Carb Jet questions

Hi all,

I have come to believe that my carb is a bit on the lean side, and am looking for someone to double-check my reasoning and lend any advice. I have an O-320-E3D (150HP), that has the 5130 carb, which is supposedly already the richer carb.

On takeoff, I am only seeing a max of about 11 GPM. This is at about 2250RPM and 29.5" MAP. I have a FP prop. The Lycoming charts suggest I should be a little over 12 GPM at these numbers. I am also noting slightly high CHT's on full-throttle climb out, and usually ease off the power around 1000' to keep them under 400. I understand there is nothing wrong with a bit over 400 in climb (it is 85* in FL today though), but I know it is possible to keep them lower.

To confirm my suspicions I did a lean test today at 8000' DA and 75% (full throttle). I found that my rear cylinders were plenty rich (>200* ROP at full rich), but the front cylinders only 60-80* ROP. This makes sense given the orientation of the carb butterfly valve directing more fuel straight up to the rear cylinders.

I have spent a lot of time on my baffles, and they pass the light test, as well as keeping things very cool in cruise. I have also checked that throttle and mixture arms are hitting the stops. I have looked for evidence of an intake leak and found none.

So, it is apparent that part of the problem is the poor distribution inherent in carbureted engines, but I would still like to see book numbers for fuel flow on takeoff. The guy who rebuilt my carb knows his stuff, but will only do certified work, so would not entertain a discussion about drilling or replacing the jet. He is however interested in learning more about the issue.

I am not opposed to drilling or replacing the jet, but as it is a critical part, would rather have a pro do it. Any input from the experts? Thanks!

Chris

Jets

Chris,
I have drilled jets. There I said it!
The, or your problem is that a PRO won't drill it! ( most likely) because it goes against everything they have been taught in the certified world.

Lots of info in the archives on this subject.
It is very normal for normal carbs to be lean when installed in RV's.
It is very astute of you to realize this condition and address it.

Short story is in the certified world the airplane manufacturer designs an induction system and the carb designer makes a carb to match the engine/induction.

Vans has not done this design work and we are just using carbs off certified designs to get close to our requirements. So when you are doing your due diligence of analyzing your systems performance you are trying to bring the two worlds together and sometimes it requires the jet to be replaced or drilled.

It is not hard to do, more time removing the carb and reinstalling it then the jet work. Once you have the jet out....size the hole with a drill to find out what you have. Then enlarge to next size bit and try it. If you go a tiny bit too far, you just pull the red knob out a little more than before.
You can do research on the net about stock jet sizes and hole diameters etc.
This will help you understand a little more.
If you are not comfortable doing it, then find some help.
Good Luck.

Thanks Bob, I found a few of your old posts on the subject. I stopped by my "carb guy" this afternoon (Ron at Performance Carb in Clearwater), and he is going to help me drill it out after all, after remembering that my engine is experimental. He hadn't heard of this issue before working with certified planes but was eager to learn more about the issue. He had laying around the same jet as the one installed in my carb, #828, and we measured it and decided we will step it up to a #40 opening, it is just slightly smaller than a #41 as is (.095). I am going to pull the carb on wednesday and try to get it back in the air later this week.

Chris

Awesome

Chris,
That is the best of both worlds having a carb guy help you with it.
Take notes and pictures.
Let us know how it turns out.

FYI...some years ago I helped a buddy drill the jet on his Long-ez with an 0-320 and a fixed prop. For the same reasons.
I don't have the model numbers but my pictures show.
Existing was a #43 drill or .089
Drilled to a #42 at .093
He was happy.
Not sure what you read in the archives but remember the math. The larger the hole the bigger the opening but the size grows exponentially because of the circumference.

I drilled out my jet, too

I have a high-compression, 140HP, Don George, O-290-D2 in my plane, but the MA-3SPA (10-3565-1) carb was sized for a stock O-290-D2. Like you, I wasn't seeing enough fuel flow on takeoff -I was only getting 10 GPH. Also, I was experiencing poor fuel distribution in cruise.

Like Bob mentioned, no "professional" is going to drill out the main jet for you - they don't want the liability and who can blame them? You're on your own, buddy

Not knowing what I was doing, I drilled mine out in increments. My stock jet was #42 (.0935) so my first attempt was to drill it out .0025 to #41 (.096). While I was at it, I also drilled two #57 "aeration holes" to hopefully spray some fuel toward the rear cylinders. Marvel came out with a fix for poor MA-4 fuel distribution with their Peppermill jets which had some side holes, but those jets weren't available for my carb. Maybe the new jets are available for your carb, but I had to roll my own:





Anyway, drilling out the jet by .0025 only increased the fuel flow at full throttle by 0.2 GPH so I kept going. I ended up with #38 (.1015), which gave me 11-12 GPH on takeoff. I also added two more #57 aeration holes.

