Van's Air Force
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Drilling out carb. jets
- Thread starter radrv6
- Start date
snopercod
Well Known Member
I drilled my nozzle
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.
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.
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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
Well Known Member
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."
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
Well Known Member
Jets
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
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
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snopercod
Well Known Member
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
Well Known Member
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
Well Known Member
I relied on fuel flow alone. I didn't know about the 1,300 degree thing until I read this thread.
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.
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.
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
The CS guys are taking off with a lot more RPM and require more fuel accordingly.
Larry
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snopercod
Well Known Member
Here it is
Yes, yes, and yes. Airport elevation was 2165'. Well actually, it was immediately after opening the throttle for takeoff and the data is taken at 6 second intervals. (Hope this embeds OK)
Code:
DATE & GMT TIME E1 E2 E3 E4 C1 C2 C3 C4 DIF CLD BAT FF USD
9/28/2017 18:36 1331 1304 1322 1329 363 345 373 363 27 0 14.8 5.5 0.2
9/28/2017 18:37 1320 1293 1322 1329 365 348 376 365 36 0 14.8 11.1 0.2
9/28/2017 18:37 1332 1311 1322 1349 367 351 378 368 38 0 14.8 11.1 0.2
9/28/2017 18:37 1332 1322 1322 1340 369 355 381 371 18 0 14.8 11.3 0.2
9/28/2017 18:37 1332 1322 1331 1340 371 359 383 374 18 0 14.8 11.2 0.4
9/28/2017 18:37 1332 1332 1331 1347 371 362 385 377 16 0 14.1 11.1 0.4
9/28/2017 18:37 1332 1332 1351 1371 373 365 387 379 39 0 14.7 10.8 0.4
9/28/2017 18:37 1332 1325 1351 1371 373 368 387 381 46 0 14.7 10.7 0.4
9/28/2017 18:37 1332 1325 1366 1384 373 370 390 383 59 0 14.7 10.8 0.4
9/28/2017 18:37 1332 1319 1371 1390 375 370 392 385 71 0 14.7 10.7 0.4
9/28/2017 18:37 1332 1311 1385 1390 375 372 392 385 79 0 14.7 10.6 0.4
9/28/2017 18:38 1332 1311 1390 1390 375 372 394 388 79 0 14.7 10.8 0.4
9/28/2017 18:38 1325 1305 1396 1402 375 372 396 388 97 0 14.7 10.6 0.4
9/28/2017 18:38 1319 1305 1402 1412 373 372 396 390 107 0 14.7 10.7 0.4
9/28/2017 18:38 1324 1299 1402 1412 373 372 398 390 113 0 14.7 10.6 0.4
9/28/2017 18:38 1324 1299 1409 1417 373 370 398 390 118 0 14.7 10.8 0.6
9/28/2017 18:38 1319 1299 1409 1417 370 370 400 392 118 -10 14.7 10.7 0.6
9/28/2017 18:38 1319 1299 1414 1409 370 368 400 392 115 -10 14.9 10.8 0.6
9/28/2017 18:38 1327 1299 1424 1409 370 368 403 392 125 0 14.7 10.9 0.6Captain Midnight
I'm New Here
weak mixture on O-320
Well known problem with O-320 plus standard carb. Check this link for what happened to me. This report was for the LAA here in UK. It was a pain! Should be a sticky.
Stan H
Well known problem with O-320 plus standard carb. Check this link for what happened to me. This report was for the LAA here in UK. It was a pain! Should be a sticky.
Stan H
Captain Midnight
I'm New Here
weak mixture on O-320
Sorry, the link didn't work. Try this,
https://drive.google.com/file/d/0B-SOSxyKpCQoMFlmenI4WnY0cXc/view?usp=sharing
Stan
Sorry, the link didn't work. Try this,
https://drive.google.com/file/d/0B-SOSxyKpCQoMFlmenI4WnY0cXc/view?usp=sharing
Stan
carguy614
Well Known Member
Carb jets
Actual drilling of the jet is not the recommended procedure. Use jet reamer for the job, inserting from front to exit. Use shank of wire size drill bit at small end, exit, to measure size. They are very hard to find, but I have a set if anybody wants to borrow them. Long time pro in the carburetor rebuilding field.
Regards,
Chris
Actual drilling of the jet is not the recommended procedure. Use jet reamer for the job, inserting from front to exit. Use shank of wire size drill bit at small end, exit, to measure size. They are very hard to find, but I have a set if anybody wants to borrow them. Long time pro in the carburetor rebuilding field.
Regards,
Chris
bret
Well Known Member
Back in my day of playing with cars and carb mods, did a lot research on this subject, the inlet and outlet shape is just as important as the opening size, if drilled incorrectly, you can actually decrease the volume through the Jet, in the end I would just buy a manufacture jet index and know for sure the flow.
