912 throttle response hesitation

[BigDiggs88]

Hi all, 

New to the group and having some issues with my Rotax 912 in my skyfox gazelle and am requesting some assistance. 

When advancing the throttle the engine appears to be rough through the 3000-4000rpm range, however when your backing off the throttle and then re-apply the power, like in the event of a go-around (again through the 3-4000range) the engine appears the sputter, and hesitate for about 3-4 seconds before recovering. The below has been checked, just looking to see if anyone else has potential ideas.

- Fuel filters new

- Fuel lines new

- On engine fuel pump new

- Plugs all new

- carbs are balanced

- ignition drops 60-80rpm L-R

- no change between choke on or off

- no change between AUX fuel pump on or off

- Inlet connections all inspected and re-fitted

- Air filter - clean and clear

 

[turboplanner]

See if the Manual shows the settings for the second stage. Might not be metering enough fuel in second stage.

[facthunter]

Check the dashpot is moving freely and rubber diaphragm in good order and correct position. (small lug in place).  Nev

[peterg]

Are the plugs the correct ones for the 80HP ie not the 100HP plugs

[skippydiesel]

• How many hours?
• Has the engine/aircraft had its 5 year rubber replacement?
• Fuel type (RON) and age in tank?
•  "-carbs are balanced."  - Mechanical?/Pneumatic/Both? Technique?
• "- no change between choke on or off "This doesn't seem right - In my 15-18 years of Rotax 912 operation, I have never applied "choke" (an enriching device) to a running engine so don't know what happens however I would expect a very over rich condition and engine rpm to drop significantly.

 

Recomend joining the Rotax Owners Forum for specific technical assistance - https://www.rotax-owner.com/en/rotax-forum

Edited November 19 by skippydiesel

[RFguy]

sounds like carb probs.

float bowl float needles.....

 

With alot of vibration, this might flood the float bowl, or otherwise have it 'not to spec'

 

I venture a guess that vibration and the sputter are unrelated except that vibration (due to maybe  mismatched carb operation (needles, float height )   causes the sputter 

 

also, check floats actually float the correct height  while you are there. 

Nev is right with the Dashpot question. have these carbs had an overhaul kit through them ?

 

Edited November 19 by RFguy

[skippydiesel]

RFguy - try getting answers to the basics BEFOR escalating the suggestions/cost/time.

 

"With alot of vibration, this might flood the float bowl, or otherwise have it 'not to spec'" - Agreed however may be addressed by the 5 year rubber (engine mounting) replacement question.

 

"also, check floats actually float the correct height  while you are there. " - weight of float  (not usually height although strongly related) supposed to be checked at (memory?) 200 hr service interval.

 

Nev is right with the Dashpot question. have these carbs had an overhaul kit through them ? - 5 year rubber replacement - could be called an overhaul kit but usually has connotations of seats/needles etc and should only be required (if at all) with a very high hours engine, hence my questions above. 

 

Rotax 912, Maintenance Manual Line, Service Schedule,  Chapter 05-20-00, Page 9, onwards

[RFguy]

Skippy, really ????

 

? I think I provided quite useful advice , having seen this sort of fault PRECISELY AS DESCRIBED !

[IBob]

With the 912 standard throttle cable setup, throttling back is pulling the cables, but throttling up is pushing the cables, and relying on the springs on the carb throttle arms to pull the arms and cable at the other end.
So it may be worth checking that the cables and linkages etc are all moving freely on both carbs.

Edited November 19 by IBob

[Area-51]

If fuel related, 3000rpm and upward is the main jet circuit of the bing64 carbi. Check for incorrectly positioned tab or ruptured diaphragm or asymetric needle position height between carbis. (The diaphragms would be my first and easiest go to if everything outside looks correct). Check for a broken throttle arm or choke spring.

 

If electrical, check the resistor plug connectors for correct resistance and ignition leads for arcing through insulation to earth at the inlet manifolds or cylinder heads. Verify plug gaps are set correct.

