Acceleration Hesitation

lr172 · #1 09-05-2015, 06:34 PM

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

mosquito · #2 09-05-2015, 06:47 PM

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

-jon

Dbro172 · #3 09-05-2015, 06:49 PM

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.

lr172 · #4 09-05-2015, 08:39 PM

Quote:

Originally Posted by mosquito

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

-jon

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

lr172 · #5 09-05-2015, 08:42 PM

Quote:

Originally Posted by Dbro172

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.

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

PCHunt · #6 09-05-2015, 08:47 PM

Quote:

Originally Posted by lr172

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

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.

petersb · #7 09-05-2015, 09:33 PM

Quote:

Originally Posted by PCHunt

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.

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

Bugsy · #8 09-05-2015, 10:02 PM

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.

lr172 · #9 09-05-2015, 10:12 PM

Quote:

Originally Posted by petersb

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

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

PCHunt · #10 09-05-2015, 10:24 PM

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.