914

[rhtrudder]

Rough running engine between 4800/5200 , new throttle cables , Carbys done, balance good , runs fine either side of those revs, 

[RFguy]

suggestion = @RHTRUDDER- please use more descriptive subject titles- this is not email with your mates ! (rather than '914' and 'hash') 

 

example 

"914 rough running 4800-5200rpm  in ..... aircraft"
"Hash interference on microair radio " etc

Edited November 23, 2022 by RFguy

[rhtrudder]

Okay, I’ll have to pick up my act, rather than write a novel I thought I would keep it to the point

[RFguy]

914s are genuine complex beasts. could be alot of things.  What is the MAP during this rough running ?

[rhtrudder]

Found the needle clip out on one carby, right hand carby should run one notch lower to make it a bit richer something to do with the boost favoring the side, they were both the same, but noticed when I advance the throttle one side gets ahead of the other , they both finish against the stops , not sure what’s going on , have set them up with a carb mate not easy with the spinning prop , bit hot and windy to test maybe later

[RFguy]

good stuff !.  good info for those searching. my MAP question came out because the carbs behave differently when there is any boost going on....  there is a SB about it soemwhere in the rotax doco. make sure you wear your chainmail-mesh suit near the prop..

[facthunter]

You don't have to run the engine to check the carbs open evenly.   Nev

[rhtrudder]

All that and no difference, did notice at 5000 revs when I done a mag check the engine didn’t like it , both the same ,  pulled the plugs and were all very black , swapped them with new ones about to try again and thought maybe check the air filter which is mounted in the bottom cowl , easy to overlook, caked solid and beyond cleaning as it would fall to bits, after much driving found found a good enough replacement at the fendt dealer , fingers crossed will fly again tonight, looks like I may overthought the problem again.

[RFguy]

front more black then rear ?

enrichment circuit not slightly on ? I have heard of Bing carb enrichment shafts getting a bit sticky with the rubbers- causing enrichment never to truly  disappear and runs hell rich. remove and pollish enrichment shafts  and reinstall.  No accidental carb heat ? no mouse in the intake ducts ? EGT drop on carb heat applied? EGT does as expected as WOT and as expected (gets hotter ) at cruise throttle ?

[rhtrudder]

Going to have a go at installing a O2 sensor, seen it done on a 912 , YouTube, still not running like it should , good idea or not ?

[RFguy]

mmm dunno that will help that much you can get an idea of mixture just from the EGTs ... what did you find re my above questions ?

[rhtrudder]

Carbys just repaired by wal , floods, everything replaced, not sure how accurate the egts are don’t have carby heat, I guess because of the turbo although it has a intercooler fitted, previous owner,  feels like it’s rich around those revs , winding some pitch on the prop makes it run a bit better

[RFguy]

if you have not as yet, do consult the book on recommended loading (manifold pressure, fuel flow ) versus RPM in the 914 ops manual, 

[rhtrudder]

Yep, prop fine, 4800 rpm, map 28, haven’t got fuel flow , air speed around 90 that’s where it runs rough which I now check every time I take it up , 

cruise is 5400 , map 35 gives close to 120kn, runs okay,  just thought a O2 sensor might show exactly what’s going on , cost $400, haven’t ordered it yet, , the YouTube video looked impressive, showed where the engine was lacking , maybe I’ll just put up with it, 

[BrendAn]

On 25/11/2022 at 2:52 PM, RFguy said:

front more black then rear ?

enrichment circuit not slightly on ? I have heard of Bing carb enrichment shafts getting a bit sticky with the rubbers- causing enrichment never to truly  disappear and runs hell rich. remove and pollish enrichment shafts  and reinstall.  No accidental carb heat ? no mouse in the intake ducts ? EGT drop on carb heat applied? EGT does as expected as WOT and as expected (gets hotter ) at cruise throttle ?

Carb heat on a rotax ?. Mine doesn't have it and the tecnam I have been training in doesn't either.

[RFguy]

Carb heat is common oncarbed Rotax,  Brendan, depends on manufacturers implementation of WHERE they get the inlet air from.

Most Tecnam seem to have dual air filters on the carbs inside the engine bay. so, somewhat preheated air.

