My engine hesitates -- need ideas

dwilson · #1 05-25-2008, 07:45 PM

I have a carburated O-320 B2C in my 9A. I have nearly 300 hours on it and it works great except that when I advance the throttle it "hesitates" or "stutters" between 1150 and 1200 RPM. If I advance real slow I don't notice it.

Using one mag and one LightSpeed electronic ignition.

We have checked the accelerator pump and the intake manifold and they seem to be fine.

One other symptom: When leaned out at 8000 feet WOT runs great, but if I pull the throttle back engine stumbles noticeably for a second.

Does this sound familiar to anyone?

Thanks,
Duane

n5lp · #2 05-25-2008, 08:10 PM

Quote:

Originally Posted by dwilson

I have a carburated O-320 B2C in my 9A. I have nearly 300 hours on it and it works great except that when I advance the throttle it "hesitates" or "stutters" between 1150 and 1200 RPM...

My O-360 has always done this (9 years). I swapped carburators early on but it did it with the new carb also.

Strange thing is that it actually doesn't do it anymore because I have adapted. I no longer advance the throttle fast enough in the relevant RPM area to get the behavior. I'm sure it is still there if I look for it though. It has never been an actual problem.

osxuser · #3 05-26-2008, 12:17 AM

1 piece venturi carbs started doing this, it's nothing to be alarmed about. Our brand new Precision carb doesn it on the O-360.

asav8tor · #4 05-26-2008, 12:27 AM

http://www.sacskyranch.com/eng33.htm

JCFL · #5 05-26-2008, 01:56 AM

My O235 in my Lancair does this. Lots of work including new mags and rebuilt carb did not help. I've come to think of it as normal.

hngrflyr · #6 05-26-2008, 04:42 PM

Is the accelerator pump working? Sounds like maybe not well enough.

rvaitor87 · #7 05-26-2008, 04:56 PM

My -3 has done it for 20 years. I've re-built the carb 2 times, it still does it.

My engine came out of a Tri-Pacer, and I talked to a RV owner who had the same type engine also from a Pacer, and he has the same problem. Now I just take it as normal operation.

Where did your engine come from?

Mike Bauer
RV-3 N87LB flying
RV-4 N742MC on the gear!!!!

dwilson · #8 05-26-2008, 05:08 PM

Where did the engine come from?

I bought it from Bart as a rebuild of one that came out of a Robinson helicopter. It has been flawless for nearly 300 hours, except for this one little thing.

Yes, I did check the accelerator pump.

Thanks for all the input. I really appreciate it.

Duane

RV6ARoger · #9 05-27-2008, 02:02 PM

A while ago, I posted the same problem. Mine would stumble and miss a few cycles. It would usually do it only on take off at high RPM. After having our ace A & P go through the entire engine, we traced it down to the mechanical fuel pump. Even with the boost pump on, I would see 5 PSI of fuel pressure, so I thought things were getting into the carb. I guess not. Here is what I found when I tore down the engine.

1. The fuel pump was old and came with the engine when I got it. The mechanical arm on it would press about 1/3 to 1/2 way through its stroke before it would actually do any pumping. When the new fuel pump arrived from ASSCO, it would pump at the top of the stroke.
2. When I pulled off the pump, I noticed there was no rod in the back cover to press down the arm. Some idiot who rebuilt the engine at the overhaul, put the rod in upside down-- probably too lazy to remove the rear cover after it was already on, and he just installed it with the ball on the bottom side. Of course this meant pulling the pan, removing all of the accessories, and pulling the back cover. A good time to check everything anyway. All checked out OK and I installed the rod correctly.
3. This led us to surmise that the pump at a 1/2 stroke was blocking the fuel from the electrical boost pump from getting through. Not sure if this is the exact answer, but after buttoning everything back up and retiming the mags, it doesn't do it anymore. Maybe this helps. Might be worth to pull the fuel pump and checking it out. Just a thought. Roger.

apatti · #10 07-03-2011, 09:41 AM

All,
This thread came up when I did a search on a problem I am having with my newly flying 0360-A3A. Since the thread is 3 years old and has some conflicting posts, I thought I would ask for some updated replies.

Just like the original post, my engine hesitates when I add power for the take off roll. I also noticed it once at 3000 ft (D.A probably 5300') when I was adding power. When it happened at altitude I also noticed the fuel pressure down in the 3.2 range (on the ground at idle my fuel pressure is around 6.0). In the air, after I added power (and turned on the electric fuel pump) the pressure slowly (over a minute) built back up to the upper 5.x range.

Some additional (maybe unrelated) info... The engine was built about 3 years ago by Penn Yann Aero and now has about 3 hours of flight time and an accumulated ground run of an additional 1 - 2 hours. I am not using any oil to speak of. But, I am also not convinced I am developing full power. I have a Catto 3 blade 76" pitch prop and only get around 2120 on take off roll. I have had it up to about 2450 rpm at a D.A. around 5300'. I checked the throttle control and it is throwing the carb lever full up against the stop. The aircraft is an RV8A with no wheel pants or gear leg fairings.

Thanks for any comments.