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How do I drill a jet?

PCHunt

Well Known Member
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I'd like to know the best practice technique for drilling a carburetor jet. :confused:

I bought an RV-6A with an O-320-E2D, FP, MA-4SPA carb. It was running overly rich when I bought it (had to lean for takeoff at SL), so I elected to rebuild the carb myself. I replaced the main jet and tube with the same P/N, but noted at the time that the new jet was considerably smaller than the old one, which had apparently been drilled out.

Now it is running too lean, as evidenced by high EGT and very little rise to peak EGT.

Can I just use a standard numbered drill, or should I get some small reams? Any advice on how to calculate how much to enlarge the jet, or just go in little steps and test it each time?

I understand that some jets are "stepped", and that drilling them out can cause major changes in fuel flow with only small increases in drill size.

Any and all ideas are appreciated.

Here is a very good article on carbs from AMT online: http://www.amtonline.com/publication/article.jsp?pubId=1&id=1024
 
Pete, I am sorry I cant answer your question. I also have a 'too lean ' jet that needs to be enlarged.

Out of interest do you know how large your 'too large' one is? If you dont, just gently try to slide a drill through until you find the right size.

I hope you get plenty of info. Steve.
 
Check with your local FBO/mechanic.

You can also find jet drills in hot rod shops.

Be careful when drilling, I would not use a air or elect drill, try using a pin vice and just twist by hand.

Be careful not to scar up the bore of the jet.

Good luck.
 
Is it possible to buy the correct size jet from a carb shop? I had a too lean problem at one time, found that if I would add carb heat that would lower the EGT. I stopped by a shop and the mech. looked it up in some manual and simply turned the air screw out about 1/2 turn and much to my supprise that fixed the problem. I have worked with engines all my life and I dont know how that could fix the problem. I have driled out a lot of jets when I ran a motorcycle shop. I found that a gas or propane supplier has a great selection of drill bits. check with one of them.
 
You can get good results by drilling the jet. Go carefully, I used one number drill size larger at a time. My jet (same engine as yours, but with 160hp cylinders) was 0.097" (I think), so I went to #39 (0.0995). I probably need to go one size larger. Fly for several hours before deciding to go up to the next size. The jet will drill out very easily.

Pete
 
Drilling jets

If you want to believe the original manufacturer Marvel Schebler, you shouldn't drill any jet. Get the proper one for your engine. The reason is a jet is usually not a straight constant diameter hole and may be tapered or stepped. A jet is designed to give the proper mixture throughout the full throttle range for a particular engine. I know this is experimental etc. etc., but do me a favor and don't screw with the only thing keeping you in the air. A slam excel on a go-around is a bad time to have it cough. Just my VH A&P/IA been around for awhile O.
Don
 
jets

If you want to believe the original manufacturer Marvel Schebler, you shouldn't drill any jet. Get the proper one for your engine. The reason is a jet is usually not a straight constant diameter hole and may be tapered or stepped. A jet is designed to give the proper mixture throughout the full throttle range for a particular engine. I know this is experimental etc. etc., but do me a favor and don't screw with the only thing keeping you in the air. A slam excel on a go-around is a bad time to have it cough. Just my VH A&P/IA been around for awhile O.
Don

Don- I would like to not have to drill the jet, but several folks have told me that there is no factory jet that will not run lean when the engine is in an RV, which has increased airflow compared to C and P installations, mostly due to the FAB and the higher speeds. If you know of a source for jets, I'm all ears. Thanks.
 
You can get good results by drilling the jet. Go carefully, I used one number drill size larger at a time. My jet (same engine as yours, but with 160hp cylinders) was 0.097" (I think), so I went to #39 (0.0995). I probably need to go one size larger. Fly for several hours before deciding to go up to the next size. The jet will drill out very easily.

Pete

Yup, one drill size at a time, unless your bits are ground for brass. went from #42 ---> #39 (O320-D2J) An easy Job.:cool:
I used a drill press.
 
