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Short version of a somewhat long story... or at least convoluted:

Me and partners just purchased a Tecnam Bravo P2004 with Rotax 912ULS.
Engine built in factory in 2005 and first put in service by Tecnam factory in 2006.
One owner,now 315 hours (only 15 put on by us).

Despite plug change, oil change, filter cleanup, carb sync, replacement of a split O-ring on a adjustment jet detected by experienced Rotax mechanic, engine still won't spin up to specified 5100 RPM in static test at WOT.
And (at least before all the "tune-up" type attempts in level flight at any altitude won't spin up past 5100 RPM at any altitude.

In short, overall performance/power is about what we'd expect if the engine's max power was in the 60-65% output range. Substandard.

We have 100% ruled out that it has the wrong prop (pitched too high, was first thought).
Not only by specs on prop and contact with Tecnam, but also by fact that if prop was pitched a bit too high we'd be seeing impaired climb and takeoff but probably improved top end cruise.
In fact cruise peaks at 5000 to 5100 RPM and is about 15 knots IAS below claimed sustained cruise (intended to be tat 5500.) And reports from other flyers of this model report getting better than claimed factory cruise rates.

Despite the seller believing it had never been substantually ran on avgas our mechanic looked into the pistons and says it looks like it's seen a fair amount of leaded avgas during its lifetime. (Don't think there was any bad faith.... Owner died and friend of owner was helping surviving wife sell it.... may have had wrong info on avgas use history.)
Given that, and the fact that the compressions on leakdown test are low (within spec but low) and leaking back through carb (intake valve leakage) our prime suspicion at this point is that lead on valves is preventing good seal.
(See---
Figure 8: Valve after 1000 hours Avgas use
http://www.jmfk.net/rotax%20maint/Rotax912Maintenance.pdf)

(I'd publish the most recent and most trusted leakdown results here, but log book is with the mechanic 2.5 hours drive away waiting for a nice flying day and someone to drive me there to fly the bird home.
Will say, that in engine log leakdown tests looked real good to about 170 hours then down.
Second to last by guy who did leakdown at 300 hours a year ago at previous annual was
1-80/87, #2-81/87, #3-82/87, #4-82/87.
Definitally far lower than on our 900 hour 15 year old 912UL on our other plane we're selling.)

So a few question I'd like responses too:
1) Any chance that running Decalin Run-Up through it with the mogas we'll be using from now on that it can retroactively remove any of that lead?

2) Can a valve-job be done on it?
If so, similar to the way it could be done on a car.... remove head, send them to reputable shop and say "do a complete valve job?"
Or if not that way, how? And at approx what cost is a valve job?

3) plug colors and exhaust pipe color are not screaming "too lean" or "too rich" but could be a tad off.
Yet,At 315 hours and 9 years of age what's the chance (a) it _needs_ carb overhauls and (b) chance overhauls of carbs could restore substantial part of the missing power.

Thanks for any help in this frustrating situation..

Al
  • Re: Is it possible to do "just a valve job" on a 912?

    by » 5 years ago


    Hi Al,

    Removing the cylinder heads is relatively straightforward, it just requires draining the coolant and removing the manifolds. Make sure you don't drop the push rods when pulling off the heads.

    The valves can be removed and lapped back in but I would take it to a machine shop where they have the equipment to clean up the ports properly.

    Have a close look at the valves when they are removed in case any have lost their case hardening and have become distorted. It may also be worth closely inspecting the heads, cylinder and piston crowns whilst you are in there.

    Kevin

  • Re: Is it possible to do "just a valve job" on a 912?

    by » 5 years ago


    Declin run-up and Alcor TCP do a great job of preventing lead build up but they will not do much cleaning of old lead out.
    I would take the heads to a Rotax overhaul shop if they need more than valve lapping.
    The piston rings and lands will need the lead cleaned out.
    Scrub the cylinders with varsol and red scotchbrite. Do not hone.

    Thank you said by: Al C

  • Re: Is it possible to do "just a valve job" on a 912?

    by » 5 years ago


    Thanks,

    If it comes down to it we'll do the valves

    As for ---
    "Declin run-up and Alcor TCP do a great job of preventing lead build up but they will not do much cleaning of old lead out":

    I spoke directly to reps at both Decalin and Alcor. Even have email from Decalin (See below.)
    Both claimed that they will, although slowly, clean out the old lead.
    Alcor even says they have independent third party test data from Embry-Riddle Aeronautical University supporting with tear down tests that support that claim.

    Maybe not, but at $30 for a bottle of Decalin or $45 for Alcor TCP you can bet we're going to try that first.
    If we get no results, some results, or good results with that "cure" I'll try to remember to post that here.

    Al
    ----------------------



    On Fri, Jun 5, 2015, at 04:21 PM, Decalin Chemicals LLC wrote:
    You left a message on our answer machine concerning the removal of lead deposits from using avgas in an aviation engine by using Decalin RunUp.

    The short answer is yes it will remove deposits.

    Our product contains an advanced deposit removal ingredient which will remove combustion deposits in the combustion chamber and from inlet valves, especially if they contain carbon. Metallic deposits on exhaust valves and spark plugs, if they have been there for some time, are much harder to remove.

    Normally deposits are removed in one or two tankful’s of unleaded gas.

    I hope this helps and answers your question.

    Regards,

    Chris Lowery
    Decalin Chemicals LLC
    decalin@sbcglobal.net
    650-353-5062

  • Re: Is it possible to do "just a valve job" on a 912?

    by » 3 months ago


    Where in that email does it say the lead will be removed ? “Metallic, much harder to remove”

    I’m guessing that “metallic” is lead.


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