What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Reference Material for Jetting Carb

B

BrianMatice

Member
Good morning all. In order to avoid the risk of receiving many conflicting opinions and not knowing who to follow, I'd like to ask the group for suggestions on a book or other reference for jetting a carb for my RV-7A O-320-D3G.

I'll include my story as well for those that are interested, however I'm only looking for the reference material.


The Story:
Initially (first 10 hours) I installed the carb I had which was an MSA MA-4SPA 10-5009. After fighting relentlessly with super high CHTs I turned to fuel for my cooling needs. This led me to do many many (many) hours of research into all possible carb models / jetting combinations available for the O-320. I have learned that the 10-5009 carb configured as stock, comes with the smallest jet possible. Stands to reason as it was designed for the 150 HP variant O-320. Research also led me to the 10-3678-32 which comes with the largest bore main jet in the MA-4SPA line. I also contacted Mark at MSA who informed me that most RV guys go with the 10-3678-32 when running an O-320. So I ordered a new MA-SPA 10-3678-32 from Spruce. I do understand I could have drilled the jet on the old carb to achieve the same result. I made the decision to purchase new as the old carb was last overhauled ~ 15+ years ago (I think) and I have no paperwork.

With the new 10-3678-32 carb installed, I will say that on takeoff and WOT the engine cooling effect is amazing compared to my experience previously. As with every change however, it introduces new concern. I'd like a way to gage if it is actually running with too much fuel. I know it is, but I'd like a method to take measurements to prove it.

I always taxi with a lean mixture completely strangled so it will not run above 1200RPM. This will prevent me from accidently taking off while leaned. WOT I am observing about 14.7 Gal per hour flow max at full rich. When reducing manifold pressure below 20 inches, the engine begins to pop lightly, and gets worse as I reduce throttle. I can clean up the backfire instantly by leaning the mixture. I was fouling plugs like crazy until the other day. Working with my mechanic friend, we opened the plug gaps a bit which has eliminated the fouling problem. Backfire remains unless leaned. When landing I must maintain the leaned setting all the way to engine shutdown. When setting full rich (traditional training sets this on down wind) the engine loads up by the time a go around is executed. After the full rich setting is used from down wind to final, when applying full power again the engine stumbles as it works through clearing out all that excess fuel in the cylinders.

My options are:
1 - Learn to manage this engine/carb combination by immediately leaning any time below WOT. This includes during landing. If executing a go around, I must remember to hit the red knob as well or the engine will overheat instantly.

2 - attempt to find/make a smaller bore jet which will not backfire/load up the engine at full rich but still afford me the cooling effects I am seeking on takeoff.

3 - sell the plane and go fishing instead.

Again, I'm looking for any reference material I can read to learn about re-jetting a misbehaving O-320 if it is out there.

Thanks
 
With your engine loading up, fouling plugs badly, backfiring, I think I would not be flying that plane. Sounds like something is wrong with that carb. What is your rpm when burning 14.7 gph? Constant Speed prop? What is your fuel flow in cruise?
 
I had the -32 on my 320 (160) and would take off with a specified amount of mixture out from full rich to avoid excessive fuel consumption. IMHO, just learn the EGT profile for your engine and use that to make quick mixture adjustments on the fly. For example, I never like to go richer than 1300 (highest EGT peaks a bit over 1500) on my 320 and a quick peak at the highest EGT box on the EFIS is all I need to decide if the mixture knob needs a turn.

Larry
 
What Larry said

I have the -32 carb on my 150HP O320E2A and have flown it for the last 700+ hours exactly as Larry flew his. I made an easily removable spacer for the mixture control to see 1300+ EGT at full throttle on takeoff and then lean to 1400+ at 55%.
 
Zero4Zulu said:
With your engine loading up, fouling plugs badly, backfiring, I think I would not be flying that plane. Sounds like something is wrong with that carb. What is your rpm when burning 14.7 gph? Constant Speed prop? What is your fuel flow in cruise?
Click to expand...

This is the 2nd carb (new from MSA). RPM is 1700 at WOT I get 14.7 gph. Yes I have a constant speed prop.

