English French German Italian Portuguese Spanish Swedish

 

914 idle rpm surge.

  • Brian Mollerup
  • Brian Mollerup's Avatar Topic Author
  • Offline
  • Fresh Boarder
  • Fresh Boarder
More
6 months 4 days ago - 6 months 3 days ago #18257 by Brian Mollerup
914 idle rpm surge. was created by Brian Mollerup
Hello Gents.

I have this 4 year old 914UL with 180 hours only and looking brand new witch has begun to surge, Hunt or osscilate in idle.

I made 2 videos of the behavior if anyone would like to see them.

The engine oscillates only in idle.
Idle is set to approx 1800 with about -0.5bar on the Syncro dials and it is dropping to 16-1500 and pressure increases to -0.4 bar on the Syncro dials for about 15-20 seconds with rough running and then it raises back up and stabilize smooth idle.
After about 30-60 sec it drops again and it continues like this.

1 or two fuel pump on, 1 or the other or both ignition systems on doesn't change anything.

I have tried to clamp the cross over tube but then the rpm drop becomes permanent and even more rough running. Added throttle and back wont just raises it again. The clamp must be removed and it needs 25-3000 for 10-15 sec to stabilize again.

Maybe the rough running with the clamp on causes the floats to bounce and flood the chamber because Sometimes fuel is coming from the chamber vents ones the low rpm rough running is going on. I'm thinking this is the result of the rough running and perhaps not anything to do with the fault. Maybe.

I have looked for components that could leak, like different big and small hoses, manifold, MAP indicator ect. None seems to have any damage.
I have suspected the map indicator as this is a new installation but this was disconnected and plugged on the engine.
Carb Syncro checked ok.
Idle jets checked ok.
Idle mixture checked 1,5 turn out ok.
We have visually checked the floats for damage, no smoking gun there. I have not weighed them yet.

I have also been thinking about Carb ice but I doubt it can be that even the ambient temperature is just below freezing.

The map increases when the rpm drop occurs.
The waste gate actuator doesn't move when it happens.

Can it be a leaking carb diaphragm?
Thinking its not.

Looking at the video it seems to start on the left carb side.?? and then pulls the other with it.

We will replace the spark plugs and check slipper clutch friction and ignition pickup gap though I don't think those are the cause..
I will check the fuel pressure if above inspections doesn't show anything.

Any other ideas is much appreciated?

/Brian
Last edit: 6 months 3 days ago by Brian Mollerup.

Please Log in or Create an account to join the conversation.

More
6 months 2 days ago #18270 by RobSeaton
Replied by RobSeaton on topic 914 idle rpm surge.
sounds like carb contamination in the idle circuit.

Please Log in or Create an account to join the conversation.

  • Brian Mollerup
  • Brian Mollerup's Avatar Topic Author
  • Offline
  • Fresh Boarder
  • Fresh Boarder
More
6 months 2 days ago #18277 by Brian Mollerup
Replied by Brian Mollerup on topic 914 idle rpm surge.

Rob Seaton wrote: sounds like carb contamination in the idle circuit.


Hi Rob,
Well the carbs including the idle circuit has been checked and found OK.
I did however find the cause I think.

/Brian

Please Log in or Create an account to join the conversation.

  • Brian Mollerup
  • Brian Mollerup's Avatar Topic Author
  • Offline
  • Fresh Boarder
  • Fresh Boarder
More
6 months 2 days ago - 6 months 2 days ago #18278 by Brian Mollerup
Replied by Brian Mollerup on topic 914 idle rpm surge.
Yesterday evening I manage to check the weight of the floats and it was quite surprising findings.
All Floats weigh about twice as much as they should. Normal weight of a float is 3g +/- 0,2g right?
Right Carb float weight: 5,7g and 4,3g = 10g total
Left Carb float weight: 6,9g and 6,0g = 12,9g total.

The scale has been checked so the values are correct.

What puzzles me is that there are no real sign of wear marks or damaged surface on the floats that could cause the floats to soak fuel.
4 years and 180 hours old floats w. 2 marks.

New floats have been ordered.
New spark-plugs installed. Needed replacement anyway.
Slipper clutch friction torque in free rotation checked OK 25dg/60Nm

Lets see if this helps. BRB

/Brian
Last edit: 6 months 2 days ago by Brian Mollerup.

Please Log in or Create an account to join the conversation.

  • Brian Mollerup
  • Brian Mollerup's Avatar Topic Author
  • Offline
  • Fresh Boarder
  • Fresh Boarder
More
5 months 2 weeks ago #18385 by Brian Mollerup
Replied by Brian Mollerup on topic 914 idle rpm surge.
Okay, the floats was replaced and the engine now run as normal.
Sigh.
/Brian
The following user(s) said Thank You: admin, Gyrochook.

Please Log in or Create an account to join the conversation.