Mike,
I think that among the answers you'll get will be some useful information. I've never owned a C-90 in a Coupe but, from what I've heard, I don't thing 6 gph is out of line (23 lph). But I perceive something wrong in that at 2200 rpm, you are only getting 82 kts , IAS at 2,000'. My Ercoupe, with a very flat climb prop on a C-85 engine, would fly 100 mph (87 kts) and an Ercoupe with a "normal" 7150 prop (on a C-85) will normally get 108 mph (94 kts). Normally, any Alon will just walk away from us in cruise. They are faster than 94 kts. Many have claimed cruise speeds as high as 115 mph (100 kts.). I presume that's at the C-90 maximum recommended cruise. (One resource says C-90 max recommended cruise is at 2250 rpm and I've got another source that says 2350 rpm.) Here's a chart from Paul Prentice's book Fly About Adventures and the Ercoupe: http://edburkhead.com/Ercoupe/enginepropeller_efficiency.htm I'm wondering what the pitch is on your 2-year-old prop. I've seen a few planes with "cruise" props that were so steeply pitched that the engine couldn't turn them effectively at cruise and the plane ended up with a very slow cruise rather than the desired fast cruise. I guess it's possible to have a prop pitched so flat that you'd only get 82 kts (94 mph) at that rpm. But you'd have a really fantastic climb if that were the case. Test question: Does it climb really well or very poorly. Great climb would match a very flat climb prop. Very poor climb would match a very steep "cruise" prop or a weak engine. Possibly you have a weak engine but I'd look first at other causes, like prop pitch. Also, I'll agree with John that you should get your tachometer checked (a model shop digital tester is cheap but many airport mechanics have them). Also, you should consider doing a test of your airspeed indicator. This website has a good procedure: http://www.csgnetwork.com/tasgpscalc.html Ed Burkhead http://edburkhead.com ed -at- edburkhead???.com (change -at- to @ and remove "???")
