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Thread: Help!! Hirth 2706 driving us crazy!!

  1. #1

    Help!! Hirth 2706 driving us crazy!!

    Hi all!!


    I'm coming here looking for help, and I'm pretty sure this is the right place . Some months ago my father bought a quad city challenger clipped wings with just 35 hours of flight in both engine and airframe. This sounds good, but the previous owner sold the plane for one reason: the engine became a headache... long story short, he bought the engine brand new, fuel injected. Since the begining the things went wrong when one side of the ignition failed and he had to replace the armature plate. At this point the engine ran well. Based on this event and on some histories on internet, he decided to get rid of the injection and replaced it with two bing 54, with the jetting according to the factory (He sent the injection to the dealer and received the bings..). Once this done, the engine start to gave problems to accelerate. He worked on it for months and at the end he decided to sell the plane to us...


    The problem is very weird: If you push the throttle slowly, the engine go up to 6200 RPM easily. But if you push it in one shot from idle, it get stuck stumbling in 2500 RPM's, until you close slowly the throttle at certain point where it pick up again and you can open the throttle fast and it will go to 6200 again... In any case, the EGT goes up almost to the limit (1256F) if you stay around 4400 RPM for some time...


    We have tried several things to fix this, for some months... We have tested all the combinations of jetting that you can imagine, and all the positions of the jet needle, and adjusted the mixture all ways possible... and the problems remain the same... at some point, we took two carburetors from a 503 in perfect working condition and installed on it, and the engine did the same...


    Recently, we started to suspect about the timing, so we checked the manual and found out that it can be adjusted, so we moved it in all the ways possible and the things went the same...


    Now we have the suspect that maybe the E-Boxes, which are suposed to be programed, could have and advance timing map for and injected engine, and this don't mach with the carb behaviour.. however, looking at the part list, the E-boxes have the same part number for both, and injected and carbureted engine...


    All this being said, and sorry for made you read all this, we have no clue what to do next...


    Any advice will be highly appreciated, greetings from Costa Rica,

    Esteve.

  2. #2

    Join Date
    Jan 2012
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    290
    The problem is that your are running lean in the mid range. When you open the throttle quickly the air responds more quickly than the fuel. Automobile engines solve this with the accelator pump which most aircraft do not have. There are several things you can do to minimize the problem but you may not solve it completely. These are things to look into.
    1. increase the idle rpm.
    2. Richen the idle mmixture slightly.
    3. Richen the mid range mixture.
    4. Lean the high part power mixture but keep the same WOT mixture if you are at the EGT limit. The engine can operate above the published WOT egt limit at lower power settings. WOT= Wide Open Throttle.
    Hirth has a good fuel enjection system. I would look to reinstall the fuel injection and get it operating correctly. In the US we have a good Hirth dealer. I do not know about your situtation. If necessary go to the company for assistance. I have found them to be very helpful and dedicated to solving the problems.

    Operations wise, if you will advance the throttle to off idle, about 3,000 rpm and hesitate slightly, and then go full throttle The engine should respond nicely. This technique will allow you to keep a nice low idle and still have good acelleration. I have found that having a second throttle cable to allow for a low or high idle speed can be handy to obtain a low idle speed for short field operations. With the low idle the engine is temperature sensetive and cools excessively and eventually dies. Returning to the high idle or adding power about every 30 seconds will remidy the situtation. This technique is not unique to the 2706 but is a general two cycle characteristic as the engine will tend to load up with excess fuel in the crankcase with prolonged low speed idle. All of this can be fixed with a better carburator or fuel injection which is why the fuel injection works so much better than the carbs but it needs to be set up properly. Hirth makes a quality engine but it is more high tech than the Rotax and needs to be understood and treated accordingly.

  3. #3

    Join Date
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    Quote Originally Posted by jedi View Post
    The problem is that your are running lean in the mid range.

    2. Richen the idle mmixture slightly.
    3. Richen the mid range mixture.

    That's what I would do, leave the main metering jet where it is for now but change to the richest needle and needle jet before the engine gets damaged. Then start working to a leaner setting from there. At the same time:

    Verify the idle is not too lean.

    Check the fuel level in the float bowl.

    Reset the timing exactly to spec.

  4. #4
    Thank you very much marty and jedi for the quick response. I'm going to do the testings that you both are suggesting. However , one thing that i have done while the engine is stumbling at 2500 is push the primer to see if with more fuel the engine pick up again and it doesn't... It is jetted accordingly to the manufacturer... What i'm missing? Do you have the contact of the u.s dealer to write him?

    Thanks again!!

    Esteve.

  5. #5
    Sam Buchanan's Avatar
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    I would make sure the crankcase seals aren't leaking. If air is pulled through the seals no amount of jetting changes will make the engine run properly, it will always have problems with throttle response.
    Sam Buchanan
    The RV Journal RV-6 build log
    Fokker D.VII semi-replica build log
    YouTube Channel

  6. #6
    Thanks a lot Sam! Any tip to check this? the first thing I would do is check for oil leakage marks, but if there is a more technical test I would do it. thanks again!!

  7. #7
    Sam Buchanan's Avatar
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    Quote Originally Posted by chacalextreme View Post
    Thanks a lot Sam! Any tip to check this? the first thing I would do is check for oil leakage marks, but if there is a more technical test I would do it. thanks again!!
    It has been a few decades since I messed with two-stroke motorcycles but here are a couple of links I found via Google:

    http://atv.off-road.com/atv/tech/pre...ing-19386.html

    http://www.dirtrider.net/forums3/sho...ght=home+depot

    These should give you some ideas about how to pressure test the seals in your engine. Hope this helps.
    Sam Buchanan
    The RV Journal RV-6 build log
    Fokker D.VII semi-replica build log
    YouTube Channel

  8. #8

    Join Date
    Jan 2012
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    US Hirth Distributor

    Us Distributor is Recreational Power www.recpower.com

    The primer does not have enough capacity to make a difference. You can try restricting the air intake to make a choke effect. While I am on the subject, make sure the air cleaner is not making a restriction that messes up your mixture settings. Try running with the air cleaner off but realize that you will need a slight retune when you reinstall the aircleaner.
    Last edited by jedi; 12-08-2012 at 09:32 PM. Reason: More details

  9. #9
    We are going to try changing the jet needle and needle jet, as some of you mention and I have read, this control the middle range circuit. I will be posting if it has some result.
    Thanks a lot!

  10. #10

    rotax question

    We have dealed with the hirth a little bit more with no luck... still have the same flat spot no matter what we do with the jetting, carb adjustment timing or whatever. Now we are pointing in another direction... We have a friend wich is selling us a rotax 503 with a gearbox with electric starter... the question is (I haven't made any measure yet) could be possible to install this engine upside down with the rotax gearbox? is there any problem with the prop cleareance? I haven't seen any challenger with this configuration yet...

    Thanks a lot!!


    Esteve.

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