Stumbling at low cruise rpm

[Griphos]

Hello all,

I've got a problem that puzzles me. The Chief has always run well. It's got the A-65 upgraded to 75hp. Recently we changed the glass and seals for the fuel gauge (Model A stuff). When we did that, we discovered that the fuel gauge had been left loose to serve as a vent. (See Jack's post on his exciting engine out landing adventure!) We now have a venting gas cap on it, but after the engine out that was caused by tightening the fuel gauge when replacing it, the engine has run rougher. Initially, it seemed to run rough just off of idle. It was fine at idle, and fine once you got up in the mid-teens, but right around 900 rpm, it sputtered a bit.

So, we took off the Stromberg and had it cleaned and ordered a new steel needle and seat and put it back on. It needed cleaning. It was dirty inside. I don't know when it was last cleaned. Now, it runs fine at idle but starts running rough around 1700 rpm. It runs great at full throttle, but pull it back to 2100 and it'll sputter and cough. Pull on carb heat, and it smooths back up.

So, this makes me think it's running a bit lean, since pulling carb heat helps it and as soon as I push carb heat off in that 1800-2100 range, it roughens up again (so I don't think it's carb ice because it happens immediately). Carb heat enriches the mixture, of course. But why is it only happening in this narrow rpm range? I flew it an hour yesterday and cruising at 2200 was great. Pulling throttle back with carb heat on was fine. But pulling throttle back to 2100 or 1900 would mostly make it run rough, sometimes coughing significantly. I flew 4 patterns to a full landing and it did fine all the way through, but of course, she's running pretty much full throttle during climb out and downwind, and then 1300 or less on base and final with carb heat on.

I'm pretty much stumped as to what could be causing this. Any ideas?

[rkittine]

You and Dan (see his post on Mag Leads) should talk as he was having the same problem and sent his carburator off to be cleaned and inspected.

[rkittine]

See Dan's Post - trouble with advancing throttle a week or so ago and all the responses to him.

[Griphos]

Hi Robert. Yeah, I've read that thread. That's my plane Jack is talking about in the next to last thread. But Dan seems to be thinking he'll send his carb off to be cleaned/rebuilt. I already did that.

But reading the thread again made me think of something. When we put the carb back on, I noticed that the gasket to the airbox wasn't a tight seal. I don't know that there was a tight seal before I took the carb off, because I just didn't notice. I don't think this could be the problem, though, since the carb is open to the airbox anyway right there. How could it hurt to have just a little more air leaking in around the seal to it? However, when the carb heat is pulled on, it basically shuts off the airbox inlet, and perhaps this reduction in airflow is part of what makes it run a bit richer. In other words, it doesn't smooth out because of the heated air (although that also enriches the mixture), but because of a lower airflow. Does this make sense? Is there a way to just seal around the connection of the carb to the airbox? Perhaps with silicone sealant?

The other bolts and connections are tight. I'm sure of that.

[Paul Agaliotis]

Phil,
If you're sure you got the carburetor cleaned up good, take a look at the intake elbow gaskets. Over time they tend to shrink up a little and you start to get a little intake leak. This causes pretty big fits for the engine, rough running and back-firing at the RPM mid ranges. I don't know why the engine problem would change with RPM differences. Did you reset the idle mixture?
Paul

[Griphos]

Hey Paul,

Yeah, that's puzzling me as well about the rpm range. I get backfiring at idle when on final, of course, since that's when the engine tends to run a little rich. I did adjust the idle mixture 1/2 turn richer just before this last flight (but after the roughness had already moved up the rpm range after cleaning the carb) to see if that would help. It didn't. It's set pretty lean to begin with, about 1 1/2 turns from full lean. That was what it was set at when I got it. I'm going to set it back though, as it backfired even more on final after the 1/2 turn.

I'm pretty confident about the carb being cleaned well. Whether it's adjusted fully correctly is a different question, right. I don't know where the intake elbow gaskets are. But when I figure it out, I'll check them.

[Paul Agaliotis]

Phil,
The backfire on the glide is generally a bad exhaust gasket. For the idle mixture I rapidly push the throttle forward and the engine should respond. If it backfires or hesitates I richen it up until I'm satisfied. Then I run it at 2000 RPM more or less and rapidly pull the throttle to idle. It should respond with no backfiring to a stable idle. If it under shoots idle, stumbles, and recovers lean it some. Now this is assuming the throttle valve is set correctly. Some carbs have the throttle valve too far closed and lean the idle mixture to compensate, and vise-versa.
Paul

[rkittine]

Hi Phil,

Yes, Dan sent out his Carburetor, so it will be interesting to see if he has the same issue after the cleaning and inspection and re-install as you apparently did.

Look forward to hearing what the outcome of your problem and Dan's is.

Ciao - Robert

[Griphos]

Update: I flew her again today. Before flying I adjusted the idle mixture screw a little at a time until she ran well on the ground, both advancing and retarding throttle rapidly. That required a full turn rich, and she's now at 3 turns from full lean.

She ran better in the air, but still has a problem in a 200 rpm range between 1950 rpm and 2150 rpm. At 2000, she'll cough and sputter. Pulling on carb heat helped at first, when the engine was cooler still (ambient was upper 50s/lower 60s), but less so as she warmed up. In fact, in the first ten minutes or so, when I pulled carb heat, rpms actually climbed 50 or so and she smoothed out (in that rough band). I could advance the throttle smartly without hesitation, and retard it quickly without backfiring. In fact, backfiring on glide was almost eliminated. Flew the pattern great. I could fly around with carb heat off at 1700-1900 and just purr along, but push it a little more and she ran rough. Push it past 2200 and she settled down and ran like a top.

Any ideas appreciated. I'm deeply puzzled. I'm going to advance the idle set screw one half turn more the next time I fly to see if it makes any difference.

[Paul Agaliotis]

Phil,
It sounds like something in the carb is plugged. Idle mixture won't help this situation.
Paul

[Griphos]

Hey. I think you may be right. Adjusting the idle mixture did help (at least it narrowed the range of roughness), but that's probably just because it wasn't set well in the first place. The remaining roughness may be something blocked in the carb. It was thoroughly and well cleaned, but we may have to take it off and look at it again.

[Griphos]

Update: Well, it was intake leaks. Intake tube rubbers were hard and cracking. Going to go fly it today and test it out.

[rkittine]

Hey Danny, Did Bill send your carb back yet? Hope everything is fixed. - Ciao