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Runs rough to 3000 rpm

in General Tech Discussion 9 years ago
I have two new 912 uls engines on a just built aircam. The right engines carbs balanced with no problems. It runs very smooth thru all rpm ranges.
The left is a different story. I could not get the carbs to balance in any rpm range using a Carbmate. The best I could do is to get it to run smoothly from 3000 an up. Idle set at 2000 it runs rough to 3000 rpm an stumbles a bit accelerating from 2000 thru 2500.
As per Lockwood recommended, I cleaned the idle jet, needle jet, starting jet an idle mixture screw. Blew air thru housings an jets. All are clean.
Engines run fine from 3200 rpm on up
Lower plugs on cyl 1 an 4 clean. All the rest of the plugs were either sooty, or wet an sooty, after flying
Any thoughts anyone
Thanks Ron
Also the problem engine burns 1.5 gph more than the right

  • Re: Runs rough to 3000 rpm

    by » 9 years ago

    Hi Ronald

    I would remove the top of the card and check the diaphragm is located correctly in the grooves and is undamaged and the slide is undamaged and operates smoothly, espesially if you have used compressed air to clear the jets and carb housing.

    Mark

    I have just re read your post and it is obviously an ignition problem. but if you have used compressed air on the carbs, still check the diaphragm.

    Mark
      
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  • Re: Runs rough to 3000 rpm

    by » 9 years ago

    Maybe check the needle circlips too.
    I would think one notch different (left vs right carb) would be more evident at lower rpm?
      
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  • Re: Runs rough to 3000 rpm

    by » 9 years ago

    I did check the circlips today, an their both on the same notch. # 2an4 are running noticeably cooler than 1 an 3. Ie. 1an3 are 165 degrees an 2an4 are 120 degrees.
      
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  • Re: Runs rough to 3000 rpm

    by » 9 years ago

    Ronald,

    I had trouble with the Carbmate until I used hose clamps to tighten the adapter hoses to the engine. It was sucking in just enough air to throw it off.

    I'm helping my father resolve this same issue. If the problem engine is using more fuel, I don't think its electrical related, sounds fuel related.

    I would remove the carbs from the problem engine, and check the float movement for interference, and the proper setting which is level while the carb is inverted. I would also check the weight of the floats. They should be about 7 grams per pair. For $15 you can buy a small kitchen scale in walmart (the small square one) it is digital and measures down to 1 gram. It is surprisingly accurate for its price, and is in the kitchen wares section.

    If jets were blocked, it would be using less fuel all others things being equal. I would be suspicious that the fuel level in the float chamber is getting too high. Also check the hose clamps on the cross over pipe, and the carb to intake boot. These wouldn't make it run richer, they would make it actually run leaner, but could be a factor when trying to balance with the Carbmate.

    Please keep us updated as to what you find. I'm not going to be able to help my father with his for a few weeks, but it sounds very similar to your situation.

    Kevin K.
    Sonex Jabiru 3300
      
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  • Re: Runs rough to 3000 rpm

    by » 9 years ago

    Thanks Kevin
    I changed carbs from left eng to the right an problem went away. One float was heavy but works normal on good engine. I have no idea what I did but it's improved.
    You could be right about the carbmate. I thought I had them sealed good, but maybe there was just enough air to mess it up
    Also the cyl temps went back to normal on both sides. I haven't checked fuel usage yet
    Thanks Ron
      
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