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Rouigh running at high RPM under load  


SEQFTA
(@seqfta)
Active Member
Joined: 2 years ago
Posts: 5
Topic starter  

Hi Doug
Any idea's ?
Jab160D Hydrolic lifters 2.2 4 cylinder
I suffer a "slightly" rough running motor between 2700 to 2900 RPM , but only under load not in descent
So
Below 2700RPM totally smooth , 3000RPM and above totally smooth , All altitudes .

The "cruncher" is that I can get it to run smooth again by increasing to full power .
So Far I have
Changed Bing Carby with overhauled kit and really cleaned the needle jet and the main jet , added the fluted insert from Jabiru at the carby end of the induction pipe to stop the air from becoming turbulent .
New air filter , new NGK D9EA x 8 gapped correctly
Glued rotor buttons
Had ohms checked with auto electrician on leads (Yes , I know "not under load") but still good ohms reading
Leakdown 74'PSI all round
Checked the distributor caps
After all that , still the same
No , I haven't individual EGT's reading's
Jabiru say's a possible fix is to cut a hole in the cowling directly beside the air box and fit a scat from the cowling to the air box to stop turbulent airflow but my aircraft never used to have this problem and it's always had the normal induction on the side of the cowl , so why now ?
Anyway , any idea's Doug ?
SEQFTA


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Doug Smith
(@doug-smith)
Member Admin
Joined: 3 years ago
Posts: 82
 

Hi, 

"Slightly" rough running eh?  Well it sounds like you're already across all my stock replies so I'll try to be inventive...

1. Sometimes you can get intermittent vibrations from the engine rubbing on the cowl or similar, but based on the description it sounds like you're satisfied that it's something within the engine itself...  

2. For a while there my engine had a really soft stumble or burble just where the needle went from "cruise" to "power".  I always put it down to a bit of confusion within the carby as it made that transition and - seeing as I had to be listening pretty hard to even notice it - I didn't do anything about it.  Eventually, when I changed the propeller, it went away.  But sounds like yours is more severe than that.

3. Try turning off one ignition at a time while it's missing and see if anything changes. 

4. Try turning on caby heat and see if anything changes.  Heating the air tends to richen the mix slightly, and change the flow through the air box so I'd expect that to do something... if it makes it worse then chances are your mix is a fraction rich.

5. Any issues with too-high fuel pressure (fuel leaking out of the bowl etc)?

6. Induction system seals all ok?

7. Tried a different fuel?

8. Rubber diaphragm of the carby is in correctly and not damaged?  No sticky spots or other damage inhibiting the movement of the slider within the carby body?

From what you're describing, it's a particular power setting that's the problem; a particular combination of manifold pressure, flow rate etc.  However you've made some substantial changes to the intake and carby that should have at least affected the problem.  Which makes me wonder if it's something downstream of the carby (induction leak) or a curve-ball like a funny lifter.  It would be nice to swap out the whole carby and see if that changes anything, if you can borrow one from somewhere.  That would at least confirm if it's a carby issue or not.  I have had carby bodies go "bad" on old motorcycles before (probably an internal gallery blockage that I wasn't able to clean out or wear between the slider and the body) so something like that isn't out of the question.

Regards,

Doug.


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SEQFTA
(@seqfta)
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Joined: 2 years ago
Posts: 5
Topic starter  

Doug

It's Wayne  , from one of your classes a few years ago .

Anyway

In the last 45 minutes

I don't believe it "Nulon Engine Flush"  , I followed the instructions on the bottle

All else I have done is change the spring on the carby heat mechanism from the middle screw to the end screw  , squirted some WD40 up the carby heat cable and as it wouldn't close in the box before because the cable would wouldn't support the movement under the dash board  , it works fine now with my little adjustment .

Anyway

I wanted to get the carby heat working so as to try it and see what happens when I get into that negative range RPM , perhaps even get over the airfield and try each CDI to see if there's any excessive drop 

So anyway

Did that , changed the oil and filter and  , took it up for a fly and "No rough running" !!!

Well  , put it this way 100% better than it was between 27 and 2900RPM

Could it have been the Nulon Engine Flush ??????

