Carburetor Jets

The following topics are discussed in this article -

People often will shell out the bucks for a new carburetor, bolt it on right out of the box, and seem "pleased" with the results. Others are upset by a backfiring, or smoke belching monster. In either case, spending the time to properly jet your carburetor is a very worthwhile task. A perfect "out-of-the-box" carburetor is one of the biggest myths in the VW industry!

So why worry about the jets? Well, if the fuel/air mixture is too rich, the extra fuel will wash the film of oil off of your cylinders and wear out your pistons, cylinders, and rings quickly. Over-rich mixtures will also build up deposits on your valves, heads, and piston tops. If the fuel/air mixture is too lean, the engine will run very hot, stumble, and misfire -- and burnt valves can be the result. It's important that you get it right.

And fuels! much has changed in recent years. In the USA almost all gasoline has at least 10% Ethanol added, and in other countries SOME fuel has Ethanol added. Ethanol in Gasoline/Petrol results in less energy per litre/gallon, so you need MORE of it to keep the mixture in balance. That's fine for modern cars with engine computers which can adjust the mixture on-the-run, but what about our old fixed-jet carburettors?

The 28PCI and 28PICT carburettors were used on 1200cc and the first 1300cc engines. The 30PICT/1 carburetor was used on most 1300cc engines (about 1964 to 1967, and in some countries, up to 1970). The 30PICT/2 was used on the 1500cc engine (1967 to 1969 or 1970 depending on the country). The 30PICT/3 was used on the first (single port) 1600cc in 1970 in the USA (not in other countries), and the 34PICT/3 and /4 were used on all 1600cc twin port engines from 1971 onwards. The /4 version was used in California and had additional features to reduce emissions. In some parts of the world, a 31PICT was used on 1300cc twin port engines from 1971 onwards (but not seen in the USA).

The modern H30/31 carburetor replaces the 28, 30 and 31 series carburetors and can be used on 1200cc, 1300cc, 1500cc and 1600cc engines with appropriate jetting. On the twin port engines, it has to be used with a 30/34 adaptor, since the twin port manifold has a larger mounting flange than the single port manifold. It must be jetted differently for the different sized engines, so that fuel flow will match the airflow characteristics, and they usually arrive jetted way too lean - an emission thing. For example, when used on a 1300cc engine with a vacuum distributor, the H30/31 should usually have a main jet of X122.5 and an air correction jet of about 140Z; but when used in a 1600cc single port engine, it should have a 125 main jet and a 125 or 130 air correction jet.

The 34PICT/3 carburetor is usually used on 1600cc or larger engines. Assuming a vacuum distributor, the 34PICT/3 carburetor is usually jetted with a X127.5 main jet, and 100Z -120Z air correction jet.

For the 1600cc engine, you need more fuel flow, so a larger X127.5 main jet and 100Z or 120Z air correction jet are needed.

Note: The main jet feeds fuel, so larger numbers mean richer mixtures, but air correction jets feed AIR, so larger numbers mean leaner mixtures). "Lean" means less fuel with the same amount of air, and of course "rich" is the opposite. The optimal stiochiometric (balanced) ratio is 14.5:1 -- that is, 14.5 parts air to one part fuel. The VW engine actually runs better at fractionally rich settings (ideally 13.8:1 rather than 14.5:1 according to Bob Hoover).

More x's mean that jet is working more.

The main jet provides fuel from about 1500 rpm right up to maximum rpm. The air correction jet prevents the mixture going over-rich at high rpm (again, smaller numbers means richer for this jet).

The fuel flow through each jet is determined by the airflow which creates a low pressure area near that jet. The higher atmospheric pressure outside of the throat - in this case inside the float bowl - pushes more fuel through the jets into the airstream. The effect is not linear though, so as the airspeed increases, the main jet will start to deliver a richer mixture (too much fuel), so the air correction jet is needed to add air and stop the mixture going over-rich, then the air correction jet starts working too well at higher airflows (engine revs) so the Power Jet (Aux jet) is need to add more fuel again. It's a complex problem that faces ALL open throat style carburettors.

