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Thread: Rough running engine when throttled back from full power at circuit height

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  1. #1
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    Default Rough running engine when throttled back from full power at circuit height

    HI Folks - new Forum member / First Post / Visiting from Australia for 2 months

    I have a 2013 CC SS that I recently purchased. Just finished 2 weeks training at Tac Aero (which was great) but ran into a weird engine problem. Engine runs perfect on taxiing, warm-up, run-up and take off.

    Engine Roughness after throttle back
    However around the time we get to 1000ft and smoothly throttle back, we get noticeably rough engine running, rough enough to keep close to the rwy but NO engine shudder, NO miss and NO sign of quitting - more like a v bad mag check.

    Also the engine analyser data show that CYLs 1 and 2 run approx 100 degree cooler CHT than 3 and 4 during the rough running episode. The Savvy site is super cool - highly recommend.
    See graphs below

    Jon D, Don from CC's and Tac Aero has been super helpful. We think its Ignition related - unfortunately Klaus declined to spk with me (did send me the standard troubleshooting list on a Saturday) which as a customer is really disappointing.
    Jon D mentioned he had a customer in N/S Carolina that had near identical issue and it was the Mini-sensor.
    However we pulled the Mini-sensors and the internal solder all looked Ok - grrrhhh !!

    Actions
    So far we have
    1) replaced the coils with new - all 4 (initially we swapped the coils so each spark plug had a different coil - this didn't move the low temps from CYLs 1 and 2 as we hoped plus we still had the rough running at circuit height - so we DONT think its the coils - but just in case we have replaced them).
    2) replaced spark plugs, ALL Ignition leads
    3) replaced both the Mini-sensors with 2 new which is a bear of a job but Hayden from Tac Aero did a super job.

    Next is to install the intake spacer for the carby -while we are at it - for better fuel air mixture .

    Test flying the a/c tomorrow 15th May with these changes - so will send an update.
    If new Coils, new plugs, new ignition leads it only leaves one or both of the Ignition modules unless we are completely on the wrong track - which is always a possibility.

    So couple of questions
    1) Does this look like an Ignition system issue - problem effects 2 cylinders almost identically , only happens at throttle back after full power climb to circuit height
    2) Has anybody else had a similar issue as described , (the engine runs close to perfect at taxi, warm up and run up and climb out.)
    3) Is there any other suggestions to troubleshoot (checking main ignitions circuit breakers tomorrow. )
    4) What symptoms do failing coils normally exhibit
    5) What other possible issues could cause 2 CYLs to drop in temperature (CHT and EGT) like this.
    (We are assuming less or degraded spark to CYLs 1 and 2, less combustion, less heat causing rough running)


    Engine Monitor Graphs
    Green line is RPM

    First graph shows Abnormal engine performance (of which we now have several the same) shows CHTs 1 and 2 diverging sharply downward from CHTs 3 and 4 around the time we throttle back and see onset of noticeably rough running.

    Second graph is a normal engine performance - CHTs and EGTs move together up and down - as you would expect
    Even at 2450 PM - we see CYLs 3 and 4 CHT's of around 360/370 degrees but CYLs 1 and 2 are only 290/300 degrees




    The difference between the 2 is quite stark.

    Welcome all input and safe flying.

  2. #2
    Senior Member Dan L's Avatar
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    Default Re: Rough running engine when throttled back from full power at circuit height

    Two things come to mind right away. If the engine has over 500 hours change the spark plug leads. But you’ve done that already.

    Second, have you leaned it aggressively when you throttle back?

    With full rich my O-340 will run rough under those conditions too. Even worse if I pull carb heat thinking it is ice. Leaning is all it takes for me.
    Flying Carbon Cub EX #11 since 2011

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    Default Re: Rough running engine when throttled back from full power at circuit height

    Quote Originally Posted by Dan L View Post
    Two things come to mind right away. If the engine has over 500 hours change the spark plug leads. But you’ve done that already.

    Second, have you leaned it aggressively when you throttle back?

    With full rich my O-340 will run rough under those conditions too. Even worse if I pull carb heat thinking it is ice. Leaning is all it takes for me.
    +1 with Dan L. Also make sure your primer is properly seated and locked.

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    Default Re: Rough running engine when throttled back from full power at circuit height

    I think if found the problem! Looks like a service advisory was put out on a subset of carburetors for the CC340. I took the advice of others and had my problem analyzed via Savvy and they were great.

    See attachment for details on the carburetor nozzleN92LG CARBORATOR NOZZEL.pdf

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    Default Re: Rough running engine when throttled back from full power at circuit height

    Quote Originally Posted by Asher View Post
    I think if found the problem! Looks like a service advisory was put out on a subset of carburetors for the CC340. I took the advice of others and had my problem analyzed via Savvy and they were great.

