17 apr 2022

Tuning the Weber carburetor for (my) non-standard application

For the Rambler 195.6 OHV (3200cc) inline six

reference: Top End Performance, highly recommended by me.

reference: Reline/Weber IDF adjustments

reference: Weber float level

reference: Weber factory tuning manual (the final word, as disorganized as it is)

reference: VM flat-four, but many useful ideas and tips

reference: tim's roadster, weber tuning

reference: 240 260 280 weber tuning

reference: SAFRtool.com, target A/F ratio values

reference: Innovate Motorsports, relationship between spark advance and fuel mix

reference: Rambler 195.6 OHV inline six

Below I describe how my experience differed from "standard advice" found elsewhere. RTFM first, then this. I assume you know how a carburetor works and can figure out how to setup a Weber.

The final authority on Weber carburetor operation and tuning is the factory manual above.

Old inline motors are not sports car motors

Older american-type inline sixes like ours differ from european sports cars in how they meet their design goals. There are some terminology differences too. Though this written specifically for my warmed-over Rambler 195.6 OHV, this would apply to small Ford Chevrolet and Dodge/Chrysler engines of the 50s, 60s and 70s.

The common sports-car application is one or two venturi per cylinder. The path from throttle plate to intake valve is short. Most sports-cars are oversquare, have relatively hot cams that accentuate intake pulsing. Most carbureted sports cars seem to have no PCV or other air-injecting accessories. While RPMs are higher the volume of air drawn per-stroke is lower.

In contrast my engine, like a lot of 50's and 60's american inlines, is undersquare, six cylinders, and has a very mild cam. All cylinders are manifolded to a single carb. The fuel/air intake is likely considered to be a "flow" (it's still bumpy, but less so) and manifolding, mild cam and long stroke means the carb sees a much stronger, smoother draw through the venturi. RPM range is lower than sports-engines so the rules of thumb built into standard advice ("main jet operation begins at XXXX RPM") is often just plain wrong.

These differences manifest in tuning details, in progression circuit(s) operation, and the resulting state of tune.


This info is all over the place, but I found this nice summary from SAFR tool, succinct and to the point. Here is a copy of TARGETING AFR VALUES in case that page goes away. Note that the IDF series don't have high speed enrichment eg. power valves; they are assumed to be for performance use and so are always set somewhat rich. A better way to think of the IDFs and DCOEs is that they lack "economy" circuits. And they don't generally have chokes.

Spark timing


I have an Autometer in-dash A/F ratio meter installed. Tuning by ear and the seat of your pants is still important, but I now can't imagine tuning without an A/F meter. Best $200 i ever spent. I have a homemade datalogger system and it reads the analog voltage off the meter and stores it as AFR once per second.

I have found that the meter is required for highway cruise jetting. For low speed and acceleration/WOT, it jives with by-ear and seat-of-the-pants where so far it seems to want one-jet richer than what the meter says, which also fits with general carb advice for other engines.


Set float level first!

First thing, take the top off and set the float level. I have not yet had one Weber carburetor with the float level set even approximately correct out-of-the-box. My 44IDF was supposed to be 10mm and it was more like 5mm. The 40 IDF... well see for yourself, below. The float is resting on the gasket (float is set with the carb top vertical).

Adjustment requires some care as the tabs you have to bend are small and stiff and attached to delicate plastic floats so you pretty much have to pull the pin, bend, reassemble, check, remove, repeat, ad nauseum. While you're in there verify what jets are actually installed, including the squirter return jet.

I made tools from a pieces of aluminum sheet; cut and filed to exactly 10mm and 32mm high, used as a feeler gauge.

Fuel pressure

American cars of a certain era have mechanical pumps that deliver 5 - 7 psi. This is too much for Webers that want 3 - 4 psi. I run a Carter P60504 electric (vane) pump. They're cheap and work great. Put a filter on the inlet to the pump, grit will ruin it. OEM diaphragm pumps are happy pulling fuel; electrics push. pulling (often, air) will burn up the pump. Put the pump out back and, best, low, so gravity primes it.

My tuning experience

The above is a pretty great chart, found on at the 240/260/280 Z website listed above. Where it says "RPM" think "relative air mass flow" (there's that sports-car-engine assumption). As flow increases from idle to maximum, the contribution of fuel from each circuit -- idle mix screws, low speed jets, main jets -- changes. The crossover can be quite distinct.

the chart is just a rough guide, and it understates the effect of the idle mixture which affects much "low speed" driving.

