From Adam; IO360 engine in a Super III:
Well, after an oil change today, compression check showed all cylinders in mid 70’s, and one at 67. This probably rules out oil fouling. I flew a couple patterns today and did not develop power on the first take-off roll. Adjusted mixture and was able to develop power.
I’m thinking that this mouse is really touchy WRT mixture, especially at low RPM. I kept it lean during taxi and run-up, and it seemed to help (i.e. it took longer to foul), but it still eventually fouled.
Any ideas would really be appreciated.
Good compression does not rule out oil fouling on the Lyc IO360. Have you read the FAQ posts on BAC about oil consumption, starting, and leaning this engine? I put a lot of time into them, and it is too much text to copy and paste into email unless there is no other option. What is your oil usage rate? Rates as high as three hours per quart are not uncommon on this engine, with compressions in the high seventies.
Assuming you have read the BAC FAQ, cheap things first. Locate a set of four used but serviceable fine-wire spark plugs (new ones are quite expensive, but unlike the massive-style plugs, they usually last to TBO). Some valid numbers are REM38P, REM38S, and SR83P. You can usually find some good used ones at a spark plug reconditioning company, such as Aircraft Spark Plug Service, 818-787-5680, and sometimes on eBay. Try putting these into the lower plug position on each cylinder. Perfectly legal and safe to mix plug types. If the problem mostly disappears (which I doubt), it is primarily an oil fouling problem (which predominantly affects only the lower plugs).
If it is oil fouling, you should be able to taxi back in while it is happening, pull a lower plug, and see wet oil on it. If the plug is pretty dry, but is black with a soft black coating, the engine is running too rich. Sometimes (but not always) this will also show up as a lengthy delay in idle shutdown using the mixture control. Instead of slightly rising and then stopping “clean”, the engine RPM rises quite a bit (as much as a couple of hundred RPM or more), and sort of burbles and tries to keep running before it finally stops.
If it is a rich mixture problem, the fine wire plugs may help, but they won’t fix the problem. It will probably mean that the Bendix Fuel Injection Servo will have to be pulled and sent in for overhaul (not a cheap proposition). If you reach this point, make sure that the A&P has carefully checked your magnetos for spark quality. If you are getting a weak, yellow, or intermittent spark from either magneto, it will make the engine much more sensitive to mixture.
Lycoming list of recommended procedures to prevent spark plug fouling. Be sure to cut and paste the complete link:
CAUTION! You CANNOT use the REM37BY long-nose spark plugs in the 200 HP high-compression IO360. They are not approved for that engine, for several reasons.