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Help with engine stutter on Final?

Dbro172

Dbro172

Well Known Member
I have a predictable "pause" or "stutter" in RPM when applying power on base or final. I usually cut power back to idle on downwind to slowdown and add flaps. Then I usually find myself adding a touch of power back in on either base or final. When I add the power back in after being at idle, I always get very slight stutter or pause before the RPM catches up, maybe a half a second. Scared me at first but is now predictable. Looking for a smoother transition, especially this phase of flight. I've seen this in spam cans.

My thoughts are either too rich or lean idle? But don't know which way to go. Anyone familiar with this?
 
I get the same behavior in my carbureted O-320. I guess I've learned to expect it and minimize it by moving the throttle quickly through the region. It also happens when I first put in power for take off. I don't know why it happens though.
 
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Boy, this dredged up a distant memory from my Grumman days - seems that we all had a it of catch with our carbed O-320's, just like you describe. I seem to remember that it was accepted as a "feature" of the power plant.

I haven't noticed this on our IO-320 in the -3.
 
Sounds like the normal transition region within the carb's idle jet-to-power jet circuit. Make sure your idle mixture is set properly -- that may help ease the transition.
 
Acceleration Stutter

Could this be a carb accelerator pump problem? This sounds just like the C152 when it first came out, that carb did not have the accelerator pump and normal engine behavior was a pause when adding power from low settings.


Don Broussard

RV 9 Rebuild in Progress
 
Never had the problem, but I agree with cajunwings. It sounds like a faulty accelerator pump.
 
idle circuit check

Derek:

I don't recall - do you run a carburetor on your 320?

If so, concur w/ David's suggestion to confirm idle rpm & richness, i.e the idle circuit mixture. I'm embarassed to admit how long I had the same issue - a stumble at low rpm- before I adjusted mine, & removed the stumble.

In my Lycoming O-320 Operator's manual, the procedure is at Section 5, para 2.e.

Ron
 
Once upon a time, my carb was slightly loose on its mount to the sump. The tell-tale sign was a slight stumble when transitioning from idle to a higher throttle setting. While investigating, I noticed the carb and airbox had a slight wiggle when I touched them. I tightened he nuts holding the carb to the sump and the problem went away.
 
Check what Kyle said

I too had the carb come lose because my snorkle rubber seal was too rigid. It caused a similar low RPM stumble, and upon uncowling I found the carb losened from the engine torquing within the cowl and the airbox not moving freely.
 
A suggestion: I know all landing checklists say mixture rich, but we have found that keeping the mixture leaned avoids the stutter when adding power on final. We lean at all times when below 20 inches mp. Keeps the plugs clean and saves money. Just have to remember to push all knobs forward if need to go around.

LeRoy Johnston RV-6A Esperanza 860 hrs. (O-360 A1A/CS)
 
Mine did this until I read something that suggested the idle mixture was too lean. Enrich the idle mixture a quarter turn and go fly. If that doesn't cure it, go another quarter turn. Mine is cured now. Smooth all the way and a nice 100 rpm or so rise at cutoff.
 
DHeal said:
Sounds like the normal transition region within the carb's idle jet-to-power jet circuit. Make sure your idle mixture is set properly -- that may help ease the transition.
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I would look to be sure the idle mixture is correct. This can cause a stumble when coming off idle and is more likely than the accel pump. This is a very common symptom of improper idle mixture. Do you get the stumble when going from idle to half throttle or ful throttle? If not, your acell pump is not likely the problem.

Larry
 
Idle mixture

lr172 said:
I would look to be sure the idle mixture is correct. This can cause a stumble when coming off idle and is more likely than the accel pump. This is a very common symptom of improper idle mixture. Do you get the stumble when going from idle to half throttle or ful throttle? If not, your acell pump is not likely the problem.

Larry
Click to expand...

That's what I was hoping. I'll reset idle mixture and rpm and give it a whirl. Only 16 hrs into phase one so still have a couple bugs. No stumble coming off idle during takeoff and the accelerator pump will load up the air box with fuel no problem.
 
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