So That Was What It Was!

The story? I was flying the Extra with the new GP38 engine. I had given it a bit more lean on the low jet to reduce the burbling at low throttle. After couple of flights it seemed like the exhaust note was a real bark now - a bit too noisy I thought. Checked exhaust, nope still nice and secure. Spoke to Jon Jacobs later and said I think I need an OS33GT exhaust, better baffled but expensive. Jon says there is one on BMFA for £60 incl P&P. Went home and secured it. Great! Today I did the change over but had to remove 20mm of downpipe to allow the cowl to slide back on. Sorted but will probably need to use 19mm ID silicon tube extension to keep the carbon off the fuselage underside.

Has anyone got a spare 22mm clip by the way?

Anyway, tidying up and putting the original exhaust into a bag for storage noticed that the "smoke blanking plug" was missing. :ohmy: Aha! That's why it was noisy! I feel really stupid now. £60 for nothing. No not really, the OS is far superior compared to the Heinz bean tin supplied by GP.

Should this be in Stories of Stupidity?

ive got the spring clips Phil - give me a call

Kevin Ross wrote: ive got the spring clips Phil - give me a call

Thanks Kevin, will do. Should only need one. Not desperate though due to weather. When are you next up the field?

hoping to go this week but awaiting a rendering contractor to start work.

Well tuned it up today with new exhaust. Running nice. Installed cowl, came back a few hours a later to give it a whirl. Doh! Take-off distance extended, top throttle does not leap forward like it used to, sounds rich now. Same issue that Jon Jacobs had. So, add a tube to the carb cover into the fuselage and make some form of access from the top of cowl to the carb jets. A large fuel dot would look the part and not out of place.

If it sounds rich, it almost certainly is rich, why not try tuning it again before trying to fix a problem that doesn't exist??

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.