Fuel System OverPressure 912IS

[Jaguar56]

Your Rotax sender sounds suspiciously like it's sensing atmospheric pressure rather than "gauge". Not a problem if the electronics adjusts for it.

I am really starting to feel it is nothing so complicated. I flew for almost 2 hours on Friday with no issues. But I was very closely watching any throttle movements. I worked the throttle full stop multiple times prior to engine start and all throttle movements during flight were much slower than normal.

Considering how quickly the events would happen and then settle down I really think it is the throttle cable bunching up slightly and then snapping to full throttle quickly as the bunch straightens out. I did see this happen once with the cowl off while working the throttle. This would explain the sudden rpm change and the increase in fuel pressure as the engine suddenly asked for a lot more fuel. And it would explain the warnings to the system and things settling down quickly after the event. And it would explain why the rpms would go up without a throttle movement in the cockpit.

I will keep working it with this in mind and post here with findings.

[efwd]

Im glad your posting this stuff. I installed my throttle a couple weeks ago and right away noticed that when the throttle cable is pushed forward I can see the end thats attached to the throttle arm takes on an arched shape even without the arm being full throttle. The pivot point inside that arm that the cable is bound too is not free to swivel. Of course if you loosen the screw that holds the cable in you risk having the cable come free and then your wide open throttle until you shut down the engine. My cable is what directly in line with that throttle arm. I don't really know what needs to be done to correct it but having read your posts I realize maybe the engine vibration is not sufficient to keep it from binding up as I thought might be the case. Maybe thats a spot where the LP1 dry lube is needed.
Eddie

[jrevens]

Could you guys post close-up pictures of the linkage at the engine? That would be helpful.

[Jaguar56]

Could you guys post close-up pictures of the linkage at the engine? That would be helpful.

Post #26 has an image of mine. Let me know if you don’t see it and I will post again.

[Jaguar56]

Not having built the plane I am not sure how the spring mechanism works. As the throttle is pushed forward it seems to push the throttle lever. Is that action with or against the spring?

Looking at the throttle image I am betting throttle up is with the spring action while throttle back the spring will help close the throttle. Guessing this is a safety measure in case the throttle linkage fails it will close the throttle?

[efwd]

throttle at idle, full throttle which the spring assists, and the way it probably should be if the bushing rotated freely and not binding.

[efwd]

Thanks John, for inspiring me to get off the couch this morning and get in the shop.

[Jaguar56]

throttle at idle, full throttle which the spring assists, and the way it probably should be if the bushing rotated freely and not binding.

You nailed it. That is what I was seeing.

[efwd]

Actually, the spring IS a safety measure but the opposite than what you stated. It fails with the throttle open so that you can get to a landing spot. Take the engine off line for the approach.

[jrevens]

The cable "bushing" should absolutely rotate freely in the throttle arm. It looks like it either isn't installed correctly, needs a little "work", or it's a poor design.