I fly a Fox .15, .19, .29, two .35's all very old but hardly used, and a new .45. I haven't had any 'burp' no matter
what I did to them. At what point does the burp happen?  When I invert the .45 it sags badly ,rich I think. I don't
stay inverted long enough to kill the engine. The tank, homemade square wedge uniflow 4 1/2 oz., is 1/4 in.
above engine center on a profile fuse. What do I do to correct this. Red Max 15% with 25% castor 3% syn.
Perry Rose

The "infamous" burp is the tendency of a side mounted (profile) Fox .35 to quit, or nearly quit in the performance of an outside loop. Most annoying! 


W.

I've been flying one of my old Fox 35's mounted upright.  When doing OTS horizontal eights it has a tiny (one revolution?  ) burp transitioning from insides to outsides.  Never noticed that before.  No burp anywhere else.   

Perry,

You could try searching (that input box at the top right next to the magnifying glass..) under wig-wag  for a post I put in a while back. Helps reduce 'burp' and getting in a first flight w/o starving or richening off the engine.

The reason for the burp may be related to the change of g-loading when the model kicks over from inside g to outside g at the intersection in the figure eights. Why it takes the form of an audible misfire or non-fire for some Fox 35s could be a very complicated combination of many things. If the tank, prop, plug and fuel are all good, and the setting is about right, you may hear a one or two step stutter without losing noticeable power, and without the engine shutting off.

The low round loops in the round Eights require about 10 to 11g for their shape, and the effect of gravity adds 1g at the bottoms and about 3/4 g at the top (trigonometry - at the angle that gravity works on the model at 45° line elevation.) The intersections are supposed to be vertical, so lift and gravity are at right angles - they don't interact at that split second... So the loads on the fuel are from 10-11g toward the wheels (in the inside) to the same range away from the  wheels (in the outside) and that's suppossed to happen in a hundredth of a second or less. Quite a change.