Rough Running at Full Power

[magman]

Re: Rough Running at Full Power

Quick way to check 4 intake leaks is to pressurize the exhaust with the throttle closed & squirt soap & water at all areas.
You may find something you can't see.
I just did.
I presume you have pipes & no muffler.
On a Piper the symptoms could lead to lose exhaust baffles.


Ron

[rotaxflyer]

Re: Rough Running at Full Power

I have been doing a bit of research and have come across pictures of the different types of carburetor needles....

I have a neoprene tipped needle. I use premium mogas which I buy from the local flying club and it is supposed to be guaranteed alcohol free. They use it in all the club planes. But who knows if their supplier is vigilant.

I suspect the needle has swelled up and is restricting flow and leaning out the engine.

I'm going to change this out to another type of needle. I'll let you know how it goes.

[Rex Havis]

Re: Rough Running at Full Power

This may help. a easy way to check the fuel induction system, start the engine let it warm up, then use a squirt bottle spray diesel fuel on all of the induction system fitting. The rubber hoses on the induction tubes is the normal problem.The engine rpm will increase if there is a leak.
Also checking the gascolator to make sure that it is installed correctly is a good idea. If it is installed backwords it will slow or stop the flow of fuel.

[Guest]

Re: Rough Running at Full Power

This may help. a easy way to check the fuel induction system, start the engine let it warm up, then use a squirt bottle spray diesel fuel on all of the induction system fitting. The rubber hoses on the induction tubes is the normal problem.The engine rpm will increase if there is a leak.
Also checking the gascolator to make sure that it is installed correctly is a good idea. If it is installed backwords it will slow or stop the flow of fuel.

That doesn't make sense. Please explain.

Fluid flow is bidirectional in the absence of a check valve and these systems do not have check valves

[NY86]

Re: Rough Running at Full Power

Get rid of the neoprene needle and check that you have adequate needle travel, 0.048" minimum. There are different thickness washers for the seat so you can dial in the float level correctly.

[Guest]

Re: Rough Running at Full Power

I have been doing a bit of research and have come across pictures of the different types of carburetor needles....

I have a neoprene tipped needle. I use premium mogas which I buy from the local flying club and it is supposed to be guaranteed alcohol free. They use it in all the club planes. But who knows if their supplier is vigilant.

I suspect the needle has swelled up and is restricting flow and leaning out the engine.

I'm going to change this out to another type of needle. I'll let you know how it goes.

I looked up neoprene in a materials handbook and online and in each case it claimed to be ok (ie little to minor effect, the best rating) with alcohol (ethyl & methyl) 0-5% swell.

Isopropal has a minor effect (5-10% swell). I don't think that Isopropyl is in gas.

My bet is not on expanding rubber in the needle.

BTW, what rubber is in the hoses? Neoprene?

[flyguy]

Re: Rough Running at Full Power

I think the reference to the gascolator stopping the flow is if (on mine at least) it is hooked up in reverse (it was, by an AI, Whoa!) any debris from the tank or whatever will collect on top of the screen, cannot be removed by draining, and may eventually block the flow if that happens.
That is what I was referring to anyway.

DC

PS: Andy, I have yet to hear you say that you have checked out the fuel delivery system ahead of, but not including, the carb. May I assume that you did that first? Goes back to the 30 second business.

[rotaxflyer]

Re: Rough Running at Full Power

Based on this http://www.bowersflybaby.com/tech/st...20needles1.pdf and the note in the aircraft spruce catalog advising not to use the neoprene needle any more, I'm changing the needle and seat regardless of whether it is the cause or not - but I'm hoping it is.

One of the first things I did was to confirm plenty of flow through the fuel line up to (but not including) the carb. It is in the first post.

Thanks for all the input folks.

Andy

[Howard Wilson]

Re: Rough Running at Full Power

Did you check the fuel flow with the tail up and the engine running? That's where mine would cut out, and it turned out to be flat crud in the gascolater that you couldn't see when the engine was off. It would let plenty of fuel through just sitting there. The only way I found it was to take the gascolater apart. Maybe you've done that? just sayin' ;-) h

[Guest]

Re: Rough Running at Full Power

I think the reference to the gascolator stopping the flow is if (on mine at least) it is hooked up in reverse (it was, by an AI, Whoa!) any debris from the tank or whatever will collect on top of the screen, cannot be removed by draining, and may eventually block the flow if that happens.
That is what I was referring to anyway.

