[NSRCA-discussion] Fuel Regulation in 2C Engines

[Bob Richards]

Dean,
   
  On a similar note, some carbs that are designed for muffler pressure will not work well with the Cline or Iron Bay regulators.
   
  I tested an MVVX .61 w/ Cline regulator years ago for the Kfactor.  The needle valve could not be opened up enough to run properly. Long story short, the carb was designed for muffler/pipe pressure, and needed to be modified for the Cline. The modified setup worked quite well, but to be honest that was not a long-term test.
   
  I thought about possible modifications to the Cline regulator, which would have been simply to put a pressure fitting on the regulator, and connect it to the pressure tap on the muffler. This way, there would be muffler pressure on the backside of the diaphragm instead of atmospheric, so the carb should see the same pressure it would if the regulator was not present. Never had a chance to test it, however.
   
  Also, the Tee method did not work at all on my OS 1.20 Surpass II. The problem was that the regulator was built into the carb, completely separate from the pump. I understand that the Tee worked well with the earlier OS 1.20 pump.
   
  Bob R.
  

Dean Pappas <d.pappas at kodeos.com> wrote:
  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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.nsrca.org/pipermail/nsrca-discussion/attachments/20060306/cc4f2081/attachment.html