Rough running when mixture is rich

Bob_pipedream · #1 10-03-2010, 05:25 AM

Hi all,

Just wondering people's ideas on what could cause the following:

Background - Fixed pitch prop, new XP-O320 now completed 9.8 Tacho hours - all parts new. No flight has gone above 5000 feet due to airspace restrictions.

Engine ran fine other than occasional rough running for few seconds during initial 5 hours. However engine seemed to dislike certain rev ranges, but this appeared to cure itself as testing continued. Mixture was always full rich as the engine had to be run in.

5-8 hours she ran really well.

At 8 hours, Fuel Pressure sender stopped reading. I assumed it was a sender failure as everything was still good. I did a 1 hour flight at cruise and everything ran perfectly with no rough running at all.

Then on a short flight the following weekend, after a low pass and climb out, engine began running very rough. I changed tanks, played with the mixture and this appeared to cure things. I landed sharpish and the following weekend, I pulled the cowls off and inspected the entire fuel system. I found a big chunk of fluff covering the outlet of the gascolator which in itself could have caused engine failure! Still no Fuel Pressure reading ater cleaning so have ordered new sender. I did a flow check of boost pump and it was fine.

However on the subcesquent test flight, I found the following:
Engine starts fine, take off fine, climb is fine. However after levelling off engine ran rough. This was cured by leaning mixture - but at any rev range above 2000 RPM, using full mixture now causes rough running which is only cured by leaning (this is a new condition).

EGT on number 2? (rear lefthand cyl) is very much the highest at up to 1420. It is quite difficult to lean properly as pulling mixture causes rise in all EGTs but on number 2, reducing by 50-75 degress would put the mixture almost all the way back in.

My thinking is that one of the following may be responsible:
*Blockage/restriction still present somewhere in fuel system before gascolator (but not sure this would actually be improved by leaning mixture and would think that take off and full power would be restricted?)
*Leak on induction tube on number 2? (rear left hand cyl) causing cyl to run very lean and others to run too rich (This would give strange EGT and on take off engine revs are lower as engine under load so may only become an issue as engine picks up speed and manifold pressure increases?)
*Idle Mixture screw setting incorrect (but it does not feel as if it has moved or could move)
*Float level in carb
*Mag timing wrong - but mag drop is okay on run up (the Slick SB regarding weak cams etc. is applicable)

Sorry for long post, but just hope someone may have an idea or have had a similar experience. Fluff in gascolator really does emphasise the need to check gascolator and carb fuel strainers after first few flights in new aircraft. Also really should not fly aircraft without FP gauge working...

TS Flightlines · #2 10-03-2010, 05:43 AM

You said that you found some "fluff" at the gascolator. What was it? Actually something foreign, or parts of the hose liner coming apart, assuming that you might be using 701 type hose. I cant speak for everyone else, but I would think that I'd be rechecking all of the fuel system from the tanks to the gascoloator.
Tom

Quote:

Originally Posted by Bob_pipedream

Hi all,

Just wondering people's ideas on what could cause the following:

Background - Fixed pitch prop, new XP-O320 now completed 9.8 Tacho hours - all parts new. No flight has gone above 5000 feet due to airspace restrictions.

Engine ran fine other than occasional rough running for few seconds during initial 5 hours. However engine seemed to dislike certain rev ranges, but this appeared to cure itself as testing continued. Mixture was always full rich as the engine had to be run in.

5-8 hours she ran really well.

At 8 hours, Fuel Pressure sender stopped reading. I assumed it was a sender failure as everything was still good. I did a 1 hour flight at cruise and everything ran perfectly with no rough running at all.

Then on a short flight the following weekend, after a low pass and climb out, engine began running very rough. I changed tanks, played with the mixture and this appeared to cure things. I landed sharpish and the following weekend, I pulled the cowls off and inspected the entire fuel system. I found a big chunk of fluff covering the outlet of the gascolator which in itself could have caused engine failure! Still no Fuel Pressure reading ater cleaning so have ordered new sender. I did a flow check of boost pump and it was fine.

However on the subcesquent test flight, I found the following:
Engine starts fine, take off fine, climb is fine. However after levelling off engine ran rough. This was cured by leaning mixture - but at any rev range above 2000 RPM, using full mixture now causes rough running which is only cured by leaning (this is a new condition).

EGT on number 2? (rear lefthand cyl) is very much the highest at up to 1420. It is quite difficult to lean properly as pulling mixture causes rise in all EGTs but on number 2, reducing by 50-75 degress would put the mixture almost all the way back in.

