Recommended Main Jet Size for O320

[lr172]

I am rebuilding my carb and plan to drill the jet pre-emptively to avoid Lean conditions. I am building an RV-6A.

I have an O-320B1A (160 HP)with a 10-3678-12 carb. The stock jet is a #40 drill size. I have researched all of the posts on this subject and see drilling to #37-39. I have taken it to #39 and am thinking of going to #38. I was hoping those that have done this on this particular carb/engine size might share their experience/recommendation. It would be nice to not have to pull the carb apart later, but I don't want to wind up over-rich either.

I appreciate your input

[Kyle Boatright]

Originally Posted by lr172

I am rebuilding my carb and plan to drill the jet pre-emptively to avoid Lean conditions. I am building an RV-6A.

I have an O-320B1A (160 HP)with a 10-3678-12 carb. The stock jet is a #40 drill size. I have researched all of the posts on this subject and see drilling to #37-39. I have taken it to #39 and am thinking of going to #38. I was hoping those that have done this on this particular carb/engine size might share their experience/recommendation. It would be nice to not have to pull the carb apart later, but I don't want to wind up over-rich either.

I appreciate your input

I have no idea of the model number of my carb, but it is the one that came with my engine core from a C-172. I've never seen any jetting problems and would suggest you leave yours alone until you've flown. Once you drill it out, you can't un-drill it.

[jrs14855]

A new jet is $100 plus, so you don't want to trash it. There have been numerous postings by Mahlon, here and on Lycoming forum. I think his recommendation was maximum of two number drill sizes. I have a file at home if I can remember to check. I am going to start with stock, if necessary I will go one drill size at a time.

[patterson]

What I did was some research on the resizing of the main jet the Thorpe guys did many years ago. They discovered you could go slightly larger to get lower EGT's at full power TO and still have good performance overall. But, going too large produced problems with stumble on application of throttle and lowered fuel economy. Use the aerated jet.

I don't recall where I learned it, but using a reamer on a drill press is preferable to using a drill bit. Much cleaner/smoother/straighter hole. YMMV
Ron
N8ZD

[penguin]

My engine came from a 150hp 172, at overhaul I fitted 160hp cylinders. The CHTs would climb to 430*F by around 1000' on climb out on a Spring Texas day. My main jet was 0.097". I ended up at 0.101" (#38) to keep the CHTs around 400*F in the climb. With the original jet the engine was definitely running too lean as leaning at 6000' gave almost no rise in EGTs. I am now getting a 200*F rise in EGTs from full rich to peak.

However, I have since found that much better control of CHTs is possible by installing louvres in the bottom cowl to increase the effective cowl exit area (6A).

So there are definitely 2 problems - the jet size must be adequately large to put the engine at the correct fuel/air ratio and to allow some leaning to take place, but CHTs can be controlled more effectively by getting the proper air flow through the cowl.

Pete

[lr172]

Thanks for all of the input here. I ended up putting it back together with the Jet at #39. It's only a couple of hours to pull it apart later. I am still thinking I'll need a #38, but there are too many open variables to go too far too early.

Larry