Read this article before you drill the jet

I am sorry that I forgot about the following article:

http://www.kellyaerospace.com/articles/Accessory_AMT.pdf

Read page three about enlarging main jet. I read it after I have drilled my jet and forgot about it until I re-write my own record. The major problem is over rich between full and idle throttle. If you don't lean aggressively (whenever under 75% power) your engine will run too rich. I have engine monitor and have been leaning as soon as I pull the power back below 75%.

My own record:
http://3limafoxtrot.com/maintenance.htm#highcht

Steve,

Split the carb is pretty easy. The jet can be taken off easily with a wrench (see pictures below).
http://picasaweb.google.com/tc1234c/8222007CarbMod#
 
Drilling

is not a problem once you source the correct "pilot orifice drill kit". I do a lot of business with the propane folks so that is what our kits are called. The trick in drilling jets is to make sure that you are drilling concentric with the bore and that there are no bures or tailings to alter the direction of the fuel.
 
Don- I would like to not have to drill the jet, but several folks have told me that there is no factory jet that will not run lean when the engine is in an RV, which has increased airflow compared to C and P installations, mostly due to the FAB and the higher speeds. If you know of a source for jets, I'm all ears. Thanks.

Pete,

I come from the certified world so that's been my experience. What little I know about them is there is more to the science of a carb than meets the eye. I often read here that this guy or that has a lean engine so they drill the jets the manufacturer says not to. The other thing that affects mixture is the float level. No one ever mentions that from what I've seen. That's how the mixture is controlled normally; by varying the float level.

I got a strong feeling that many experimental guys take their carbs apart and don't put them back together per the overhaul manual, but I've been known to be wrong. If the float level is not correct then the mixture isn't either. Much easier adjustment than screwing with the jet.

I don't think I buy the FAB / pressure / speed thing either, but could be wrong there too. The physics of the thing is that mass flow through the venturi is what sets the amount of fuel that is mixed and not the pressure it is at. A certain size engine at a certain RPM can only draw so much air no matter the pressure at the carb.

My very humble suggestion is to contact someone that are experts in the field and get their wisdom. As good as we are at what we do, we can't know everything about everything. If it were my butt I'd call someone like the people at www.aircraftaccessoriesofok.com instead of talking with guys hanging around the local airport or lurking here. I've had my A&P since 1971, worked on many types from Cessna to Boeing and won't mess with one. Like Dirty Harry said "A man's gotta know his limitations". If it were mine I wouldn't take any chances to save a few bucks and send it to a qualified shop for an overhaul or exchange. Personally I don't experiment with engines, props, injector systems or carbs. YMMV Maybe I'm just getting too conservative in my old age as I like my gliders to have a 17 meter span.

Take it for what it's worth and good luck.

Don
 
Pete/Don, looking back through the archives lots of folk have had good experience increasing the size of the jet.

..... My very humble suggestion is to contact someone that are experts in the field and get their wisdom. .....

Don, My engine was put together by Aerosport power. I think they would fall into your catagory of expert?

This is what they say.

"We have found that enlarging the main nozzle from .096" to .104" works well. We find that some airbox designs have a tendency to make the carburetors run leaner. ........ if you want to do it there[in the UK], it is not complicated. You would need the two gaskets for the main nozzle and four new bowl screw locks. The machining should be done in a lath to ensure accuracy."

Mostly my discussions were in conversation, but the above, is a quote from an email in the same vein. They also noted that no two set ups appear to give quite the same results. The one thing to add is that they particularly put the problem down to when running a c/s prop. (I have wondered if it is compounded by the simple exhausts. Mine is a 4-pipe. Just conjecture on my part.)

I havnt made the change yet, but Deakin (Pelicans Perch) does not believe that the fuel flow at Lycos spec is adequate at full power. I am right on the Lyco spec., but clearly running lean.

I have to say I am slightly nervous of increasing from .096 to .104 in one go, since that is a 17% increase, but Brad at AP is pretty confident that is what it will need.

