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Troubleshooting Rotary Valves

Discussion in 'Gas Engines' started by Bartman, Jun 8, 2019.

  1. Bartman

    Bartman Defender of the Noob!

    Hi All,

    The ZDZ 80 problems continue. :(

    Opened up and cleaned the carb, put in a donor diaphragm, and the engine will run off the primer charge from the choke but then die after a second or two.

    The engine has rotary valves which I understand a bit but is there anything that needs to be checked with them? Reed valves can be checked and tuned to work better but what about rotary valve engines?

  2. 49dimes

    49dimes Damn I'm hungry

    My YS 4cycles are rotary valve induction. Virtually maintenance free. The only thing I can think of is to check the clearance of the back of the rotary disc to the back plate of the engine with a feeler gage. You would need to know what the factory clearance is. But I really think you should keep looking at the carb as being your culprit or an air leak externally or internally (ie stuck open/closed check valve inside carb). Is the carb mounting block cracked? Does the engine die when trying to advance throttle or just run a bit and die as you stated. Another consideration might be that the engine will start but suddenly richen and flood out the spark plug because of fuel pooling inside the engine. Did you try pulling the plug after a "start and die" to see if it's excessively wet with fuel? or dry because of a lean out and die? Also the pump side of the carb can prime up everything for starting but if the "leaflets" or flappers don't properly seat will cause a lean and die condition. I personally would not tinker with the engine internals until I knew for sure the carb is not the problem. Do you have another carb that fits to see if the engine will stay running even though the carb may be for a smaller displacement engine? (40-60cc).

    BTW....When I first got the Cub the engine (RCGF26) would do the same thing....Start and run for a bit then die with the needles at the factory recommended start settings. I backed out the HS a bit more and LS 2 more full turns and the engine would stay running but was very unreliable and would lean and die while at WOT. I replaced the pump gaskets and metering diaphragm and had the same issue. I then checked the operation of the internal check valve ahead of the HS jet by squirting WD40 through it both ways. Got WD40 coming out both ways telling me the valve no longer operates as it should. Purchased a brand new carb and VIOLA! And no it was not the same exact carb. It was a 30-40cc for a DLE but a Walbro carb. Engine runs strong and reliably now. Chris is the new owner of it.
    Bartman and Alky6 like this.
  3. Bartman

    Bartman Defender of the Noob!

    Thanks for the reply. Reading about rotary valves it didn't seem like there was much there to check but figured it didn't hurt to ask.

    If the engine is coming off the plane again it is definitely getting a new carb before it goes back on. The only other thing I might try is to raise the tail so the excess fuel is forced to stay in the engine instead of running out of the carb. They say rotary valve engines need to be very wet to start but who knows at this point.
    Alky6 and 49dimes like this.
  4. 49dimes

    49dimes Damn I'm hungry

    Your most welcome. Trying to find out problems on line is very difficult because "like symptoms" can cover many different issues.
    Small engine repairs really need "hands on" evaluation by experienced personnel. But look at the bright side. You got the plane and engine for a song :). And like anything you get for a song...:rolleyes:
    Bartman and Alky6 like this.
  5. Bartman

    Bartman Defender of the Noob!

    Woo hoo! Problem solved!

    I admit it, I can be a dingbat. The engine was running great wide open but wouldn't idle and wouldn't recover once it started to die. I was about to start looking for another engine but thought to reach out to ZDZ USA first and they offered one last tip that fixed it......the butterfly was backwards! Not sure how I let that slip past me but I changed the linkage, reversed the channel and it started right up and held solid at idle with great transition back to higher RPM's. It is a frickin' monster! I can't believe how dumb I am!

    It definitely needs a big prime charge and so far it seems to like starting at mid throttle instead of idle but man it runs strong. So happy. :)

    @Pistolera when's the next IMAC contest???? lol
    Pistolera, AKNick, Alky6 and 3 others like this.

  6. :woohoo!:
    Alky6, dhal22 and Bartman like this.
  7. dhal22

    dhal22 GSN Sponsor Tier 1

    Did you check your butterfly valve.jpg
    AKNick, Alky6, Bartman and 1 other person like this.

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