 

Try opening throttle slowly and see if issue is still apparent; better or worse

[Blueadventures]

Worth dropping off the carb bowls and check closely to see if any slithers of fuel hose is present (you said fuel lines replaced) while off check the floats (type and weights).  Next I would do a mechanical balance as per the maintenance manual.  Then watch the movement of the throttle arms through the travel.   Then run engine up again; if vibration still present and same and when in the vibration rpm range pull the carb heat and note what occurs.  Cheers and I have some other checks but do these first.

[BigDiggs88]

8 hours ago, turboplanner said:

See if the Manual shows the settings for the second stage. Might not be metering enough fuel in second stage.

Thanks buddy, if it wasnt metering the second stage properly would it not be across the entire upper RPM range where it becomes an issue?

[facthunter]

When you cut fuel hose there is always the chance a sliver of rubber will lodge in the hole and not be detected. A  CV carb should cope with sharp throttle applications better than most other types but It's still not good practice.. Quite a few people don't really understand HOW they work. The dashpot only rises if enough suction is applied above it to raise it up (open)  Before the new diaphragm is finally tightened the dashpot should be manually lifted all the way UP a few times and then fully tightened. The choke only works when the throttle is closed or nearly so.  IF you left it on you'd only affect very low power settings. Nev

[facthunter]

 As your altitude increases the dashpot will rise less and less the higher you go.. This matches the fuel flow (Tapered needle) with the reduced air volume going into the engine. There's always a drop in pressure through the area of the dashpot similar to a venturi..  Nev

[turboplanner]

45 minutes ago, BigDiggs88 said:

Thanks buddy, if it wasnt metering the second stage properly would it not be across the entire upper RPM range where it becomes an issue?

No, Bing call the three stages Idle Range, Mid Range, Full throttle.

The needle usually plays the most important part in Mid Range so coming into Full throttle range where jet size plays the most important part, the engine should be clearing if it is fluffy in Mid Range.

 

This explains the sequence: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://electricmotorglider.com/wp-content/uploads/2017/02/CV-Carb-Web.pdf

 

[onetrack]

Correct link in Turbo's post is here....

 

https://electricmotorglider.com/wp-content/uploads/2017/02/CV-Carb-Web.pdf

 

[turboplanner]

6 minutes ago, onetrack said:

Correct link in Turbo's post is here....

 

https://electricmotorglider.com/wp-content/uploads/2017/02/CV-Carb-Web.pdf

 

tks

[BigDiggs88]

9 hours ago, peterg said:

Are the plugs the correct ones for the 80HP ie not the 100HP plugs

yeh mate, double checked the the manual before purchasing the replacements and installing them

[BigDiggs88]

5 hours ago, Area-51 said:

If fuel related, 3000rpm and upward is the main jet circuit of the bing64 carbi. Check for incorrectly positioned tab or ruptured diaphragm or asymetric needle position height between carbis. (The diaphragms would be my first and easiest go to if everything outside looks correct). Check for a broken throttle arm or choke spring.

 

If electrical, check the resistor plug connectors for correct resistance and ignition leads for arcing through insulation to earth at the inlet manifolds or cylinder heads. Verify plug gaps are set correct.

 

Try opening throttle slowly and see if issue is still apparent; better or worse

if you advance the throttle slowly the engine responds but you can feel slight hesitation during normal operation. more prominent when you stab the throttle quickly. Hesitates for about 3-4 seconds and pulls through the rev range just fine. Not a vibration, more so a sputter as the engine responds to the throttle advancement.  Plug gaps should be fine as i replaced them all. cheap and quick but no real improvement. throttle arms are all good and choke cables all connected with no breakages. ill have a play later and see if the others show any signs of causing the issue

[BigDiggs88]

3 hours ago, turboplanner said:

No, Bing call the three stages Idle Range, Mid Range, Full throttle.

The needle usually plays the most important part in Mid Range so coming into Full throttle range where jet size plays the most important part, the engine should be clearing if it is fluffy in Mid Range.

