Fuel issues: O-360

gerrychuck · #1 12-03-2011, 01:05 PM

I am a new owner of an RV6A with a carbed O-360 A1A and fixed pitch Sensenich prop, dual standard mags. Little over 300 hrs on factory engine, good compression at last condition inspection in late July. Plane has been running like a top for the 12 hrs I've flown it. Disclaimer: as noted, I am a new RV guy, and NOT a builder. I have 24 years of flying, but all in certified aircraft, so I have a primarily theoretical knowledge of aircraft power plants, with very little hands-on time with these engines.

As mentioned, the engine has run perfectly since I got the plane at the end of the summer. The only slight anomaly was that the idle was a bit faster than I would like; about 900 RPM or a bit faster with the throttle pulled all the way out. Otherwise, smooth and sweet. Today, on startup (cold day, less than 20 degrees F, thoroughly preheated with CHTS in the 70's and oil temp the same) the engine seemed to be idling a bit rough and didn't respond well to the throttle initially. I switched off the boost pump to taxi and do my runup, and the engine smoothed out fine. No issues on runup; normal drop on both mags, normal response to mixture and carb heat. Pulled to check idle and found the idle speed below 700 RPM; normal idle, but not for that plane. Flipped the boost pump back on for takeoff; instant rough engine again. Pump off - smooth. I ended up taking off with the pump off; engine ran great and the plane climbed out at 2200 fpm, so it was making full power. Everything was normal throughout the .8 hr flight, including some mild aeros. No problem with leaning the engine properly, and EGT's responded normally to movement of the mixture control. Coming back into the pattern, tried the boost pump again for landing; rough again. Ended up doing 3 circuits, and felt like the engine was just a bit rougher than usual with the throttle pulled back to slow down and descend. Otherwise it was great. After the last landing, I taxied to the fuel pump. Idle still about 700; did my dead mag check (normal) and pulled the mixture, but the engine continued to run happily at a low idle. Ended up shutting the fuel selector off to get it to quit. Restarted normally after refuelling, idle still low, taxied back to the hangar, and again had to shut the fuel off at the selector to shut down. I did try turning the boost pump on and off after the flight as well, with the same result; instant rough running; seemed like it was getting way too much fuel with the pump on, although the fuel pressure didn't change.

I haven't pulled the cowling yet; thought I'd consult a bit and give this some thought before diving in and doing something dumb. Any thoughts on what could produce this kind of behaviour in the engine? As I mentioned, everything had been completely consistent with the engine right up to this flight. Any thoughts on the problem would be greatly appreciated. I'll probably call up a friend who is a journeyman AME (you guys would call him an A & P) as well to get his thoughts. Definitely not flying again until I get this sorted though.

Thanks in advance, and please be gentle

Pat Stewart · #2 12-04-2011, 07:09 AM

First I would not fly with this condition. I'm sure the engine experts will jump in here and help you quickly however this sounds like a leak allowing air into the fuel system to me. It must be effected by the pressure the pump provides. The idle cut off stop should have killed the engine unless something has moved or changed like fuel air mixture. I realize this did not help much but the experts will join in. Don't fly it until its resolved.

Pat

gerrychuck · #3 12-04-2011, 10:38 AM

Thanks Pat; definitely no intention of flying until this is dealt with. I was wondering about a leak as well. My first thought was the mixture control somehow coming out of adjustment, which would explain the inability to shut the engine off, but I don't see how it could also create the rough running with the fuel pump on. The other thing that doesn't make much sense with the mixture control scenario is that it was fine on one flight and messed up on the next; no maintenance or anything else in between, and the mixture control wasn't moved between flights. The only time the airplane was touched in between flights was to pull it out of the hangar to get my hangar partner's 8 out for a flight. I am a bit nervous about getting into the cowling; we've had a bit of a mouse problem the past 2 weeks, and that could explain a leak. I was very conscious of that possibility before flying yesterday and had as good a look as I could up the exhaust outlet, but didn't see any signs. Makes ya wonder, though...

gerrychuck · #4 12-04-2011, 11:30 AM

On further thought, I suppose the mixture coming out of adjustment could explain the whole thing; if the mixture is richer than normal with the knob all the way in, adding fuel from the pump could push it into running rough, and at the other end of the adjustment still be supplying enough fuel to prevent the engine from shutting off. Occam's razor; go with the simplest possible solution first, I guess!

Pat Stewart · #5 12-04-2011, 12:49 PM

Again, not an engine expert but my bet is the mixture adjustment has not changed, the fuel air mixture has. I will bet on a leak or even a bad engine drive fuel pump diaphragm but we need the experts to jump in.

Pat