Engine stumble at full mixture / throttle

SixPapaCharlie

SixPapaCharlie

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Was flying to the avionics shop yesterday (7 miles as the crow flies)
Takeoff was total perfection. I suspect books will be written about it

I executed a perfectly coordinated crosswind and downwind turn.
Just out of my home field, still climbing, full rich, just under redline, my engine does that thing it does when you are leaning and go just past the peak.

It did it for just a half second and resumed normally. was half way so kept on truckin. Just prior to downwind at the destination field, it did it again.

Just a little shudder and then everything is normal.

I am wondering if it could be a mag issue. I am having a heck of a time getting it to start. turns and turns, sometimes will sort of catch and spin fast but not quite fire up. Eventually it starts.

Annual is next week but Wanted to get your take on what might be happening.
 
Not sure about the stumble, but I had my mag timing adjusted a while back and my plane has started much easier ever since.
 
Yes Mags, get them rebuilt at upcoming annual its worth every penny for the piece of mind.
 
Most likely mags,when where they serviced last.
 
SixPapaCharlie said:
...
I am having a heck of a time getting it to start. turns and turns, sometimes will sort of catch and spin fast but not quite fire up. Eventually it starts.
Click to expand...

Yep that likely sounds like mags/ignition related.

But if instead you'd said it always starts quick and easy, I'd say it could've been a bit of carb ice. (TB9 = O-320 engine, correct?) The humidity we've had in northern TX the past couple days is conducive for carb ice to form.

Also does your plane have a pump style primer in it? If so, was it fully in and locked? I once forgot to lock the primer in my old Cherokee and the extra richness fouled the spark plugs in the cylinders the primer lines were connected to, and gave me a similar roughness/stumble episode.
 
A sticking valve will do that, too. Does the engine experience morning sickness?

Dan
 
My first thought was carb ice.

What were conditions like, and how long did you stay on the ground at idle?

When this happened to me, my first thought was a bad mag. So, I gave it a run-up on the ground afterward, and it was fine.

Basically, I had sat in line for takeoff just after marine layer had burned off (so, essentially 100% humidity) on a nice day. Ice melted at 400 AGL and the engine stumbled and came right back to life. Exactly once.

400 AGL is a bad place to have your engine burp.
 
It is the O-320
PITA to start and getting worse

No primer, just pump the throttle before starting.
Was probably Idle for 15 min on the ground.

Called the A&P and told him. He is going to take a look while in annual.
 
15 minutes is enough to build carb ice, if it's humid (even if it's warm).

The starting issues can be a lot of things. The engine needs spark, fuel, air, compression, and sufficient RPM to get all those things moving. I'm sure you can rattle off a dozen things that could screw one or more of those up. Mags definitely qualify, but a bunch of other things do, too. One really, really obvious one is a vacuum leak.

My carbureted truck cranks and cranks if it has been sitting more than a week or so. Why? The fuel evaporates out of the bowl through the vent, and you have to let the engine driven fuel pump fill it up. That also defeats throttle priming, and it behaves quite a lot like you describe.

I'm certain your A&P will get to the bottom of it.
 
Could be a mag, but full rich is a problem also. Lean that sucker out! My bet is your plugs are fouling.
 
MAKG1 said:
15 minutes is enough to build carb ice, if it's humid (even if it's warm).

The starting issues can be a lot of things. The engine needs spark, fuel, air, compression, and sufficient RPM to get all those things moving. I'm sure you can rattle off a dozen things that could screw one or more of those up. Mags definitely qualify, but a bunch of other things do, too. One really, really obvious one is a vacuum leak.

My carbureted truck cranks and cranks if it has been sitting more than a week or so. Why? The fuel evaporates out of the bowl through the vent, and you have to let the engine driven fuel pump fill it up. That also defeats throttle priming, and it behaves quite a lot like you describe.

I'm certain your A&P will get to the bottom of it.
Click to expand...


A vacuum leak makes a constant roughness or power loss.

Carb ice causes a steadily decreasing RPM. It doesn't burp and fix itself.

If the airplane has a gravity feed system, like most high-wing airplanes, the bowl doesn't dry out unless the fuel is shut off, and even then it takes a long time. As soon as the fuel is turned on, it will fill up.

Many engines don't want to start using throttle pumping. There's just not enough fuel being squirted into the airflow, especially if the accelerator pump is sticky. Use the primer. The accelerator pump nozzle also doesn't atomize the fuel worth a hoot; it relies on the high-speed airflow going through the venturi at operating RPMs to do that. It will just splash fuel on the walls of the intake system and the cylinders get nothing but weak fumes, which are insufficient in a cold engine.

One mag failing intermittently is often almost undetectable. The other mag is making spark, after all, so a large RPM drop isn't likely. More often a mag will get weak enough that the weakest spark plug stops firing.

Dan
 
Geico266 said:
Could be a mag, but full rich is a problem also. Lean that sucker out! My bet is your plugs are fouling.
Click to expand...

I lean to taxi and I lean at altitude
I was only flying a few miles

This was from 52f to KDTO so out of one pattern into another
Rich the whole time in flight
 
MAKG1 said:
15 minutes is enough to build carb ice, if it's humid (even if it's warm).

