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I was doing a carb sync for my first time today using a CarbMate. Mechanical sync performed first. Pneumatic sync started out in the 2cm range and it was way out at -4. So I closed back a little on the left carb and it came to zero. Switched to the 0.5cm range and it was out just a little positive. Smooth enough to continue a warmup before final tweeking. About 2 minutes into the warmup at 2000rpm, it suddenly dropped to 1600 or so rpm, running very rough. The CarbMate reading didn't change. I tried turning off one mag at a time and couldn't tell if it made a difference. It did respond to a small increase in throttle but continued to run very rough so I shut it down. I pulled each carb bowl and they were full and clear. Floats looked good, they are pretty fresh. All linkage connections looked good with proper travel. I'm stumped.

Any ideas what I should look for?

Wade

  • Re: First time carb sync and then this happened??

    by » one month ago


    Not an expert, but I would check to see if one of the vacuum hoses became disconnected.   Check the security of the bolts holding the cables.  Also, one of the carbs has a throttle position sensor on it, and you probably want to adjust the other carb cable. Right or left depends on perspective.  Looking from the engine towards the prop, the TPS is on my left carb.


  • Re: First time carb sync and then this happened??

    by » one month ago


    Hey Chris,

    I'm assuming by vacuum hoses you are referring to the CarbMate hoses. Yes I looked them over but also realized if one of them was loose or damaged that the indication on the CarbMate would have changed dramatically. It did not change. I did look over the cable connections and throttle movement through the entire range on both carbs and it was proper and equal. I don't think I have a TPS (912ULS). I guess I need to do a little studying to make a plan on troubleshooting the ignition systems. But it does make more sense that it would be in the fuel system since that's what I was working on. I just don't see anything though. Weird. 


  • Re: First time carb sync and then this happened??

    by » one month ago


    You said the carb bowl's were full, that's an indication of flooding as they should be half full. Look for bad floats or needle valve leaking. Keep an open mind.


  • Re: First time carb sync and then this happened??

    by » one month ago


    Hi Sam,

    I suppose misleading but by full I meant to say full to normal which in my experience is probably more than half full. They were not spilling over when I removed them. I have seen them spilling over before. Last fall I had a fuel contamination issue that caused flooded carbs. They looked ok yesterday I think. It was so sudden. The rpm change. You're right though. I need to remain open minded. And a good part of the running up to that point was pretty rough until that last adjustment that smoothed it out. Maybe that roughness caused a carb issue like we are discussing. 


  • Re: First time carb sync and then this happened??

    by » one month ago


    I pulled and checked each spark plug today. Some were wet and all were a little dirty. Cleaned each one and verified good spark gap. Also pulled bowls, dumped dry, and weighed floats. They were good. Started it back up and the sync was way out. Back in the opposite direction that I had to adjust before. So I'm wondering if maybe I adjusted it before with the carb bowl level higher than it should have been. Either way it ran fine today and I warmed it up and got it synced in perfectly. Taxi tested. Looked good so I put about 45 minutes on it and seems to be running great. Tomorrow is another fly day! Thanks Chris and Sam for your advice.


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