I can manage it to never backfire, however I need to be super diligent with the mixture.
 
Carb

No mention on here of the so called pepper box nozzle which is the ultimate solution for the MA4SPA issues. Improved fuel distribution and other benefits.
 
jrs14855 said:
No mention on here of the so called pepper box nozzle which is the ultimate solution for the MA4SPA issues. Improved fuel distribution and other benefits.
Click to expand...

I drilled my nozzle to be the “pepper box” style. It helped enormously. My climb temps are great.
 
BrianMatice said:
This is the 2nd carb (new from MSA). RPM is 1700 at WOT I get 14.7 gph. Yes I have a constant speed prop.

I can manage it to never backfire, however I need to be super diligent with the mixture.
Click to expand...

You need to figure out why it is running so rich. 15 GPH at 1700 RPM is insanely high/rich. I think I only pull around 12 or 12.5 or less at 2500 WOT. There is an old RVator article that talks about the FAB creating turbulence at the input of the carb throat. The older carbs had a main metering channel bleed port right at the throat opening and the turbulence there was messing with the flow through the main circuit. They discuss how to move that opening to a spot further into the throat. The carb meters fuel, in part, from both the gravitational pressure of the fuel in the bowl (correct fuel height in bowl is critical) and balanced by the ambient air pressure at the bleed port. As little as 1/64" in fuel heigh can change the balance, so no surprise that slight change at the ambient side can make big changes in flow. The venturi negative pressure (directly related to airflow, as in more air flow=lower pressure) then pulls from this balanced circuit. I did this modification on my -32, as that vintage of carb had the bleed port at the opening. Later carb models had this port moved down into the throat, presumably to address the problem that I mentioned. There is a restrictor deep in the passage, so a precision hole is not required at the bleed port.
 
Last edited:
jrs14855 said:
No mention on here of the so called pepper box nozzle which is the ultimate solution for the MA4SPA issues. Improved fuel distribution and other benefits.
Click to expand...

While that helps a lot with better atomization and better fuel distribution and mixing, it does NOT change or impact the metering of fuel, which is what sets the ultimate fuel qty delivered for a given volume of air moving through the carb. That said, if the jet removed for the pepperbox was of a smaller size, then yes this jet will flow more. The OPs issue seems to be excessive richness as opposed to imbalanced AFRs across cylinders and the -32 carbs has one of the largest jets in the 4 carb line.
 
Last edited:
You asked for reference material so here is some of that:

http://www.insightavionics.com/pdf files/MA-4 Carb Manual.pdf

I suspect you were looking for less opinion and more facts that would lead you to resolution. I'll offer a bunch of facts, and a single opinion:

1. Aircraft engines are air, oil, and fuel cooled. Your wide open setting will use an enrichment circuit that will add additional fuel to cool the engine. Your carb also has a mid range circuit as well as an idle circuit. It's important to know what jets, settings, parts, screws etc adjust which circuit so that every part of the throttle range gets the right amount of fuel.

2. Lycoming engines tend to have a fairly specific fuel consumption. From data I saw on the internet your engine is around .42lb per HP per hour. That means that in theory everything above 11.2 gph (160x.42/6) is probably going to be cooling, so you are tossing an extra 40% of fuel at it. (All this is assuming max HP, leaned, at sea level)

3. There are many indications that tell you how rich the engine is. Deposit on plugs, poor/lazy transition, backfiring when reducing throttle, carb heat killing the engine, etc. For instrumentation you likely have an EGT gage, but remember that the probe position will radically change the temp, so you can't compare your EGT with someone else unless their probes are in the same location.

Now for the opinion:

I think that having an airplane that requires constant attention to be reliable is well, a liability. You should be able to go full throttle and full rich and have enough fuel to cool the engine, as well as be able to idle and full rich and sea level and have it idle normally.

If it where me, I'd play with the mixture until I figured out what the engine wanted based on what others are using for full power GPH and confirming my CHTs remain reasonable, then look at what mixture settings need to make it idle correctly with it only popping with carb heat on at sea level then record the EGT temps that get you there. Now you have a target EGT to try and hit by changing the jets or carb settings.
 
You must log in or register to reply here.
Back
Top