The starting issues can be a lot of things. The engine needs spark, fuel, air, compression, and sufficient RPM to get all those things moving. I'm sure you can rattle off a dozen things that could screw one or more of those up. Mags definitely qualify, but a bunch of other things do, too. One really, really obvious one is a vacuum leak.

My carbureted truck cranks and cranks if it has been sitting more than a week or so. Why? The fuel evaporates out of the bowl through the vent, and you have to let the engine driven fuel pump fill it up. That also defeats throttle priming, and it behaves quite a lot like you describe.

I'm certain your A&P will get to the bottom of it.
Click to expand...

I would lay bucks on it your truck has a Q-jet carb on it. It is not loosing the fuel from the carb from evaporation. Those carbs have a problem with this and there is a fix.
The fix for this carb cost about 5 cents. But finding the person whom knows how to do this fix....priceless....

Tony
 
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1600vw said:
I would lay bucks on it your truck has a Q-jet carb on it. It is not loosing the fuel from the carb from evaporation. Those carbs have a problem with this and there is a fix.
The fix for this carb cost about 5 cents. But finding the person whom knows how to do this fix....priceless....

Tony
Click to expand...

It's a Rochester but not a Quad.

I've had this carb apart several times. What's the fix you're referring to, and what is the mechanism if it isn't evaporation?
 
Called the A&P and told him. He is going to take a look while in annual.[/QUOTE]

Bad choice, let him send them in at the start of the annual and they will be back when he's complete you wont be waiting on them to arrive.
 
Couple years ago I had a similar problem. Engine actually quit on takeoff roll out of SAV! Got rid of my electronic ignition (right side) and the other mag. Got two brand new mags from Champion. Ran great for a couple weeks and then it became hard to start again. Got an SB from Champion saying they sent out a bad batch of mags! Sent both in for overhaul (free of course) and now she runs like a champ.
 
MAKG1 said:
It's a Rochester but not a Quad.

I've had this carb apart several times. What's the fix you're referring to, and what is the mechanism if it isn't evaporation?
Click to expand...

I knew you where going to tell me this. All Rochester Carbs will do this after time and there is a fix. You have had this apart so I will tell you the fix.
First I love this carb. They are great carbs if you know how to handle them. When you get your carb off, take the bottom plate off. Take the center section and turn it upside down. You are going to see two silver looking things. These are there to plug the drilled passage in the bottom of the float bowl.
Take some J-B weld and mix it up. next coat this silver portion you see or cover these plugs with the J-B weld. and let it dry over night. Now you will need to get a file and smooth this J-B weld so the bottom of the carb can be put back on. Make sure it goes together flat with out force and do not uncover those plugs. If you see silver you went to far and will need to re-apply the J-B weld.
These plugs start leaking and slowly drip. if you do not start the engine for a week or two your float bowl becomes dry and you have to crank and crank as the stock fuel pump squirts a little fuel into the bowl and each revolution. Its hard on the starter and a lot of other things. Fix the carb.

Tony
 
SixPapaCharlie said:
I lean to taxi and I lean at altitude
I was only flying a few miles

This was from 52f to KDTO so out of one pattern into another
Rich the whole time in flight
Click to expand...

The engine doesn't care. Too much fuel, it too much fuel. Lean and soon as you are at pattern altitude and when landing. Landing full rich is old school, wasteful, and hard on your engine.

http://www.eaavideo.org/video.aspx?v=2274677932001
 
1600vw said:
I knew you where going to tell me this. All Rochester Carbs will do this after time and there is a fix. You have had this apart so I will tell you the fix.
First I love this carb. They are great carbs if you know how to handle them. When you get your carb off, take the bottom plate off. Take the center section and turn it upside down. You are going to see two silver looking things. These are there to plug the drilled passage in the bottom of the float bowl.
Take some J-B weld and mix it up. next coat this silver portion you see or cover these plugs with the J-B weld. and let it dry over night. Now you will need to get a file and smooth this J-B weld so the bottom of the carb can be put back on. Make sure it goes together flat with out force and do not uncover those plugs. If you see silver you went to far and will need to re-apply the J-B weld.
These plugs start leaking and slowly drip. if you do not start the engine for a week or two your float bowl becomes dry and you have to crank and crank as the stock fuel pump squirts a little fuel into the bowl and each revolution. Its hard on the starter and a lot of other things. Fix the carb.

Tony
Click to expand...

Bottom plate? You mean the throttle body? This is NOT a Quadrajet. There is only one jet and it's more or less right next to and below the bowl.

I don't recall there being any plugs below the float bowl, but I'll look next time I have it apart. There is only one hole in the bottom, and it's for the needle and seat. Of course, some of the other passages do have plugs and I guess a pressure test might be a good idea next time it's apart.

It makes me quite nervous putting epoxy on that side of the fuel filter. I'd rather just replace the plugs. Or maybe SOLDER a seal.
 
GeorgeC said:
I thought the point of landing full rich was in case you needed to go around, it would be one less thing to remember to do?
Click to expand...

That's rote thinking. Some engines will load up and try to quit on you if you do that and glide too far, especialy if it's carbed and the carb heat is on. A hot/high day is bad, too. You want that engine to respond instantly if you need to overshoot, not belch and stutter and fool around. One can always go richer after the throttle is opened up.

Dan
 
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