The International Cessna^® 170 Association

This is one of those problems that is hard to pin down. On final approach sometimes when I add a little throttle my engine hesitates (or even drops a hundred RPM or so) before accelerating. It only happens a very rare occasions. This will be a little long winded but I wanted to detal everything that happens.

I set up for landing: mixture rich, carb heat on, throttle to about 1500RPM. Trim for approcah, first notch of flaps. Turn base, airspeed 65-70, deploy flaps as needed. Engine RPM still in the 1500-1600 RPM range. As I turn final, IF I feel that I need to slow my rate of decent, I add a little throttle. This is when it happens!! As I advance the throttle I hear the engine speed decrease for just an instant before accelerating. It's very smooth, no backfiring, no cutting out, it is as if I had reduced throttle rather that advanced throttle. So far it has not been a danger, except to my cardiac heath when it happens LOL.

This is the only situation where it happens. And it only happens once every 10 approaches or so. Not during taxi or takeoff. Otherwise the engine is a dream, C-145-2, about 700hrs smoh, 70's compressions, very smooth, easy to start, Mag checks are good, I get about a 25 RPM rise at shutdown.
If the accelerator pump were bad I think I would have problems on take off. Carb heat is on so I'm not sure Ice is to blame. Maybe the mixture is too rich with heat on.
Has anyone had a similiar expereince?

Patrick Mahaffey
N1410D
Across from Joe (have to keep an eye on him)

Do you get a good RPM drop when you check carb heat on runup? If it only does it occasionally, it might be a little carb ice. If its a small amount of ice, it might be enough to reduce RPM without causing the engine to actually "stumble". Things to check on carb heat would be security of the shroud around the muffler, integrity of the scat hoses and connections, and full travel on the flapper valve.

Just my 2 cents...

Miles

Yes Carb heat check is good. I get a noticable RPM drop on the ground AND when I apply Heat on Downwind. Muffler, shroud, and hoses are in excellent conditon.

Patrick,

That's a stumper. I would take a real close look at the carburetor and induction system. On my last annual, the AI found a loose nut attaching the carb to the sump (or was it heat box to carb?). It hadn't caused any problems, but could have. There's a slim possibility that there's something (I couldn't hazard a guess as to what) that could be just loose enough to only cause a problem occasionally when every thing is up to temp. Try checking before, then just after a flight.

Let us know if you find anything.

Miles

Sounds like the carb's accelerator pump is weak.

N1277D wrote:Sounds like the carb's accelerator pump is weak.

This was my first thought. When was the last time your carb was overhauled?

Could be the engine is loading up a little. When you add power it takes a few seconds to for the engine to burn the extra fuel off.

When Patrick is low he "adds a little throttle". Would the accelerator pump be kicking in for this? I thought it kicked in only when you gave the throttle a pretty good jolt.

Eric

One thing the Continental operator's manual says to look for when diagnosing a hesitation is the idle mixture. Seems to me that a 25 RPM rise on shutdown is a bit on the low side, indicating the idle mixture might me on the lean side of optimum (assuming the main mixture control is full rich just before you're shutting down). I don't recall how sensitive the idle mixture screw is, but you might try backing off on it (turn left/out to richen) a few clicks at a time until idle cutoff gives you about a 50 RPM rise. Make sure the engine is up to normal operating temp when setting idle mixture.

Miles

With limited info concerning the history of the Carb, time since Ovh etc. And with the intermittant nature of the problem.The place I would head would be the little brass bulb on the bottom of the Carb below the accelerator pump shaft. There is a small screen and Check Ball there that can cause problems. Possibly a piece of "dirt" occasionally jams the check ball where the fuel goes back the way it came in instead of out the nozzel and to the engine.
On Carbs that have been flooded, or water contaminated and not cleaned properly, a jell forms and then gives you fits later on. I found out a Piper Pacer can be taxied with the primer if you are a mile from the hanger when it quits. Any way something else to consider. Brian

This may help

http://www.dgsupply.com/trouble.html

The Carb was overhauled at engine overhaul which was about 700 hrs ago (but also 15 years ago too.) Since the hesitation occurs at 1500 RPM the idle system is being overidden by the main metering Jet. It may be just that it is simply loading up at bit. My primer DOES on occasion leak a bit indicated by the stain behind the left exhaust stack. I suppose that may be enough to cause it to hesitate as it is clearing the excess fuel out of the system.
And since all of the other parameters; idle, acceleration on take off, fuel comsumption, smooth engine running, are all good, I am hesitant to start tearing into the carb. But my annual is coming up in a month or so, and I will take a closer look at the induction system as a whole. Thanks for all your input. And that weblink is great too.

Patrick
N1410d
Across from Joe

I've seen this sort of mid-throttle hesitation on a worn out carburetor that leaked air at the throttle shaft. (Pushing on the throttle moves the shaft forward in an "egg-shaped" hole allowing air to leak past the shaft and into the carb, until additional movement actuates the accelerator pump, which makes up for the momentary lean-condition.) The fix was to re-bush/replace the shaft. To check yours, try to "wiggle" the shaft to observe if excessive movement is possible. Do this not by operating the throttle lever, but by grabbing the lever and without rotating it, attempt to lift it and/or move it fore/aft within the carb body casting while observing the clearance at the shaft. This is difficult to see with it installed but can be done with a good light and with the lower cowl removed.
If that doesn't prove to be the case, then I'd inspect the accelerator pump seal and pump bore. They may have a "flat" spot. (Not actually a flat place, but a position where the spray-rate is flat due to a worn/hard seal or place it no longer conforms to the bore.) They can be easily replaced, but most people send them out for overhaul if the shaft is loose. TexAir in Ft Worth and several other suppliers can provide overhaul kits but usually it's better to send it to a shop that has a flow bench.

I'd disconnect the primer lines and cap them (especially if you think you have a leak in the system already), then fly it. the primer could be either leaking extra fuel into the induction (too rich, loads up) or leaking air (induction leak). either could give you a hitch in your getalong.

also look at your carb like George says, worn out shafts/bushings can happen over 15 yrs and that gives you induction leaks too. or the accel pump is dried out, I just rebuilt my carb and was amazed to see how dried out (read not sealing) things had gotten due to not flying enough. then, how dried out are your induction rubber connectors? and on and on until you find it.

trouble shooting is the best part, paying to fix it is the bad part

http://www.cessna120-140.org/forum/viewtopic.php?t=1959

This thread and the other on carb operations seem to discuss a few times on the accelerator pump. Above link will put you to our site regarding this on our Marvel carbs when used on the 120/140's. Interesting stuff on operation and problems FWIW.

Matter of fact, you might want to browse all our tech topics and our tech forum to see if there is anything useful to you!