O-360 Backfiring and Low CHT

Ironflight · #1 08-20-2010, 07:01 PM

OK, here's a puzzler for the peanut gallery....lookign for ideas!

Louise aborted an evening flight today after noting considerable backfiring at low RPM's, accompanied by both a very low CHT AND EGT on #2 Jug. I ran it and confirmed the situation. AS I ran it up, the temps came up, but stayed power than the other cylinders. Mag check was OK - a bit rough, but no significant differential drop. I believe that I have noted a slight increase in backfiring as Louise taxies in from her daily commute the past couple of weeks, so the problem may have been progressive.

We pulled the cowl - compression check was good, no difference between it and the other cylinders - 72 or 74 over 80, not terribly hot. Upper and lower plugs were very clean. Mag timing is perfect. Checked all the plug wires and Magneto caps - there were fine. Pulled the rocker box cover, and nothing is amiss under there. No apparent leaks (exhaust, oil, or other) from the jug. About the only thing I haven't done is replace the intake coupling tube/hose - might be an intake leak, and the hoses are all pretty old. I tightened up the hose clamps, but don't have any spares to test by changing them out.

Before I try and find some intake couplings, anyone else have any ideas?

Lycoming O-360-A1A, stock carb, mags, nothing unique.

Paul

rbibb · #2 08-20-2010, 07:10 PM

I'd suspect the intake tubes lacking any other obvious issue. A leak would screw up the mixture on that jug and maybe lead to the problem.

Steal one off your other plane and check it out.

Some Lycoming expert may have a better idea....

John Clark · #3 08-20-2010, 07:18 PM

I'm voting for an intake leak, one cylinder only and the symptoms indicate lean.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

aerhed · #4 08-20-2010, 07:22 PM

Could be a sticky valve my friend. They stick, they unstick, then stick again. Not sayin' that's it for sure, but if you don't find something else, maybe try a rope trick. Unspring the valves & operate the valves by hand. If they are leaded, you'll be able to tell. If they were bent or burnt, you wouldn't get compression.

AltonD · #5 08-20-2010, 07:30 PM

I vote for the intake leak. I saw the same thing, only difference, the leak was not on the odd ball cylinder, it was on another one.

az_gila · #6 08-20-2010, 07:42 PM

...or valve sticking...

From Sacramento Sky ranch...

14. Sticky valves. Occurs most frequently in the hot summer months. Frequently occurs on the first start of the day when the engine is cold. Remove rocker gasket. Turn engine backwards until valve is fully closed. Using both thumbs, depress valve springs. Valve should open and close without any feel of dragging. Valve movement should be smooth.

Time to perform the SB 338 valve wobble test...

http://www.lycoming.textron.com/supp...dfs/SB388C.pdf

Has it been hot in TX recently...

vic syracuse · #7 08-20-2010, 08:07 PM

Paul, I agree that most backfiring at LOW rpm's are the result of sticking valves. Intake leaks usually are recognized by higher CHT's at higher rpm's.

Vic

David-aviator · #8 08-20-2010, 08:14 PM

SI-1485 might help in determining if the wobble test is necessary.
http://www.lycoming.textron.com/supp...fs/SI1485A.pdf

FWIW, my nearly new engine backfires on occasion at idle power in the traffic pattern. I've about concluded it is normal, but who knows? Engine seems to run fine otherwise.

dd

az_gila · #9 08-20-2010, 08:54 PM

Quote:

Originally Posted by David-aviator

SI-1485 might help in determining if the wobble test is necessary.
http://www.lycoming.textron.com/supp...fs/SI1485A.pdf

FWIW, my nearly new engine backfires on occasion at idle power in the traffic pattern. I've about concluded it is normal, but who knows? Engine seems to run fine otherwise.

dd

...my Tiger O-360 engine needed a TOH at 600 hr - on Lycoming rebuild with new cylinders with the hi-chrome guides - so the 1000 hr first check recommendation is not very foolproof.

If you have the correct Lycoming tool, it is pretty non-invasive if you check the guides only and don't clean the lifters. The valve springs do not have to be removed with the correct tool - but if you have the AC Spruce tool, then the valve springs do need to be removed...

Ironflight · #10 08-21-2010, 06:26 AM

Great ideas folks - I hadn't thought about sticky valves (even though those used to be discussed all the time in the Grumman world, as Gil well knows) - I'll look at that this morning.

Paul