How to properly drill carb jets?

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MopaR&D

MopaR&D

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I have a tuning issue with my 750 Street Demon where the tuning range I need can't really be had with the primary jets and metering rods available from Holley. The primary jets jump from a #82 up to #87 and after lots of messing around with both sizes and all of my metering rods (which are basically all the ones they make, got the full kit) I could really use a #85 jet. I have other primary jets from the tuning kit which I will never use (far too small, like #76-80), is it possible to carefully drill them out and not mess them up? I've heard just using a normal drill bit will leave burrs on the inside surface which can screw up the fuel flow? I would just buy the correct jet if I could but they aren't available and I have at least 2 spare pairs of jets to mess with.

Basically is there a special drill bit that can drill out carb jets properly?

EDIT: And no regular Holley or Edelbrock jets won't interchange, only the secondaries use regular Holley jets. Primaries are specific to the Street Demon only.
 
Don't drill them. Drills are about as accurate as measuring you cylinder bore with a ruler.

If you are going to modify an existing jet, you need a reamer. Drill the jet to ~.010 under the size you want and ream it to finish. Only run the reamer through the hole forward, and use only one pass.
 
I believe it's the length and the diam of the hole not just the diameter.
 
yellow rose said:
Don't drill them. Drills are about as accurate as measuring you cylinder bore with a ruler.

If you are going to modify an existing jet, you need a reamer. Drill the jet to ~.010 under the size you want and ream it to finish. Only run the reamer through the hole forward, and use only one pass.
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So if I have a jet that's 0.076" I can just ream it to 0.085" and not use a drill at all? Is that something I could do in my garage or should I just perhaps take the jets to a machine shop?
 
I drill mine all the time and go in small steps using a hand drill. Diameter and depth determines flow. Using an AFR meter and reading plugs after 1/4 mile WOT works for me. All four jets on my car are different sizes.
 
yellow rose said:
Don't drill them. Drills are about as accurate as measuring you cylinder bore with a ruler.

If you are going to modify an existing jet, you need a reamer. Drill the jet to ~.010 under the size you want and ream it to finish. Only run the reamer through the hole forward, and use only one pass.
Click to expand...


brian6pac said:
I believe it's the length and the diam of the hole not just the diameter.
Click to expand...


They radius the hole on the inlet and outlet to blend it when they make the jets, it's nearly impossible to duplicate that at home in our garage... Best practice is not to drill jets, but buy the proper size from Holley...

Holley rates the jets after the hole is drilled and blended according to how it flows, not the hole size...
 
MopaR&D said:
So if I have a jet that's 0.076" I can just ream it to 0.085" and not use a drill at all? Is that something I could do in my garage or should I just perhaps take the jets to a machine shop?
Click to expand...

You can probably remove .011 with the reamer but be damn careful. Brass is grabby **** and you can stick a reamer, although that small a reamer with that short length of engagement should be doable. Just use a good lube.

Again, you also need to account for the radius at the entrance and exit of a jet. If you drill out that out, you can have a jet with a bigger hole that flows LESS and you will go wild trying to sort it out.

In the late 1980's I was working with mechanical FI and alcohol. I was buying jets and nozzles that were said to be flowed. I was at the track and it was hot Hot HOT and the DA was so high the birds were wearing oxygen masks. Anyway, I increased the return jet .010 (on mechanical injection the jet, or pill is in the return line so a bigger jet is LEANER which is opposite of a carb but the principal is the same) and the car went slower. And the plugs looked like the engine was a bit more fat. I said W in TF is going on. Threw another .010 at it and it slowed down more. I said piss on it and went home. If I was so stupid I couldn't pull fuel out and my tuning was slowing the car, I should pack it in.

So I called a buddy and he hooked me up with a local guy who had a flow bench for fuel injection. I ran down there after work and I watched him flow all 20 of my pills. On 2 or 3 actually flowed as marked. The pills I was using that should have been leaner were in fact, flowing LESS than they should have because the company that made them had ZERO standards. All that junk went in the shitter and I order Kinsler nozzles and jets.

The moral of the story is drilling jets is a bad idea. BUT, you have to do what you have to do. Actions have consequences. Once you remove materiel it's near impossible to put it back.
 
Used to be SOP was to drill jets since aftermarket stuff had not been invented.

I can't imagine Gartlis, Mickey Thompson, The Summers Bros. and thousands of others waiting for Jegs and Summit to be available so they could get their carbs.

Like YR said...do what you have to do.
 
What's the threading on Carter and Eddy main jets?
 
Krooser said:
Used to be SOP was to drill jets since aftermarket stuff had not been invented.

I can't imagine Gartlis, Mickey Thompson, The Summers Bros. and thousands of others waiting for Jegs and Summit to be available so they could get their carbs.

Like YR said...do what you have to do.
Click to expand...
Wax works great as a lube. You are metering fuel in a carburetor not a main rocket engine booster. You have multiple unused jets so go at it IMO.
 
I bought a bunch of brass blank plugs from McMaster Carr that fit my Holley carb. Then bought a pin vise and the numbered drill bits to drill it by hand to the sizing I needed. I was able to make any arrangement of jetting I needed at the time. Might look at their site to see if they off brass blanks in that thread pattern and go that route?
 
I think for now I'll get my fuel system sorted out, I'm starting to feel like my fuel pump is giving out... it is just a parts-store stock replacement mechanical piece. If I do try to modify my carb jets I'll take them to a machinist I know of in town who does more oddball misc. stuff.
 
straightlinespeed said:
I bought a bunch of brass blank plugs from McMaster Carr that fit my Holley carb. Then bought a pin vise and the numbered drill bits to drill it by hand to the sizing I needed. I was able to make any arrangement of jetting I needed at the time. Might look at their site to see if they off brass blanks in that thread pattern and go that route?
Click to expand...

