10 april 2019

tuning the Weber carburetor for (my) non-standard applications

specifically here, the Rambler 195.6 OHV (3200cc) inline six

reference: Top End Performance, highly recommended by me.

reference: Reline/Weber IDF adjustments

reference: factory manual

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

having tuned only three Weber carburetors, and two of them hardly at all, i'm no expert. but i've consistently found that some procedures given elsewhere as must-do simply don't work on my engine. the main difficulty i had was setting up low-speed jetting "by the book". i had to work it up from first principles and from procedures taken directly from the Weber factory tuning manual. a PDF copy of this is above.

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.

caveat: at least one of the people at the redline/weber help line quite impatiently insisted that the "idle mix screw one turn out" rule is fundamental, that i was wrong, and that the carb can't be tuned further until this requirement is met. i politely disagree and have measurements and data to back me up. i am fully willing to be wrong here, and it makes me uncomfortable that i'm having such different results from conventional advice. but i want facts and reason, not asserted authority and claimed rules. also, i can find no reference to this "rule" in the weber factory manual i have a link to above. in fact, that manual is from where i derived my tuning. whatever -- i am getting my desired A/F ratios at all loads/throttle positions, going by my Autometer A/F ratio gauge, which matches and affirms "seat of the pants" feel. it's also just plain running great.

i go where the data takes me.

old inline motors are not sports car motors

older american-type inline sixes like mine differ from european sports cars in how they meet their design goals. there are some terminology differences too.

the common sports-car application is one venturi per cylinder, or maybe two. 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 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 the draw through the single carburetor is different.

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.


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 precise 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.


these aren't Rambler-specific, but FYI stuff i experienced.

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. 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 you might as well verify what jets are actually installed, including the (IDF) squirter return jet.

i made a "tool" from a short piece of aluminum sheet; cut and filed to exactly 10mm high, used as a feeler gauge under the float.

accelerator pump (squirter) stroke

i don't think i had to do anything at all on the 32/36 DGEV or 38/38 DGV, so it took some time for me to pay attention to it on the 44 IDF. turns out that most of my initial high-speed troubles were mostly squirter -- mine was originally set for minimum stroke and i need, at least for now, maximum stroke. once i thought to check, it was blunt-object obvious: zero squirt for any throttle position past about 50%! pressing the gas pedal in 5th on the highway was like turning the key off.

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 it's inlet, 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 the net (sorry i forget where) and annotated in red, by me. where it says "RPM" think "relative air mass flow". as flow increases from idle to maximum, the contribution of fuel from each circuit -- idle mix screw, low speed jet, main jet -- changes. the crossover can be quite distinct.

the chart is just a rough guide, and it understates the effect of the idle mixture, which greatly affects much "low speed" driving. in my application, at very light ("cracked") throttle flat and level between stop lights, say, half the time i'm driving on the idle mixture.

the red annotations are for my engine. the effects of low speed vs. main jets was hard to puzzle out until i worked out the two transition RPMs. i did so by accident; i had a too-rich low speed jet and a too-lean main jet. a long slow accelleration from 20 mph to 70 mph returned 13:1 at low speeds; right around 2200 rpm A/F ratio dropped fairly suddenly to 15:1, my main jet. right at 60 mph, a slight incline (tiny bit more throttle) ran lean; faint downhill (lighter throttle) it went rich. this was my First Enlightenment.

SURPRISE #1: nearly all driving is done on the low-speed circuits

the low-speed jet and even the idle mixture dominate driving up through highway light cruise, at which point it transitions to the main jet. in my car, 5 to 15 mph, flat level cruise is off the idle mixture screw; 10 to 50 mph or so, entirely on the low speed jet.

the low speed jet meters fuel to the transition holes that are located in the bore just above the close throttle. from just above closed-throttle (idle) to the point where there is sufficient air flow to activate the booster venturi, the low speed jet and these transition holes determine A/F ratio.