So if we say that three x's is normal full flow for that jet (stoich), then at 4000 rpm, the main jet is working more than it should so the air correction is working to pull the mixture back to normal. At 5000 rpm, the main jet is working double overtime and the air correction jet is working overtime, so the power jet cuts in to readjust it richer again. Nothing is exactly that linear of course as the jets are all different sizes, but maybe it helps visualise the problem of keeping the fuel flow balanced at different rpm/airflows.

To summarize: idle jets operate from idle to 2500-3000rpms or so. Main jets are primarily from 2500-4500rpms, and Air jets affect the engine from 3500/4000 on up. Lastly, you have emulsion tubes to change "where" it's rich, lean, etc. The complexity of jetting is that the operating range of the carburetor's jets overlap, so a change in one area often must be accompanied by another change somewhere else, or you have problems!

Paraphrasing from John Muir* -

Rob wrote in this regard - See our article on Reading Spark Plugs. The center electrode should look quite clean and grey/white. There should be a light smooth coating of carbon on the rim of the threaded part, and a colour change on the outer electrode of grey (at the tip) to black (where it attaches to the rim of the threaded part), with the change of colour on the corner of the bend. All this indicates a good mixture and correct heat range.

Rob's note regarding modern fuels: There are two major issues with modern fuels. First - Muir above was writing in the 1970s, before the USA changed their octane rating system, and also before Ethanol became common in fuels. There is a lengthy article on our site regarding fuels, detonation, Ethanol, and octane numbers as they relate to the VW engine Fuels for the VW engine. Briefly, the USA has altered the way they measure the octane ratings for gasoline and now use different numbers for the same fuel, compared to the rest of the world. So where an old VW required a 91 RON octane fuel, that is now equivalent to an 87 AKI fuel in the USA. 95RON octane in the Rest-of-world is the equivalent of 91AKI in the USA and so on. And the second issue is the presence of Ethanol (alcohol) in gasolines. Most gasoline in the USA now has at least 10% Ethanol added (E10), and in other countries, SOME fuel grades have 10% ethanol added. There are a few fuels which have up to 85% ethanol in them (E85). Ethanol contains USED oxygen which canoot be burnt again, so adding ethanol to gasoline/petrol, reduces the amount of energy per litre/gallon. That means that the jetting requirement for any VW using ethanol-added gasoline/petrol is different to those using straight hydrocarbon gasoline/petrol. Ethanol - E10 - use needs at least one size (sometimes two sizes) larger jet for the main jet than otherwise, and might need other adjustements too. E85 - forget it for the aircooled VW engine!

Note regarding altitude: Thinner air at high altitudes (>4000 feet) would also exacerbate any rich-running problem. An altitude of 5000 feet is roughly equivalent to a one size main jet difference. For example, the X130 main jet is a tad rich for the vacuum dizzy at sea level, so if the car is to be operated at a high altitude it will be running very rich, and you might get a smoother engine and less exhaust soot with an X127.5 or even an X125 main jet (the "right" size for 5000 feet altitude).

The 1200cc engines usually used a X120 or X122.5 in the 28PICT carburetors, the 1300cc engines used either X122.5 or X125 in the 30PICT carburetors, the 1500cc engines usually used a X125, and the 1600cc engines usually need a X127.5 in either 30PICT, H30/31, or 34PICT, but MIGHT get away with an X125 in the smaller 30PICT and the H30/31.

Rob notes -- I have a 1500cc single port engine with it's original 30PICT/2 carburetor. The carburetor has a 55 idle, X125 main and 125 air correction jet. I've used this particular carburetor with a 1600cc dual port engine and it works fine with the same main jet, but seems to like a fractionally larger X130 air correction jet (it runs a little rich with the X125 air correction jet). Others might find that the 30 sized carburetor on a 1600cc engine works better with a larger X127.5 main jet. There is enough variation in fuel types, wear and tear on engine and carburetor, altitude and so on that you might need to experiment a little. All suggestions here are just that - suggestions.

