I think I have a carb ice issue...

cjensen · #1 03-07-2011, 01:24 PM

Searched the forums today, and didn't find anything *SPECIFIC* to my setup...so, I'm asking for 1) General thoughts on what I'm experiencing, and 2) SPECIFICALLY, anyone with my setup, which is O-360, MA-4 carb, and Sam James intake, to chime in.

I have 35 hours on my 7 now, and I get this exact same problem from time to time. That problem would be, after a cruise configuration power reduction (doesn't matter if the mixture is already full rich or not), I am getting a VERY noticeable engine stumble. I don't know if it would actually quit because I always push the throttle back in when it happens, and it smooths right back out. It's an attention getter, but I half-expect it now. As I get closer to my 40th hour, I need to get this corrected. It does not happen on every flight. In fact, it hasn't happened at all in the last five or so hours. Yesterday while pulling the power to enter the pattern, it did stumble. It was humid, and temps were in the high 30's/low 40's.

It has never done it while in the pattern shooting stop n go's. This sort of leads me to believe that it's a carb ice issue. After a 15-20 minute cruise set up (or longer), it occurs, but not all the time.

After my pattern entry, and subsequent take off's and landings at that airport, it did not do it in the pattern, and it didn't do it on my return leg to my home airport.

Thoughts?

Mel · #2 03-07-2011, 01:28 PM

It may be too rich. When you close the throttle, you are within the idle circuit of the carb. If the idle mixture is too rich, the windmilling prop can force an extra rich mixture.

cjensen · #3 03-07-2011, 01:31 PM

I'll double check that Mel...my A&P buddy asked me that same question, and I shut it down too quickly to check that yesterday. But, I do check the shutdown RPM increase from time to time, and I get the 50rpm rise or so. It could change tho, so I'll confirm it...

hydroguy2 · #4 03-07-2011, 01:33 PM

I'm probably no help, but I too have a random engine stumble issue(20.2hrsTT). It has happened 2 times now on the ground while taxiing and once running rough in the air.

I fairly certain the roughness while airborne was a fouled plug. I have FI, so no carb ice is ruled out.
My taxiing stumbles have been after landing from significant high cruise with a rich mixture. bump of the throttle and all is smooth and I can't reproduce it. I'm thinking it's an overly rich mixture situation.

just saw Mels post, I'll need to check that again, I haven't check my idle rise since we first ran the engine. I don't shutdown with my mixture but use move the purge valve to ICO.

Radomir · #5 03-07-2011, 03:00 PM

I used to fall into #2 there and never had the issue you're experiencing.. flown in all kinds of temps and humidities..

my carb was 10-4164-1, if that helps any...

I'd be looking along the lines of what Mel suggested first..

cjensen · #6 03-07-2011, 03:13 PM

Wouldn't it tend to do it on every flight if it was an idle mixture adjustment tho?

elippse · #7 03-07-2011, 04:12 PM

Usually the Lycoming with its heated carb riser, buried in the oil pan, has very little trouble with carb ice unless you're in a region of really high absolute humidity, not relative humidity. With your Sam James intake system, what would you estimate the total length of your inlet is from the opening to the intake valve. Sometimes you will find on some planes that the engine will stumble going through a certain rpm, usually about 1400 rpm to 1800 rpm. This is thought to be due to anti-resonace of the inlet system. You should see some of the videos of a cloud of gasoline-air particles sitting right above a carb at certain rpm. Some induction systems claim that their filter box design eliminates this.

Ironflight · #8 03-07-2011, 08:10 PM

Although I know of a few cases of carb ice in Lycomings, I have personally never experienced it Chad. I would lean towards mixture adjustment as well. It has always interested me that if you have the engine leaned perfectly at cruise power, you have to richen it up to prevent stumbling when you pull the power back for descent. Counter-intuitive, but seems to hold true for the 320's and 360's I have owned. Didn't work that way for the 235, interestingly enough.

Paul

LOGGY · #9 03-07-2011, 10:17 PM

Are you using 100 LL.......?

Alan Carroll · #10 03-08-2011, 05:35 AM

Quote:

Originally Posted by cjensen

Wouldn't it tend to do it on every flight if it was an idle mixture adjustment tho?

Is there any correlation to OAT? At a given setting the mixture will be tend to be richer in warmer temperatures (because the inlet air is less dense).

I have the same engine/carb and have never (?) experienced icing.