You'd want carb heat if deriving inlet air fresh from front of aircraft. 

disadvantage of internal air source  is you leave performance on the table with the warmer air intake . 

 

rudder, I'd suggest persisting in finding the problem. I'd be looking for the enrichment circuit sticking, or carbs not well balanced in those RPM ranges. After thoroughly cleaning all the plugs, did the behaviour change ?

The 914 runs tricks on the carb bowl pressure when in boost.  at MAP28- that's a transiiton area where you like are starting to get the boost of coming online. Maybe the float bowl pressure boost compensation is not working currectly.

Is it generally smooth  at any setting where there is a couple of inches of boost ?

 

 

 

[facthunter]

In my humble opinion the whole concept is too complex. Pressurising the Carburetters is not the way to go.  Nev

[skippydiesel]

2 hours ago, BrendAn said:

Carb heat on a rotax ?. Mine doesn't have it and the tecnam I have been training in doesn't either.

Due to the locating of the Rotax 9 carburettors/inlet air, above and to the rear of the engine,  carburettor icing is (in many installations) a low risk due to ingesting warm air from the engine bay.

 

Turbo charged varient have even less risk of carby icing as inlet air heated by turbo .

 

Fuel injected 9's don't have the venturi/fuel vapour condition present in a carburettor, so are also resistant to icing.

 

BUT

 

If your instillation has ambient (cold) air ducted  directly to the carby's the risk of icing increases, so in these installations carby heat is often designed in.

[RFguy]

This is a good read

https://www.rotax-owner.com/pdf/UNDERSTANDING THE 914 ROTAX.pdf

 

Particularly sections : 

the ENRICHER SOLENOID SECTION (9)
and FUEL REGULATOR- AIR BOX PRESSURE (14) 

 

Rudder - take a good look at those two sections. I reckon you will find your demon there. 

[Blueadventures]

3 hours ago, BrendAn said:

Carb heat on a rotax ?. Mine doesn't have it and the tecnam I have been training in doesn't either.

I experienced carb icing on a 912 in a skyfox ca21 and we only just got over the fence onto the strip.  Like having carb heat to use if experience rough running at low throttle and descending.  A worthwhile option.

Edited November 29, 2022 by Blueadventures

[rhtrudder]

48 minutes ago, RFguy said:

This is a good read

https://www.rotax-owner.com/pdf/UNDERSTANDING THE 914 ROTAX.pdf

 

Particularly sections : 

the ENRICHER SOLENOID SECTION (9)
and FUEL REGULATOR- AIR BOX PRESSURE (14) 

 

Rudder - take a good look at those two sections. I reckon you will find your demon there. 

Yep, I do understand how that all works, had trouble once before with Leakey bowls , engine hesitation in the boost range, the enricher valve opens at full throttle to direct higher pressure into the bowls, tapped into a port closer to the turbo, fuel pressure is kept just above air box by the regulator, I have the understanding the 914 saved , spell’s it out pretty good 

[RFguy]

rightio then , sounds like just needs methodical approach to figure out what is going on. Yeah the O2 sensor (ULP only - no lead) would be a handy diagnostic I think.

[rhtrudder]

Hope so, just paid for it, I’ll keep you posted, I’ve ordered a bung as well so I can plug it off, was told they don’t like lead, mostly run 98

[BrendAn]

6 hours ago, skippydiesel said:

Due to the locating of the Rotax 9 carburettors/inlet air, above and to the rear of the engine,  carburettor icing is (in many installations) a low risk due to ingesting warm air from the engine bay.

 

Turbo charged varient have even less risk of carby icing as inlet air heated by turbo .

 

Fuel injected 9's don't have the venturi/fuel vapour condition present in a carburettor, so are also resistant to icing.

 

BUT

 

If your instillation has ambient (cold) air ducted  directly to the carby's the risk of icing increases, so in these installations carby heat is often designed in.

i realise the carbs on top pick up warm air, the tecnam will ice up a little bit when starting on a cool morning but  comes good after a minute or 2. mine probably will too. i did not think about ducted air , used to looking at the filters on the carbs.

[RFguy]

I've had the jabiru ice up on the ground at high idle on a foggy cold morning. ...