If anyone has recorded how their fuel flow changed when they went from size a to size b I would be very interested?
 
Ted, thats great. the pictures are really useful. If you were doing it again would you take the carb off the engine or split it in situ?

Thanks so much.

FWIW, I left the carb on the engine. The jet is in the lower half which is pretty easy to remove (at least for my setup). I used a reamer by hand in a "T" handle. Reaming (or drilling) "backwards" makes it much easier to keep the bit properly aligned- just keep it centered in the tube. I increased my jet by .006" and am happy with the results.
 
Drilled main jet

I just enlarged mine about .004 I did it with a cordless drill and noticed that when I was done that I did scar the inner wall slightly on one side. Do you think that might create a problem?

Engine XP-0360
 
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drilling carb jet

Mahlon Russell from Mattituck is one of the leading experts on these engines and he basically says go ahead and drill the jet. There is no other realistic option. Check out groups.yahoo.com/group/lycoming for Mahlons posts on this subject. Brass is a very free cutting material. If you consult a machinists handbook you will find that very small drill bits should be turned at quite a high speed, far higher than most battery powered drills. Some type of lubricant will minimize scoring. Number drill bits are available in most of the larger hardware stores but I have not found them at Lowes or Home Depot. MSC Supply if no local sources. If you prefer a reamer MSC probably has these in number sizes.
 
49clipper

On my O-320-D1A, I can only get 125 ROP at normal cruise (2400 rpm) at full rich setting. Fuel flow is slightly less than the lyc manual says. All the gurus think it should be about 180 to 200 ROP (at full rich setting) for keeping out of the "red zone". Cylinder heads run very cool at cruise (about 320-340) on a typical 80 degree day. Very seldom ever see over 350 in cruise, but takeoff is a different story. I see 410 or so on #1 & 2 on TO until I reduce power, then it drops quickly. #3 & 4 stay about 380-390 on TO. Am I running too lean? I don't get much rise on pulling mixture to idle cutoff, so that indicates being lean. For the first 70 hrs, I have been leaning aggressively below 65% power, but now think maybe I am too lean. I know it cannot hurt the engine leaning aggressivly below 65%.
 
Do we know if those having to to drill are equiped with economizer valves in the carb (i.e -32). I'm runing a O-320-B1A with a -32 carb and see leaning especially at sea level. Most of my ops are above 3000 and see no significant problem. I do see a rise in egt in winter with power reduction but still have mixture control.
 
49clipper

Did the lean cutoff test and saw no rise in rpm, so its definetly lean. Guess I will have to drill the jet also. Also checked it at 2000 with little rise.

The bigger question is why do I see a fuel flow increase just by turning on the electric pump. It jumps at least .5gph flow with the pump on. How can it use more fuel with noting else changing. EGT stays the same, CHT stays the same, rpm stays the same, just fuel flow increases. No return line to the tanks either. HMMM.
 
Question - when you take the nozzle out of the carb, will it be obvious which end to drill?
Do you pass the drill right up through the entire length of the nozzle?
 
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Question - when you take the nozzle out of the carb, will it be obvious which end to drill?
Do you pass the drill right up through the entire length of the nozzle?

It'll be obvious. Only the end that's screwed into the carb body will be small enough to fit a #40 or similar size drill/reamer. The end that sits in the air stream is much larger diameter.

Don't forget the gasket on the end that's screwed into the carb body. The gasket is tiny and may or may not come out of the carb body with the jet. I would recommend fishing the gasket out of the carb body and placing it on the jet before re-installation, just so you know it's not buggered up inside. Better yet, throw it away and use a new gasket.
 
Thanks!
I'm going to try to put together a YouTube video when I do my jet, and I'll post it up here. I'll include a parts list for gaskets, screws etc, and torque values from the Precision Service manual, in the video. Hopefully this will demystify it and provide a single reference source for future builders who want to do this mod.
 
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To help matters out...

