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Rough Engine after Mx

KALEWIS

KALEWIS

Well Known Member
Good Morning VAF land -

Will post on rotaxowner.com as well, but thought there may be some ideas from the braintrust here too.

After normal mx, our 912uls (approx 700 hrs) is running extremely rough. So rough that a carb sync is not possible. Local AP IA is assisting, not a rotax guru, but is familiar with them. He thinks a carb is not functioning.

Here is a summary -

-Gearbox overhaul by Lockwood - all ok and re-installed on a/c
-Both carbs had the "5 year" overhaul by Lockwood
-New air filters
-New spark plugs, gapped properly
-Replaced a carb drip tray
-New engine mount isolators
-Throttle cables reinstalled on carbs per KAI (never removed from carb mounting bracket)
-Did not change oil, so no need for oil purge
-Floats weighed within spec (each around 6g per set)

Engine was running fine when last run (about a month ago before the condition inspection)

Pulled plugs and all look to have been firing

Fuel is getting to both carbs equally

As rough as it was running, no fuel overflowed into the overflow tubes or drip trays

More troubleshooting to commence -
 
I'd take a look at the ignition wire/spark plug terminal connectors to make sure they are all in good shape and properly installed. I made a little gauge out of aluminum that verifies that the distance between the bottom of the rubber connector and the spark plug hexagon shell is correct and consistent for all eight plugs. However, I suspect that your "extremely rough" running engine may involve more than merely an incorrectly installed or damaged connector.

Another consideration is that following a hose change (if you changed any hoses) pieces of the old or new hoses may have flaked off internally and mucked up the various orifices in the carb/fuel system.
 
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I know you said throttle cables were never removed from the carb brackets.
Regardless you should do a mechanical carb synchronization first. Will get you in the ballpark prior to the pneumatic synchronization.
 
In addition to the above, ensure wires were not swapped between cylinders. It can and has happened!
 
Swoda said:
I know you said throttle cables were never removed from the carb brackets.
Regardless you should do a mechanical carb synchronization first. Will get you in the ballpark prior to the pneumatic synchronization.
Click to expand...

It's so rough that a carb sync is not possible......with the engine running

The cable lengths are equal from the exit of the throttle cable housing at the bracket.
 
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Check the basics, correct wiring to plugs, carb sync without running the engine. Something has been disturbed.
 
Check for a cold cylinder

If you feel that the engine is safe to run, I would check for a cold cylinder. I only have two CHTs and two EGTs so I don't know the temps on the other two cylinders. If you have monitoring on all 4 cylinders, disregard this post. If not, I would run for a couple of minutes and check temps of the cylinders with a temp gun. An old school way to check for a cold cylinder is to run for a couple of minutes, shut off engine and then touch the cap (pointy is best) of a ball point pen (still on pen) to each exhaust stack. Cold cylinder will not melt or barely melt the cap. This will at least help you isolate which cylinder/s may be causing the problem.
 
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Rubber Flange

If you just installed the carbs after rebuilding, I would check the rubber flange assembly between the carbs and the manifolds.
 
Make sure the idle jet screws are adjusted 1.5 turns out...switch from A to B ignition to see if improves, check compression for sticking valve.
 
bobg56 said:
Make sure the idle jet screws are adjusted 1.5 turns out...switch from A to B ignition to see if improves, check compression for sticking valve.
Click to expand...

checked that - all good

no sticking valves, checked by thumb compressions
 
KALEWIS said:
It's so rough that a carb sync is not possible......with the engine running

The cable lengths are equal from the exit of the throttle cable housing at the bracket.
Click to expand...

Doesn't matter if the cable lengths are equal.
I suggested a mechanical carb sync, this is accomplished without the engine running and is performed prior to the pneumatic carb sync which is done with the engine running.
 
Check that L-R choke (enrichener) is assembled on correct carb. Sometimes a shop will disassemble both carbs and install L-R choke on wrong carb...
 
KALEWIS said:
Good Morning VAF land -

Will post on rotaxowner.com as well, but thought there may be some ideas from the braintrust here too.

After normal mx, our 912uls (approx 700 hrs) is running extremely rough. So rough that a carb sync is not possible. Local AP IA is assisting, not a rotax guru, but is familiar with them. He thinks a carb is not functioning.

