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Acceleration Hesitation

L

lr172

Well Known Member
I am in Phase I with my 6A / O-320 (160 HP) (FP prop). I have my idle set around 700 RPM and I think that I am still a bit lean on the Idle Mixture (no rise on ICO shutdown). I notice that during a smooth increase from idle to full power (i.e. take-off) I get a hesitation/stumble in the early part of the throttle application. I assumed this was more of an acceleator pump issue than an Idle mixture issue. During the rebuild of my MA-4SPA carb, there were three possible enrichment settings on the accel pump linkage. I put it back where I found, which I believe was the middle setting.

Any suggestions here? Do you think this is too much or too little accel enrichment? or could it be related to the idle mixture?

Thanks,

Larry
 
Plug Fouling?

Do you lean during taxi? If not, or you have very long taxi, could be some plug fouling?

-jon
 
Read the instructions on how to adjust the idle mixture /speed (they go together) and do it, it's easy. 700 is too fast, making it more difficult to slow down in the air. I'd get er down to about 550-600, static idle on the ground and 50-100 rise on the RPM when pulling the mixture for idle cut off.

I found the rpm rise was barely detectable compared to Cessna I flew, but could see it on the G3X.
 
mosquito said:
Do you lean during taxi? If not, or you have very long taxi, could be some plug fouling?

-jon
Click to expand...

I don't think it is fouling. I do lean aggressively during taxi and I do get this problem doing touch and go's as well.

Larry
 
Dbro172 said:
Read the instructions on how to adjust the idle mixture /speed (they go together) and do it, it's easy. 700 is too fast, making it more difficult to slow down in the air. I'd get er down to about 550-600, static idle on the ground and 50-100 rise on the RPM when pulling the mixture for idle cut off.

I found the rpm rise was barely detectable compared to Cessna I flew, but could see it on the G3X.
Click to expand...

I know this needs to be done and I will make it richer the next time I have the cowl off. However, it wouldn't seem that the idle mixture could be causing my stumble. I have not been having problems slowing down. I don't believe that I can run 500 RPM. My prop is too light. I would need more rotating mass to support that idle speed.

Larry
 
lr172 said:
I know this needs to be done and I will make it richer the next time I have the cowl off. However, it wouldn't seem that the idle mixture could be causing my stumble. I have not been having problems slowing down. I don't believe that I can run 500 RPM. My prop is too light. I would need more rotating mass to support that idle speed.

Larry
Click to expand...

The idea is that you set your idle speed to 500-ish, which is too low. But then you don't actually close the throttle all the way on the ground. But when you are on final, and in the flare, you can close the throttle all the way, and you get less residual thrust from the prop. The engine won't even slow down to 500 RPM because of your airspeed.

It really helps in reducing the landing distance, and you might even need less braking.
 
PCHunt said:
The idea is that you set your idle speed to 500-ish, which is too low. But then you don't actually close the throttle all the way on the ground. But when you are on final, and in the flare, you can close the throttle all the way, and you get less residual thrust from the prop. The engine won't even slow down to 500 RPM because of your airspeed.

It really helps in reducing the landing distance, and you might even need less braking.
Click to expand...

He is right about the rpm setting, however it has nothing to do with your stumble. I don't believe idle mixture is the answer either. I have the same exact problem with my o360, tried idle mixture, had no effect, learned to live with it
 
New carb

I had th same problem with my O-360. I changed out the carb to the new model and the problem fixed and the he plane launched down the runway. Do a search on O-360 stumble and you'll find postings on carb upgrades.
 
petersb said:
He is right about the rpm setting, however it has nothing to do with your stumble. I don't believe idle mixture is the answer either. I have the same exact problem with my o360, tried idle mixture, had no effect, learned to live with it
Click to expand...

I am pretty sure it is the amount of fuel added by the acell. pump. I just am not sure whether it is too much or too little. Thought someone here may have figured it out. At some point I will change the linkage to the other settings to find the optimum. I am guessing that it is too little fuel.

Larry
 
A while back there was an AD note on the venturi. The old ones were two-piece venturis, and a few of them failed and the parts went into the engine and caused damage.

A new one-piece venturi came out, and there were lots of reports of "stumbling" on acceleration after that.

As stated, most just "get used to it".

I'm assuming that the stumble is just a brief one, nothing prolonged.......?

And that might not be the cause in your case, either.
 
It is a brief stumble and it is not limiting. It happens at a critical phase of flight and therefore has been raising my blood pressure (10 hours into Phase I and haven't flown in that last 10 years (got lots of training recently).

I am familiar with the issue from work on auto engines. The accelerator pump discharge is necessary to let the carb aerodynamics to catch up in transition. I have done this tuning a few times on Holly 850's (different arms and jets give an endless set of options for fine tuning, as each egine is a bit different in what they need). I just hasn't happened enough times for me to get a bead on whether it is a lean or rich stumble. It feels a bit like a rich stumble, but these engines seem to handle filthy rich conditions without too much fanfare.

My auto tuning background doesn't help with complying with Aviation standards. :) By my second flight, I was pulling the mixture out at least 1/2" I just wasn't comfortable with how poorly the engine ran on T/O at full rich. It runs much better leaned a bit from full rich. I have no overheating problems doing this, even with the 90 days we have been having.

Larry
 
Last edited:
petersb said:
He is right about the rpm setting, however it has nothing to do with your stumble. I don't believe idle mixture is the answer either. I have the same exact problem with my o360, tried idle mixture, had no effect, learned to live with it
Click to expand...

