Lean Carburator R&D - thinking out loud

Lycosaurus · #1 01-21-2009, 07:52 PM

I've been flying for 25 hours now, and my carb is showing that it is lean for my setup. I have 11.5 gph at full throttle, full rich, -10C, 2000 ft. According to the o-320/160hp manual, I should be flowing at least 13.5 or 14 gph (from memory).

Many on VAF have had success drilling their jets one or two drill sizes larger. Bart at Aerosport Power offered to do the drilling of the jet for me on a lathe, so I sent it off yesterday.

Meanwhile, little wheels are spinning in my head, trying to understand what is different with our RV setups. Here's some of my out-loud thinking :

For one, we have a 'more efficient intake filtering'. While that is true, the greater air flow, should also result in greater fuel being sucked up through the nozzle.

Another thought is that propeller pulses are causing resonances in the airbox, that mess up the fuel metering. Given our highly pitched propellers, there may be something to this. Going to carb heat should resolve that .... and it does a little bit.

Some say that the lean carb is due to our low restriction exhausts. Maybe, but I have dual mufflers on mine, and still have a lean condition.

Yet another idea, and this one I found on the Matronics site, dates back to 1988: maybe the carb float chamber vent is not seeing the proper pressure. Imagine, if you will, that the ambient pressure is decreased around the chamber vent hole due to some strange turbulence, then the amount of fuel that is being sucked out of the jet will decrease. Hmmm.... that sounds kind of right to me. It would seem that this vent can be found in different areas for different versions of our carburators. Could this in fact be the root cause of the lean carb jet issue?

Interested in other people's thoughts / ideas on this.... (Mahlon, any thoughts on this?)

[Please pardon my ramblings .... I am suffering RV flight withdrawal until I get my carb back, and temperatures/weather get back in line.]

asav8tor · #2 01-21-2009, 08:39 PM

I know if you have put that much thought into it you have already confirmed the mixture full fwd pushes the arm against the stop on the carb, right? And there is a little bit of cable throw left so in the heat of the battle when everything else is going on a little push fwd does in fact push the mixture arm to the full rich stop?

Just checking.

It has happened before to others but I knew it couldn't happen to you

Lycosaurus · #3 01-21-2009, 09:03 PM

Quote:

Originally Posted by asav8tor

I know if you have put that much thought into it you have already confirmed the mixture full fwd pushes the arm against the stop on the carb, right? And there is a little bit of cable throw left so in the heat of the battle when everything else is going on a little push fwd does in fact push the mixture arm to the full rich stop?

Just checking.

It has happened before to others but I knew it couldn't happen to you

Yup, mixture hits the stops at the carb, and still room to push further the control cable at the panel. My situation is not unique.

No problem on the checking .... all info needs to be put out there.

John Clark · #4 01-21-2009, 11:35 PM

Exhaust systems can play a big part in the mixture issue. The systems on RVs are far superior to the ones found on production airplanes. Remember that most of carb setups were "tuned" for some really inefficient exhaust systems.

John Clark
RV8 N18U "Sunshine"
KSBA

Lycosaurus · #5 01-22-2009, 03:28 AM

Quote:

Originally Posted by John Clark

Exhaust systems can play a big part in the mixture issue.

Agreed, however I believe the Mooney also has a richer jet (the richest, as manufactured by the carb company?). I don't think they have anything special for an exhaust, but they certainly have a similar high end speed... as our planes. What happens when they install one of these super flowing exhaust systems that is supposed to dramatically increase horsepower in these planes? Do they have to modify the carb as well to enrich the mixture?

We have a static pressure line for our instruments. You would think that we would need something similar for our carburators to use as a pressure reference. Not as critical, mind you, however given the tight cowling we have, what are the pressures surrounding the cowl. Would be interesting to measure with a manometer in-flight.

tc1234c · #6 01-22-2009, 05:33 AM

Alfio,

Here is an article from Kelly Aerospace on Marvel Schebler carburetor (Kelly owns it):
http://www.kellyaerospace.com/articl...essory_AMT.pdf

The last section on the last page is entitled "Choosing the Correct Carburetor". It mentioned the complex interactions that affect the fuel/air ratios. So according to the manufacturer the causes are many.

Quote:

Airframe performance, cowling configurations, baffling, and choice of air-box, all contribute dramatically to fuel/air ratios and to
engine cooling.

I am no expert on the issue and this is just what I read.

RV10inOz · #7 01-22-2009, 05:36 AM

Few Champagnes on board so be gentle....... get fuel injection, and an all cylinder CHT/EGT monitor, GAMInjectors if need be and learn to run LOP.

http://www.avweb.com/news/pelican/182179-1.html

And read the rest of the series!

OK ...I know, hat...coat....I am outa here!

DB

mahlon_r · #8 01-22-2009, 07:15 AM

I think there is something wrong with your carburetor if you are seeing real fuel flows that low out of it. That is pretty lean. What are the egt's like? They should be really high, if the fuel flow is that low. Many things affect the fuel delivery out of the carb. 320's tend to be leaner rather then richer. I think most drill their jets because they have a good carb but may need for more fuel because they are battling temps or may be they just want to richen things up a bit from a standard fuel flow. Your flow seems pretty lean to start with, making me suspect the accuracy of the fuel flow reading or that maybe the carb has a problem.
Good Luck,
Mahlon

?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

Lycosaurus · #9 01-22-2009, 08:52 AM

Quote:

Originally Posted by mahlon_r

I think there is something wrong with your carburetor if you are seeing real fuel flows that low out of it. That is pretty lean. What are the egt's like?

I see 110 kt climb temperatures of close to 400F, with one cylinder going to 410F (#3?). In flight , at about 75%, I see an average of about 370F (I should download my EMS info so I don't have to work from memory... I'll do that this weekend. This is all at approx -10C to -20C air temps.

I expect that my cylinder temperatures will get hotter in summer, so that is why I am looking at enrichening the carb.

Quote:

Originally Posted by mahlon_r

I think most drill their jets because they have a good carb but may need for more fuel because they are battling temps or may be they just want to richen things up a bit from a standard fuel flow. Your flow seems pretty lean to start with, making me suspect the accuracy of the fuel flow reading or that maybe the carb has a problem.

Fuel flow was checked wrt to one calibrated fillup at the pumps. Came in within 1.5%.

My understanding is that one numbered drill size increase in the jet gives you 1 gph increased fuel flow. Most people seem to be drilling 2 to 3 drill sizes larger. If I have the carb drilled 2 sizes larger, that would bring the fuel flow to 13.5 gph, which would be within Lycoming minimum fuel flow spec.

Aerosport will also be looking over the carb, and see if there is anything wrong with it.

What kind of fuel flows did other builders see that have a carb'd o-320 (before and after having the jets drilled)?

Lycosaurus · #10 01-22-2009, 08:55 AM

Quote:

Originally Posted by tc1234c

Alfio,
Airframe performance, cowling configurations, baffling, and choice of air-box, all contribute dramatically to fuel/air ratios and to engine cooling.

Had read that article a while ago.... good catch on the above quote.