Here is a summary -

-Gearbox overhaul by Lockwood - all ok and re-installed on a/c
-Both carbs had the "5 year" overhaul by Lockwood
-New air filters
-New spark plugs, gapped properly
-Replaced a carb drip tray
-New engine mount isolators
-Throttle cables reinstalled on carbs per KAI (never removed from carb mounting bracket)
-Did not change oil, so no need for oil purge
-Floats weighed within spec (each around 6g per set)

Engine was running fine when last run (about a month ago before the condition inspection)

Pulled plugs and all look to have been firing

Fuel is getting to both carbs equally

As rough as it was running, no fuel overflowed into the overflow tubes or drip trays

More troubleshooting to commence -
Click to expand...

Is there fuel in both carb bowls after the rough run? I would assume if one is not full of gas then you found your culprit.

Any change to prop angle or did you re-attach the prop with out measuring the proper angles? (believe it or not I managed to do this once)

Bad soft start ignition module?

Check the mechanical linkage and operation of the choke circuit and both throttle cables? not mech synched could cause rough running.

Balance tube hooked up? Blockage in balance tube or maybe a bug built a nest while the carbs were out?

might have gotten a bad carb rebuild possibly? seems unlikely. gearbox rebuild issue? also unlikely but i guess stranger things have happened.
 
cactusman said:
Is there fuel in both carb bowls after the rough run? I would assume if one is not full of gas then you found your culprit. See above - yes fuel equal to both carbs

Any change to prop angle or did you re-attach the prop with out measuring the proper angles? (believe it or not I managed to do this once)Resest prop correctly

Bad soft start ignition module? Not likely, soft start seems to be working fine, both ignition modules were working prior and within spec on "mag check", you can tell when the soft start engages/disengages

Check the mechanical linkage and operation of the choke circuit and both throttle cables? not mech synched could cause rough running. Checked all this on install, installed as described in KAI

Balance tube hooked up? Blockage in balance tube or maybe a bug built a nest while the carbs were out? balance tube was never removed, except to hook up carb mate for sync.

might have gotten a bad carb rebuild possibly? seems unlikely. gearbox rebuild issue? also unlikely but i guess stranger things have happened.
Click to expand...
This is a possibility, the idle jet looked to be contaminated and carbs have been sent back to lockwood.....
 
Mechanical sync

Thanks Swoda, will try that on next attempt after getting carbs back from the shop.
 
KALEWIS said:
Good Morning VAF land -

Will post on rotaxowner.com as well, but thought there may be some ideas from the braintrust here too.

After normal mx, our 912uls (approx 700 hrs) is running extremely rough. So rough that a carb sync is not possible. Local AP IA is assisting, not a rotax guru, but is familiar with them. He thinks a carb is not functioning.

Here is a summary -

-Gearbox overhaul by Lockwood - all ok and re-installed on a/c
-Both carbs had the "5 year" overhaul by Lockwood
-New air filters
-New spark plugs, gapped properly
-Replaced a carb drip tray
-New engine mount isolators
-Throttle cables reinstalled on carbs per KAI (never removed from carb mounting bracket)
-Did not change oil, so no need for oil purge
-Floats weighed within spec (each around 6g per set)

Engine was running fine when last run (about a month ago before the condition inspection)

Pulled plugs and all look to have been firing

Fuel is getting to both carbs equally

As rough as it was running, no fuel overflowed into the overflow tubes or drip trays

More troubleshooting to commence -
Click to expand...

Well coming out of condition inspection I too did several of these services and had similar issues when I started up. After much checking of my right (1-3 side) carb I changed the diaphragm. With counsel I determined air/vacuum leak was getting in and I knew I was using a diaphragm I got from a local carb shop, one the shop said was just like Bing used in the 64 carbs. But when I installed the diaphragm it just didn’t fit right. It was close but would winkle a little bit. The engine ran rough like you reported but also noted the EGT, read on the #3 cylinder, would look okay on start up but would drop very low.
Once I replaced the diaphragm it ran close to normal. A mechanical sync followed by a sync on the gauges and the 912 was running well again. Several ground checks and one flight so far and everything is looking good. Even a 50 degree difference from one EGT to the other has come closer to each other.
Hope this helps and I’m sure Lockwood will get your carbs perfect. Best of Luck!
 
update

Quick update after looking at carbs again and a call to lockwood-

Removed idle jets and inspected the ports/holes and on one carb, the idle jet "top port" looked out of round and the overall description of the idle jet is "worn". There also appeared to be some very small debris in one of the small holes, enough that it was blocking some light.

Carbs are being sent back to lockwood for a closer look and idle jet replacement - which one would think would be part of a 5-year overhaul, fwiw.
 
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