low power

[hawaiiJim]

BC12-D A-65 8 f with low compression. Now has 2200 hours, overhaul at 1200, another at 1653 ( reported cylinders already bored, installed ,015 oversize rings.) Sent one cylinder for overhaul at 1940 hours. Now 3 cylinders have low compression readings.

I overfilled the crankcase and flew it couple hours before correcting the error. Aircraft not flown , but prop turned, for 5 months in 2003.

What to do, where to have it done?? Any thoughts appreciated. Aloha, Jim

[alwaysoar]

If you compression is low it could be valves. Carbon can form on your exhaust valves. Have you pressurized your cylinders to find where the blowby is coming from?

[Robert Lees]

...or worn valve guides or gummed up/worn rings. Sounds like a bit of invasive surgery required.

[ROllerton]

Maybe a shot of MMO?

Prefered by 5 out of 4 old british cars.

[hawaiiJim]

thanks for replies, MMo? Hmmmm after more testing, ordered 4 new Millennium cylinders from API @ $3500.! The firewall back is show quality, restored by Robert Ross, "Taylor" award winner. will post results. Jim

[Jim Hartley]

MMO...No Joke!

[Tcraft Classics]

Check your valve springs, too. Weak ones give you an aweful rpm drop. Should be less of a problem if you rotated the prop.

[NY86]

I'm sure I'll get a lot of flack for this, but, IMHO, the only thing rotating the prop does for an idle engine is scrape any oil film you may have had off the cylinder walls. Better to just leave it alone, or fly it. Ground running has its own set of issues, too (water vapor in the oil...)

[Tcraft Classics]

I was once told that turning the prop 1 rotation every week takes the load off the valves on their respective intake and exhaust strokes, thereby decreasing the likelyhood of a valve spring losing its strength due to being constantly compressed. What John says makes excellent sense also.

My C-85-12 suffered from a long-term idle condition.( 1 year +) It would run well (sometimes) until I made altitude, and then drop 100-200 rpm. It wasn't very consistant. Very scary to say the least. I tried intake boots, fuel, carberator rebuild, fuel system clean - just about everything. The an old-time cub pilot told me to check the valve springs. I ordered new springs ($80) and, viola - no more problems. Compressions on the cylinders came up also.

[ROllerton]

The idea of rotating the load on the springs is a good one; but I think its offset by scraping the oil film on the cylinders and the cam lobe. In fact I worry about the cam and lifters the most.

I thing the big thing thats missed is to get that dirty oil out, get clean oil in there and then run the engine to circulate it so it coats the metal to metal parts with something that hopefully will last during storage. If you take an engine apart thats been sitting with old dirty oil you can see pitting and corrosion on the polished surfaces from acid and perhaps water thats in the oil.

bob.