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  • #1

    Massive flat spot

    T-Rex 700 - Helibug conversion - 270RC Zenoah - all set up - big day arrives for the engine test and may be maiden.

    Motor starts 3rd pull out of the box!!

    Both needles 1 3/8 out - None synthetic 25:1

    Burbles away beautifully - reliable tickover sorted on tranny etc.

    Spooled up and just sat in the hover like a well sorted heli that had been tweaked and tweaked - amazing!! Almost no vibes.

    But when I landed to tweak radio settings - each time I spooled up again a mid range flat spot appeared that got worse and worse until punching from 1/4 stick to 3/4 stick was the only way to get past it.

    Any ideas????

    The bottom end is quite rich the top end is a little rich and maybe needs to richen by 1/8th. The fuel was filtered before filling the tank etc.
    T-Rex 450 DFC / AR7200BX

    ALBA SAILING - West Coast of Scotland's Premier Yacht Charter Company and RYA Sailing School - on site Chandlery, Hotel, Restaurant and Bar - www.alba-sailing.co.uk- it's another world.....
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  • #2
    PM Asid, he has probably the most experience with gassers on here mate.
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    • #3
      What oil? 25:1 is quite high and if running rich with some oils you could stick the ring quite quickly. The idea is to run it correctly tuned using higher oils in case you go too lean to help protect and keep checking those temps.
      Conventional or Malorie-type linkage? These gasser curves are weird anyway.
      At one time I was advised to use XR77 instead of the TT I had used before and ended up decoking my new engine after only 1 gallon.
      Also how are you venting the tank?
      PGK
      450Pro Clone fb, Trex500 fbl beastx, Trex 600N fbl beastx, Trex700N fbl msh brain, Spectra G Hanson 26 3dmax fb, Blitz Avro fb...Futaba 8FG

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      • #4
        Hi PGK

        Its BelRay - none synthetic S2 - always used it to break in Zenoahs for the first gallon then fully synthetic at 40:1 from thereon in.

        Tuning wise its a little rich low needle and about bang on high needle

        Its a conventional linkage but Malloryised via the 7 point curve in the tranny ie 50% stick is 25% throttle (0, 10, 20, 25, 35, 70, 100)

        The tank is vented via a loop from the top vent which is led up and then down to the skids so it cant leak when inverted - it vents freely.

        Any ideas??
        T-Rex 450 DFC / AR7200BX

        ALBA SAILING - West Coast of Scotland's Premier Yacht Charter Company and RYA Sailing School - on site Chandlery, Hotel, Restaurant and Bar - www.alba-sailing.co.uk- it's another world.....

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        • #5
          im pretty sure Ma88ut has Castrol TTS
          Chris 'CHUNK' Newby
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          NAHC-Newton Abbot Heli Club, check out the website at WWW.NAheliclub.org.uk

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          • #6
            I ran mine in on halfords cheap stuff at 33:1 then TTS at 40:1 after a gallon. Needles are stock and she's going great will look up curves tomorrow


            Sent from my iPhonings
            Last edited by Martind; 24-04-2011, 09:45 PM.
            Oxy4+, tgy servos, hw60a, spirit fbl on 6s 🤓


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            • #7
              Logic doesn't always work :-( .....but you're comfortable with zenoah tunes and you have good clunk/lines (and new they have to be) and the tank vent is usual (rather than those horrid one-way valves) then then only sensible reasons for it flatting mid range when OK low and high is air/carb or exhaust. An oily clogged filter, old mucky exhaust, or the carb not pumping right.. can't be spark 'cos grounding or poor seating would be worse low down...or bad mix old fuel settling out (I had some that must have got some condensation into it..was fine whenever shaken up with fresh)...
              PGK
              450Pro Clone fb, Trex500 fbl beastx, Trex 600N fbl beastx, Trex700N fbl msh brain, Spectra G Hanson 26 3dmax fb, Blitz Avro fb...Futaba 8FG

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              • #8
                Thanks PGK - its a brand new air filter and brand new Zimmerman.

