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Engine surge - flaky floats?


BMcCand - N248CT

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Sorry, long. flying 2006 CTSW, 344 hrs.

 

I was flying from OR to Corning,CA to take the LSRM course. At 11500, 5200 rpm I got a brief drop of about 500rpm.

Later, near Red Bluff I got two deep drops down to 4000 rpm. Made a precautionary landing at RBL. Examined

plug wire ends, connectors to ign boxes, drained gascolator, examine carb mount and cables. No issues.

Full power runup - no issues. Started takeoff roll, and just after rotation engine went rough. Abort

takeoff, stay at RBL and get a car. A mechanic comes up from Corning the next day, we repeat the

exam plus drop gascolator - it was clean. Again a runup, fly the mechanic around the pattern, plane

is Ok. I then take off to fly to Corning, circling about red bluff to climb. Two engine stutters. I had enough

alt to glide to Corning, so off I went. This time I try each ignition during stutter, seems worse on#1 but

this is a transient problem. I did find that throttle back to 4500 reduced frequency, and 4000 the

engine was steady. Slipping rt or left at 5200 rpm had no effect, no climb or descent attitude.

We replaced plugs, test flew still had problem.

 

Dropped carb bowls and saw several small 1/16" black fragments in both. Cleaned those out. We think is is pieces of float.

sprayed carb cleaner liberally, did a test flight, only one rpm drop in 20 min. I was not trained, then,

to pull jets. Next step is to remove carbs to a clean area, pull jets and carefully examine, clean,blow

out. Replace floats.

Fuel had been 1 week old Arco mogas, I put in 10G 100LL just before the incident.

Could summer fuel formulation be causing float issues?

 

Question - has anyone else seen float degradation, and black flakes in the bowl? Has it caused deep,

but intermittent RPM drops?

 

Thanks, Bill McCandless

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This sounds familiar. Over the last few weeks I had been trying to debug an engine that won't start. It was water that I hadn't seen in the gascolator.

 

My theory is that wile the engine is vibrating the water is in small droplets that might make it through the jets. When you let the engine sit, water will accumulate in the bowls. Flying immediately after buying bad gas (and not having the water settle out) is what got me in trouble.

 

When I had my bowls dropped, water continued to drip back through the jets for a couple of hours. A puff of air through the clear plastic hose that goes to the air filter box can speed this up. You want to make sure this water evaporates or you haven't solved anything.

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Hi Bill,

 

You have a carb debris issue. (not electrical) You can try and determine which carb with a carb sync. You are looking for a large vacuum difference. The one with the largest vacuum is the culprit. It sounds though you may want to pull both carbs, then the jets and blow them out with a little jet spray of carb cleaner followed by some higher pressure air from a compressor. Not only clean all the jets, but blow out the passage ways in the carb body. Do a good job and don't miss any holes so you don't have to do it a second time. This isn't hard just time consuming. I feel fairly certain this will take care of your problem. The answer to the float flaking issue, yes that has happened, but you would be able to see it on the floats. Did the 100LL you got come from a tank truck or a ground tank? Trucks are notorious for debris in the fuel just as you described.

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Thanks, Roger. I will be doing a thorough examination and carb cleaning under Brian's supervision.

The fuel was dispensed by a truck, it was 100LL BP. I examined the fuel filter behind the panel

and found very little, some red sealant about 2mm x 1cm . Some blue dye from 100LL. None of the black flakes.

as noted before, gascolator was totally clean.

 

Rubbing the floats would cause more black plastic to come off, so geting new ones.

This problem has not been duplicated on the ground, even duriing carb sync or full power runup.

I don't have a D120.... but now I'm thinking about adding one. I already have D100.

Looked at another CTSW, it had only 2 EGT probes on aft cylinders. Is this normal?

I'd want 4 egt. Engine has only 2 cht points. Couldn't locate the fuel flow sensor on other plane!

 

Current IPC says float is 861-184. CPS has 861-183 in catalog, Something changed.

due to holiday weekend I'm unable to order parts until Tues. Instead of CPS or Lockwood,

or Leaf, does anyone use the Bing vendor in KS ? Is that legal for SLSA?

 

Bill McCandless

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Hi Bill,

 

Brian will do a good job. All CT's only have 2 EGT 's from the factory, but the Dynon can handle 4 and only 2 CHT's. I don't think 4 is really necessary. If you put in a D120 you need to get a "Flow Scan" fuel flow sender. You can buy from Bing, but I would bet any money that CPS is cheaper. Floats are 861-183, but unless you see the float has actual flaking I wouldn't replace them to just replace them. Garbage doesn't have to pass through the filter or gascolator it could have come from after the gascolator, i.e. pieces of hose.

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...

You can try and determine which carb with a carb sync. You are looking for a large vacuum difference. The one with the largest vacuum is the culprit....