 

This explains the sequence: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://electricmotorglider.com/wp-content/uploads/2017/02/CV-Carb-Web.pdf

 

thanks mate ill have a sus when i get a chance

[Blueadventures]

18 minutes ago, BigDiggs88 said:

if you advance the throttle slowly the engine responds but you can feel slight hesitation during normal operation. more prominent when you stab the throttle quickly. Hesitates for about 3-4 seconds and pulls through the rev range just fine. Not a vibration, more so a sputter as the engine responds to the throttle advancement.  Plug gaps should be fine as i replaced them all. cheap and quick but no real improvement. throttle arms are all good and choke cables all connected with no breakages. ill have a play later and see if the others show any signs of causing the issue

Are you moving from idle to full throttle over a four second period?  The Rotax does not like fast throttle (like it sounds when you refer to "stab the throttle quickly") which is not correct application of throttle according to Rotax.

Edited November 19 by Blueadventures

[BigDiggs88]

13 minutes ago, Blueadventures said:

Are you moving from idle to full throttle over a four second period, the Rotax does not like fast throttle (like it sounds when you refer to "stab the throttle quickly" which is not correct application of throttle according to Rotax.)

yea ok, interesting you do say that through because the manual does state about RPM variations after start, more so about rpm stability before retarding the throttle to look the reduction shock absorber but cant find anything about the advancement of throttle. I am aware smooth steady power adjustments are best. major concern was in the event of a go round you would want power on hand to recover, but the 3 -4 second hesitation was the concern. the advancement on throttle to have the issue is definitely quite aggressive. aircraft has only done high speed taxi runs at this stage till I'm happy with the gremlins. if you would have manual ref i could view just to put my mind at ease then id happily accept it. just want to determine if its operational error or a genuine issue.

 

[BrendAn]

12 hours ago, BigDiggs88 said:

Hi all, 

New to the group and having some issues with my Rotax 912 in my skyfox gazelle and am requesting some assistance. 

When advancing the throttle the engine appears to be rough through the 3000-4000rpm range, however when your backing off the throttle and then re-apply the power, like in the event of a go-around (again through the 3-4000range) the engine appears the sputter, and hesitate for about 3-4 seconds before recovering. The below has been checked, just looking to see if anyone else has potential ideas.

- Fuel filters new

- Fuel lines new

- On engine fuel pump new

- Plugs all new

- carbs are balanced

- ignition drops 60-80rpm L-R

- no change between choke on or off

- no change between AUX fuel pump on or off

- Inlet connections all inspected and re-fitted

- Air filter - clean and clear

 

no mention of stripping and cleaning carbs. worn needles will do that to a 912.

[turboplanner]

6 hours ago, BigDiggs88 said:

yea ok, interesting you do say that through because the manual does state about RPM variations after start, more so about rpm stability before retarding the throttle to look the reduction shock absorber but cant find anything about the advancement of throttle. I am aware smooth steady power adjustments are best. major concern was in the event of a go round you would want power on hand to recover, but the 3 -4 second hesitation was the concern. the advancement on throttle to have the issue is definitely quite aggressive. aircraft has only done high speed taxi runs at this stage till I'm happy with the gremlins. if you would have manual ref i could view just to put my mind at ease then id happily accept it. just want to determine if its operational error or a genuine issue.

 

In the material in the link I supplied it explains how the throttle is opened by vacuum createwd by your throttle demand (accelerator pedal in off-road  vehicles, hand throttle on snowmobiles and jetskis, and throttle lever on aircraft. If you're descending to land and meet wind shear (I've done it twice) immediate wide open throttle is required to stop you sinking into the ground. Both my times were with the Bing carb, so the vacuum is fast enough for hand throttle with the vaccuum system in correct specification in the carb. On the other had I had one on a 450cc engine that defied all efforts to stop that fluffy fail in mid range and I replaced it.

[skippydiesel]

To be human is to jump to  the most expensive/complex conclusion's/solutions to any situaton - I am staggered by the demonstration of this very human failing.

 

What of the BASICS????

 

We know very little about this engine, other than a few replacement parts have been fitted.