So That Was What It Was!

Chris, the carb jets are on top so access means cowl off everytime. It would seem that the change in atmosheric pressure is the problem. The plan is to create an access hole in the cowl much to my digust.

Considering that the Extra was tuned with cowl off and back on with no noticeable difference with the original exhaust then I am assuming that air pressure in the cowl is not the problem. The OS exhaust is properly baffled therefore increasing back pressure and changing the scavenging characteristic of the engine making more possibly sensitive to fuel/air mixtures. Today I have bored a 18 mm hole directly over the jets (yes, I hate having to do this but necessary). it will now take 20 mm electrical grommet once tuned up. At a later date it is planned to fit a fuel dot to replace the grommet to give the impression that the fill line is accessed from the top of the cowl. much like some General Aviation has an oil fill cap in the cowl.

Phillip Ford wrote: The OS exhaust is properly baffled therefore increasing back pressure and changing the scavenging characteristic of the engine making more possibly sensitive to fuel/air mixtures.

Definitely changed the engine characteristics, gagged it. See Topic wimbornemac.org/forums/roll-calls/2827-saturday-26th-june#24831

Today tested the Extra with the original exhaust. Reset jets to factory. Start easily enough, leaned the low a tiny bit. Took off and Yep! after half throttle it really pulls now. Did a second flight and just raced it around with the bit of aerobatics and flies like it used too before swapping the exhaust. With OS exhaust 6,600 rpm. GP exhaust 7.600 rpm. Left it a rich on bottom end for now.

My setup is exactly the same and works perfectly.Can’t understand what the problem might be but I’d give it another go because the GP silencer is REALLY NOISY.

Jon, here is part of a topic on Flying Giants is verifies my thoughts - www.flyinggiants.com/forums/showthread.php?t=287709&page=5

and www.flyinggiants.com/forums/showpost.php?p=3116127&postcount=73

At present there appears to be no good answer to the muffler issues with GP engines, at least in the USA and Canada.