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The last time I flew my turbo Onex, I had the servo actuator adjusted so I got very close to 40" MAP with full throttle. Yesterday on takeoff, suddenly only 27" MAP → aborted takeoff.
Took the filter off and verified the aerocarb slide was full open at full throttle.
Turned the servo actuator fitting 1 turn tighter.
Put everything back together and tried it again today; same result: 27" at full throttle. Gage reads field barometric with engine off and ~ 15" at idle rpm. After this event, I took off the tailpipe assembly and found the waste gate & its seat had some kind of crud on them. A fine tooth file revealed that this was all carbon deposits (both surfaces). Turbine turns freely and there is a good deal of tension on the actuator shaft to hold the waste gate closed.
I've checked all the usual stuff; intake sealed, oil to turbo, etc.
Help

I hate to say it, but your symptoms sound exactly like the few others that have had turbo seizing issues, although if you checked the impeller shaft and it spins freely I'm surprised. Since you spun it by hand, I would reassemble the downpipe and try it once more to see if you get proper boost pressure. If you don't, I would still suspect the turbo is having issues causing your problem.

How many hours do you have on your turbo at this point?

This is the same symptoms I had only I would get 35 in. map at full throttle. When i took off the tailpipe I could freely turn the turbine but I suspect it was not as free as it should have been. When I test ran the engine after replacing the heads I could only get 2500 to 2700 rpm static. When I removed the turbo to send to Sonex, the turbine was seized completely. Sonex's turbo supplier said the shaft was seized due to coking of the oil in the bearing and found scoring in the bearings from metal in the oil. I'm afraid your symptoms are a harbinger of doom seen by the other three turbos that have had problems. I, unfortunately did not check the turbo when new to see just how freely the turbine would spin, when checking it in May it turned freely but I suspect it was coking up but just hadn't seized although it was dragging the turbine down for the reduced map.

What oil are you using?

After listening to the latest podcast and hearing about the turbo oil coking problems I’ll share what I learned today. My job takes me into many auto repair and dealership shops servicing wheel alignment equipment. I was into a McLaren dealer and they had a motor removed. I was able to get a good look at the turbo and was discussing what is going on with the coking in our turbo and how it’s installed.

This tech has many years of experience racing Porsche’s and servicing turbos. He questioned why we are using a scavenge pump he recommended just returning the oil to the sump. He also said we could use the pump to pull some oil from the sump and running it through the cooling jacket in the turbo center housing. From there it could be sent to the valve cover or back to the sump.

The McLaren had a shroud covering the exhaust manifold and turbo but was not fitted tight so air could circulate around the turbo housing. He said we should do the same and could also force some air to it.

I have not run mine yet but have it all plumbed up. The return to the valve cover is higher than the turbo drain (it’s a tail dragger) so I can’t see how the oil will drain from the turbo after shut down.

I have some other shops that specialize in turbo installation and dyno tuning so I’ll check with some of them.

Ed Grasso