    See attachment for details on the carburetor nozzleN92LG CARBORATOR NOZZEL.pdf
    Asher hi

    My 2013 CC 22 SN 271 has the AvStar carby and consequently I can not use the the improved fuel nozzle for the MS carby as per SB-22. Its a bugger when the manufactures chop and change vendors for critical components like this.

    The new plenum and spacer certainly seemed to have helped. The other issue with the CC 340 is that they - on all accounts from more experienced owners - do run very rich and need to be leaned at the earliest opportunity.

    From SB-22



    <<<Note: The new nozzle is compatible with and may only be installed in MSA carburetors having solid blue epoxy float part number 30-864. Do NOT install the new nozzle in a carburetor equipped with a metal float or any other type float except a solid blue epoxy float. If your carburetor is not equipped with the solid blue epoxy float, part number 30-864, please comply with Marvel-Schebler® service bulletin SB-2. Additionally fuel nozzle 47-862 is NOT approved for use in carburetors manufactured, overhauled, or serviced by AVStar Fuel Systems even where the carburetor model and part number appears to be the same or is similar to the MSA carburetor designation. >>>>

  6. #6
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    Default Re: Rough running engine when throttled back from full power at circuit height

    Quote Originally Posted by Dan L View Post
    Two things come to mind right away. If the engine has over 500 hours change the spark plug leads. But you’ve done that already.

    Second, have you leaned it aggressively when you throttle back?

    With full rich my O-340 will run rough under those conditions too. Even worse if I pull carb heat thinking it is ice. Leaning is all it takes for me.
    Hi Dan

    We did lean the mixture during one of these episodes but it didnt make any difference.
    I was wondering would too rich a mixture cause only the front 2 CYLs (1 + 2) to run that much cooler.

  7. #7
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    Default Rough running engine when throttled back from full power - RESOLVED!!!!

    I just got my carburetor back form AvStar, the replaced nozzles to the #1 and #2 cylinders (I think). Anyway when I described the problem, they immediately knew what the issue was. Unfortunately, they did charge me, even though the problem was a manufacturing defect. Oh well. $400

    That aside, the fix works GREAT. If done several flights now and absolutely no problem. Cylinder head temps are more uniform and as an added bonus there is significantly more power and a noticeably shorter takeoff roll.

  8. #8
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    Default Re: Rough running engine when throttled back from full power - RESOLVED!!!!

    Quote Originally Posted by Asher View Post
    I just got my carburetor back form AvStar, the replaced nozzles to the #1 and #2 cylinders (I think). Anyway when I described the problem, they immediately knew what the issue was. Unfortunately, they did charge me, even though the problem was a manufacturing defect. Oh well. $400

    That aside, the fix works GREAT. If done several flights now and absolutely no problem. Cylinder head temps are more uniform and as an added bonus there is significantly more power and a noticeably shorter takeoff roll.

    Asher hi Thats great news. Who did you deal with at AvStar.
    Do you still lean at circuit height / lower power settings.
    Do you have an engine monitor ?
    I didnt think carburetted engines had nozzles but am not an expert.

    Rgds Charles

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    Default Re: Rough running engine when throttled back from full power - RESOLVED!!!!

    Quote Originally Posted by CharlesTym View Post
    Asher hi Thats great news. Who did you deal with at AvStar.
    Do you still lean at circuit height / lower power settings.
    Do you have an engine monitor ?
    I didnt think carburetted engines had nozzles but am not an expert.

    Rgds Charles
    I do have an engine monitor. Before I had to lean aggressively to to prevent precipitous cooling of the #1 and #2 cylinders and a rough running engine. Now, perfect. Big difference. I was surprised that carburetors had nozzles but apparently this one does, I'm no expert either. But this fix was well worth it! I'm trying to figure out who I dealt with at AVSTAR and I'll post it if I can figure it out. I called their main number and gave them the serial number to the carburetor, they put me through to a gentleman who knew exactly what I was talking about and walked me through arranging the repair.

  10. #10
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    Default Re: Rough running engine when throttled back from full power at circuit height

    Quote Originally Posted by Dan L View Post
    Two things come to mind right away. If the engine has over 500 hours change the spark plug leads. But you’ve done that already.

    Second, have you leaned it aggressively when you throttle back?

    With full rich my O-340 will run rough under those conditions too. Even worse if I pull carb heat thinking it is ice. Leaning is all it takes for me.

    Only test flew the a/c wit the new Coils x 4, Spark Plug Leads x 8 and new Mini-sensors x 2 yesterday late afternoon.
    All went well but need further hours.

    Thanks Charles

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