When you work out the area of the hole in each jet, the steps are roughly 2% increase in fuel flow for each size up (X125 to X127.5 for example). But engine sizes have much more variation, 1200cc to 1300cc is about 8% more, 1300cc to 1500cc is nearly 14% more, and 1500cc to 1600cc is about 7% more. You'd think the jet sizes would follow the capacity more closely, but they don't.

All Solex carburetors tend to run lean at high power settings (high rpm and open throttle). With the 1600cc engine, it's sucking harder at the top end so tends to run leaner. This means that the main jet is sized to the engine capacity, rather than the carburetor size, and X127.5 is the more common size for the 1600cc engine, rather than say the X125.

Bentley (page 20 of the 'fuel' section -- table of 'carburetor setting and jets') says the X130 was normal for '71-'72 34PICTs, and X127.5 was normal for '73-'74s (non-California models). We have heard it said that the X130 (or X127.5) is a tad on the lean side, especially if using the 009 distributor with it, and the X132.5 gives a fractionally rich mix (which VWs have always liked better than lean).

The X127.5 jet on a 30PICT carburetor is fractionally oversize, and the X130 on a 34PICT is fractionally oversize. As indicated previously, the VW engine actually runs better at fractionally rich settings (ideally 13.8:1 rather than 14.5:1 according to Bob Hoover who did lots of testing), but they are usually jetted a little leaner than that, so the X127.5 is really a minimum jetting for the 1600cc engine.

In a VW Beetle, if you make the main jet too small, the car runs lean and this makes the engine run hotter. Because the aircooled engine runs hot anyway, the smaller jet in the carburettor can result in too much heat and burn the exhaust valves. You can also get detonation (pinging/knocking) if the engine gets too hot.

The main jet is accessed by removing the big brass bolt on the left side of the PICT carburetor, right at the bottom of the carburetor bowl. Unscrew this bolt and look at it -- if there is a jet screwed into the bolt (older models), you can deal with it in hand, so to speak. But if you find a solid brass bolt, the main jet is further in the float chamber (later models) but still right in the back of it on the right-hand side. In these later models, you can easily change the jet with a medium screwdriver either with the top of the carburetor removed or with the carburetor completely off the car. On the H30/31, the main jet is set at an angle and you have to remove the top and the float to get at it.

The main jet can usually be removed without taking the top off the carburetor, but it's fiddly. Remove the brass plug with a hex head on the left of the carburetor -- bottom of the float bowl (all the fuel in the carburetor will rush out). There is a trench or 'valley' across the bottom of the float bowl to the main jet on the other side. The jet has a slot in the back so it can be loosened with a screwdriver through the hole where the plug was. The fiddly bit is pulling the jet back through the hole. It's actually much easier to do if you remove the top of the carburetor and the float. Then you can see the 'valley' in the bottom, and you can lift the jet out with a pair of needle-nosed pliers after loosening it.

Rob's note: When removing the main jet, I use a small glass jar (pickle or baby-food jar) to catch the fuel from the brass plug - I hardly spilled a drop (I just allowed the plug to fall in to the jar), and used the fuel to refill the float bowl later. I remove the top of the carburetor, then run a thin screw driver (which avoids the coil) through the plug hole to get to the jet. Once the jet is loose it sort of falls on it's side, so then I use the screwdriver to nudge the jet out along the trench to the plug hole in the side (rather than using needle-nose pliers). Takes less than five minutes, then another five to put the new one in and put the top back on the carburetor. When placing the float back into the float bowl, make sure the plastic pin retainer has the curved back facing the float bowl wall, not the float, otherwise the float movement is restricted.

