[NSRCA-discussion] Fuel Regulation in 2C Engines

[Rcmaster199 at aol.com]

Dean, Vicente and all,
 
The high volume arrangement, with pressure up to around 6 psi, was  available 
from the YS 2 stroke set-up we used to fly 15 years ago. And as I  recall, 
that arrangement wasn't really much better in fuel delivery than the  better 
set-ups of today. The 2 stroke still tended to wind up on downlines,  less 
brakes, etc. BUT, it was a little easier for anyone to play with it and  arguably a 
bit more reliable.
 
It seems that it's a built-in characteristic of 2 strokes. I think Dave's  
approach of attacking a different part of the power system is a means to an end. 
 Variable pitch props have been tried before but perhaps it's time to 
re-visit  them with the programming features available in today's radios.
 
On the other hand, I might never try it at least in competition.  Although I 
am a tinkerer by nature, I much prefer KISS for competition  set-ups.
 
regards
Matt
 
 
In a message dated 3/5/2006 8:31:45 PM Eastern Standard Time,  
d.pappas at kodeos.com writes:

Hello  Vicente and All,
When you put a Perry or OS 140 pump on a carburettor that  was originally 
intended for suction, the metering slot may be very wrong. The  likely symptom 
would be mid-range richness. If, and only if, the pump is  capable of delivering 
substantially more volume than needed, at the desired  pressure, then the 
loopback method will work, but ...

A restriction in  the return line will increase the pressure, and reduce the 
surplus volume  requirement. This has been tested! The idea of a needle valve 
or other  restriction in the return line is a good one, and in the past, I 
have used a  simple piece of brass tubing filled with solder and drilled out with 
a small  drill. You could even partially crush the tube with pliers, to get 
an initial  setting. Keep crushing until the egine runs with the needle valve 
only a half  turn in from the setting on normal pipe pressure/suction. This 
still may not  get you the characteristics you desire.

The general rule is that excess  pump pressure means that the needle valve 
and idle adjustments are set lean,  and the metering slot still leaves you rich 
in the middle. This is fixed by  lessened pressure, but if a return path is 
used to reduce pressure, then the  pump needs to be able to deliver much more 
volume, or the top end needle has  to come out too much, and there will be 
sporadic leaning. It will make you  crazy. Oh, I forgot: we fly toy airplanes, so 
the craziness is  fine.

Later,
Dean  P

-----Original Message----- 
From:  nsrca-discussion-bounces at lists.nsrca.org on behalf of 
vicenterc at comcast.net  
Sent: Sun 3/5/2006 8:13 PM 
To: NSRCA  Mailing List; NSRCA Mailing List 
Cc: 
Subject: Re: [NSRCA-discussion] Fuel Regulation in 2C Engines


John:

Thanks for your advice.  I already worked the prop.  The best I  found is the 
3 blade re-pitched.  I just want to improve it more.   Probably very 
difficult task base on the feedback I am getting.

I like your idea to tune the return line.  I  have been thinking to go that 
route with a Perry external needle valve in the  return line.  The process 
would be, in the OS 160, to adjust the return  needle valve until the high end 
needle valve is about 2.5 turns open at full  power.  That is the normal setting 
when I run the engine with no  pump.  In this way,  we will be close to the 
original design  pressure.  Normally, the OS 160 with the Perry pump sets within 
1 to 1.5  turns open.  Than means that the pressure is too high. 

Vicente

--------------  Original message -------------- 
From: "John  Pavlick" <jpavlick at idseng.com> 

Yeah, I tend to agree with Nat here. After  some discussion about the Perry 
loop idea, I know someone who actually tried  it. It came down to the theory 
vs. reality thing i.e. "only one way to find  out..." It seemed like a good 
idea, but it didn't seem to work on our big  motors. I think that in order for 
this to work properly, you would need a pump  that is capable of way more volume 
AND pressure than you need. In addition to  this, the regulator should be an 
external device, not inside the pump like the  way it is with a VP-30, or the 
internal regualtor needs to have an eaxternal  "return" line. This is how a 
fuel-injected automotive system works, so it is  possible. The Perry loop thing 
works for R/C cars and smaller motors but I  think the pump needs more volume 
to feed our big motors with that big gaping  "hole" in the system (the return 
loop). Maybe tuning the size of the return  loop wi! th a restrictor would be 
worth a try. Is it worth the effort?  Generally a VP-30 that is properly 
adjusted will give reliable performance  with just a vent line and no tank pressure 
/ no return loop. Props have a big  effect on what happens when you return to 
idle too. If you want to help  downline braking, look at props before you 
spend a lot of time re-designing  the fuel system. Also, try setting the idle a 
little bit richer. The slow  transition to idle condition is sometimes 
aggravated by a lean idle mixture.  Not trying to discredit anyone's theories,  just 
passing along some of my  observations.

John Pavlick
http://www.idseng.com  <http://www.idseng.com/> 


 
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