im building a GP cap 232, and i have a 25% sig extra 300, the extra has a saito 180 mounted upsidedown, and i love the looks rather than having it stick out the side of the cowling, as well as other reasions, i also plan on mounting the engine upsidedown i put in my cap, what are the side effects to this, as i notice most modelers go by the recemended sideways mounting.

Not much difference...

When you mount the engine inverted, the glow plug will get soaked with oil, when the engine hasn't run for a while, that's all.

Plus you have to be careful when priming the engine, because all fuel will flow staight into the cilinder. Make sure there isn't too much fuel in the cilinder before you attempt to start the engine, because if there is, it isn't very nice on the con rod...

the engine will load up and die a lot quicker at low idle, especially considering it is a four stroke. However, with a fix it is much better than mounting sideways because of balance issues. you are flyinga cap, which is far more sensitive to these things becuse it does not self correct. i would mount it facing down and get a glow driver. they are a lot more convinient, and if you are low on dough, you can make one. mechanical switches fork great, but you cane also make one. here is a url: http://www.uoguelph.ca/~antoon/gadgets/rc-sw.htm

If you have the low speed end adjusted properly, the engine will run and idle as well upside down as in any other position. If the engine is loading up and dieing at long idles, the low speed is set to rich.

Yep, take the time to lean out the low end and you should be fine. Lean it out so that the rpm only drops slightly when you remove the glow plug igniter. Don't go too lean that the engine quits when you go to full throttle from an extended idle.

Well, as far as i know, all of the oil runs tward the plug because of gravity, and i have leaned it out almost a half turn, to where it coughs and dies at spool up, but it still loads up and dies. Partly because i am running the engine at about 100 rpm at idle for a .91, but still. I would take the precaution anyway. Espacially if it is an expensive plane

The biggest problem most people run into when mounting an engine upside-down is fuel tank placement. Just make sure the tank centreline is in-line with the carb and you shouldn't have any problems at all.

Note that fuel tank placement is not even an issue if you have a YS engine. My tank is about an inch above the carb and at least seven inches away from it, and so far no problem. All my YS engines are mounted inverted.

my os engine has a fuel pump so i could drag the fuel tank behind me if i wanted to

No, don't do that! You'd look silly

Dittos on the no problem with inverted 4strokers. I was worried too, lots of opinions out there, 3 engines later, and a bit of time setting the low end, never one problem!

i like doing odd things like that, but it would ruin my fuel tank