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Hi everybody, I am new here,as well as my 503 dcdi (10 hours).We have been working that bug since few weeks by now.Symptom:while flying or on the ground the RPM will drop or surge anything from 20 to 300 RPM.Somehow ,this condition,which was highly intermittent at first,now can be replicated after 2 or 3 minutes.Like somebody playing with the throttle.
Short history:
Break-in was done properly,except that the wooden prop was vibrating due to imbalance.This prop was used for the first 4 hours.Heads were torqued after 2 hours,Bings carbs set to 'standards'.EGT'S between 975 and 1050 in all range except for the break-in were it was higher at top end but still under 1150.Prop was a wooden prop which was previously on the same kind of motor and reduction B (2.58) but was cut to 64 inches due to ground clearance.Top RPM on ground was 6300 and flight level 6700.
SET-UP motor is in traction in a closed cowling with 2 NASA scoops connected to 3 inches duct hooked up to the fan through a round canister.2 scoops in front to give fresh air to the carbs and force the radiant heat from the exhaust to the bottom outlet in the cowling.Fuel supply is by the diaphram pump and the smallest Facet pump in serial.This one is right at the tank which has 2 vents.In the first few flights, we had vapor locks (very warm and sunny days) specially after stopping the motor and leave the craft out with the sun.The motor because of all the heat from the exhaust was going up when it was stopped was very hot.We did add-up 2 vents in the top cowling ,put a reflective shield over the muffler and this problem is gone . Now,I'll summarize in short what we did....
Change all exhaust , manifold, canister for another stock exhaust.
Pushing slightly the choke to enrich did not change anything except lower RPM and EGT's.
Changing the needles position to first notch (to lean) ,no go except higher EGT's.
Spark plugs changed, chokes changed,diaphrag pump was bybassed, facet pump was bypassed, gas tank bypassed and use of other mogas and oil...no go
Pistons and cylinders and Spark plugs visual check OK
Rubber intakes were checked for cracks,OK
Static test ok loose less than 1 pound for 15 minutes ,vacuum or pressure.
Prop was replaced,( Ivoprop, variable pitch, 64 inches,set-up for 6300) no go...
Both carbs were changed including needles and all jets...no go
Charging coil from the motor disconnected in case of an unknown load in the aircraft...no go
Decided to put back all the original parts,put back the exhaust,adding 2 new gaskets (the outside ones) to the exhaust,decided to retorque the heads one more time and changed the prop again for a GSC variable 68 inches and since it was not pitched properly,I started to open up the throttle slowly not to over RPM.First thing I noticed is that my cyl 2 EGT was higher than usual (1100) but cyl 1 EGT was normal (around 990).Thinking that needle jet in carb # 2 was still in top position
,I stopped the motor and checked things out...Both needles were in top position from a previous test,strange, I usually had from 10 to 40 degrees higher in cyl 2 and the average was 20 degrees.So, what the heck, decided to put both at position 2 from top (standard) and try again.Now both were higher than usual (about 1050 compared to the usual 1000. I throttled up to 6700.So I added 2 degrees of pith an now top RPM was about 6350 and guess what...The 503 was running GREAT like and electric motor with a very stable RPM in all ranges!!!WoW !!!And that test was for at least 15 minutes long...This was yesterday, today I decided to do the same test but adding the K and N filter which was not there for testing purpose (clean new filter) and repitch the blades slighly to less than .5 degrees difference and checked the tracking..and also clean the blades which were full of bugs.Started the motor , top RPM was still 6350 and ...the problem came back!!!Removed the filter...no go.So somehow , gremlins in the motor took a break yesterday night but came back !!!HELP!!
Guy from Montreal
  • Re: 503 rpm fluctuations from 4900 to 5800

    by » 11 years ago


    Since you checked the fuel delivery then I would suspect an air leak.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: 503 rpm fluctuations from 4900 to 5800

    by » 11 years ago


    Hi Roger,we did an air leak test which is removing the exhaust, plug the ports with a plate,removed the carbs ,plug in the rubber mounts and inject 6 pounds of pressure through the vacuum port and we were loosing less than a pound for a 15 minutes period,same thing with vacuum.Is there any other air leak test that we can do ??? Also the fact that our EGT are averaging 990-1050 suggest that we are not running lean.
    Guy

  • Re: 503 rpm fluctuations from 4900 to 5800

    by » 11 years ago


    Is it possible that your 2 fuel pumps are giving to much pressure and over coming the needle valves in the carbs.

    Nick.

  • Re: 503 rpm fluctuations from 4900 to 5800

    by » 11 years ago


    I had a similar problem with my 582 greyhead and a wooden 68" GSC prop. RPM varied slowly and irregularly between 5300 and 6200. Checked everything in the fuel system and carbs like you did. Finally we discovered that the prop was far out of balance. After dynamically balancing the prop, using a Microvib tester, the engine runs smoothly and the problem never returned!

  • Re: 503 rpm fluctuations from 4900 to 5800

    by » 11 years ago


    Hi Nick
    That would have been possible but this set-up is used by some guys in Europe running the same ship,the Choucas motorglider.The main intent was to back-up the diaphragm pump in case of a failure and also eliminate the primer.It does fill the carbs cuves before starting.It does the job fine as far as starting the motor without having a primer.The Facet pump provides and average of 3 pounds of pressure and pushing that gas trough the diaphram does not exceed 6 or 7 pounds,We, however did by-pass the Facet pump by connecting the diaphragm pump straight to a canister full of fresh gas. We also made the contrary, we removed the diaphram pump and delivered directly to both carbs from the facet.
    Thanks
    Guy

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