Drilling out carb. jets

[radrv6]

I have a MA-4SPA carb on my 032 Lyc. Have put in high comp. pistons, and think I need a larger jet. Any ideas on drilling out my existing jet ?, or better to just install larger one. Need part number.

Thx Richard

[snopercod]

Against the advice of just about everybody, I drilled out the main nozzle of my MA-3SPA carb because I wasn't getting enough fuel flow on takeoff. I, too, have high compression pistons and my carb was sized for a "normal" O-290-D2. The stock nozzle was .0935" and I ended up with .1015" - .008" larger. I drilled it out in stages and flew the plane at each stage to see what the fuel flow was. When drilling brass, you'll want to round off the corners of the drill bit so it doesn't grab. I couldn't find a vice to hold the nozzle when using a drill press, so I just used a regular bench vice and my battery drill with no problem. You can buy reamers, too. I also drilled four "aeration" holes in the shaft of the nozzle to obtain better fuel distribution, but that's another story.

[DanH]

Originally Posted by radrv6

Any ideas on drilling out my existing jet ?

Sure. Don't do it unless you establish less than 200F between full rich and peak EGT.

Drilling the jet to make it real rich will lower CHT, but compared to all other cooling choices, it's expensive and dirty.

[Ed_Wischmeyer]

There's more to the carburetor designation than just MA-4SPA.

My RV-9A had one cylinder that always had high EGT, hence high CHT on climb, and I contacted Marvel-Schebler about replacing the jet after being dissuaded from drilling it out. Turns out that there are various kinds of MA-4SPA carburetors, and I ordered a replacement that gives more fuel flow than the MA-4SPA that came stock with the engine from Van's. I got a 10-3678-32, and they flowed it on the bench at the rich end of the scale before they shipped it to me.

I'm certainly no expert on carbs, but I'd strongly recommend that you find out exactly what kind of carb you have and then call M-S and ask for advice (855-MSACARB). They were extremely friendly and helpful when I called them.

"It's an experimental and you can do what you want, but the laws of physics don't care what you want."

[Bob Martin]

Richard,
I have drilled out 3 main jets over the years, 2-0360's and one 0-320.
As others have replied, do not just jump into this option.
There is plenty of info in the archives.
That said, sometimes there is a need or an option to drill rather than replace.
Changing jets sizes is clearly the recommended way once you know you have known workable model carb to start with. But drilling a jet does work and can be cheaper.

As an A&P, you know the manufacturer installs or builds a model airplane with a carburator built for their airframe/induction set up. They do the design and testing to create optium unit. That is why there are so many different models out there.
Vans doesn't or didn't do this testing and they make educated guesses at the appropriate carb when they sell an engine to a builder. Thus we get something close but possible not perfect, based on the fact that most FWF, prop choice, setups are not all the same. This may be changing with newer kits.
So after you check the leaning, baffeling, fuel flow, appropriateness of existing carb or jetting, etc....it might be time to drill.
Easiest way is to check original opening hole size with drills and just drill one size up. Fly and see what difference it makes. Go slow because for every thousands of an inch bigger the volume of the diameter is exponentually larger.
The main reason we might drill a jet is to get more fuel flow at full rich, otherwise we are leaning and the main jet orifice size doesn't come into play.
Full rich is normally associated with take off performace at sea level. Too lean will cause increased CHT's. A wise man named John Deacon wrote that a properly set up engine should have an EGT of approximately 1300 degrees on takoff and if it runs hotter, add fuel to cool. Again, this is after checking all else........Good Luck

[snopercod]

Originally Posted by Bob Martin

A wise man named John Deacon wrote that a properly set up engine should have an EGT of approximately 1300 degrees on takoff and if it runs hotter, add fuel to cool.

That's some good information right there. I was always told that you should have 1 GPH of takeoff fuel flow for every 10 HP but, in retrospect, I think that's too much. I just went back and looked at my last takeoff (yesterday) and my EGTs were between 1320 to 1340 so I guess I'm good. My fuel flow at the time was 11 GPH on my 145 HP engine.

I used to know John Deakin (and George Braly) and "chat" with them on an aviation forum, but haven't seen either of them in almost 20 years. Upon his retirement from ANA, JD had more hours in the left seat of a 747 than any man alive. He was famous, but was always gracious to us "little guys" Deakin, Braly, and Walter Atkinson got together and formed https://www.advancedpilot.com/ .

Bottom line is that if one chooses to drill out the nozzle, one is taking full responsibility for the results. My carb is so old (1946 Piper Pacer) that MS doesn't support it with different nozzles any more so I pretty much had to do what I needed to do as best I could. I figured the worst that could happen if I drilled out too much was that I would have to buy a new nozzle (about $100). Nozzles are still available.

[alpinelakespilot2000]

Originally Posted by Bob Martin

A wise man named John Deacon wrote that a properly set up engine should have an EGT of approximately 1300 degrees on takoff and if it runs hotter, add fuel to cool. Again, this is after checking all else........Good Luck

Originally Posted by snopercod

I just went back and looked at my last takeoff (yesterday) and my EGTs were between 1320 to 1340 so I guess I'm good.

I don't doubt the expertise, but also I don't understand the reliance on absolute EGT values. EGT is HIGHLY sensitive to where the probe is located, thus every individual aircraft with slightly different installations are going to have potentially radically different absolute EGT values. Mike Busch has talked about this. Relative EGT values can help for sure but how do absolute values help?

[snopercod]

Originally Posted by alpinelakespilot2000

I don't understand the reliance on absolute EGT values.

I relied on fuel flow alone. I didn't know about the 1,300 degree thing until I read this thread.

[PCHunt]

EGT's should never be used as an absolute number. The actual temp displayed is dependant on how far the probe is from the exhaust valve (among other variables). Each aircraft is different.

EGTs are Relative. No two aircraft are the same. You can't use your buddy's EGT number for leaning. That's why the modern Engine monitors have the "peak" feature.

As Professor Einstein said, rather famously: It's all relative!

OK, maybe he didn't say that, but you get the idea.

[lr172]

To help others, I thought I would point out that take off is not the critical area for the FP crowd. On my 320 with FP, I set my fuel flow around 12 GPH on take-off and initial Vy climb. I am well over 200 ROP. As I transition to cruise climb at pattern altitude, the FF goes up to 12.3 or so for 150 ROP. I am slightly under-pitched, so my FF's may not match others.

The CS guys are taking off with a lot more RPM and require more fuel accordingly.

Larry