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C90 not taking throttle at times

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  • C90 not taking throttle at times

    During takeoff and throttle advancement, it hesitates at about half throttle while I'm going towards full throttle. I am not pushing it forward fast, almost a 3 count. I've tried keeping carb heat on while advancing power, but it does it off and on. I also don't feel that I should have to have carb heat on...when I want full power I want it/may need it. I've made sure I'm on the tank with the header. I'm pretty green on the stick, have asked the guy I bought it from and he is looking into it as well(A&P). It is a marvel so it has an acc pump. I've only been flying since early Nov so I don't really know where to look, even though I'm pretty sure it carb related. Any ideas? Thank ya much

  • #2
    Couple things come to mind. I ran a C-90-8 engine for 10 years in a PA-11.

    1. The bolts holding the carb assembly to the bottom of the engine are loose and letting some air around the flange. This would make you run too lean while accelerating. ( A bad gasket down there will do the same )

    2. The Carb-Float assembly is sticking. This would do the same thing and make you run lean at high power settings and power changes.

    3. The accelerator pump is FUBAR.

    4. You might have gunk from car gas, water goobers or a bug in your Carb bowl.

    5. The bushings on the throttle flapper are worn and letting air in the sides while making the flap stick or flutter.

    6. Carb heat flap is loose or goofed up and it is flapping around closing and opening at certain power ( air flow) settings.


    You can also have the same thing happen if one of your mags is going bad or going out of time. Although this is less likely.
    Float-CFI, Photo Guide, Fishing Guide, Remote Kayaking
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    • #3
      Oh yeah here is what a bad mag looks like. This bad mag was still working just fine....But only by pure magic. It would have failed at any minute. the only symptom was shaky acceleration and a little bit of static over the radio.

      The left photo shows a new mag cap compared to the old one. Note that the center spring loaded contact is completely burned away. The next photo shows the contact point on the rotor as being all burned and melted. It was arcing.
      Attached Files
      Float-CFI, Photo Guide, Fishing Guide, Remote Kayaking
      Guest Cabin, Flight Reviews, Aerial Tours


      • #4
        Have you heard of the lil baffle that some guys put in front of the air filter? I guess guys that are having same symptoms as me( especially in 'colder' temps) have done this and cured the issue..the carb has about 50 hrs on fresh rebuild as does motor...


        • #5
          Here is a pic of what I'm referring to
          Attached Files


          • #6
            1. How cold is it where you are trying to fly? Here in Homer it has not been below +20F this winter. I have flown mine at 20 to 30 below zero with no troubles. Other than the fact that I was freezing to death.

            2. I just had to rebuild a carb in my C-172 that had less than 80 hours since rebuild. The I.A. who rebuilt it used an illegal carb float and old buggered up screws. So being recently rebuilt in the logs does not mean a thing. It all depends on who did the work.

            There are baffles over oil coolers, and I used foam chunks to block some airflow in my Cub during the winter months to retain heat. But I never had to goof around with the carb intake air.

            My C-90-8 had notches on the carb heat so it could be set at partial carb heat settings. But that is a matter of moisture in the air being turned into ice INSIDE the carb venturi. It is not a matter of the air itself being too cold. In fact very cold and dry air does not produce carb icing.

            A. What are you seeing for idle speed ?
            B. What do you see for max throttle rpms?
            C. Have you used an optical tach to check the accuracy of your cable drive tach?
            D. What is your fuel burn per hour at cruise.
            E. Do you have leading or soot on your plugs?
            F. Do you have dark sooty exhaust residue?
            G. Have you checked the mag timing since this problem started?
            Attached Files
            Float-CFI, Photo Guide, Fishing Guide, Remote Kayaking
            Guest Cabin, Flight Reviews, Aerial Tours


            • #7
              It had been 15-30 degrees. I've got a small exhaust leak on a clamp, but nothing like the pic you've shown. Havent actually calculated fuel burn but I know it's 4-5gph. Idles at 600-700. 2500 on take off. I only have 20hrs so I'll have to check out all the ideas you've mentioned.


              • #8
                Or could this just be the infamous c90 'cough' I've been told about ?


                • #9
                  The inter web just ate my gooder response so we'll do cliff notes

                  Take it to a mechanic that knows what they are doing.
                  This is not that big of a problem to figure out but could end your day in a very bad way.
                  The internet is a poor place to troubleshoot airplanes. Be real careful.

                  Where are you located?
                  Valley? Fish Creek in Palmer would be a good place to start.

                  My three guesses - in this order-
                  air box
                  But until i saw looked it overů..thats almost worthless.

                  Be conservative till its FIXED. Your family may want you home.

                  Merry Christmas!


                  • #10
                    Second plug for Fish Creek in Palmer.

                    A about three or four years ago I had a client crack a jug on my C-90. ( left it at full throttle for his whole check-ride)
                    The crack caused a loss in power and rpm. And it stumbled during certain RPM settings.

                    A couple of A&Ps could not find the problem since it only happened when the engine was warm. I even had good compressions while cold.
                    Joe Benallo an I.A. down here in Homer figured it out. We ran the engine until hot and then pressurized the cylinders with air. Then he strayed windex with a pump sprayer until we found a small leak next to a spark plug hole. Attached is a photo of the inside of that jug. You can see the tiny crack heading out from the spark plug hole. The total fix was around $1200 or maybe a little more. I bought a rebuilt jug from Alsworth.

                    As usual AK-Hunt is correct, internet advise is worth what you pay for it...
                    Attached Files
                    Float-CFI, Photo Guide, Fishing Guide, Remote Kayaking
                    Guest Cabin, Flight Reviews, Aerial Tours


                    • #11
                      Adjusted idle mixture a couple turns, got idle 650ish and seems to really help. Very minimal hiccup but stop to stop in a lil over 2 secs


                      • #12
                        Hmm, mine was set down to 500-550 rpm for float-plane taxi and it never grumbled or stumbled when it was running correctly. It would run even slower on one mag and with carb heat. ( no brakes while taxiing on the water )

                        Did you play with the economizer setting or the cable linkage setting? Maybe don't want to know... just find a good IA.
                        Float-CFI, Photo Guide, Fishing Guide, Remote Kayaking
                        Guest Cabin, Flight Reviews, Aerial Tours


                        • #13
                          Hunt/trap, I hope you are working on a experimental aircraft , if not your plane is no longer airworthy unless you are working under the direct supervision of a A&P. IMHO you are playing with fire, there are several things that could be wrong and loosing the engine in flight with twenty hours flight time may be a lot to handle.


                          • #14
                            Hunt/trap. Sounds like my c-85, same carb low hours on 1940 taylorcraft, also 25 hours or so in craft, ran perfect until cold temps. Seems like you and I have been pouring over the same material to cure it. Wondering if you used that cover yet, I also was thinking about closing up lower rear cowling gap to help with engine heat, I also have a oil pan muff on the way. Any experiences, help would be appreciated. Having engine "cough" adds to the experience😳


                            • #15
                              Problem solved. Adjusted idle
                              mixture a few turns and she now runs like a raped ape. Very slight hesitation if it idles for more than 20-30 secs. Can slap it to the stop pretty much as fast as I want. Idle about 650-700 now


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