Lean return to idle - wierd

[TK5310]

Last few outings I've had a wierd issue come up with my picco .28:

As it comes back to idle it leans out, gets that high lean idle thing happening, but after a few seconds it settles down to it's normal idle, blipping the throttle also settles it.
At normal idle it is not lean, nor is it lean anywhere else in the rev range - it tunes fine & reacts normally to tune changes.
It's not running hot.
An air leak would normally cause a lean idle, but would not clear up after a few seconds or a throttle blip.
Lean HSN would have it running hot.
I've tried to tune it out, but I can richen up the LSN untill it stumbles when you hit the throttle, I can richen up the HSN untill it flat in the top end & the issue stays the same.

I thought it was due to a missing baffle in the fuel tank allowing it to suck air under brakes, but I've replaced the tank & it does it with both full & near empty tank.

I've replaced the pipe seals & fuel tank with no change.

From here I'll replace the fuel lines & look over the carb for leaks - but I'm not holding out much hope there is any problems there.

Motor is getting on a bit, over 10 gallons on it, but still has a little mechanical pinch & enough compression that it can't be turned over by hand.
It'll idle almost indefinately & still makes very good power - I've had it at a low idle for up to 5 minutes without it stalling, which tends to indicate that the motor is fine.

Motor has been sealed - but a long time ago.

Just not sure where to look next!

 

[dub c]

sounds good to me. It may be just how she is, growing old and grumpy. After 10 gallons.... its getting real loose in there. Or perhaps your idle gap is a little too wide. A nicely set idle gap will "choke" your engine to a low idle quickly.

 

[sv250]

10 gal, might be getting some blow by. Most engines are shot by then.

 

[NitroHawk]

Blow by as SV250 pointed out is most likely.

Low pressure on the tank at idle is another one that does exactly what you describe, check the pressure line feeder on the pipe for obstructions just in case.

Last one, check the o-rings on the needles or if possible: do a full leak test on the engine.

 

[TK5310]

I think I found it, plastic plate that holds the tank seal was badly bent, so it wasn't compressing the O-ring seal, which wasn't sealing very well as a result.
Fixed with new plate, seal seems good now.
I replaced the lines anyway & gave the motor a quick once over - looks to be all good.

Going by the condition of this picco after 10 gal I have no doubt it'll do 15 or more before it starts getting sad, hasn't got much mech pinch when it's warm, but still has a decent amount cold & hot compression is still so good that it can't be turned over by hand - I still need to warm it up with a propane torch to get it started in cooler weather.