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Drilling out carb jets

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flyingvranch

Member
TimGuy mentioned in another thread about drilling out the jets using machinist drill and this has got my curiosity up. I have a spare 35HP type II carb laying around that I want to rebuild and try out. Are you drilling out the high speed orifice in the float bowl or are you drilling out the three small holes for the low speed in the venturi throat? My motor has an occasional idle speed lean sneeze and I am looking to correct that. It's not too bad but it does sneeze a bit from time to time. Since I have slightly larger displacement I may need to increase the amount of fuel I guess? Is that correct thinking?

Also how do you determine how much to drill out? On my carb there are three small holes in the venturi under the top welch plug. The rear hole is slightly larger and they each get a bit smaller it looks like. Any ideas or tips from you would be much appreciated! Thanks!
 
You NEVER EVER drill a jet!!!! You use a reamer and ream to size. Drilling can leave swirl mark in brass and change the flow of jet...sucks if it makes it leaner.
 
It works properly only if you drill carefully and with cutting oil. I have done this for 40 years. You can't see any swirl marks even with my 30 power jewelers scope. My drill set was 120 bucks in 1978, it is a Brookstone. I use it exclusively for jetting work. What works for me may not work for others. However, all my repairs carry a full 1 year warranty. I HAVE to be very careful.
 
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just to add a little something or question, seem to remember, larger jets for high speed jet(bottom of float bowl) for more fuel, and smaller jet in low speed jet(top of carb) because it an air inlet jet, so less air more fuel. but changing ether one is not normally done with out good reason, and i don't recall reading anything about changing the low speed jets in any forums. and those little holes being changed ????
 
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Just take a stranded copper wire and put it through the small holes and the bend it a few times then quickly pull it through the port. That will knock off the corrosion that probably developed from years of down time. Just running a small drill bit in and out with your fingers will do the same. I have been doing that for years with pilot assys for gas fireplaces with 100% success. Those ports are a certain size for a reason just clean them up and you shoud be good. High pressure water works well for scouring the ports as well soak it in carb cleaner and blast it with a pressure washer then blow dry with air. When installing the welsh plug use a socket half the size of the plug and gently tap it close to flat and seal it with loctite!
 
I myself have been doing this for 40+ years and the factory schools will tell you not to drill jets especially main jets. I have no concern on air jets as not as critical.
 
And then there is me (sigh!).... Back in the early 1960's (1961)when brass fixed high speed jets first came upon the OMC scene and I was doing my thing in a serious manner in a little town in southern New Jersey, I came upon one 1961 V4 75hp Johnson that (right out of the box) that was extremely hard to start (wrong jets). I had visions of OMC completely doing away with adjustable needle valves, dreading the boating future.

Then, still in NJ came the early 1970's and the slow speed jets... many hard starting engines, lean running engines, engines that simply wouldn't idle. There were no service bulletins stuffed in those engine crates and ingenuity became part of the serious minded outboard mechanic's procedure (yes, mechanics, we didn't become technicians until many years later!).

At that time of the procession of the outboard engine engineering bit, we (my fellow mechanic and I), didn't have time to give it too much thought excepting in having the mindset to know from experience that we had a fuel problem that had to be enriched, and that the I.D. of those small brass slow speed fuel jets needed to be increased, and not having a supply of various size jets on hand... the solution was to drill them out, that is when we found the affected cylinder so as not to drill a proper acting jet.

Fortunately our local tool supplier was on hand and we simply purchased a set of drill bits graduated in thousands along with the tool to grip them. No time to get fussy, thinking of a possible mark the bit might make and the consequences that may follow... it was just a case of that big bruiser standing there with his family, a frown on his face, waiting to get that rig in the water, with his new engine shaking from side to side going spit spit, cough, sneeze... and he wanted that cured before he drove that rig out the gate... and we (my fellow mechanic and I) did so want him to smile and be happy.

As it turned out, the malfunctioning slow speed jets (two of them) were size .029 ... one size bigger, hmm, a little better, enlarging it to .031 cured the starting and idle problem. Such a small increase of fuel but a huge effect. And we had a happy family going boating.

