped on the 2/4 choke shaft), gasket in the top of the slide housing is not leaking (dunking carb into cleaner will destroy this gasket) atomizing ports are clear... general carb cleanliness.
    Rob
      
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  • Re: #2 Cylinder running cold.

    by » 13 years ago

    Hi Rob- This is Ken Dunn. We talked awhile back when I was working in Redmond, Oregon. Carbs had been rebuilt, all looked clean. Checked to make sure both needle valves were adjusted the same.hey are. Owner took number 2 cyliner and head off, no damage to valve train. Installed all parts, started up the engine and initially it did the same thing. Then everything started working correctly. The engine has a 1,000 hours on it. Mostly with 100LL. Could this be a lead build up on the valves, valve train, etc?
      
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  • Re: #2 Cylinder running cold.

    by » 13 years ago

    Hi Ken,
    how much lead in the exhaust port? I guess I got distracted by the plug color but it could be morning sickness (sticky valve) if it is really plugged up with lead.
    Rob
      
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  • Re: #2 Cylinder running cold.

    by » 13 years ago

    Hi Rob- I dont know about how much lead. Owner did the MX himself with a freind to save money. They too it apart, didnt find anything wrong, put it back together. Short story, engine now runs good according to the owner. He says he is switching back to nonethenol automotive fuel.

    Ever hear of lead sticking the valves or valve train mechanism?

    Ken
      
    Report

  • Re: #2 Cylinder running cold.

    by » 13 years ago

    Hi Ken,
    Yes, lead can stick the valve. Mostly in engines that run at low power settings all the time.
    You just need to ream the valve guide to fix it (7.01mm straight reamer). If you take the exhaust pipe off and the lead build-up is pronounced in the port area you can assume you need to remove the cylinder and clean up the lead from the ports, rings, cylinder wall, valve stem and valve guide. The lead is pretty easy to see and you just keep scrubbing away with parts cleaner (varsol) until it is gone. Don’t hone the cylinder wall; just scrub it with fine scotchbrite until the dull-grey lead is gone and you see the original crosshatch.
    Rob
      
    Report
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