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Old 05-20-2020, 05:40 PM
pilotmansam pilotmansam is offline
Join Date: Apr 2011
Location: Newberry, SC
Posts: 62
Default Engine Rough/Rich?

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?
Sam Hart
Newberry, SC
RV-6A N7228Z

Last edited by pilotmansam : 05-21-2020 at 07:32 AM.
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Old 05-21-2020, 04:53 AM
fixnflyguy fixnflyguy is online now
Join Date: Jan 2006
Location: Winston-Salem, N.C.
Posts: 1,135
Default 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.
Bill E.
8A7 / Advance NC
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Old 05-21-2020, 07:35 AM
pilotmansam pilotmansam is offline
Join Date: Apr 2011
Location: Newberry, SC
Posts: 62

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.
Sam Hart
Newberry, SC
RV-6A N7228Z
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Old 05-21-2020, 08:24 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 4,868

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.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 05-21-2020 at 08:29 AM.
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Old 05-21-2020, 04:11 PM
dalemaher dalemaher is offline
Join Date: Nov 2019
Location: Kamloops, BC
Posts: 7

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.
2002 RV6A C-GZVD
Bought, not built

Last edited by dalemaher : 05-21-2020 at 04:14 PM.
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Old 05-21-2020, 04:46 PM
Scott Hersha Scott Hersha is offline
Join Date: Jan 2005
Location: Cincinnati, OH
Posts: 1,386

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 isnít 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 canít 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.
RV6/2001 built/sold 2005
RV8 Fastback/2008 built/sold 2015
RV4/bought 2016/sold/2017
RV8/2018 built/Sold(sadly)
RV4/bought 2019 Flying
Cincinnati, OH/KHAO
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Old 05-25-2020, 08:09 AM
patterson patterson is offline
Join Date: Mar 2005
Location: Queen Creek AZ
Posts: 513
Default 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
RV-4 N8ZD Sold
Phat -4 under construction
regular VAF donor despite being exempt
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