Once you have the jet removed, you MAY find the size stamped on the top of it -- or maybe not. Dave bought a Pierburg model 34PICT/3 carburetor from Aircooled.Net, and none of the jets had the sizes stamped on them. The proprietor of Aircooled.Net thinks the main jet is a size X130, and the idle jet is size 50.) Dave replaced the main, idle and air correction jets so he knew what he had. Dave jetted his new carburetor as follows: main jet - X130; idle jet - 55; air correction jet - 80Z.

Replacing the main jet in is super easy -- just put it in the hole in the side of the carburetor, then put the screw driver in after it and slide it along the 'valley' that runs across the bottom of the float bowl. Look down into the top of the carburetor to make sure that the screwdriver is squarely in the slot in the jet and that the threads line up squarely, then just screw it in (the small screw driver sits deep in the slot in the jet and helps line it up).

It's possible to develop partial blockage in one or more jets. There are three (or four) removable jets. Unscrew the idle jet from the carburetor and the cut-off solenoid, and blow through it. Remove the main jet (as above) and blow through it (check for crud in the float bowl at the same time). Have a look at the size stamped on it too. Recommended sizes are given in this article.

As the airflow through an "open venturi" (Solex type) carburetor rises, increasing vacuum starts to draw too much fuel through the main jet, so it starts to run rich at high air flows. To compensate for this, an air correction jet is inserted in the fuel delivery circuit after the main jet, which starts to add air to the fuel emulsion as the airflow increases. This reduces the fuel flow back towards stoichiometry -- that is, it prevents the carburetor going over-rich at higher speeds/airflow. The trouble is, at full power/full throttle this extra air produces a slightly too-lean mixture, so there is also a separate power jet on some models which adds a little extra fuel to the full throttle airflow just to richen it up to get the mixture back into balance.

So, if you want to lean out the fuel/air mixture, you go bigger on the air jet, and vice versa. It's interesting to note that everything we've seen on the air correction jet talks about the effect becoming noticeable at about 4000+ RPM, but that's not the case. The effect is clearly noticeable from 3000 RPM upwards (just under 60 mph), and probably a little lower than that. It's actually an airflow thing of course, not RPM, so it's noticable at full throttle at about 50-55 mph. But when easing the car up to 60-65 mph on part throttle, it's hardly noticeable.

Note: The air correction jet size is very important when using a smaller carburetor on a larger engine (e.g., a 30PICT on a 1600cc DP engine). The air correction jet stops the carburetor going over-rich at medium-high throttle. The 1600cc DP can flow more air than the 1600cc SP and smaller engines. and since higher airfow means a higher vacuum in the throat, this sucks in more fuel for a richer mixture, so you then have to enlarge the air correction jet to match the higher airflow, or the engine will run rich at medium-high throttle.

The air correction jet is also changeable. Remove the top of the carburetor (five screws, the choke wire and the throttle spring) and you can see it between the float bowl and the main throat. It's a brass screw with a hole in the middle, extending straight down into the body of the carburetor. Be careful pulling it out -- it's about an inch long. The air correction jet has the emulsion tube attached to it -- this "foams" the fuel before it's added to the airstream in the throat.

The air correction jet doesn't affect idling very much since it helps control the mixture at higher rpms, but worth a look to see if you have a fuel blockage problem in the carburetor. What you are mainly looking for is the condition of the carburetor. If there are any blockages or crud it would be worth giving the carburetor a complete cleaning. See our Carburetor Overhaul Procedure.

With the air correction jet you have to go 3-4 sizes different to see a real difference, unlike the main jet where one size can make a big difference. Air is much less dense than gasoline, so that makes sense. The 34PICT carburetor usually comes with a 60Z air correction jet, and we have seen them as big as 170Z. So there is a HUGE difference in hole size compared to the main fuel jet.