I too had the same thing with my right-out-of-the box Lycoming O-360 and like you lived with it.
 
Lean Idle Mixtur?

lr172 said:
I am in Phase I with my 6A / O-320 (160 HP) (FP prop). I have my idle set around 700 RPM and I think that I am still a bit lean on the Idle Mixture (no rise on ICO shutdown). I notice that during a smooth increase from idle to full power (i.e. take-off) I get a hesitation/stumble in the early part of the throttle application. I assumed this was more of an acceleator pump issue than an Idle mixture issue. During the rebuild of my MA-4SPA carb, there were three possible enrichment settings on the accel pump linkage. I put it back where I found, which I believe was the middle setting.

Any suggestions here? Do you think this is too much or too little accel enrichment? or could it be related to the idle mixture?

Thanks,

Larry
Click to expand...

Probably apples to oranges, but on my injected IO-360 when I had the idle mixture set too lean I had a very noticeable stumble when advancing the throttle after extended idle, such as on a touch and go. Richening the mixture adjustment to achieve the "50 RPM rise" completely cured the problem. I normally lean during ground ops and all other engine behavior was perfectly normal when the idle setting was too lean.

Skylor
RV-8
 
skylor said:
Probably apples to oranges, but on my injected IO-360 when I had the idle mixture set too lean I had a very noticeable stumble when advancing the throttle after extended idle, such as on a touch and go. Richening the mixture adjustment to achieve the "50 RPM rise" completely cured the problem. I normally lean during ground ops and all other engine behavior was perfectly normal when the idle setting was too lean.

Skylor
RV-8
Click to expand...

Thanks for posting your experience. I have had so many little things to do that I keep forgetting to bump out the mixture needle when I have the cowl off. I will make this a priority and see if it helps. I did some research last night and this hesitation seems to be relatively common on carb'ed Lycomings especially with high ambient temps. That helps to confirm my suspicion that it is too much squirt. If I remember, the different settings on the linkage had different taper levels, meaning less pump travel in the early opening phase. I may try this some day.

Larry
 
It sounds like this may be happening at the point where the carb is transitioning from the idle circuit. Other threads have dealt with the issue and the way people have resolved it. It's not exactly normal and isn't supposed to be happening, but like some have indicated, they just live with it and the airplane doesn't come out of the sky.

On the issue of low RPMs, before getting too low, you might want to check your mags to see if there is a minimum RPM to prevent the impulse coupling from engaging. (Could stress the coupling and throw off the timing.)

Dan
 
Carb

The stumble is standard with Marvel carbs. It may vary from carb to carb and you might find a carb that does not do it at all. My 0 320 with 10-1 pistons does not stumble at all.
The stumble occurs when the carb transitions from the idle fuel distribution to the higher power distribution.
If you want a real education on this and about any piston engine question you can think of, go to the Lycoming yahoo forum and search for posts by Mahlon Russell. Many of his posts are pre 2012.
With the one piece venture polishing the rough spots off the venture MAY eliminate or reduce the stumble. I have not personally tried this.
The carb shop at Hagerstown MD is good at "tweaking" the venture.
 
Stumble

Hi Larry,
I have the EXACT issue with the ma4-spa carb on my o-320 engine with FP wood prop. (Non RV plane, though.). Have you tried repositioning to a new hole on the mixture lever? Have you resolved the issue?
--Jose
 
The last mixture enrichment (1/4 turn or so) eliminated the stumble completely. I still was not getting any RPM increase on ICO shut down, so went out another 1/4 turn or so. I am now at least 3.5 turns out on the mixture screw and still don't get any RPM drop. I am not sure I want to be any richer. It idles very nicely now and the stumble is history.

Curious if any one knows how many turns out their mixture screw is? I added a washer, as the spring was getting loose. Not sure how far out I want to take this screw, wondering if I am just not seeing the RPM increase. I really don't want to make it too rich and start having fouling issues or stumbles due to over-rich.

Larry
 
Radioflyer said:
Hi Larry,
I have the EXACT issue with the ma4-spa carb on my o-320 engine with FP wood prop. (Non RV plane, though.). Have you tried repositioning to a new hole on the mixture lever? Have you resolved the issue?
--Jose
Click to expand...

I am pretty sure the mixture lever has no impact on this condition. I had considered moving the accelerator pump linkage to a different hole, but glad that I waited until further mixture testing. I have had this issue on many engines and once you are close, a 1/4 turn or less can make a big difference, even eliminate a stumble. This took me a while, as I was unwilling to change more than 1/4 turn at a time and had no method to change while running.

Larry
 
Carb

The stumble occurs at the transition form the idle fuel circuit to the higher power fuel circuit. My 0 320 with one piece venture does not do this. Just a Marvel quirk that some carbs have and other identical carbs do not. The issues with the one piece venture can allegedly be resolved by polishing irregularities out in the bore of the venture.
You can remove the carb that has the stumble and replace wiht with an identical carb, same venture, same pn etc and the stumble might go away. Or it might take four or five carbs before you find one that does not stumble. One possible source for resolving this is the carb shop at Hagerstown MD if it is still there. Owned and operated by a lady. She had the business for sale a few years ago so it might be gone.
 
Carb

Aircraft Fuel Specialists Ltd
TerriLee Bell
301-797-8106
Be careful backing the idle mixture out to far. They have been known to unscrew completely. Adding a washer is not a good idea . Longer spring would be only reasonable fix. I don't know how many turns is too much. Find an expert who knows.
 
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