O-320 trouble

Howdy,

I am having an issue with a moderate high time (1800 hrs) O-320. I realize its time for a rebuild, but not sure if this is a minor thing or not.
was on short final and had a pretty good backfire, landed and started to trouble shoot. symptoms: backfiring at low RPM..goes away at about 1600 rpm runs smoothly. backfire is consistent between L/R/Both mag switching. bottom plugs all good. compression checks all in the upper 70's. Took apart carb, 2 piece venturi solid as a rock. did have a broken primer lead into #4 cylinder..fixed it but problem is still there. Took off rocker covers and no broken springs. Haven't checked valve lateral movement tolerances but that is prob next on the list. Valves seem to be moving freely. Not bad gas..tanks/sumps clean, carb clean. Not a plugged exhaust either, checked that. Just trying to bounce this off you guys for anything obvious I may have missed. Thanks for the help!

Maybe idle mixture is running too lean, would explain why it’s a low rpm issue...


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Blown/leaking exhaust gasket sucking air

Glenn

On motorcycles a lean idle mixture (burns slower than normal/too rich does also) plus an exhaust leak can ignite any unburned air and fuel left in the exhaust. The popping fix is to richen the idle mixture if it's lean and seal any exhaust leaks upstream of the muffler outlet. Airplanes can have two big tires so maybe the same deal.

Gary

Intake valve seat leaking.

Backfire = from intake or exhaust in this case?

Gary

cubdriver2 said:

Blown/leaking exhaust gasket sucking air

Glenn

Click to expand...

I see this quite a bit... check all of your mounting hardware and look closely at any exhaust staining on your lower spark plug bodies indicating a blown gasket..

Couple of months ago had a Lyc powered 170B land that was backfiring on final... owner also complained about a noise that sounded like a small hammer banging on a pipe... When I looked at it found both exhaust nuts missing on #3 and two more missing on other cylinders... replaced them and tightened the rest and the backfire and noise went away..

Brian




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intake leak,

check intake gaskets at cylinder..

check aluminum tube in oil sump, they are just swaged in there, and can become loose...

hose & clamps on intake

X2 what mcs said. My O320 was popping in the pattern. Had the intake tubes "re swaged" where they go into the oil sump. All fixed.

On takeoff I would say exhaust leak. On idle I would say intake leak.

If it were mine I'd check for an idle cutoff rise with a warm engine. But again, are we to assume the "backfire" is from the intake or exhaust? Makes a difference.

If an intake leak is suspected carefully point a flowing but unlit propane torch at potential leak sites while idling...any rise in rpm means that fuel is being sucked into the engine.

Gary

Edit: I have an adapter hose that fits over the propane torch burner and ends in a long piece of copper tubing. Safer than holding a bottle bomb next to an engine. Not my idea tho. Harley Davidson's. Recommended when searching for induction system leaks near the throttle body.

Sounds like the old carburator cleaner trick

trying to remember... do cylinders crack from exhaust into intake port?

mike mcs repair said:

trying to remember... do cylinders crack from exhaust into intake port?

Click to expand...

Sometimes.