                I am starting to think air leak somewhere around the carb - tickover gradually increased during the tank - too soon for it to be freeing up but an air leak would lean the mixture.....
                T-Rex 450 DFC / AR7200BX

                ALBA SAILING - West Coast of Scotland's Premier Yacht Charter Company and RYA Sailing School - on site Chandlery, Hotel, Restaurant and Bar - www.alba-sailing.co.uk- it's another world.....

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                • #9
                  A gasser is a different beast to an electric or nitro. You want to run it on the power (loaded).

                  The throttle curve is different. As pgkevet said use the "Malorie" mod on the throttle to add the differential needed. These engines have a butterfly carb that is not linear so 50% movement isnt 50% throttle. 25% can be typical in the hover.

                  I run in the engine at the same needle settings as running normal but with a higher oil content. Prolonged rich running can lead to a stuck ring. Semi synth or mineral oil is fine for a gallon. 25:1 then 32:1 upto 40:1

                  Then switch to the oil you will be using. It will take a few gallons to break in. Its not a nitro lol.

                  That carb can be run with the low down to 1:1/4. High can be richened slightly. (double check this)

                  Things to check:
                  • Air leak between carb and isolator (i use an alloy aftermarket with teflon gasket)
                  • Air leak between exhaust and engine.
                  • Blocked or fouled air filter
                  • Fouled plug
                  • Run correct gear ratio
                  • Don't run a gov until its tuned.
                  • Make sure carb is clean
                  • Make sure clunk line is correct diameter for proper fuel flow.


                  This oil seems to be working very well : PENNZOIL MARINE 2-CYCLE OUTBOARD ENGINE OIL TC-W3

                  Hope this helps

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                  • #10
                    Had a look at mine and the needles are 1.5 on both running castrol TTS at 40:1 using the malorie method on the throttle.
                    My curves are 0,19,25,28,38,58,100 and this gives me 1800 in normal on the 20t pinion.
                    Oxy4+, tgy servos, hw60a, spirit fbl on 6s 🤓


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                    • #11
                      Originally posted by Skipper Billy View Post
                      Thanks PGK - its a brand new air filter and brand new Zimmerman.

                      I am starting to think air leak somewhere around the carb - tickover gradually increased during the tank - too soon for it to be freeing up but an air leak would lean the mixture.....
                      A new filter can get clogged fast if the carb is spitting back. They tell me that to check for carb air leaks then spray a bit of damp-start like stuff around the manifold..if engine note changes markedly then it's sucking in through the leak. I also have the alloy insulator/teflons.. others don't like them and just use stock or stock with double gaskets+/- rtv

                      My gasser throttle curve is extremely odd even for a gasser especially with the malorie linkage something like 0,5,11,24,100
                      PGK
                      450Pro Clone fb, Trex500 fbl beastx, Trex 600N fbl beastx, Trex700N fbl msh brain, Spectra G Hanson 26 3dmax fb, Blitz Avro fb...Futaba 8FG

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                      • #12
                        I never touched either of the needles on my gassers I just ran them straight how they came out of the box, 35:1 to break in and took it out from there using fully synthetic from the word go after sticking a ring in my first gasser and wrecking the engine using mineral based oil to break in - personal preference was Putoline MX9 fully synthetic which burns very cleanly and gives the fuel a nice blue colour.
                        Gaui Hurricane 425 FBL
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                        • #13
                          Just a quick update in case anyone else has the same problem.

                          Fault traced to an air leak between the manifold and the carb.

                          Light smear of blue hylomar (sealant) on both sides of both gaskets.

                          Flat spot gone - totally - now responds to both needles excatly as it should and is running superbly!!!

                          Many thanks to everyone who responded and tried to help.

                          PS Final needles setting for the first gallon on this particular carb (990) are 1.5 turns out (H) and 1.25 turns out (L)
                          T-Rex 450 DFC / AR7200BX

                          ALBA SAILING - West Coast of Scotland's Premier Yacht Charter Company and RYA Sailing School - on site Chandlery, Hotel, Restaurant and Bar - www.alba-sailing.co.uk- it's another world.....

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