Use a FLAT (both sides parallel) piece of wood, plastic or other suitable material.
Lock it in place on the drill press table.
Be certain that the surface of the material is perpendicular rto the axis of the drill bit.
Drill (I believe if I remember correctly) a 1/4 inch hole (or the necessary size to hold the jet in firmly place).
I'd drill to #41 first even if you aren't going to reassemble and test at that size. Just because
It may be less likely to mess up the hole.
 
I put mine in a vice, took a deep breath, took it easy and drilled it with an air drill ...

Pete
 
Parts list

I believe that this is the parts that we order to do the work...
Qty Ship B/O Item Unit Price Total Price

1 0 07-66224 66224 CARB GASKET 1.280 1.28
1 0 08-06617 16-222 TEMPEST MSA NOZZLE GASKET 6.950 6.95
3 0 08-06631 16-A107 TEMPEST MSA PWR JET GASKET 2.950 8.85
3 0 08-06640 16-B75 TEMPEST MSA GASKET 3.650 10.95
4 2 08-06854 78-A110 TEMPEST MSA WASHER 3.200 12.80
3 0 08-06880 82-11 TEMPEST MSA COTTER PIN 1.150 3.45
1 1 08-06990 500-1 TEMPEST MSA WARNING LABEL 0.000 0.00
2 0 21323 CARB GASKET CONT # 21323 (BOT) 1.880 3.76
 
Yep, that's the same shopping list that I got from Aero Sport power. Parts on-order!

One part I don't see in the list is the tabbed washer that the nozzle goes through... I think the Precision document calls this the "nozzle gasket safety washer". I don't see a part with this name on the Spruce website either. Is what they are referring to as the "Nozzle gasket" (part number 16-222) ??

Thanks MauiLvrs for writing-up the method for creating a jig to hold the nozzle under the drill press. I had something exactly like that in-mind.

Additional useful info - torque values from the Precision Airmotive Carb service manual:

TORQUE SETTINGS FOR MA-3 AND MA-4SPA CARBURETORS
Nozzle 45 - 60 in-Ibs
Screw - Throttle bodv to bowl 35 -45 in-Ibs
...
 
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One part I don't see in the list is the tabbed washer that the nozzle goes through... I think the Precision document calls this the "nozzle gasket safety washer". I don't see a part with this name on the Spruce website either. Is what they are referring to as the "Nozzle gasket" (part number 16-222) ??

I think that it is this one ... but I'll have to check.
08-06617 16-222 TEMPEST MSA NOZZLE GASKET 6.95
 
NEW FUEL NOZZLE ? MARVEL - SCHEBLER ? AIRCRAFT CARBURETORS

Found this interesting ...

http://msacarbs.com/pdf/SB-22_O New Fuel Nozzle.pdf.

3. Background: In some instances, operators have reported high and/or split cylinder head temperatures (CHT) and/or high and/or split exhaust gas temperatures (EGT). A new carburetor nozzle has been developed to improve this condition.
head temperatures (CH) and/or high and/or split e
 
VERY interesting! But I wonder whether this new nozzle would solve the problem of excessively lean mixtures (low fuel flow at WOT) seen by many of us.
Although this may not be suitable for my carb. Mine is a model 10-5217SP. The SB says it applies to 10-3678-32.
I've emailed MSA and asked what the diameter of the jet in the new nozzle is.
 
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VERY interesting! But I wonder whether this new nozzle would solve the problem of excessively lean mixtures (low fuel flow at WOT) seen by many of us.
Although this may not be suitable for my carb. Mine is a model 10-5217SP. The SB says it applies to 10-3678-32.
I've emailed MSA and asked what the diameter of the jet in the new nozzle is.

With an interesting twist (lawyer generated?) in the SB -

Additionally fuel nozzle 47-862 is NOT approved for use in carburetors manufactured, overhauled, or serviced by AVStar Fuel Systems even where the carburetor model and part number appears to be the same or is similar to the MSA carburetor designation.
 
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