Lycoming stumbles on acceleration

newtech · #1 06-23-2009, 09:00 PM

My new rebuilt O-360-A1A has a slight stumble when accelerating through about 1100 rpm. Slightly leaning the mixture seems to minimize or eliminate the stumble. The carb has the single piece venturi which has somewhat of a reputation for stumble on acceleration. We will be pulling the carb apart tomorrow to perform the float SB. We will set the float level then but is there anything else we should be looking at? It is a parallel valve 180 hp Lycoming with a 3 blade Catto. I am wondering if the combination of the one piece venturi and the 18 pound prop are the reason. We are ready to put this puppy in the air as soon as we convince ourselves we don't have a problem here.

Geico266 · #2 06-23-2009, 09:04 PM

Does the carb have an accelerator pump on it? The pump can cause a stumble until the excess fuel is cleared.

IMHO if you can eliminate the stumble by leaning it out alittle this is normal. Hot weather can cause this also.

MNForrest · #3 06-23-2009, 09:10 PM

Idle mixture might be too rich, assuming the issue crosses over to my automotive experience. Somewhere in this forum there is a discussion on the subject, I believe.

newtech · #4 06-23-2009, 09:17 PM

Quote:

Originally Posted by Geico266

Does the carb have an accelerator pump on it? The pump can cause a stumble until the excess fuel is cleared.

IMHO if you can eliminate the stumble by leaning it out a little this is normal. Hot weather can cause this also.

It is in the the 90s here. It does have an accelerator pump and it was verified to be working, i.e. fuel in the face when advancing the throttle while looking up into the venturi. Interesting thought about the possibly of too much fuel being squirted.

N661DJ · #5 06-24-2009, 05:07 AM

Steve,
I had a similar problem with my 0-360 Lyc. Installed fine wire plugs, problem fixed.
Dick

carguy614 · #6 06-24-2009, 05:18 AM

My 320 stumbles a tad thru 11 or 1200 RPM. Usually gone when engine gets good and hot. Although I get it on the runup from time to time, I never notice it in flight, so I have not addressed it yet.
Watching post with interest.

Regards,
Chris

MarinePilot · #7 06-24-2009, 06:14 AM

I would check for an intake leak, since you just received the engine from the factory call the tech reps at Lycoming they can help you troubleshoot over the phone or be e-mail.

Geico266 · #8 06-24-2009, 06:52 AM

Quote:

Originally Posted by newtech

It is in the the 90s here.

There is your answer. Try leaning her out a little more on the ground, and on take off. You can't lean it enough on the ground taxing. Try different settings for take off, but watch the EGT's & CHT's closely on take off, but the "situation" (lets call it a situation rather than a problem) is more than likely the accelerator pump. Be glad you have one though! Makes starts MUCH easier.

JMHO. I'm certainly no engine expert, I just fly alot with several different engine set ups. I would not worry about the stumble in these temps with a accelerator pump dumping fuel into the carb.

Maybe someone else can chime in for guidance also.

mahlon_r · #9 06-24-2009, 08:18 AM

It is very common to have for the carb to have an Idle to power transitional flat spot( hesitation) as you come out of idle. This normally occurs between 1000 and 1400RPM. This is when the carb idle tube transitions from fuel delivery at idle, to being the air bleed for the main discharge nozzle at power. Almost all engines with carbs have a slight hesitation somewhere in this operation range, if you really look for it. The severity of the hesitation can vary but in most cases it is negligible and not super noticeable. You can try to readjust the idle mixture a bit leaner or richer and you might lessen the intensity but it is unlikely you will make it go away entirely. If it is really bad, a different carb might be less severe but it will still likely be there.
Hope this helps.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

jonbakerok · #10 06-24-2009, 04:33 PM

I had the same issue years ago with the O-320 in my old Cherokee 140. It finally got bad enough that I took it to the local A&P. He thought it was a carb issue, so he sold me a rebuild and in the process complied with the AD concerning replacement of the two-piece venturi with the one-piece. Afterwards, it was MUCH worse, and he re-installed the old venturi, which fixed it -- so that it was only as bad as when I first brought it to him.

So I wound up the same as I started (only $300 poorer). I decided it was just something I had to live with.

About a month later, I was pulling through the prop on my preflight (as I always do), and discovered a dead cylinder. I called the A&P, who promptly found a stuck exhaust valve. Luckily, I hadn't started the engine and ruined the valve. He reamed the guide, told me to either mixing in some mogas or use TCP fuel additive, and I was back in business (another hundred dollars poorer).

But the funny thing was, the hesitation was gone and never came back.

I'm not saying you've got a stuck valve, but one thing I've learned about Lycomings is that when something changes, pay attention.