Weber carbs are tuned one circuit at a time; idle, progression, and only then, main jets/air jets. Carburetor tuning is necessarily iterative; make it idle, determine low speed jet, then high speed jets, and chances are you will be back to revisit idle and progression again, especially new-out-of-the-box.

STEP 1: Curb idle

The first thing to do is to get it to idle. Start with idle screws 1.5 - 2 turns out from bottomed (likely rich). Back off the idle speed screw (throttle stop) until it doesn't touch, then turn it in 1 to 1.5 turns. Start the engine, blip the throttle as necessary to get it warm enough to idle. Unless you have a big vacuum leak this will idle, even with wrong low speed jets. It might run for shit but it will run.

The idle speed and mixure adjustments interact and require iterating. Set speed with the speed (throttle stop) screw, then adjust idle mixture screws (both out exactly the same amount) for fastest idle. If it speeds up too much adjust the throttle stop screw, repeat mixture adjustments until it's perfect. These are initial settings, you'll probably want it slightly richer for driving.

STEP 2: Progression A/F ratio

With a candidate pair of low speed jets, engine warmed up, adjust best idle speed and smoothness as above.

You need to know the range of throttle opening over which the progression circuit is controlling A/F ratio. The range begins when the throttle plate begins to open, and ends when flow is sufficient to pull fuel from the booster venturi. See the paragraph below on how to determine this range.

With a wideband A/F meter

The process is very straightforward with a wideband A/F meter.

If you change low speed jets you will need to redo curb idle settings.

Without a wideband A/F meter

Without a wideband meter, this method is based upon "standard advice". The curb idle is first adjusted for ideal idle, then the actual mixture in the progression circuit is compared to it.

If you change low speed jets you will need to redo curb idle settings.

This non-wideband meter method is taken directly from the Weber factory tuning manual page 43 and conforms to all information in the Weber factory manual. It is critical that the progression circuit mix be correct, and the only way to adjust it is with the low speed jet. With the progression circuit correct, you can always get curb idle mix right by turning the idle mixture screws.

Observations on idle mixture screws settings

Redline documentation dogmatically insists that idle mixture screws must be exactly one turn out; there is no such rule in the factory tuning manual. It is true however that the mixture screws will end up approximately one turn out once you have selected the low speed jets based upon their behavior in the progression circuit.

Standard advice does not account for the air added by PCV systems. Even with wrong low speed jets you can make the engine idle correctly by fiddling the idle mixture screws.

If you get progression right, but the idle mix screws are less than one turn out, you have some unwanted source of fuel; did you set the floats right? If the mixture screws are out two or more turns, eg. adding a lot of fuel to idle, you probably have a vacuum leak somewhere.

How to locate the progression/main circuit transition

Imagine the car on a long straight flat road. Engine running, put it in gear, smoothly open the throttle and accelerate up to maximum speed. The throttle goes from closed ("curb idle") the idle circuit, through the progression circuit, into the main jets.

The chance of an out-of-the-box Weber doing this smoothly and at desired A/F ratio is zero.

The first step to tuning is to figure out which circuit is operating when. The idle circuit to progression circuit was tuned by the procedure above.

With an IDF series carb it's incredibly easy -- remove the two jet/emulsion tube assemblies from the carburetor. The carb will then have only idle and progression, and a huge dead lean hole above that. The VW flat-four page above suggested this and it works great. This does throw off the the progression circuit's A/F ratio (leans it out) so you might have to go up one low-speed jet size just for this test. But it's worth the effort -- a half hour's drive without main jets will provide you with a feel for where progression ends and main jets begin. I assume the same is true with DCOEs.

For the 32/36 and 38/38, or the IDF, another method is to install crazy-lean jets to get the same effect. It's much easier to feel too-lean (bog, engine dies) than too-rich (feels "soft"). An A/F meter helps here, but a too-lean jet will provide the same feel as the above IDF-no-jet.

Work in progress

STEP 3: Main jets, air jets

The flatspot: idle transition holes

With the throttle closed (idle position) the edge of the throttle plate must just block the first transition hole. Off-idle flatspot/bog is caused by the throttle plate being below the transition holes. i don't know what engine will end up with them magically in the right place but mine wasn't one of them. (engines that need more throttle plate opening at idle, which would then uncover transition hole(s) is another set of symptoms and fixes that I did not experience. this is also covered in the weber manual.)

My engine idles at 500 rpm with the idle speed/throttle stop screw 1/2 turn in, which leaves the throttle plate below the first transition hole, creating a brief but sharp flatspot immediately off-idle. The only cure for this is to file a bevel in the edge of the throttle plate; see the procedure below.

this drawing from page 10 of the factory manual is illuminating:

my current state of tune

This is the current (april 2019) snapshot of the state of tune. carb, spark, axle, gears, chassis, usage, etc are all tangled together in a multi-dimensional space.


