Initial runs wouldn’t exceed 3800rpm, 28 inches M.P.(is this normal M.P. For this power?),3.5gph, throttle about 50%, runs smoothly but lower rpm all the way to 115%.(differential fuel press vs. airbox has always been 4.0psi)

Now about 3.5 hours total time on engine. 

Using TCU software, static ambient reads 906 millibars (at 2200ft field elevation), static airbox (not running) reads 30 millibars higher, idling maybe 10 more millibars, operating at 3800rpm about 50 millibars higher than idle, never gets to 1100 target. 

The throttle position has been calibrated and verified with software while moving throttle.

I have an Airmaster electric constant speed propeller. Fine pitch has been verified. About 44” in fine pitch mode. Settings: takeoff and automatic. 


Initially installed fuel pumps backwards which, I later learned, damaged them and after hooking up correctly sent debris to carburetors which leaked fuel to airbox and turbo housing. Removed float chambers to clean out debris. 

After damage was detected replaced both fuel pumps and turbo. Fuel and oil lines, along with fuel pressure regulator, were removed and cleaned. 

During oil purge, after changing turbo, left sump off of turbo, to verify oil to new turbo bushings, then connected and completed purge. 

After replacement runs were identical. 


The waste gate set to eliminate cable slack (in contact with compression spring, then additional 1.5 turns of nut) TCU off. TCU does cycle at startup with no problem (light turn off). Waste gate arm observed moving. Turned off TCU mid cycle (per representatives advice) to verify free and easy movement of turbo arm.  I have old turbo still to verify arm position with waste gate closed. 

Have tried removing cable, and wiring waste gate closed…

Runs still identical. 

Waste gate wired open

Maybe a 50-100 less rpm


Just sent carburetors in for rebuild (cleaning) to verify no fuel pump contaminates got further into the carburetors than float chamber. 

Verified fuel flow by removing return line and running in gas can. 3&1/2 minutes just over 1.5 gallons. Fuel flow meter to engine read 28 gph on either pump, just over 29 running both. 

No change to operation. 


Mag checks at 2000 and 3800 rpm are smooth, and equal, about 150 rpm.

Vent hoses from airbox checked with 3.5 psi. No detectable leaks. 

Bypassed vent hose with new clear rubber tubing with “T” (mostly bypassed solenoid), from airbox to carburetor vent fittings (plugged original hoses to verify no back leakage from solenoid). 

No change in operation. 


Disconnected turbo and carburetors, put plugs where they were connected along with airbox drain hose’s, hooked up air hose regulated to 3psi prior to connecting to airbox, pressure gauge retained 2.5psi (keeping in mind my air nozzle didn’t have the best repeatability 2.5 - 3.5psi in testing for setup). Sprayed soapy water over all connections, hoses, solenoid, fuel pressure regulator, no observable leaks. I think the .5psi could be explained by one of my plugs to inlet side if airbox being loose, wasn’t seated the same when remove as when installed. Wasn’t looking for air leak at my plugs. 


Ran with bypass hose straight from takeoff power vent port to carburetor vent fittings, same rpm, but egt about 100 degrees cooler than other test runs.

Then removed vent hoses all together, straight atmospheric, started to cough and die about 2700rpm.

Not sure what that means as far as troubleshooting, but thought it may give someone else a clue. 


Did compression test, all 86 out of 87. 

Trigger coils all 247 ohms. (Should be 220-240 according to new manual, okay with previous, up to 250 ohms). Could be meter. 

Primary side (double ignition coil) .3 ohms








  • Re: 914 won’t exceed 3800rpm

    by » 10 months ago

    I have no experience with turbo charged petrol engines.

    When a non governed engine is unable to accelerate beyond a certain rpm, the cause is usually:

    Partial restriction to either induction or exhaust.

          Check for free flow of exhaust gas / collapsed muffler/silencer baffle

          Check air induction for free flow - dirty/damaged filter, foreign object, collapsed component

    Fuel supply not keeping up with demand

         Check for adequate fuel pressure

         Check for partially blocked fuel filter

    Ignition issues are less likely, in a system without rpm synchronised ignition advance/retard. Others may have a better understanding of the Rotax ignition modules and their potential to stop delivering spark above a certain rpm.

    Thank you said by: Dave Smith

  • Re: 914 won’t exceed 3800rpm

    by » 10 months ago

    I had a similar problem after changing my prop on a 914.  Turned out the new prop pitch was way too coarse and had to be adjusted a lot to get the max takeoff figures 39” and 5800rpm. Suggest. You try fining off the prop pitch before fiddling with the engine. Good luck.

  • Re: 914 won’t exceed 3800rpm

    by » 10 months ago

    Check all and I mean all of the pressure hoses especially the ones from the three way valve that go to the carbs.  I had the same problem and found an almost invisible crack in the hose that goes out of the three way valve.  That pressure enriches the mixture above approx. 3800 rpm.  Without that extra pressure the engine won’t accelerate.  Could also be the fact three way valve is failing to open. 

  • Re: 914 won’t exceed 3800rpm

    by » 10 months ago

    Thanks for all your replies.

    Sean was the closest. My exhaust piping from the muffler - exiting the cowling was too long and within 3/32” of baffling.  I removed that pipe, did a run up, all was well, just cut off 2.5”, and reinstalled. Have been fighting with this for a long time. With a lot of different people.

    because of Sean’s post, and my lack of anything else to look at, I removed that pipe to verify nothing had crawled in there, and immediately spotted the problem. 


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