Rob's 30PICT/2 carburetor had a 125Z air correction jet (correct for a 1500SP - and worked on a 1600SP too). Rob replaced the air correction jet with a 140Z, and at low-middle speeds there is no discernible effect. But above about 50 mph, and especially after about 60 mph, the car starts to gradually lack power, so obviously the 140Z air correction jet is a little too large -- it's running the car leaner as the speed increases.

The larger 34PICT carburetor should have the larger X127.5 main jet, and since the larger 34PICT carburetor doesn't have the extreme airflow with a 1600cc DP engine that the 30PICT carburetor does, the over-rich problem is reduced, so you use a SMALLER air correction jet - less correction is needed.

Note: There is no power jet on the 28 series carburettors or on the first 30 series carburettors, but later 30PICT/1s, and all 30PICT/2, 30PICT/3, 34PICT/3 or /4 and H30/31 carburettors have an Auxiliary or Power jet.

The auxiliary (power) fuel jet is a small brass jet screwed into the right side of the carburettor, sittin in a protrusion running up to the top. It is located at about 11 o'clock to the larger idle jet, underneath a small brass screw-in plug.

The power jet only works at higher throttle positions and higher rpms. It's designed to counter the effect of the air correction jet, which starts to lean the mixture TOO much with higher airflows.

To explain, the main jet supplies the fuel, and it supplies too much as the rpms increase (a function of the increasing vacuum in the main venturi). The air correction jet starts leaning the mixture to bring it back into balance, but as the rpms (airflow) rises to near the maximum for that carburettor, the air correction jet works too well, running the engine lean. The power jet counters the leaner condition at higher rpms, richening the mixture back into balance again.

Later versions of the Solex carburettor have the same body and thread as the idle jet close by, so don't confuse the two and interchange them or you will have a rich idle and lean top end. The most common power jet size on all Solex carburettor versions is 65 (except for the H30/31 which usually has a smaller power jet - around 47.5). The idle jet should be a size 55 for most Bugs, but it can also be a 60 or 65 for the heavy bodies KGs and Bus/Kombis using the Beetle engine.

Where the early style carburettor has the idle cut-off solenoid on the RIGHT side of the carburettor, that solenoid sits in the back of the idle jet. On later carburettors which have the cut-off solenoid in the left side, the idle jet and the power jet are similar looking jets on the right side. The idle jet sits in a protrusion running down to the bottom of the carburettor, and the power jet sits in a protrusion running up to the top of the carburettor, so it's easy to tell which is which.

There is an pilot (idle) jet on all carburetors too - these are normally left at 55 for most carburetors (28, 30, H30/31 and 34 series), on any engine size (since it's only feeding the engine at small throttle settings anyway). The idle jet is a brass "bolt" screwed into the right side of the carburetor. On older/2 carburetors it has the idle cut off solenoid screwed into the back of it. On the /3 and /4 carburetors - it looks like a bolt with a tiny silver plug in the center. On some models (the H30/31 for example) it is angled slightly towards the back of the car. On the H303/1 it is often oversized too - a 65, used to stop the engine stalling at low rpm as the main jet supplied with these is often VERY lean 117.5 or 120.

The carburetor idle circuit controls how much fuel you have at idle. Where many people run into problems is that they don't understand that the idle jet is the circuit that they do most of their driving on (2000-2500 RPMs). This jet is critical to engine flexibility and economy.

Most carburetors come out of the box with a 55-60 idle jet. Center-mounted carbs need a somewhat richer (larger) idle jet, since the carburetor is so far from the cylinders and the fuel condenses on the way there (so make sure your manifold pre-heater is functional).

If your car is having a problem with spitting/sputtering/cracking, this is an indication of the fuel/air mixture being too LEAN. The speed of the engine (RPM) when it's doing this tells you which jet you need to change. If the engine is stumbling at 2000 RPMs and lower, then you need to richen up your idle jet (go to a larger size) - but check carefully for air leaks around the throttle shaft first!. If the problem occurs at higher RPM (2500-4000), then you need to richen up your main jet. (Some of this material was adapted from John Connolly's article on How to Jet Your Carburetor.)