Current state of tune, Los Angeles, sea level.

40 IDF, venturi 28mm
low speed jets 50
idle stop screw 1/2 turn
idle mix screws 1.5 turns out
accelerator pump adj nut 0.400" in
main jets 170
air bleeds 150
float level 10mm


Spark advance is entangled with carb tune. Pinging (mild detonation; too much spark advance) can be "cured" with excessively rich mixture, and conversely, a rich mixture allows more spark advance than would be possible with a truly correct mixture. for years I persisted with the "maximum spark advance" myth, described very nicely here in Innovate Motorsports' relationship between spark advance and fuel mix.

Working with the A/F meter resolved this for me. as I got my cruise mixtures more correct it got pingy with my high-advance spark map. To separate carb jetting from spark timing issues I installed very conservative spark timing (5 - 10 degrees less at all load/rpm) and continued with carb tuning. Only when jetting was correct/close did I work on spark timing -- and found that far less advance was required for maximum power.


I make and maintain one of these spreadsheets for every car I make. The Excel file is on my website somewhere. gear, tire, engine tell me RPM which tells me where I need to tune. I used this to choose the axle ratio for my cruise speeds and driving habits and prior experience with this engine.

i used this spreadsheet to choose axle and tire to match what I thought the engine was happy with. now, After my experience with this 44 IDF and spark changes, I plan on regearing to 3.73.

Throttle plate and transition holes

Correcting off-idle flatspot

With all circuits tuned fairly closely, I finally got around to working on the off-idle flatspot. in my case this is caused by the idle stop screw position required for correct idle speed and mix: 550 rpm at 14:1 mix, which puts the throttle plate below the transition holes, so that throttle tip-in increases air flow, but not fuel flow, causing the lean stumble.

The fix for this particular condition is to file the edge of the throttle plate where it interacts with the transition holes. This procedure is in the Weber factory manual. clearly it's a last-resort fix, if you remove too much metal, replacing throttle plates is no small deal.

The drawings in the manual are misleading in how much material must be removed: it is far, far, much less than the drawings implied. lucky for me I tend towards extreme conservatism when modifying delicate and expensive objects.

I removed metal using 10 strokes of a fine, and dull, needle file in the tiny area over the transition holes. I did not photograph the result because the change is not visible.

below are photos of how I visualized the problem. Here the carb was removed after tuning, with the idle speed stop set at it's desired half-turn in. I made two reference black spots with a marker and with a fine mechanical pencil traced the edge of the closed throttle plate. Opened here to show the relationship of throttle to holes.

Closed, pencil mark drawn
Open, pencil mark visible

There are two pencil marks, the one closer to the hole (lower in the photo) is with idle stop in 1/2 turn; the higher one is with throttle completely closed, for visual reference. As you can see the scale of things here is very small and effects of tiny changes subtle.

The first transition hole edge was less than a millimeter from the bottom edge of the throttle plate. Make sure you click the image above, then again to get highest resolution to see the scale of things. I took the aforementioned 10 strokes off that trailing edge. It nearly, but not quite, eliminated the dead spot. It will take one more iteration to get it "close enough".

Throttle plate, transition hole relatioships made visible

The photos below show the relationship of throttle plate and transition holes.

If you look carefully at the high-resolution photos below (click each image) around 1 turn open the throttle plate is barely cracked open, about .010" gap perpendicular to the shaft. At about 2 turns open the edge of the first transition hole becomes visible. The flatspot occurs because as the throttle is opened, the crack opens to leak more air but the transition hole isn't uncovered until later opening.

The last photo shows all of the transition holes in the 44IDF, the throttle opened by hand.

As a measure of how much air the PCV system injects, after plugging the PCV the idle stop screw must be turned in (open) one-quarter turn more.

Idle stop screw, turns in(arrow points to where transition holes appear)
.75 turn
1 turn
1.5 turns
2 turns
2.5 turns
3 turns
3.5 turns
4 turns

Here's why you buy a real Weber, made in Spain. A lot of the cheap eBay clones look OK, but are often not very precise. and carburetors need to be extremely precise. This 44IDF from racetep.com shows how nicely the critical alignment of throttle plates vs. transition holes align. This means a precise relationship between: throttle shaft bore in the body, throttle shaft, the very precise position of the plates in the shaft, the bores of the tiny (half millimeter) transition holes, all working in concert.

idle stop screw 2.5 turns in after contact
left bore
right bore