Then came the "Idle Air Bleed" jets (sigh!). Yes, I had the foresight to acknowledge the "air" jets needed a decrease in I.D. size to enrichen the mixture (a complete reversal of our former "fuel jet" procedure). Many mechanics didn't at first which led to their actions adding to the problem. However, as time passed, knowledge increased... and all was well in OMC-ville once more.

Bottom line... As crude as simply drilling a fuel or air jet may be and throwing caution to the wind be pertaining to nicks, swirls, digs, etc... it's always worked for me. And when it gets too complicated and I feel I may be getting into a bit of something that's over my head... I'll quit!

Oh wait, Damn!... I already did, didn't I ? :cool:
 
Sounds like I don't need to mess with it as I have no provisions in my shop for small precision drilling like that. I might add a tip I learned from building airplane engines. Buy a set of electric guitar strings to use for cleaning the ports. The smallest high E string is perfect in size and stiffness. If it is too small you can go up one size till you find the one that works. Keep them coiled for easy storage and they will last a lifetime.
 
Then when all else fails, you can go play your guitar.......I do that too.
Mr Reeves, you have made my day. You have probably forgot more than I know.
 
timguy said:
Then when all else fails, you can go play your guitar.......I do that too.
Mr Reeves, you have made my day. You have probably forgot more than I know.
Click to expand...

And there Tim, you have brought up another matter... the guitar! I haven't played and picked (country) since sometime around 2004... BUT... having good intentions (and 2 guitars sitting around collecting dust), I did buy a new amplifier, mike stand, other items a few years ago... and there they sit, still in their boxes. I really got to get back at that as not to pat myself on the back... I used to be pretty good. And I got to thinking where George Burns got to be a country star when he was in his eighties (I Wish I Was Eighteen Again)..............

I might just give it a shot ! :)
 
We should open a new thread on guitar pickin', but yes.....I love to sing and play and don't get enough time.......maybe we should "cut back" on our forum time......but its fun to try and help people here. Sometimes our old methods conflict with modern ideas, however, you should see the "condenser tester" I built from an old fuel oil furnace igniter. You probably have one yourself. How many high and low speed needles have you refaced on your drill press? I have a special stone that I use too. My boy has no interest............this has been a great work at home business for me, over 45 years now. Its not too bad a job to walk out the kitchen door with a cup of coffee in the morning and start work.........right here in the middle of an 80 acre forest.......still got 'em standing in line......gotta turn down work.
J.Cash...........had that engine runnin' just like a song.
 
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You guys are a hoot! I play blues guitar also. I love the electric and I have a Telecaster that is like my right hand. I am just retired (Geologist) so I have time now to pursue my hobbies - flying, woodworking, boat building, etc. I own a cattle ranch in Texas and where I live is surrounded by deep forests also. I have a runway on my ranch and a beautiful shop attached to my house. My wife likes to keep an eye on me in case I blow something up.... Picture_065.JPG
 
flyingvranch said:
You guys are a hoot! I play blues guitar also. I love the electric and I have a Telecaster that is like my right hand. I am just retired (Geologist) so I have time now to pursue my hobbies - flying, woodworking, boat building, etc. I own a cattle ranch in Texas and where I live is surrounded by deep forests also. I have a runway on my ranch and a beautiful shop attached to my house. My wife likes to keep an eye on me in case I blow something up....View attachment 18616
Click to expand...

You forgot to mention RC Aircraft! (unless it falls in with that "flying" mention) I used to own and run a hobby shop in South Jersey when RC was pretty much in its infancy (middle 1960s)... recognized your unnamed hobby immediately. When I closed the shop up around 1975, I sold out of RC and Control Line stuff but held onto a bunch of HO Tyco Trains for some reason... still got them, in their original boxes (collector items now I suppose), ambroid cement, balsa wood, track switches, transformers, other hobby supplies... for what, I'll never know. Gotta get that stuff on eBay!
 