Carby heat at 2800RPM no noticeable difference  , CDI drop at 2800RPM  , yeah , but well within normal range and both sides the same amount of drop .

It's just that I notice in your reply Doug  , you mentioned "lifter"  , could the engine flush have cleaned that out ????


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scsirob
(@scsirob)
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Joined: 2 years ago
Posts: 8
 

From the 'out of the box' department, could it be prop vibration/resonance? Has the prop been disassembled and perhaps re-assembled in a different position? Are all the prop bolts in good shape and properly torqued? Have you had it dymamically balanced? If this is a carbon prop, is the hub in good shape without cracks or loose nuts/bolts? Any sign of fretting? Crack in the spinner maybe?

 


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Doug Smith
(@doug-smith)
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Joined: 3 years ago
Posts: 82
 

I'd have expected prop resonance to be felt more as a vibration than a miss, but it's not impossible!

Yes, there was the odd chance that a lifter might be a little clagged up or have some sort of similar issue... I'm not familiar with engine flush and generally using automotive additives in an aero engine can be risky, but I'm not going to argue with results !

Regards,

Doug.


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SEQFTA
(@seqfta)
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Joined: 2 years ago
Posts: 5
Topic starter  

Yes Doug

Well there seems to be a marked improvement  , but that said  , Doug who does the dynamic balancing ?

I still run the original wooden prop" the plane came out with in 2010 and iv'e done almost 1300hrs now so the prop is looking a little tired  , so who can do a dynamic balance for me ?


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Doug Smith
(@doug-smith)
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Joined: 3 years ago
Posts: 82
 

I couldn't name anyone in particular... I've seen dynamic balancing kits that you attach to the prop for a ground run - I think they're not an uncommon tool amongst LAMES so you could ring around and see who you can find.  

D.


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SEQFTA
(@seqfta)
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Joined: 2 years ago
Posts: 5
Topic starter  

Hi again Doug

Well i'm afraid that i'm suffering the same problem again  , rough running at between 2800rpm and 3000 .

I went better than the Dynamic balance  , I ducted up to Bundaberg and Cody sold me a new prop  , one of those composite new age             fan-dangle black ones hahahaha , anyway  , I put it on early this morning  , 12ft/lbs torquing instead of the old 6 with the laminate wood ones  , and I love it  , i'm back to just under 3000rpm on static full power at the beginning of take off where the old hoop pine one I was down to 2800

But , as nice as the new prop is  , on a cross country with a student today , I had to work the throttle for him to keep it out of that rough running range , the engine flush seemed to make a positive difference to the smoothness of the motor in that rough running range for a short time , it's so strange that to get it to run smooth again  , increase throttle up to 3100rpm which I get comfortably with my new prop" .

Doug  , i'm going to do a leak down in the morning and the lowest couple of "pots" i'll pull the heads off of and check the valve sealing and see if there's any "gunk" in the hydrolic lifters    , what do you think   ?


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Doug Smith
(@doug-smith)
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Joined: 3 years ago
Posts: 82
 

I've had rough running on a cross country like that many times which I blame on carby ice: pull carby heat on for a few minutes and it goes away, for a while at least.  In certain conditions it'll just keep coming back.

I think preemptive checks are never a bad idea 🙂

Regards,

Doug.


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James p
(@james-p)
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Joined: 2 months ago
Posts: 1
 

@seqfta did you get to the bottom of this? I am having same issues with my new gen 4 3300. It’s like it is running to rich. Go for smaller needle jet and it will start getting to the upper limit temps. Between 2900 - 3000 rpm is rough. I have a + wind divider 


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Jabiru Support
(@jabiru-support)
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Joined: 7 months ago
Posts: 76
 

Hello James,

A few questions so we can try and narrow down the problem for you.

Do you have egt monitoring on each cylinder?  Is the engine in a Jabiru airframe?  

What propeller are you using? 

It is surprising hard to get a Jabiru so rich it doesn't run smoothly.  

Have you tried any of the things Doug suggests for @seqfta above?

 

 

 


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