 

Hi Roger,

 

this is the second time, I stumble over your opinion, that you can see different air-fuel-ratio with carb sync. (If one jet is contaminated, this will result in leaning one bank)

 

Please allow me to discuss this and regard the following (mostly theoretical) question:

 

One grounds the ignition and prevents the engine from firing this way. Next, this guy uses a big belt in companion with a big electric motor and rotates the engine at, say 2000 RPM. You are the carb sync man and what do you expect to read in the following cases:

 

a.) Both carbs has clean main jets?

b.) One main jet is contaminated and blocked?

c.) One main jet is completely missing?

 

Before you answer too quick, consider the measuring point of the carb sync gauges in respect to the location of the throttle flap and the main jet <_<

 

Many greetings from germany

 

Markus

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Hi Markus,

 

Nice to hear from you again. Haven't seen much of you lately on the forum. Page, AZ is coming up again in October.

 

I can't answer your question because I have. never done such an operation and don't know how all the components will or will not effect the final out come. I can tell you have have used the carb sync gauges several times to track down a problem carb and fuel flow. I just did it 2 weeks ago. The vacuum on the idle side had 7" of vacuum difference and the high rpm had no problems and would sync okay. This told me there was an obstruction in the left carb idle circuit. The vacuum on the obstructed carb was also way more than I would normally see.

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Hi Roger,

 

oh, I was not lost in space. From time to time I read through all new postings in this forum and I find it very useful. Maybe you remember, I am responsible for the maintenance and repairs of two CTSW which are very busy. They fly about 800 hrs a year (in sum) and therefore I am busy as well :rolleyes:

 

Here I find useful information very often and I appreciate your obligation (right word??) to the forum. Many thanks for that!!

 

Back to topic:

As far as I understand, with our carb sync gauges we are able to measure the sucction, one bank generates. The sucction only depends on RPM and on the position of the throttle. It doesn't matter, weather the engine fires well or not. So it makes me wonder, how you have the experience that the measurement varies, if a jet is contaminated. In my technical understanding, this does not make sense. But all this are only my thoughts. I never tried this.

 

Maybe a very high sophisticated engineer can help us??

 

Greetings

 

Markus

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Hi Markus,

 

While the engine is developing any rpm it has a vacuum reading. If it is at idle it runs through the idle circuit and the higher rpms get controlled through the main jet with the needle. At either of these settings the vacuum changes because of the butterfly and the constant decompression slide setup. It`really won't make any difference at what rpm they are at because these are naturally aspirated engines and carbs so we pull a vacuum and when the carb is set at any vacuum. The two carbs when set properly should get the same vacuum gauge reading and they should be adjustable. If you have a fuel flow or an air flow issue from an obstruction this will change that vacuum on the one carb compared to the other. The affected carb may then not be able to be adjusted to match the other and the vacuum will have a wide margin. An unobstructed carb will not have a fuel flow or air flow issue and should be able to be adjusted to match the other and you should have a resulting smooth running engine.

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a.) Both carbs has clean main jets?

b.) One main jet is contaminated and blocked?

c.) One main jet is completely missing?

 

I have never synchronized carburetors, but I will give the theory a try. Let's assume nothing changes downstream of the carburetor, like weak exhaust springs, air leaks in the induction system, etc. to affect the amount of draw on each side of the engine. Therefore, the air being sucked in is the same on both sides and the carbs were synchronized prior to the conditions listed above.

 

I would think:

 

a. equal vacuum readings - both jets clean

b. contaminated jet would have more of a vacuum reading (less absolute pressure)

c. missing jet would have less of a vacuum reading (closer to atmospheric pressure)

 

I think the difference would be fuel flow from the jet. The pistons displace only so much at a time, so if some of the volume being drawn into the engine is being taken up by fuel, as in case c, the vacuum would be less. The opposite would be true with the blocked jet, case b.

 

Of course, I always lose on Jeopardy! and Are you smarter than a 4th grader?

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The conclusion was flaky floats. The black chips were about 1/16" square. One carb's floats were easily

scraped by a fingernail, yielding more flakes. I did a complete carb rebuild including diaphragms

and floats. I did a test of half an hour circling over the airport, no issues. I then made

a 3 hour flight home, again no issues. I drained the tanks, discarded fuel, bore-scoped tanks. Clean.

Auto fuel had been from Arco and Fred Meyers (a portland dept store). Maybe the latter was

bad auto fuel. Roughness began after adding some BP 100LL to the auto fuel.

 

Before the rebuild:

The carb slides were working correctly. The engine had the carbs in sync, as tested

by pinching off the compensation tube. I changed spark plugs; no effect.

I examined the main fuel filter - some minor debris, looked like sealant.

No black flakes there. Examined gascolator, not contaminants. Checked fuel pressure, it

was Ok; presume therefore nothing in the fuel pump internal filter.

One carb had a leaky viton tipped valve.

 

After rebuild and re-install, I did a complete mechanical and pneumatic sync of carbs.

 

This is what Brian Carpenter called on-the-job training for the RLSM-A course!

 

Bill McCandless, almost RLSM-A; still need to visit FSDO.

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Flaky floats unfortunately aren't anything new. This has happened before and it isn't usually from the fuel. I have seen it with 100LL, too. It used to happen even before ethanol. The Bing floats of today are ethanol resistant.

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