Good note Larry. RPM is one of multiple variables making fuel flow a rough approximation, at best. Here Larry has considered mixture via reference to peak EGT (a method which does not lie), and found that for his cruise climb condition, an increase in RPM leaned the mixture a wee bit, even though fuel flow went up.
Not a surprise if you think about it. Assume 2200 initial RPM at 1000 MSL, 2450 for cruise climb by 2000 MSL, standard day air density, and 100% VE. The engine is an air pump, so at 2200 it would inhale 908 lbs of air per hour, vs 981 pph at 2450. So, the engine requires 7.5% more fuel flow to maintain the same mixture, despite a 3% decrease in air density (.0721/.0743 lbs/ft^3) during the 1000 ft interval. Without an altitude change, it would need 10% more fuel based on RPM change alone. If the RPM rise brings a speed increase, and we assume a good ram inlet, pressure (thus density) would rise, and it would need even more fuel to maintain correct mixture.
Next time someone insists you need a particular fuel flow for such-and-such engine, ask "At what RPM, what OAT, and what MP?"... and watch for the blank look.
BTW, 150 ROP is good; at or close to best power mixture.
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Interesting table. In 108 seconds #1 and #2 EGT dropped 4 degrees, while #3 and #4 rose 102F and 87F respectively. Tells me fuel distribution isn't real good.
That's about all it tells. We have no idea of peak EGT for any of them, so no idea where each cylinder is actually running in terms of mixture, i.e. power or detonation margin.
Well, wait, we can say none are detonating, because the 108 second CHT rise would have been much more than the max 30F we see here.
In this little sample, absolute EGT ranges from about 1300 to 1425, but again, with reference to peak it tells us nothing.
Let's consider judgments based on flow and HP, here 11 and 145. Lycoming charts place minimum fuel flow for rated power at around 0.5 BSFC, and that's conservative, as they want to ensure detonation margin under worst conditions. If we take the above at face value, 11 gallons x 6 lbs per gallon is 66 lbs, and 66lbs / 145 HP = 0.45 BSFC, a bit on the lean side. Holy cow, bring out the jet drills!
Naaa. it simply isn't making 145 HP under the conditions present. There is no reason to believe it actually made 145 on a dyno either (assuming it was ever on one). Rated power is an approximation of corrected HP, a theoretical number which would be true if the engine was magically transported to sea level on a standard day. (I say approximation because rated power can include some wishful thinking by the marketing department. For example, a "200 HP" IO-360 angle valve typically makes around 196 corrected HP on an honest dyno.)
A midday observation in Asheville says baro was 30.05 Hg, so station pressure (think static pressure outside the intake throat) would be about 27.95 at 2000 MSL. Temperature was 74F, and humidity aboutwas around 50%. Multiply the 145 rated power by the appropriate correction factors, and actual power is down to about 129. 66 / 129 = 0.51 BSFC, which would be desirable.
Should we declare victory? Nope, because the whole thing was based on the 145 corrected HP assumption. John, did that number come from a dyno run?
snopercod
Well Known Member
More likely from Don George's hindquarters. I don't care about the absolute value of the HP, but it seems like I have plenty. Two days ago on a 10 mile final to KTYS, my IAS was 165Kts and my ground speed was 189 Kts with no tailwind. It really confused Knoxville Approach that a little "experimental" was moving that fast. (video/audio upon request)
Thanks for the analysis. My EGTs usually peak around 1550, and usually either #3 or #4.
Regarding the CHTs, I have two 2" ducts feeding my oil cooler taken from either side of the rear baffles. That probably explains why #3 & #4 run hotter. Now that fall is here, I'll probably blank off one of them (or both of them half-way).
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You're welcome. The previously posted full throttle EGTs would thus be 125 and 140 ROP, which is fine.
I'll bet closing a duct drops all cylinders a bit, not just the rear ones. Here I'd bet the CHTs spread is largely a function of fuel distribution. I suspect 1 and 2 are fat, while 3 and 4 are leaning.
Does the 108 seconds include an RPM rise? See Larry's previous comment.
I'll add that I manually lean to get these settings. I pull the mixture back a 1/2" at takeoff and dial it richer during transition as the RPMs go up. The entire climb is slaved to EGT, but I transition leaner as I go higher (lower power and reduced detonation risk). By about 3000', I am running around 100 ROP to save fuel. I have also been experimenting with LOP climbs (above 3000'), but my impatient nature seems to prohibit that for the norm
While I have to lean for takeoff, the FI is set up correctly, as I would need that extra capacity in the nozzles to handle WOT cruise (ROP) at low altitudes. I just don't need it for takeoff, as the FP simply doesn't allow anywhere near rated power in that phase of flight.
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snopercod
Well Known Member
Yes, Initial RPM was 2,400 at 2,130' (indicated) and 180 seconds later it was 2,500 RPM at 3,000'. (I retrieved those figures from my Garmin Virb camera in the cockpit.) I wrote down the OAT, Baro, and DA on my checklist, but all that's at the hangar.
