MC carb for Webra

[Brian Billings]

Keith, Matt, I have not messed with the pump yet but that is next, I had lots of mail that said don't mess with the pump and a few that said it's ok. I have triple checked everything else and adjusted needles 1/16 at a time from one extream to the other, I guess adjusting the pump is in order. Is everyone in agrement that increasing the spring pressure in the pump increases the pressure.
  ----- Original Message ----- 
  From: Keith Black 
  To: discussion at nsrca.org 
  Sent: Thursday, June 05, 2003 6:56 PM
  Subject: Re: MC carb for Webra


  Brian, have you experimented with increasing the pump pressure? Maybe the upline mid-range is starving due to not having enough fuel but WOT creates enough extra suction to pull the fuel through. 

  If you haven't already tried this why don't you increase the pump pressure by 1/4 turn at a time and see if it helps.

  BTW, you're correct that the throttle is very non-linear. Once my carb is about half open the engine is much closer to full throttle than mid throttle. I added a mix to my radio to introduce an expo behavior with the throttle and this tremendously improved how linear the throttle response is on my throttle stick. This made the plane much easier to fly.

  Good luck,
  Keith Black

    ----- Original Message ----- 
    From: Brian Billings 
    To: discussion at nsrca.org 
    Sent: Thursday, June 05, 2003 2:27 PM
    Subject: MC carb for Webra


        Bob, I was very careful running this motor in, ran several tanks through engine before flying and monitored the cylinder head temp the whole time. Motor ran great the first weekend out with the plane but the next weekend motor was having trouble from the first start of the day and I was unable to fly due to troubles, sagging mainly. I sent motor to piedmont for check out and they said it ran fine and had no problems. I can hold engine against compression with no leak down felt but the discoloration of the ring and top of piston does add credence to your theory, plus it does seem like there is less compression once engine has been ran to operating temperature. I would put a piston, ring and liner in it if I new my problems would go away but I am afraid that it is not the root of my problem but a outcome of the sagging in flight. 



    Hi

    I'm sticking my nose in here, but I've been running the Greve (Piedmont) set up for some time now and at their setting it WORKS, very well.  I agree with Jerry, by the way, the numbers are wrong, based on my theory, but it works so I have not messed with it.  Reading your symptoms, I suspect you have a bad ring, allowing blow by into the crank case, which changes with temp, which changes the crank case pressure, which changes the "pump performance", which changes the temp, which changes........  I have found that long before the ring is worn enough to cause loss of compression/power it will upset the pump/regulator (speaking of the OS here) because of combustion blow by into the crankcase.  Not enough leakage to effect peak performance, but with crankcase pressure at 8-12 lbs (I think those are the numbers) and combustion pressure at probably thousands  of lbs a little leakage could double the case pressure and screw things up.  I have found replacing the ring sometimes ring and cylinder to solve this, anyway, the theory as to way is just that, theory/speculation.

    Hope this helps

    Bob 
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