Model Forum / Radio Controlled / Air Models / July 2003

I take it that these engines have a crankcase vent line. You don't have it
plugged, do you?

Does the engine run properly at 100% throttle?  By that I mean are there any
tuning problems or running problems other than the symptoms you describe?

--
Paul McIntosh
Desert Sky Model Aviation
http://fly.mcintoshcentral.com

Hi,

I have quite a collection of 4 cycles and a few 2 cycles.  I have seen this
caused by eight different conditions that I can remember.  There are
probably more, but they haven't bitten me yet.

1) Using muffler pressure forced too much fuel to the carb and the mixture
actually got  richer as the rpm increases.  If this is coupled with #3
below, the extra fuel is sucked out of the carb.  I restricted the pressure
line.

2) The spray of fuel is actually coming from the fuel tube at the carb
nipple.  The muffler pressure caused the feed line between the tank and the
nipple on the carb to expand slightly under pressure and fuel leaked between
the inside of the fuel tube and the outside of the carb nipple.  No effect
on the running of the engine, but lots of fuel on the firewall.  I installed
a double loop, wire spring clip on the tubing at the nipple.  I use clamps
on the tank fittings and this probably saved me from fuel in the fuselage.
This is the last time I used the thin-wall tubing supplied with some ARFs.

3) The mounting position of the engine and the configuration (no cowl)
caused air to flow from prop blast to flow across the top of the carb and
suck some of the fuel from the throat.  This showed up as a wet firewall and
limited RPM because of a "lean" condition.  I used some plastic tubing from
Home Depot (plumbing dept) that was a press fit over the intake of the carb
and used my heat gun to form the plastic tube into a "ram".  This made the
high speed needle difficult to adjust.  I used some twisted ss safety wire
to clamp it on the carb lip.  It came off once in the air and the engine
went supper rich quickly.

4) Leaks at the carb to crankcase mount (2C) and leaks in the intake
manifold joints (4c).  The little "O" ring between the manifold pipe and the
carb body is easy to damage if you take the assy apart.

5) Fuel leaked around a old (worn from vibration) needle valve in the needle
valve assy.  I replaced the needle valve, but the inside of the assy was
also worn and the leak continued. I had to replace the entire assembly.

6) The crankcase vent was partially blocked.  The crankcase pressure at the
bottom of the intake stroke must have caused "reverse blow-by" and prevented
the full shot of fuel from entering the cylinder.  It took a while to find
this one - I checked the crankcase vent because I was not getting above
about 70% rpm.  Cleaned the vent and both problems went away.

7) The intake valve was not seating because of some carbon.

8) On both a horizontal and inverted mount O.S. FS .91 - the engine ran fine
at idle and high rpm.  But, I couldn't seem to get a real clean transition.
There was always extra fuel from residual pressure in the flex-tank when
dropping to idle quickly and when shutting down.  The fuel would "pool" in
the manifold just enough to cause the problem.  I replaced the "flex" tank
with a rigid tank and this reduced the problem.

Well, there are eight things to check.  Good luck.

J

Thanks to all that responded. I had forgotten about the crankcase vent.
You may have the answer. I dont know for sure. Seems likely.
This year the engine seems to start harder. Usually blocking off the
exhaust pipe, opening the throttle to full and flipping the prop 5 times
 (no battery) and it would start on the first try(now the battery).
This year it is a little cranky and on the high end(10,500) it wavers a
little. The APC I used to run on it makes the engine run hotter than
hell so I went to a 10-6 MAS. No cowl yet.  Runs cooler. I think I will
send it in for overhaul. Probably more cost effective to buy a new one
and sell the old one. I have the engine on a Hangar 9 cub. 80 incher ARF.