UPDATED 912iS Rpm drop at 100%

by DW in 912iS Technical Questions 4 years ago

Hey gents,

On a new engine just installed on a new drone we are have a RPM drop when we go to 100% throttle (as indcated by the TPS) the problem is that the RPMs drop when uncommanded. We have adjusted the prop and took all of the pitch out and there was still a uncommanded drop in RPM. I was able to pull the logs and I can see the drop in the RPMs on the graph but I cannot find what is driving the changes. If theres a good way to get the logs to someone who is willing to look at them please let me know.

Any help suggestions would be great.

UPDATE: After performing a various amount of checks we have determined the following.

Fuel Filter and gascolator had contaminants in the screens. Removed plugs and found a fouled plug on cylinder 2 lane B and plenty of black soot which indicates it is running rich. Also drained our fuel cell to help clear and debris, twice.

Cleaned gascolator, replaced filter, and replaced plugs (knowing plugs were a sympton and not the cause). Engine ran better than before however we still had the RPM drop. While experimenting with pump selection we determined that the aux pump helps create the rich condition. When on high power with one pump our EGTs were 1300 ish and when with both pumps our EGTs dropped into the 1100 which led me to believe that we had a fuel pressure regulator problem which was substantiated by a phone call to a fellow forum member. We removed the regulator and cleaned it the best we could and did have some debris come out. We reinstalled and tried another run up and our RPM drop is not as bad but still an issue. We have a new regulator on order and will update here when we perform another high power run. If you read this I appreciate your time to do so. If you have any opinions about this I would love to hear it.

Thanks again.

Re: UPDATED 912iS Rpm drop at 100%

by Jeff B » 4 years ago

Jeff B

14 601 2.18k

Is your fuel pressure going higher than spec when running two pumps? If so check the fuel pressure AFTER the regulator. Rotax specifies that the return line fuel pressure must not exceed .5 bar (7.25 psi). If you have used a check valve directly in the return line it should have a low opening pressure, typically less than 1 PSI. If the return line pressure is too high the regulator can't do its job effectively, especially with two pumps running.

Some versions of the fuel system diagram in the 912is installation manual show a .4 - .7 BAR check valve directly in the return line, but I believe that should be .4 - .7 PSI. Using a .4 - .7 BAR check valve would not make any sense in a system that cannot exceed .5 BAR. I have never seen this typo acknowledged but it was corrected in later manuals. Andair makes a .4 - .7 PSI check valve for this use. Rotax also shows a .8 - 1.2 BAR check valve in their diagram for the bypass circuits, but this must not be used directly in the return line.

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Re: UPDATED 912iS Rpm drop at 100%

by DW » 4 years ago

DW

3 7 32

We replaced our regulator and that ended up resolving our issue. I suspect there was enough debris remaining to still cause problems.

Jeff, we are not using a check valve in our return line. If we had an incident and had to rely on that check valve to stop the flow of fluid from the tank, I am certain the tank would be compromised by other means. Thanks for your suggestion though. I appreciate it.

Thank you said by: RotaxOwner Admin

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UPDATED 912iS Rpm drop at 100%

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