The pin vise and GOOD QUALITY numbered drills is a good way to go. A good finish can be had from a drill using the right technique and good lube. I'd consider polishing the entrance and exit of the holes with some 600 grit valve compound on a toothpick or formed hardwood dowel when you are done.
 
I’ve been drilling Carter jets for years. If you’re worried about polishing after drilling, just run some string soaked in brass and silver polish back and forth through the jet. Works great.
 
After doing more driving and testing it turns out my primary part-throttle is way lean to the point of bogging out (can see it with my wideband O2 AFR readout), I thought it was a tad lean but close to good... I just ordered the #87 jets from Summit they were only $6, also a Carter stock replacement pump. I'm more inclined now to add an electric fuel pump at some point too.
 
Somehow missed your thread when it mattered, but I have that same carb that I was using on my 410, had the same deal as you. You mention primary jets sized from 76-82 That is the kit for the 625 carb. You need the kit for the 750 which comes with 87/89/91 primaries, as well as 54-38, 56-40, and 58-44 rods. What rod were you running with the 82's?
 
12many said:
Somehow missed your thread when it mattered, but I have that same carb that I was using on my 410, had the same deal as you. You mention primary jets sized from 76-82 That is the kit for the 625 carb. You need the kit for the 750 which comes with 87/89/91 primaries, as well as 54-38, 56-40, and 58-44 rods. What rod were you running with the 82's?
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I got the kit for the 625 because I'm at high altitude and it needed to be leaned out from factory. Now I'm running #87 primary jets and 62-54 metering rods, I had to richen up the secondaries "back" to the factory #86 jets and now it's pretty much spot-on. I could possibly even go one step leaner on the primary side, I have 64-56 rods for that.
 
MopaR&D said:
I got the kit for the 625 because I'm at high altitude and it needed to be leaned out from factory. Now I'm running #87 primary jets and 62-54 metering rods, I had to richen up the secondaries "back" to the factory #86 jets and now it's pretty much spot-on. I could possibly even go one step leaner on the primary side, I have 64-56 rods for that.
Click to expand...
Ahh, well that explains that! I got to a point where I needed to consider drilling bleeds etc for overly rich off idle cruise and either plugging or restricting the secondary bleeds to eliminate a hesitation when going wot quickly. Not really the carb for that sort of tuning. Still a nice carb though.
 
12many said:
Ahh, well that explains that! I got to a point where I needed to consider drilling bleeds etc for overly rich off idle cruise and either plugging or restricting the secondary bleeds to eliminate a hesitation when going wot quickly. Not really the carb for that sort of tuning. Still a nice carb though.
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Seems like the rich off-idle cruise is just the way these carbs work, lots of people complain about that online. However it always seems to be "I don't like the numbers my AFR gauge is showing but it runs fantastic" so I'm just gonna roll with it. Haven't checked my spark plugs but I'm not too worried about it, others have said their plugs looked fine even though their O2 sensor showed it was rich. Not sure what exactly is going on there but I'm not going to lose sleep over it, also not going to spend any money on another carb, that's going to go towards a full MPFI conversion.

Even the owner's manual that comes with the carb says to target a 13.5:1 mixture at cruise using a wideband, seems pretty rich but that's what they say. I have a feeling it's designed to have a lot of "crossover" between the idle and primary circuits to give that excellent response.

It would be quicker, faster and more tune-able with a 750 double pumper but I'm not drag racing. I'd rather have a few extra MPG and be able to "set-it-and-forget-it".
 
MopaR&D said:
Seems like the rich off-idle cruise is just the way these carbs work, lots of people complain about that online. However it always seems to be "I don't like the numbers my AFR gauge is showing but it runs fantastic" so I'm just gonna roll with it.
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You're using your noodle. :) Tuning for performance is right. Shooting for a number on a meter only works if every assumption involved is correct. And the fact WBO2 AFR numbers are themselves interpretations dependent on assumptions about the combustion doesn't help. You're right. They're reference tool, not a targeting tool.

MopaR&D said:
Even the owner's manual that comes with the carb says to target a 13.5:1 mixture at cruise using a wideband, seems pretty rich but that's what they say. I have a feeling it's designed to have a lot of "crossover" between the idle and primary circuits to give that excellent response.
Click to expand...
Or maybe whomever put together the manual didn't really know their stuff.
Like this graphic that shows up in several of the Eddy instructions.
upload_2019-5-23_17-38-15.png

It's just plain wrong.
Mislead many of us for many a years.
Part throttle acceleration actually should go leaner than cruise at pretty much all speeds.

Go back to the text books based on testing and experimentation for the automotive and aircraft industries.
A little synopsis from Larew's here:
https://www.forabodiesonly.com/mopa...g-to-compare-to.395945/page-3#post-1971911709

More here:
https://www.tapatalk.com/groups/rac...r-characteristics-at-differen-t421.html#p3408

Some links to various papers (getting much more technical):
https://www.tapatalk.com/groups/racingfuelsystems/metering-characteristics-of-carburetors-t14.html
https://www.tapatalk.com/groups/racingfuelsystems/air-bleed-characteristics-t80.html

Carter type calibrations discussed here, including the "economizer" and "bypass air bleed"
https://www.tapatalk.com/groups/rac...ry-of-the-carter-idle-restricions--t2055.html
 
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