185 Won’t Idle and Surges to high RPM

[EdH]

IO-520. A friend has a 185 that will not idle, and will barely run at a low throttle setting. With the throttle only about a half inch in if it starts, it will want to die spits and sputters, and then surges up to almost 2K RPM (with what should be a 1Kish throttle setting), when the throttle is pulled the half inch back it dies when the RPMs run down. You can kind of get it to run, albeit terribly, by getting the throttle just right but it’s a balance between high surge and die. He tried to start/run it with the electric pump on and it might have helped a tiny bit, but hard to tell. When it surges it sounds good and pulls hard.

The flight before it flew great, and was parked and this was happening the next time he went to start it.

Things that have been checked…
- Timing
- Plugs and leads all look good
- General look over
- All fuel screens (strainer, air/fuel control, fuel distributor) cleaned out with gunk in the strainer only
- Electric pump seems to work fine
- Injectors were tested into jars and had good even flow with the electric pump
- Throttle switch
- Throttle linkage seems fine
- Probably some other small easy things

He’s parked on a hard to access lake, and him and the mechanic are getting to the end of their rope. They have the maintenance manual, but it’s not helping. Any ideas?




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[Grant]

cleaned out with gunk in the strainer only

Was it clogged?

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It sounds like an intake leak but the only time I've seen it that bad was when a intake coupling tube was left slid back off the joint after a cylinder change. It ran like crap but if you hit the fuel pump it went to full throttle. Doesn't sound like thats the case here. I would check for intake leaks.

I would also suspect the fuel pump.

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Worst part is where he is....That sucks to have to work on a broken airplane on a lake.... Been there...Hated it!

 

[aktango58]

How is the air intake? Isn't the alternate air spring loaded on that engine? (grasping at straws here). Could it be a blockage and extra throttle opens the alternate air?

Fuel system does sound like the place to look though. Our 520's flood out when we hit the pump, unless hot and we have vaporized fuel to purge.

 

[EdH]

It’s fixed. It was a leaking intake manifold boot. It was found when disassembling the manifold to check it out. One of the boots had a loose side on the double clamp and the boot had swelled up to fill the clamp. It was sucking air from all around it but was solid feeling when everything was bolted up due to the double clamp and the swollen boot keeping it stiff I guess.


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[BC12D-4-85]

I had one on an IO-520 in a C-185S that was an air leak. We were far out in the bush and grabby hands found the loose boot. No idle w/o boost pump was the first heads-up.

Gary

 

[gntw]

I had one on an IO-520 in a C-185S that was an air leak. We were far out in the bush and grabby hands found the loose boot. No idle w/o boost pump was the first heads-up.

Gary

. Easy way to find leak in shop with shop vac . Hook up plastic bag taped on air intake or air box and tape vac hose from out let throttle open blow air in use spray bottle with soapy water and spray away . Show even minor leak.