DC

PS: Andy, I have yet to hear you say that you have checked out the fuel delivery system ahead of, but not including, the carb. May I assume that you did that first? Goes back to the 30 second business.

Hi Darryl,

ok yeah I get it now, thanks.

It appears that the gascolator is clean but in reality the "inlet" is blocked by tank debris.

Dave

[rotaxflyer]

Re: Rough Running at Full Power

I'm ready for a new plane now....

I have replaced the needle and seat and set the float level. I've dismantled the gascolator and it was very clean and it is installed in the right direction. I have checked the fuel flow through the float bowl by removing the drain plug and it pours out - as a hanger-mate said "There is enough flow there for an Allison V12". I have looked in the main tank with a flashlight and there is nothing rolling around in there that might migrate to the outlet.

Next I am going to drain and replace the fuel in case it is bad. Any other ideas?

My current carb is a NA-S3A1. I have a spare carb which is an NA-S3B. Are these interchangeable?

Thanks again

Andy

[Guest]

Re: Rough Running at Full Power

I have now removed the carburator and dismantled it looking for obstructions. I checked the float level and it was a little low at 1/2" below the parting line. I removed one washer from the float valve seat and it came up to a bit higher than spec. I inspected the intake system looking for any holes, checked each of the intake elbows for tightness. Reassembled everything. Tied it down, started up - same as before. Runs great for 30 seconds or so and then starts to miss at any throttle setting above 3/4 power.

How likely is it that both mags are bad? Could I have had one bad mag for quite a while and not known it until the second mag acted up? They are eisemann's. Also, does anyone know if the gears from the eisemann's can be fitted on to new mags if I choose to try that

There is pretty decent resistance for the usual four flips before starting, so does that basically rule out valves? Could a lifter work great for while and then deflate? But only at full throttle.

I guess I'll remove the rocker covers next and compare the valve movement.

Question, help me understand this; you said here is misses and on your first post you said it runs at 2300rpm on climb out for 30 seconds then drops back to 2000rpm and is rough.

Did I get that right, is rough the same as misses? Just double checking.

2300 rpm on climb is high for mine, 2150 tops maybe even 2000.

So I was thinking this; since you run too fast (2300) for 30 seconds it seems to me that some fuel source is drying up.

Its not the carb as far as you can tell now. How about the primer? Are the seals bad and perhaps its allowing an initial flow of gas into the manifold that richens the mixture to give a boost and then when it drys up you go back to 2000?

Not sure how the rougness figures in.

Basically something is happening that takes 30 sec. to have an effect, primer comes to mind because it can take a bit to clear extra primer flow out.

But the roughness perplexes me, with this theory I would not expect it unless the primer was allowing in excess after 30 sec. and I am not sure that will even cause roughness.

Dave

[Mark Julicher]

Re: Rough Running at Full Power

It sounds to me like a float drop issue.

One time I adjusted my float drop incorrectly -- pure ignorance on my part. The engine ran up OK, so I taxied into departure position and started down the runway. Just about where I would normally lift off the engine quit. I had no choice but to pull the power and abort the takeoff, when I pulled off the throttle the engine immediately came back to life!

I had the float drop set so that the engine could sustain 1500 RPM but it could not sustain any higher RPM.

The fuel flow with the improperly set float drop could not keep up and it would drain the float bowl at anything above 1500 RPM.

Definitely change the carburetor - they are interchangeable, in fact many A's were converted to B's over the years.

[rotaxflyer]

Re: Rough Running at Full Power

Ah, yes, the rough and miss are the same problem.

I have a pretty fine prop, 74-43 I think, which explains the higher revs which is nothing new for me.

Thanks for the primer idea I'll check it out - maybe the problem is too rich, not too lean.

Andy

[Guest]

Re: Rough Running at Full Power

It sounds to me like a float drop issue.

One time I adjusted my float drop incorrectly -- pure ignorance on my part. The engine ran up OK, so I taxied into departure position and started down the runway. Just about where I would normally lift off the engine quit. I had no choice but to pull the power and abort the takeoff, when I pulled off the throttle the engine immediately came back to life!

I had the float drop set so that the engine could sustain 1500 RPM but it could not sustain any higher RPM.

The fuel flow with the improperly set float drop could not keep up and it would drain the float bowl at anything above 1500 RPM.

Definitely change the carburetor - they are interchangeable, in fact many A's were converted to B's over the years.

Good point Mark,

one could have great fuel flow and a wrong float setting would still starve the carb.