I'm happy now, and at cruise, my graphic engine monitor shows level EGTs and CHTs with just a little fiddling with the throttle butterfly. I can even run LOP if I'm really careful.

One note about drilling in brass. Your standard drill bit will want to "grab" and pull the bit into the hole, causing it to break off. To counteract that, I used a stone to round off the sharp corners of the drill bit. You can also buy inexpensive reamers at MariTool.

Since I took my carb off several times, I discovered that I could just unscrew the bottom half, leaving the top half connected to the engine, and still have access to the main jet. Here's what that looks like:



One last suggestion: I'm pretty sure there is a previous topic here on the forum, but I'll leave it up to you to find it.

OK, one more suggestion: Be sure your fuel flow indicator is accurate before doing anything rash.

Thank your for the notes and pictures! I did find all the old threads on this topic, but a lot of the links and pictures had broken, and I also wanted to see if anybody had experience with my specific setup.

The carb shop agreed to drill out the jet with me, and he has a set of proper jet drills, so the only question is whether to go to #40 or #39. I will probably start at #40 and go from there - once I do it once under his watch it shouldn't be too hard to do it again if needed.

I am pretty confident in my flow numbers, and they are backed up by difficulty getting very rich of peak at WOT, 8000'.

Thanks again

Chris

I did this

And it worked. Just go slow. You may need to take your carb on and off a few times to go down another size, but if you overdo it the jet is ruined, for your application at least. If available, it's best to use a jet that had the small holes in the side of it. (Atomizer jet?) it improves fuel delivery to the engine and helps balance your cylinders better than a straight jet. Another tip. Try using good quality reamers instead of a drill bit. Much smoother hole and better results with flow.

Good luck.
Ron

my two cents worth,

I have been down the road with the fuel flow of the Lycoming chart as well and what I have learned is that those numbers are published for use in a "Cessna 172" application more less.. I have learned through guys in discussion that if used in a RV application that the cowling is much tighter and doesnt nearly have the airflow required to cool the engine in comparision to the cownling design on the Cessna, therefore more fuel that what the book calls for is required to achieve the proper cooling. A gentlemen I spoke with early stated to me that the Cessna application didnt ever spin up to 2600 + RPM's with 200MPH ram air going into the carb.

Makes sense to me, other RV owners with our motor seem to be at 13.5-14.0 GPH on the O320 to achieve good CHT's on takeoff...

However im not expert by NO means and Im on here asking for help as well, im simply passing on some of the info folks have been giving me today on cooling this thing down....

YellowJacket RV9 said:

Thank your for the notes and pictures! I did find all the old threads on this topic, but a lot of the links and pictures had broken, and I also wanted to see if anybody had experience with my specific setup.

The carb shop agreed to drill out the jet with me, and he has a set of proper jet drills, so the only question is whether to go to #40 or #39. I will probably start at #40 and go from there - once I do it once under his watch it shouldn't be too hard to do it again if needed.

I am pretty confident in my flow numbers, and they are backed up by difficulty getting very rich of peak at WOT, 8000'.

Thanks again

Chris

Click to expand...

The common wisedom was #38 when I did mine. I went to #39 and it worked well on my O-320 (160 HP). My only issue was distribution. I still had one cylinder a bit lean, but further drilling would have been throwing even more fuel at the richer cylinders. I finally went to FI and moved my attention to poor hot idle characteristics instead.

Larry

Well the results are in - I flew today with the new setup (jet drilled to #40), and got just shy of a 1GPH increase on takeoff. Temps were definitely cooler, as I was able to climb out at 110 KIAS with the highest CHT being 404 on #1. OAT was about 75 degrees. Very slightly cracking the throttle dropped all temps below 400 where they stayed all the way to 8000'.

I did a lean test at WOT and 8000' DA, 3 of the 4 cylinders were able to go at least 150 ROP, with only #2 at about 110 ROP. Again, every so slightly closing the throttle enabled me to even out the mixtures.

I am pretty satisfied with the results, so doubt I will go any bigger on the jet as of right now. I will need to adjust the idle as it is a bit higher now (700 as opposed to 650 RPM).

Chris

Congrats

Chris,
Thanks for getting back to us and letting us know how it went.
Information is a wonderful tool.
Think of all you have learned about your plane.