Idle jets are available in small increments. Normally, shops stock them in round sizes of "5"s, like 50, 55, 60, etc. You can find 57s, etc. if you look around to really get it right on! If you have a significantly larger idle jet (e.g., 70 or so), John Connolly recommends going down until you can not get a good engine response with the mixture screws anymore. Idle jet sizes of 70 or so will "run" on anything, but they don't run WELL (unless fouled plugs or 12mpg are your idea of "well"). In addition, if you have a problem with a 55/60 idle jet, and you have the right main jet, and still have a hesitation regardless of accelerator pump adjustment, you need to play with your idle air bleed (what the fuel from the idle jet mixes with). Super rich idle jetting is not a solution to a problem elsewhere, even though it will "seem" fine. Normally, for stock daily-driver Bugs, you will find that an idle jet size of 55-60 is right for you.

The idle cut-off valve is not strictly a "jet," but this is a good place to discuss it. The idle cut-off valve sticks out the left side of the carburetor on H30/31 and 34 sries carbs (but on the right side on 30PICT series) and has an electrical connector on the end. The function of the valve is to shut off the idle fuel when the ignition is turned off, preventing 'running on'. It provides fuel at low medium revs too, so if it's not working you'll get a rough idle and lean mix at lower revs.

The idle cut-off valve receives power from the positive (+) terminal on the coil (#15) -- see the schematic which shows this wiring arrangement. "N" on the schematic is the coil. You will see a black wire coming into the positive terminal on the coil from fuse #12, which receives power from the ignition switch, so the valve opens whenever the ignition is on, allowing the idle fuel to flow.

Note the three black wires leading from the positive terminal on the coil -- one to the backup lights, one to the automatic choke, and one to the idle cut-off jet. That terminal on the coil is just a convenient place to obtain power to these components. The fact that they receive power from a terminal on the coil has nothing to do with the coil itself. The three wires could each go up to fuse #12 individually, but that would be very inconvenient. So VW chose this configuration.

If the idle cut-off valve fails in the shut position, the car will still run (roughly) on the main jets but will stall as soon as you take your foot off the throttle. If the valve sticks in the open position, the idle fuel circuit will remain open, so if the engine is still turning it will still draw fuel through the idle jet, resulting in running on if there is a hot spot in one or more cylinders. With the idle cut-off valve working correctly, hot spots in the cylinders don't occur as readily, since there is no fuel available to keep the engine running once you turn off the ignition. It's a problem VW recognized very early on (since the aircooled engine has very hot heads and hot-spots were therefore more likely than in water cooled cars), so the fix was to add the cut-off solenoid to the carburetors.

It is very easy to check the idle cut-off valve to make sure it is functioning properly. Turn the ignition on (but do not start the engine). Disconnect the electrical lead from the valve, then touch it to the connector again. If it's working, you'll hear a clicking sound as the relay pulls in. If you do not hear this clicking sound, it's time to replace the idle cut-off valve. They are cheap enough.

When replacing the idle-cut off valve in the carburetor, don't screw it in too hard ... it has fine brass threads and it's screwed into the soft metal of the carburetor. Also check that the valve hasn't worked loose in the carburetor -- they do sometimes, and then work only intermittently. And of course make sure that it's properly connected electrically to the connection on the coil where power is provided (the + connection) -- the lead can break off.

The emulsion tube mixes air from the air corrector jet with fuel from the main jet, and controls the mixture from the discharge nozzle at all engine speeds that the main jet is working. Basically, the emulsion tube changes the diameter and location of small holes in it's body.

We can only give a guide here, as engine age, type of fuel, carburetor wear, etc. all make a difference in how our old engines run, so you might need to experiment a little.

