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Engine Rough/Rich?

pilotmansam

Active Member
I’ve been chasing a problem for two or three years now, so I thought I would go ahead and throw this out for ideas:
I fly an older RV-6A 0-320 E3D L-46125-27A with a Marvel-Scheber Model MA-4SP Carb Part 10-5009-1 (nozzle drilled to 0.106 in 2002). I purchased the plane in 2010 and had no real problems, but at some point I have noticed that the engine seems to be running rich because once I take off and throttle back it gets a little rough. Leaning smooths it right out. This occurs even at low altitude.
The only change I have made that is ‘suspect’ is that I have installed the new style FAB from VANS. The problem “may” have started back around that time, but I am not sure. My A&P went through the carb and found nothing wrong. I am thinking about putting the old air box back on as a troubleshooting step (a fair amount of work).
Anyone else out there with my setup seen anything similar?
Thanks.
 
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CAB?

I'm not sure what your reference is to CAB is, but if your replaced the FAB..filtered air box, I would be suspicious of the changes made. Anytime something changes after replace/mod its the first thing to troubleshoot.More explanation may help with the issue. Did you over oil the Filter or use the wrong oil? Is the seal at the snorkel being sucked into the inlet air stream? Either there is a lack of air, or an abundance of fuel consistent with stuck float or high level float setting. I would definitely start with the items replaced and make sure all is correctly set up.
 
Correct. Used the wrong acronym. I replaced the FAB. And, you are likely correct that I should start there. But, for the life of me, I cannot determine what I did wrong.
 
Drilling the jet will do that. Most drill it to get the fuel flow they want at max power, as some carbs can be a bit lean at full power and low altitudes in the slicker RVs. That modification adds richness to all levels of operation above 1200 RPM. You need to develop a habit of leaning shortly after take off in order to optimize power and avoid excess fuel burn.

It is possible that you over drilled the jet and it is excessively rich on take off as well. If so, you can establish a method of leaning there too.

Larry
 
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Had a similar problem in my Cherokee 140 with O-320.

Mixture progressively got worse to the point I had to lean for smooth take off from 1120' airport.

Not likely the same problem if you already had a mechanic check your carb, but in my case the gasket (throttle body to bowl) was bad. Inexpensive repair, but easily overlooked. Might be worth a second look.
 
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Can you give us any details about the FAB you installed? Does it have both a carb Heat muff and flapper valve? Does it have an alternate air door? If either of those are malfunctioning, it can cause a disruption to airflow. The original alternate air door, located on the bottom of the air box was held closed by magnets epoxied onto the fiberglass bottom. Sometimes these were found to be too weak to hold the door closed in certain flight conditions. This door was designed to operate automatically if your intake scoop became blocked. The door would suck open by the intake manifold suction. The fix for it was a manually operated mechanical door controlled from the cockpit. If your carb head cable linkage isn?t set up right, it could become partially open under certain conditions. Both of these possibilities are easy to check. Just secure both of them in the normal flight position (full open for the carb Heat, fully closed for the alternate air), and lock them in this position so they can?t move. Then go fly and see if your rich condition still exists. If it still exists, look for other things that could restrict airflow, like inflight misalignment between the nose of the air box and the cowl intake scoop, including the baffle seal material used to seal the gap between the two. Since your problem surfaced when you installed your new air box, that is the place to start.
 
I drilled my jet to .098

Back when I did this I remember going very slowly with reaming the carb jet on my 0-320. I read the Thorp list then and several guys cautioned against over drilling the jet too much. I had the carb off 3 or 4 times, reaming 1or 2 thousandths at a time. .098 or .099 was the sweet spot for me. YMMV
 
Problem not solved but fixed

I wanted to post a follow-up on my issue. I finally decided to just re-install the old FAB. It uses the same filter as the new FAB. Went flying today, and the problem has disappeared. Now when I pull the throttle back there is no tendency for the engine to get rough without leaning.
So, I solved the problem, but why? Who knows?
 
Just so you know, there are just too many variables to consider with the amount of information you gave. But I'm sure you feel smarter than everyone else because you solved your problem. Here's an example in different terms.

Solve for B(sub-g) where X is a random super set of 1/376 of delta q. L2 X X X X X X y when OY is less than 49Tr.

That makes as much sense as your problem.

Oh yea, the answer: 3 goats!

I'm so smart to have solved that problem.
 
Just so you know, there are just too many variables to consider with the amount of information you gave. But I'm sure you feel smarter than everyone else because you solved your problem. Here's an example in different terms.

Solve for B(sub-g) where X is a random super set of 1/376 of delta q. L2 X X X X X X y when OY is less than 49Tr.

That makes as much sense as your problem.

Oh yea, the answer: 3 goats!

I'm so smart to have solved that problem.

I didn't read his intent that way. His wording could have been better, but I believe he was saying that he could make his problem go away with the old part and doesn't understand the cause and effect or why. He was looking for guidance from those more experienced on why the change occurred. He seemed very humble to me.

Larry
 
I wanted to post a follow-up on my issue. I finally decided to just re-install the old FAB. It uses the same filter as the new FAB. Went flying today, and the problem has disappeared. Now when I pull the throttle back there is no tendency for the engine to get rough without leaning.
So, I solved the problem, but why? Who knows?

without seeing the two FABs, I don't have much to offer other than I would be looking for something that cuases turbulent air at the opening of the carb throat at the FAB. If the air becomes very turbulent at the throat, it can disrupt the metering of fuel at the main jet, in the venturi. Some older M/S carbs had the vent for the main metering channel in a bad spot that was susceptible to turbulence in the FAB and it was later changed. Turbulent air in the FAB would cause irregular fuel metering. You will find an old article on this and a fix in the RVator series. Mine old carb had this and I drilled a new vent and JB welded up the original hole.

Larry
 
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Thanks for everyone's input. Yes, there are a lot of variables, but sometimes if you have a problem someone else has had it too. I guess it just shows me how important proper air flow into the carb can be. If I ever figure it out completely, I will do a follow up here.
 
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