MC carb for Webra

[patterndude at attbi.com]

Brian,
Now that you've got all different flavors of advice I hate to step in and tell 
you something different.  However, I want to put these comments in context.  
There are two ways to adjust midrange mixture: pipe length and pump pressure.  
Each has its own set of side effects.  If your Greve pipe is anywhere between 
27 and 28 inches then adjust the pump.  Be sure it's sagging due to leanness 
before increasing pump pressure.  It is a very rare engine that comes from the 
factory with too little pump pressure.  When adjustment is needed it is 
usually to lower the pump pressure (screw it out about 1/2 turn to start).  
I'm sure you already know how to tell if it is lean or rich in midrange so 
I'll stop here.
--Lance
> 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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