How do I diagnose vacuum leaks on a Lycoming IO-320?

[j1b3h0]

I have an airplane engine that run slightly rough at or near idle, and doesn’t respond normally to adjustments in the mixture. Seems to run okay at higher RPM but tends to foul spark plugs. I suspect one or more leaks in the induction system. How might I find exactly where?

 

[Magman]

First visually check for fuel stains

Disconnect Alternate Air Inlet duct

Carb Heat ON

Throttle OPEN

Use a CLEAN vacuum cleaner ( pressure side) to Carb Heat duct

With pressure ON squirt soapy water particularly at connections

Look for bubbles!

 

[asicer]

I have an airplane engine that run slightly rough at or near idle, and doesn’t respond normally to adjustments in the mixture. Seems to run okay at higher RPM but tends to foul spark plugs. I suspect one or more leaks in the induction system. How might I find exactly where?

Fixed pitch prop? Or constant speed?

 

[j1b3h0]

Fixed pitch.

 

[j1b3h0]

First visually check for fuel stains

Disconnect Alternate Air Inlet duct

Carb Heat ON

Throttle OPEN

Use a CLEAN vacuum cleaner ( pressure side) to Carb Heat duct

With pressure ON squirt soapy water particularly at connections

Look for bubbles!

Doesn’t have carb heat, fuel injected.

 

[j1b3h0]

First visually check for fuel stains

Disconnect Alternate Air Inlet duct

Carb Heat ON

Throttle OPEN

Use a CLEAN vacuum cleaner ( pressure side) to Carb Heat duct

With pressure ON squirt soapy water particularly at connections

Look for bubbles!

Thank you, I’ll give that a try. Pulled all the plugs, most showed evidence of running quite rich. Taxied around to get the oil hot, running very lean mixture. At runup gave 2-300RPM mag drop both mags. The old plugs were Tempest REM4-Es. I installed new Champion REM37BYs.

 

[kyleb]

Find a way to pressurize the airbox (leaf blower to air intake, sealed with shop rags works). Then spray all the intake tubes and junctions with soapy water. Turn leaf blower on, look for bubbles, rotate prop 60 degrees, repeat. Repeat a couple more times.

Alternately, decowl the airplane, chock it, start the engine, and have a friend spray carb cleaner or starter fluid at each of the junctions where the intake tube either meets the sump or the cylinder. Be careful, don't want your friend to eat a prop.

 

[j1b3h0]

Thanks, I’ll try it.

 

[Dan Thomas]

An induction leak makes the engine run lean. The plugs say it's running rich. Something else is at work here.

When were the mags last off for internal inspection? 90% of all engine performance problems. Electrical includes ignition.