SURPRISE #2: low speed jet and idle mix: REASONING, NOT "RULES"

standard lore on Weber tuning is this flat-out statement: if your idle mixture screws are out more than 1 turn, your low speed jet (idle jet) is lean. this is not correct.

i could find no such rule in the Weber factory manual. the final screw position for correct mixture will depend on a number of factors and is itself unimportant. (within reason: out at least a half turn so that you have some adjustment range; and not so far out that it's loose.)

fact: when the throttle is closed (idle stop screw) the throttle plate is below the transition holes, the idle mixture screw alone determines mixture.

fact: when the throttle plate opens and uncovers transition hole(s), the engine is fed fuel through these transition holes, the low speed jet alone determines mixture.

fact: no matter what low speed jet is installed, you can adjust the mixture screws for good idle.

this method passes the Weber Factory Service Manual test procedure

lest you think i am just making this shit up, in the Weber factor manual, page 42 (not PDF page 42) the paragraph beginning with Transition (or progression) stage check has a check procedure for the correct low-speed jet. this procedure below will produce results consistent with this test.

below is the recommended procedure from the weber factory manual reduced to step by step.

and this is the test for correctness:

for my setup, low speed jet 45 (in carb as delivered from racetep.com) passes the above test, but the engine runs better one step richer, with 50's installed. maybe as i iterate closer and closer to "ideal" tune i'll end up with 45's in there. however it's easier and safer to do fine-grained tweaks from too-rich than from too-lean, so 50's it is for now.

idle speed stop screw

if the idle stop screw is not at the ideal one turn in past where it touches the stop, you will have either an off-idle flat spot (less than one turn) or low-speed-jet-selection problems (more than one turn, "idling" on uncovered transition holes). this is described precisely in the factory manual. however my engine, and i'm sure others, does not idle correctly one-turn-in; on my engine, it sets an idle speed of 900 rpm or so, where 550 rpm is desired.

idle mixture screw

the traditional lean-best-drop method works fine. start rich, bring both screws leaner in increments until it runs worse (on the lean side); then back out until it sounds great, then out a bit more until it worsens (on the rich side). the difference between too-lean and too-rich should be fairly small, 1/4 turn or less. the correct position is in the middle, best-smoothest.

SURPRISE #3: confusing main jet enleanment with squirter issues

i spent an embarrassing amount of time messing with main jet and air bleed when the real problem was accelerator pump issues at highway speeds. "out of the box" acellerator setting worked fine on my previous two webers so it took me a long time to get around to thinking about it on the 44 IDF.

to test a jet selection i would start looking at A/F ratio initially at 65 or so mph flat level cruise, on the main jet, then accelerate slowly to say 80 mph to see the effect of the air bleed; slowly so that i wasn't seeing squirter effects.

however undiagnosed squirter issues meant that the open throttle created enleanment for the dozen seconds it took to reach the new cruise plateau, and i mistook this for a jet issue. i was uncomfortable running so lean (16, 17:1) for the time it took to get there. after mostly fixing the squirt problem i was able to discern the real effects of main jet and air bleed, and have since ended up with much closer to "conventional" jetting.

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 550 rpm with the idle stop screw at 1/2 turn, possibly due to the conservative cam (it will idle, not well, below 400 rpm). whatever the reason, the situation left me with off-idle flatspot. see 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 tanged together in a multi-dimensional space.


current state of tune, Los Angeles, sea level. this is pretty close to ideal; highway cruise about 14.5, WOT between 12 and 13, as long as engine RPM is 2400 and above; WOT at 2000 goes lean until RPM picks up. the small air bleed is odd i admit. rarely does this engine persist above 3500 RPM (rapid highway accelleration shift point is about 3200) so i suspect aboove that is when air bleed issues will appear.

44 IDF, venturi 36mm
low speed jets 50
idle stop screw 1/2 turn
idle mix screws 2.5 turns out
accelerator pump adj nut 0.400" in
main jets 220
air bleeds 170
float level 10mm


spark advance is completely 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 ("many") 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

Etc Code Objects Archive AMC Home top of page