My thinking is that one of the following may be responsible:
*Blockage/restriction still present somewhere in fuel system before gascolator (but not sure this would actually be improved by leaning mixture and would think that take off and full power would be restricted?)
*Leak on induction tube on number 2? (rear left hand cyl) causing cyl to run very lean and others to run too rich (This would give strange EGT and on take off engine revs are lower as engine under load so may only become an issue as engine picks up speed and manifold pressure increases?)
*Idle Mixture screw setting incorrect (but it does not feel as if it has moved or could move)
*Float level in carb
*Mag timing wrong - but mag drop is okay on run up (the Slick SB regarding weak cams etc. is applicable)

Sorry for long post, but just hope someone may have an idea or have had a similar experience. Fluff in gascolator really does emphasise the need to check gascolator and carb fuel strainers after first few flights in new aircraft. Also really should not fly aircraft without FP gauge working...

David-aviator · #3 10-03-2010, 06:20 AM

You did not mention what fuel system supports the engine.

If is a carb, I would focus on it. The needle valve may be leaking due to contamination. Or the float is not properly set.

If it is fuel injection, the controller may be contaminated.

jeffrj2007 · #4 10-03-2010, 10:37 AM

Quote:

Originally Posted by David-aviator

You did not mention what fuel system supports the engine.

If is a carb, I would focus on it. The needle valve may be leaking due to contamination. Or the float is not properly set.

If it is fuel injection, the controller may be contaminated.

I Had a friend that had that trouble for some time? I fixed it when i found the primer unlocked, so if it it a o-320 check!.

Bob_pipedream · #5 10-03-2010, 01:21 PM

Hi all thanks for the responses.

She has a carb and the fluff was lint-like and it was in the gascolator's filter. I checked the carb screen filter and that was all clear - completely clean with no trace of anything. So I doubt that any contamination will have reached the needle seats in the carb etc as the gascolator did its job. I also drained the tanks and checked them and they look clear.

My thought is that the contamination is a coincidence (which happens), as I doubt that a fuel restriction would cause her symptoms? She starts fine, the take off is fine and with the mixture leaned she runs fine bar the weird EGT thing on number 2. However I stand to be corrected!

Also the carb has the updated non-sinking foam float in it (as per the SB).

In regards to the hoses, she has all alloy tubes up to the gascolator, so it can not be hoses breaking down that caused the fluff.

It just really confuses me that she runs so well, but needs to be leaned just to run smooth. That does point to float level I guess? But could it be an induction leak?

The infuriating thing is that it will be another week of day job work before I can get to the hangar to investigate further (and my job is boring too).

Bob_pipedream · #6 10-03-2010, 01:22 PM

Also she has no priming system at all (too lazy to fit it yet) so fuel system is very simple standard Vans and Facet fuel pump.

penguin · #7 10-03-2010, 02:01 PM

Bob,

I don't think it is an induction leak - that would tend to lean the mixture, but in your case manual leaning improves matters. Its worth checking all the inlet hose are tight, but it seems unlikely to be the problem.

I would take the carb off and make sure the main jet is still tight and everything looks OK, it doesn't take that long to get it all apart.

I would also check the timing. Most RVs will 'take off fine' without the engine developing full power. Check that you are getting the expected static rpm at the start of the take off run (2200 for a metal Sensenich). If its lower then there is something wrong with the engine.

Are you using a new or overhaulled engine and accessories?

1420 on an EGT seems a little hot when running at full rich - I would expect near to 1200, but absolute EGT numbers don't mean much. It could be that you are running very lean to start with - which is very common with carbed O-320s (and well documented on other threads). When the carb is off find out what size main jet you have (using number drills). It should be around 0.104" diameter, its not unusual to be around 0.097" which is the standard Cesnna & Piper jet size - but RVs flow much more air so need more fuel (Steve Sampson has this documented well on his blog).

Hope this helps

Pete

Mike D · #8 10-03-2010, 07:11 PM

Bob,
Just a thought, but because you have been running full rich on a O-320 you might pull the spark plugs and check for lead deposits. This can sometimes cause rough running and high EGT?s. I am assuming you are running 100LL.

I am no engine expert, but this solved a similar issue I had.

Bob_pipedream · #9 10-04-2010, 05:38 AM

Thanks all,

She is a new engine and was running okay. I guess that it could be something as stupid as sparkplugs and I will check them when I check the timing. The thing that confuses me is that she is fine at start up and fine during takeoff. If after landing, she had played up, then I would assume that it was float height, but she sits at tickover and mag check revs (2000rpm) very nicely.

The main jet and carb are all new with the engine straight from superior.

One thing, as I slow down to approach speeds I get a popping noise for a few seconds. I assumed this was cowling/fairing related, but it could be popping from the exhaust...

penguin · #10 10-04-2010, 02:35 PM

Bob,

Could I suggest that you sit down and write out all of the symptoms, and then what you know, for sure. Not what you think, or what you assume, but what you know to be true. Then write down your diagnosis for each problem, and why you have reached that conclusion. Try to be very logical, it should be straight forward to figure out where your conclusions are not support by the data you have, or what you need to verify to be true. I think you have a fuel flow or fuel distribution problem. If the fuel and the sparks are OK, are your compressions good? Can you get a similar engined RV as a chase plane to check your t/o & climb performance?

Pete