Nitrous on a 340 100 shot

This post by @Rat Bastid is an excellent post and applicable to your situation. Read it and understand it and you’ll be on your way to razor sharp tune up.


@Rat Bastid wrote;

“Hang on before you start changing jets.

You really need to do a few things first. You either need to get a pin set so you can determine the size of the holes in this thing, or at least get some drill bits and use them to figure out the sizes of the holes.

Then you need to get some 6-32, 8-32 and 10-32 brass set screws and the required drill and taps for each size. That way you can make a change to the carb and if you don’t like it, you just screw in a new piece of brass. I get the brass from McMaster-Carr.


The very FIRST thing you need to do is determine the size of the Idle Air Bleed. That’s on top of the carb and it has the bigger of the two holes. The bleed with the smaller hole is the Main Air Bleed. Don’t worry about that one. Yet.

Once you figure out what size the IAB is, you need to find the Idle Feed Restricter in the metering block. It’s a piece of brass and it’s probably up high where it should NOT be.

You need to determine what size it is and then using your drill and 6-32 tap, drill and tap the lower IFR hole and move the IFR jet down there.

As for sizing, that IFR should be .028-.029 and no bigger. Then the ISB should be .068.-.070 to start. Moving the idle feed restricters down and getting the sizing right is critical and must be done before you ever change a jet.

Ive seen the most wacky tune ups come in these carbs. You have to do the work to get the performance and that starts with the idle circuit.

Next up is looking at the emulsion bleeds. On your metering blocks there my be 3, 4 or 5 holes. Doesn’t really matter at this point, what you need to do is deter what size the holes are and how many are open.

As a starting point for a 3 emulsion block, you want to use a .028 hole and use the top and bottom holes. Plug the middle hole.

If you have 4 emulsion holes, use the same sizing and use the top and 3rd hole from the top and block the other two.

If you have a 5 emulsion block, use the same size bleed and use the top and either the 4th or 5th hole. I’d have to see the block to tell you exactly what hole to use.

Usually these carbs have too big emulsion holes and too many. This makes the booster “slug” fuel. What happens is as the signal gets high enough to pull fuel from the booster, you get a big “slug” of fuel. And then you get a big “slug” of air. And it repeats. You can’t fix that with a jet change.

After you get that sorted out and before you change a jet you need to restrict some fuel to the T-slots so you can control how much fuel you get at a cruise.

For the most part, when you are cruising through town or whatever, you are on the T-slots. Most of them today are way too wide and way too long. So to correct that you take your 10-32 tap and correct sized drill and drill into the main body then tap it and use your set screws there.

For your application, I’d say start at .0625 (1/16 inch) and tune from there. You may have to go smaller but its pretty easy to sneak up on it from the rich side rather than the other way.

Once you do all that and you get the thing running well enough to drive it, you want to hook a vacuum gauge to manifold vacuum and see what you have for vacuum at a cruise.

Use that number and take 2-3 off of it and that’s where your power valve should start opening. Do not use 1/2 idle vacuum as that is the wrong way to do it.

If you have 15 inches of vacuum, ideally you’d want a 11.5 or 12.5 power valve, but you can’t get those. 10.5 is the highest you can get so that’s where you start.

If you have 10 inches of vacuum at cruise you’ll want an 8.5 or maybe a 6.5 power valve if you are set up to get on the boosters pretty quickly. That’s controlled a little by emulsion and then by the main air bleed.

When you remove the power valve you will see two small holes. They may be tapped already and have brass in them or not. It’s a crap shoot on that.

The number on the power valve tells you when it opens and those two holes control how much fuel the engine gets when it opens. Changing the number on the power valve will open it sooner or later depending on manifold vacuum. Those two holes control how much fuel the engine gets when the power valve opens.

You need to drill and tap those holes if they aren’t already. Once you understand the power valve circuit and what it really does you can have an incredible tune up that is crisp, clean and it will rival EFI all day long.

Like I said, you still shouldn’t change a main jet. Not just yet. Once you get all the above done as your base line, you can drive the car and see how it responds.

Also, before you drive the car, get the timing curve in shape. I’d start with 18 degrees of initial and 34 total. The curve should start adding timing at slightly over idle speed but no sooner than 1200 RPM. Ideally the curve should continue to add timing until max RPM but that can be time consuming to test. So get it all in by 3200-3400 and start there.

Also, if your distributor has a vacuum advance, use it. It will make the engine run cooler and get far better fuel economy than you will without it.

Now, back to driving the car. At this point you need to understand the relationship between the power valve circuit and the primary main jet circuit. If you don’t understand that, you’ll never get the tune up correct and you’ll suffer from the dreaded “hollyitis” which is solely on the tuner and Holley for decades of bad tuning advice.

The power valve allows the primary main jet to be much smaller than it would need to be at Wide Open Throttle. That’s because at low(er) manifold vacuum that power valve opens, allowing additional fuel to the engine. It really should have been called the “economizer” valve because that’s really what it does.

Once you are at this point, you reduce the primary main jet size 2 numbers at a time until you get a SLIGHT lean surge at a cruise on flat ground. Once you get that, go back up 2 jet sizes and do not change the primary main jet again.

Thats because once you get off the T-slot (I forgot to mention that you need to be at enough throttle opening to be on the main jet and not the T-slots) you are cruising on the primary main jet. You want that jet to be as small as possible for a clean cruise and the best fuel economy.

Once you have the primary main jet dialed in, you can work on WOT tuning.

At this point, with an .0625 restricter in the Power Valve Channel Restricter (the holes you may have had to drill and tap behind the power valve) you need to start 8 jet sizes BIGGER on the secondary main jet. This is because you don’t have a power valve back there.

No matter what, when making tuning changes for WOT you do not change the primary main jet. If you need more fuel for WOT, then add some of that fuel at the secondary main jet and some of it at the power valve channel restriction on the primary side. Make the holes in the brass a few thousandths at a time. You don’t want to add all the fuel at the secondary main jet for WOT. You split it as best you can between the secondary main jet and the power valve channel restricters.

The opposite is true if you need to lean the tuneup out at WOT. Take a couple of jet sizes out of the secondary side and a few thousandths out of the PVCR. Keep making both smaller until your WOT tune is spot on.

As long as you do it this way, your cruise tune up will always be clean and crisp. That’s why once you have the primary main jet sorted out for cruise you don‘t ever change that.

Now you can work on your cruise tune up on the T-slots. When you are hopping around town, screwing the pooch you’ll most likely be on the T-slots unless you run pretty deep gears in the rear axle.

The tune up procedure is the same as tuning the cruise on the primary main jet. You reduce the T-slot restriction until you get a SLIGHT lean surge and then add .003 to the T-slot and send it.

If you bother to do all the work your carb will rival EFI for economy and driveablity AND it will make more power.

Its rather time consuming but well worth the effort in the end.

Just changing main jets to try and unscrew a poorly tuned carb is a fools errand. You’ll never get it right by doing that.”