You know, I attached your name to a place like you just described. Very nice. I spent months and months working on a Northern Minnesota ranch over 20 years. 700 acres, mostly forest. Logging in winter. 200 head Herefords. Cutting hay with big Case and Hesston stack wagon, (2000 lb stacks), made nearly 400 per year. Stanley is gone now at 88 years old and his son and daughter are still farming. Cultivating their own grain too. Neighbor there, Hughs, had an airstrip along the Little Fork river, all grown in now. Cattle ranching in Northern Minnesota is a tough business.
To get back on topic, no we don't change the hole sizes in the tube, just the main jet. You can pick up a small machinists set to cover the jet range, on-line for the price of a couple nice ribeyes. The low speed side should stay close enough, but the .020 pistons will produce a bit more power and to ensure we don't run lean, I open the main one size.......Oh, hi Joe, we were typing at the same time. Gonna go back and read your post.
Neat, that is very collectable. I sold most of my planes already. Still have my original Cox .049 Corsair in the box. Bought it 50 years ago. Used to fly a real Cessna too, my brother was a great pilot and my dad built his own full size plane and started flying at age 16. It was a Heathkit and he had an Indian motorcycle engine in it. He helped build 2 airport terminal buildings, one in Hibbing, and the other right here in Duluth.
Quite the change of pace, Joe......we share so many interests.
 
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I've been flying RC over 50 years now. I have build hundreds and hundreds of models. I also fly full scale planes. I learned to fly before I learned to drive. JoeReeves that Ambroid cement is some of the best smelling stuff! Just a whiff brings back memories of me as a boy sitting in my bedroom on a cold winter day building stick and tissue models. Take care of that stuff as modelers are constantly looking for it.

TimGuy, I am unsure if I want to try to modify the jet sizes. My motor runs flawlessly and I am hesitant to make any adjustments like that. It does have an idle speed sneeze though and if you trim the choke just a tad it goes away. Yesterday on the river I got up to 32 MPH briefly and the motor sounds great. I am also getting an oil leak that I think is coming from the top seal. The seal is new. I thought this could be the reason for the sneeze. I tested it by putting some oil around the seal and rotating the motor, but all looked good with no bubbles. I am running 25:1 at the moment so I am unsure if the oil leak is due to the excess oil in the crankcase. It's not a bad leak, just enough to sling around inside the cowling.
 
Okay, that could be the source of lean, the upper cylinder may have a lighter color on the plug, but it is hard to say at a ratio of 24:1 if it would show much. I have a way to double check leaky top seal, but we already know that its leaking as it is emitting oil/fuel. It will have to be corrected. It will be leaning the upper cylinder, we don't want you to score that cylinder. What happened before that it needed an overhaul?
 
Well to be exact, I am not 100% sure that the seal is leaking. There was very little oil around the mag plate under the flywheel. Also the seal is brand new so not likely to leak. What is the way that you check? On other outboards I have seen a leaky seal emits quite a bit of oil. Also this is a new build. I had a donor engine but I just used the frame. I built the power head from new components as much as possible and the rest came from Ebay sellers. The motor is about 80% new.
 
I feed no more than 10 psi into the fuel pump pulse port.....
this is a special adapter that goes from my painting regulator, which can provide enough volume of low pressure air. Reed valves take care of any escape through carb. I have crankshaft seal area soaped and slowly turn motor by hand with spark plugs out. Air will alternately escape as rings move past exhaust ports. I like to check while moving because that action will challenge the seal/seals. Perhaps others have done this too, but it is a very effective test.
 
I did not think about soapy water. I'll try just the soapy water first and then if nothing I will rig up a way to pressure the crankcase to 10 PSI. Can I use the hose barb already on the bypass cover plate that goes down to the drain valve on the intake manifold? I am unclear what that hose does anyway, I suppose it drains excess fuel and oil away from the reed plate?
I have another seal on the way and if the tests prove a bad seal I will pop the old one out and push in a new one. Thank you for the tip!
 
You can drill into the old seal on opposite sides, I have a small bit fastened to a larger extended shank. Then I put two self tapping sheet metal screws into the seal and then "lift" it out with 2 screwdrivers. There are tools made from a conduit like piece with outside threads, they come in various sizes, but they don't work very well once the threads get dulled. The small bit I use is perfect and with the heavier shank, you have better control. The vacuum developed at idle will kinda suck in the soap, it can be hard to tell. The only way vacuum is low enough to cause any "escape" from the crankcase is under acceleration. Any port that you can provide a little air to pressurize the crankcase will suffice.
 
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