I've heard so many reports that support both ideas and I am in a quandary now. My latest project is a 90-100 sized Extra 300, it will be about 7 1/2 - 8 pounds soaking wet, and power is a Saito 150. It fits so nicely inverted, but the last time I tried this, nothing but problems. Are there any tricks I don't know about? How about starting the plane upside down? (Biggest problem last time)Talk to me.

No problems at all.
Make sure the tank is high enough and tyhe carb will change position. Better still, buy a pump or regulator and get the tank over the CG. Start the engine with the aeroplane inverted.
Just make sure you have controll of the cooling. The two openings at 9.00 and 3.00 oclock on an Extra cowl aint' going to cut it. Better to leave 'em uncut or block them off... and make/cut/design and new set of openings at the bottom.
Remember the outlet area needs to be 3x the inlet area.
95% of people having problems running inverted engines and track the problem down inadiquate cooling and/or/ tank position.

Sonetimes an inverted 4 stroke is a pain and sometimes not. Depends on how much oil gets past the rings to drown out the plug. I would think that an onboard glow starter would take care of that problem.

When the engine is running, oil will not drain past the ring and drown the plug. There is far too much pressure in the head to allow oil back in.

Invert the engine and have a good time!

John

I've been running my OS 91 4-stroke inverted for nearly a year now, in 2 different planes. It runs great, only recently it would lean out in a hover. I realized I bumped the needle valve almost 180 out, oops, got it right now...

It does when it is sitting up. Making it hard to start till the glow plug burns off the oil. Still the plug seems to have a lot of oil even right after running. That may be oil seperating out of the mixture. I'm not sure of the mechanics but the engine ran much better after it burned almost two gallons. Never could get it to idle as well as when it is upright. Switching to Coolpower fuel helped also. The best plug was the Thunderbolt FS plug. Next time I will try an onboard igniter.

keep the center of the tank lower lower than the carb. at least 2/3 of the tank should be below the carb. if the tank is too high, there will be a siphoning effect into the carb especially evident at idle. some 4 strokes don't like this type of "forced fuel supply" and will idle erratically with tank too high.

Most, not all 4stroke invert problems are the fuel tank position. make sure the center line of the carb is also the center line of the tank or Close. The old rule of thumb was no more the 3/8 of an inch up or down for a normal upright mount, invert should be even closer to center line if possible, if not possible use a regulator of pump.[8D]