For information -- the 1200cc engines originally came with a 28 PCI or 28 PICT carburetor, both of which have a 22mm throat. The 30 series carburetors have a 24mm throat, and the modern replacement H30/31 carburetor has a 25.5mm throat. The 30 and H30/31 carburetors were designed primarily for the 1300-1600cc single port engines, but you can get slightly better performance with the larger carburetors on a 1200cc engine, but only if they are set to run properly with the smaller engine.

The standard jetting that the 30PICT/1 & 2 carburetors came with is size X122.5 or X125 main, size 55 idle, and size 125Z air correction jets. The 30PICT/3 and the H30/31 carburetors came with size X112 main (super lean), size 65 idle, and size 125Z air correction jets, which made them very difficult to tune. The larger size 65 idle jet helped the 30PICT/3 and H30/31 carburetors overcome the very lean main jet at idle and low speeds, and thus help to reduce the "bogging" problem.

When using the 30PICT or H30/31 carburetor on a twin port manifold and any vacuum distributor, change the jets to X125 or X127.5 main, 55 idle, and leave the air correction at 125Z. Try the X125 main before you try the X127.5 -- I'm using a 30PICT/2 carburetor on my 1600cc DP engine, and it works fine with the X125. But engines do differ a little, and the sizes above are "ball park" figures rather than definitive.

Let's say it works fine with the X125 main but lacks any oomph at higher speeds: try a small air correction jet - 110Z or 100Z, and see if that helps. (See the discussion of the air correction jet above.)

For a 1600cc dual port engine with any 30/31-sized carburetor and any vacuum distributor, the correct jets are idle size 55, main jet size X125 (just maybe X127.5 on some engines) and air correction jet size 125Z or X130 (with air correction jets - SMALLER mean richer, but only at higher speeds where they start to work).

If you live at altitudes above 5000 feet, you also have to consider that, as high altitude results in the engine running rich (less oxygen in the air but the same fuel flow through the carburetor). It this case you would definitely need to reduce the main jet size to say the size 122.5 or the 120.

On the H30/31 carburetor, there are two brass jets on the right side (right is right of car). The angled one (points slightly towards the rear of the car) is the idle jet. The other one (pointing straight out to the right side of the car) is the power jet, which feeds additional fuel at high throttle/high rpm. The power jet is needed to correct a tendency for the air correction jet to work TOO well at max airflow - it leans the mixture off too much and the power jet corrects this.

If you look at the two jets you can see that the (angled) idle jet sits in a protrusion (channel) which continues downwards under the jet, towards the idle and transfer ports in the throat near the butterfly.

The power jet sticks straight out to the side, and sits in a protrusion nearer the top of the carburetor - if you remove that jet you can see a drilling straight through the carburetor body to the air correction jet/emulsion tube where the the main fuel supply is delivered into the throat of the carburetor.

The idle jet is usually a size 55 (when the main jet is normally a size X125 or X127.5) but it might be a size 65 if the main jet is a X122.5 or smaller.

With the larger main jet, you might also need to consider a size 50 idle jet in lieu of the normal 55. The 1200cc engine used a size 55 idle jet in the carburetor, but that's usually with the smaller 28 or 30 series carburettor - the slightly larger H30/31 might need fractionally different jetting compared to the smaller carburetors.

The power jet is usually a size 65. In most cases you can just leave the power jet alone. It only works at full throttle and high RPMs ? the size 65 power jet should work fine for those conditions.

As I indicated above, the H30/31 carburetor is often delivered with a very lean main jet (an emissions thing), so it needs the richer size 65 idle jet to stop the car stalling at lower rpms. If it has the small main jet, then you'll get better performance and easier tune-ups if you change it to a size X125 (for 1200/1300/1500cc engines) or a size X127.5 (for 1600cc engines) and change the idle jet to a size 55.

Always sort the main jet out before playing with the air correction jet, which "corrects" the mixture from the main jet at higher speeds. If it seems a little rich at higher speeds (plugs sooty after a plug read) then try the 130Z air correction jet.

