Engine Burps sometimes, Gets your attention

[RV6ARoger]

Maybe one of you guys has some insight as to what this might be. Flying back from Oshkosh, I took off and the engine burped or stumbled for a couple of cycles. It then was OK, but it did it for about a second on the flight home about 4 times. Now, when I take off, it will stumble for a couple of rotations then clean itself up. Here is what has been checked already. (O-360 A1D, Hartzel CS, Carb.)
1. Checked fuel for water. All new fuel in tanks and system Checked flow.
2. Carb was rebuilt by a reputable rebuilder about 50 hrs before this happened.
3. Installed a P-Mag on the left side and had about 3 hours on it before this happened for the first time. Got with the P-Mag people went through their checks with them.
4. Had the right mag rebuilt. It needed that anyway.
5. Checked all plug wires for continuity and had them megered.
6. Checked all fittings, wires, mag switch, etc. etc.
7. Run up is normal not much drop on either system.
8. Tied plane down and did a full RPM run-up. It won't do it tied down, but shoot down the runway, and will scare you to death either on the ground, or shortly after breaking ground it will stumble. Doesn't do it much in cruise, but will burp every 5 minutes or so for a couple of prop revs.

Things I am considering and I am having a good A & P look at it.
1. Could I have a sticking valve? Do you guys think that might cause the condition?
2. Any other ideas of what to check? I am about out of ideas of things to look at. Any ideas, no matter how crazy will be seriously considered. That is how desperate I am.

[az_gila]

Originally Posted by RV6ARoger

.....

Things I am considering and I am having a good A & P look at it.
1. Could I have a sticking valve? Do you guys think that might cause the condition?
2. Any other ideas of what to check? I am about out of ideas of things to look at. Any ideas, no matter how crazy will be seriously considered. That is how desperate I am.

Perform the Lycoming SB-388 Valve wobble test.

http://www.lycoming.com/support/publ...dfs/SB388C.pdf

gil A

[N395V]

Even though you checked for water this is my most likely suspect. I had a similar problem for a long time and apparently my sumps were not at the lowest poin when on an un even floor. It is humid here and when I would let the plane sit for a couple of days with half tans I would get condensate that didn't completrely drain when on an un even surface.

Sump your tanks on a level surface and fill them then sump again. Also get your fbo to fill a small glass jar before they pump any other fuel one morning.

2nd thing to look at is your fuel pump affected by a not to distant past AD.

3rd if you have an AFP FI system check the small screen at the fuel inlet to the throttle body.

Good luck.

[Geico266]

Sounds like an ignition problem to me. Most of your recent work was on the mags, suspect that area first.

Does the Pmag advance timing over the Mag? If it does then, are your plugs aircraft or auto plugs?...Have you cleaned them and pressure checked them? Try that 1st. If it makes any improvement you are on the right track. It's cheap, easy, and you probabbly need to do it anyway.

A new / rebuilt "hotter" mag may now be producing a hotter spark that is shorting somewhere in / on a bad / weak plug. Just a guess.

Don't get frustrated, thats when mistakes are made. If you can't figure it out time to call in an expert and let them fix it. Good luck! Be careful!

[scard]

I would be interested in hearing a better description of "burp". I spoke a bunch about a repeatable momentary drop in rpm that I had in past threads that was tracked down to ignition (p-mags).

[pierre smith]

..on My buddy's -4..and yeah, it'll make you sit up! I happened to be flying it. We checked for water and there were a few drops on each side. We also pulled the carburetor drain plug and found a drop or two...they probably plugged the jet until they either passed or moved.

Regards,

[tomagin]

Once I had a mag problem that was a bit harder to detect. Bad condenser. It would work fine while cold/cool and the run up would be perfect. However once the engine became hotter it would fail and fail the mag. When it cooled down the connection would again be made and the engine would run fine.

However, I join some of the others in suspecting water. Running down the runway also moves any drops of water that may be in the tank and might move them to the outlet casing a "burp" at the engine.

I'm assuming "burp" means a definite momentary power loss and "shuddering" engine which goes away within 5 seconds but as you say catches your attention.

Your carb was also worked on recently. A float problem could give similar symptoms but in my experience it has been more on power reduction rather than going to full power.

Whatever it is --- don't press your luck too far as it might be something that results in a more serious situation one day. I had some "burping" in my Maule which I thought might be carb ice only to have the engine fail one day completely abeam the runway --- carb float stuck.

Good luck!

[flyeyes]

Originally Posted by RV6ARoger

Now, when I take off, it will stumble for a couple of rotations then clean itself up. (snip)

It won't do it tied down, but shoot down the runway, and will scare you to death either on the ground, or shortly after breaking ground it will stumble.

(snip)
2. Any other ideas of what to check? I am about out of ideas of things to look at. Any ideas, no matter how crazy will be seriously considered. .

Crazy I can do ;-)

How is your induction set up? Have you looked for induction leaks?.

If it does it consistently, you can test various changes, as long as you have a long enough runway to comfortably abort. Try taking off with just the mag or just the Pmag (obviously with enough remaining runway for a comfortable landing). Try removing your airbox or filter. Try a really slow acceleration.

In cruise, try running for 10 minutes at a time on just one side of the ignition to see if it can be isolated.

Good luck.

Have you checked the resistance of the plug wires for the Pmag? They're cheap enough it might be worth making up an extra set and throwing in a set of new plugs (The good ones are almost $2 apiece though...).

[zilik]

It's one of the two. I would re-install the mag you ditched for the p-mag and see if it clears up. Next I would check the carb.

[mahlon_r]

What P/N carburetor do you have?
Good Luck,
Mahlon
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