I've been running my '68 Bug with the X127.5 main jet in the smaller carburetor, and he's running a tad rich. The exhaust system peashooters started turning black inside as soon as I put the larger jet in. I haven't run enough through the tank to get a real idea about consumption yet, but since I don't notice any huge difference in performance I'll probably be going back to the X125 soon. Useful experiment anyway.

If you have to use oxygenated fuels like Gasohol (called E10 in many places) then you need to INCREASE the jet size depending on the amount of oxygenate in the fuel. E10 usually needs a larger main jet and might also need a larger idle jet.

Some tests to help you determine proper jetting. Take the car for a run at a modest speed - say 35-40mph. Use a steady throttle - minimum acceleration. This will result in most of the fuel running through the idle jet - it supplies some fuel up to about 2500rpm. Then remove a spark plug and read it (see our Spark Plug Reading Guide). A sooty plug means the idle jet might be a little large. A clean plug with a light grey/tan look means the mixture is about right. The try the same test at a higher speed - a steady 55-60mph. This will have the main jet supplying the fuel, and read the plug again. Soot will indicate the main jet is a little too large.

If you have a 009 (centrifugal advance) distributor, you may find that the spark plugs still seem a little dry/burnt looking at medium speeds , or the car lacks acceleration in the mid ranges. If this is the case, try a larger size main jet -- size X127.5 1200/1300/1500cc engines. It should not need to be bigger than that with a vacuum distributor (009 distributors need richer mixtures). For 1600cc engines, use an X130 main jet. In both cases, when running the 009 distributor, set the accelerator pump for maximum squirt.

Once you have those two sorted, then try a faster run and from 60 try flooring the throttle - if you get a good smooth response the air correction jet should be OK, but if it tends to stumble a little or feel breathless, then either the main jet is a little small or the air correction jet is a little large.

None of these tests are definitive - just indicative. The only method of getting it exact is to run the car on a dynomometer with an exhaust gas analyser in the exhaust pipe.

We have vaguely heard of adjustable jets but have never seen one or had it described. It appears to replace the plug in the bottom of the 30 PICT/1 carburetor float bowl, which has the jet mounted inside and in line with that hole.

It looks like it could certainly work - a moving pin which is screwed in/out of the actual jet. Just start off with a larger than normal size (150 maybe) and use the screw to increase/decrease the actual hole size.

Interesting.

Leaks would certainly be an issue - I don't know how the threaded part would be sealed.

The adjustable jet would work with any of the Solex carburetors which have the jet mounted horizontally inside the carburetor. Some of the replacement carburetors (H30/31 and 30PICT/3 maybe???) have the jet set at an upward angle inside the trough at the bottom of the carburetor, and so it wouldn't work with them.

It would be interesting to try using one an adjustable main jet, but there are some dangers too ? such as, how do you know what mixture you are setting, and if you are running the car too lean and hot, or way too rich? The performance would tell you some of the answer, but inexperienced folks using one might get themselves into trouble where using a fixed (normal) jet and changing it for a larger or smaller one would give you a known change in fuel flow.

We need to do some research regarding the adjustable main jet. As we learn more, we will include it here.

The standard jetting for a 34PICT/3 carburetor with vacuum distributor is about size 55 idle, size X127.5 main, and size 100Z-120Z air correction jets. All three of these jets can be changed. Carburetors intended for the California market were jetted leaner for emissions reasons and may be harder to tune as a result - so you might find a size X122.5 main jet and size 140Z-170Z air correction jets. A loss of power is the result -- about 58hp instead of 60hp.

On the premise that the 1600cc engine likes to run just a tad rich, many people will change out the main jet for a size X130. Dave has done so with good success.

See the discussion of jetting with the 009 distributor below for needed revisions to the jetting of the 34 PICT/3 carburetor in this application.

Following are the jet and other specifications for the 34PICT/3 carburetor -