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Note on Weber carbs by Mr. Passini

General engine tech -- Drag Racing to Circle Track

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lc-gtr-1969
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Re: Note on Weber carbs by Mr. Passini

Post by lc-gtr-1969 » Sat Oct 05, 2019 4:17 pm

Caprimaniac wrote:
Sat Oct 05, 2019 5:36 am
JohnP wrote:
Tue Oct 01, 2019 11:50 am
I bought his little green book in the late 70's, while working on my '76 Fiat 131. I remember he said your plugs should be the color of Cadbury's milk chocolate. Another gem, "Suck it and see" (to find out if something will work).
If you have unsynced barrels at 3000+ rpm, the shafts or bores or blades need to be faulty. OR, the different cylinders might be different regarding how much it sucks….. based on cam lobe, valves, ignition…… alot of different factors that can vary. Either the carb or engine need to be fixed, at a larger scale...
I understand your logic here but in my experience, it is surprisingly common to see multi-carb engines read differently at higher rpm when tested with a high speed/ high vacuum synchrometer. Many people do not undergo the test with the presumption that if it is synced at idle, it will be fine everywhere. There are many simple mistakes or issues that can cause this, not always major mechanical issues (although this can be true in some instances).

1) 4 webers synchronised to 6hg at 1000rpm. The throttle screws are holding the blades open just enough to let in a trickle of air. However the linkage on one carb has some very slight slop in the rose / ball joint, which causes it to slightly trail the other 3 carbs. At 3500, this carb will read 3-5hg lower depending o the level of slop.

2) On the same system, one lever has a slightly shorter adjusted travel, which has changed the linkage geometry- the carbs that are actuated by the shorter lever will open at a faster rate and subsequently hold the carb throttle blades in a different position to the other carbs at higher rpm- this would synchronise fine at idle though.

3) Again, 4 weber system- one carb has one leaky butterfly (common). The leaky butterfly bore reads 9hg instead of 6hg like the others. The tuner drops the idle of the affected carb to make the leaky throat read 6hg (like the other carbs) and opens the air bleed on the sister throat to bring the companion throat UP to 6hg (as this would not be 4hg or thereabouts). The issue here, is that you now have the affected carb reading perfect synchronisation at idle, but in fact, its throttle blades are in a more closed position than the other three carbs (one throat has a slight but fairly common mechanical fault and the other has an intentional air-leak introduced). Basically this carb's throttle blades are now trailing the other three carbs, if tested at high rpm, you will see a mismatch between the 3 'normal' carbs and the fourth affected carb.

These are just some quick examples of how mistakes can be made, or slight issues can be overlooked when setting up multi-carbs... all of the examples given are quite easily fixable/ able to be tuned around but its amazingly common how frequently synchronisation at mid-rpm is not done nor checked. Some think I am pedantic when aiming to get every throat on multi-carb setups working 100% in unisyn, as quite often the cars can drive 'ok' even if they're not pulling the same air at mid-rpm, but when corrected they always drive better.

Another tell-tail sign that issues can exist with linkage geometry, or wear in linkage ball joints etc, is to hold the throttles to wide open (engine off) and see if all carbs are hitting their throttle stop at the same time. Quite often you will be surprised at how far a carb can be trailing the others (sometimes this can be due to other issues like slight differences in carb bodies etc but its a fairly good indicator to check everything over before proceeding to sync at idle and again at 3500).
Fumbling around in the shed...

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Re: Note on Weber carbs by Mr. Passini

Post by modok » Sat Oct 05, 2019 8:14 pm

Right on!

There are a lot of instructions and discussions about synchronizing idle, but it's just NOT very important.
It's really COOL to have the idle set low and perfectly smooth......but It's the least important thing to tune. Lowest priority. You will never damage the engine from idling uneven, and in fact you might damage the engine from idling too low or too much, especially when new. Turns out engines don't really like to idle low, not much coolant and oil flow. I believe time is better spent getting the carbs bench set even before installing, and keeping them that way initially by making equal adjustments to each, and in a lot of cases that's good enough for the long run also.

When it WAS really important was just before the emissions test. I would adjust it in the parking lot right before the test, never failed. But now those simple tests where they check idle and 3000rpm, are a thing of the past.

I have noticed that on overrun(trailing throttle), some models swing lean, others swing rich. IMO swinging rich seems to give better response after a shift. Must have something to do with how much fuel and air is coming out of where, transfer ports VS idle needle, throttle gap VS bypass. If there is a deeper meaning it might be to do with this. But wat this or that mdle will do on any particular engine.... who knows, you'll have to suck it and see :P

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Re: Note on Weber carbs by Mr. Passini

Post by digger » Sat Oct 05, 2019 9:07 pm

on a 6 cyl i bolt the pairs of manifolds using a template and ensure the surface to mount throttles is flat/planar. then mount throttles to the manifold using a precision template to ensure alignment and position (i.e. the throttle shafts need to be as concentric as possible). this seems to be importantl to maintaning the best balance across multple positions otherwise linkage effects seem to occur

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Re: Note on Weber carbs by Mr. Passini

Post by enigma57 » Sun Oct 06, 2019 1:54 am

lc-gtr-1969 wrote:
Sat Oct 05, 2019 4:17 pm
Some think I am pedantic when aiming to get every throat on multi-carb setups working 100% in unisyn, as quite often the cars can drive 'ok' even if they're not pulling the same air at mid-rpm, but when corrected they always drive better.

Another tell-tail sign that issues can exist with linkage geometry, or wear in linkage ball joints etc, is to hold the throttles to wide open (engine off) and see if all carbs are hitting their throttle stop at the same time. Quite often you will be surprised at how far a carb can be trailing the others (sometimes this can be due to other issues like slight differences in carb bodies etc but its a fairly good indicator to check everything over before proceeding to sync at idle and again at 3500).
Agree 100%. And as for being pedantic about such things...... Kleinigkeitskrämer Mom would say (she did all the mic work when she and Dad had their automotive machine shop from the '30s through the war years)...... Guilty as charged, your Honour! :D

After I complete some other things, my next task will be to put a baseline jetting in the Webers and then work out the throttle linkage. Because this is an inline 6, the throttle shafts are oriented parallel to the crankshaft. And because its a home built intake running DCNF downdraught carbs rather than DCOE sidedraughts, I'll need to build the linkage from scratch. May end up with a Rube Goldberg sort of affair, as direction of pull to all 3 carburettors must be redirected 90 degrees and each carb must be individually adjustable. I'll let you know how it works out.

Best regards,

Harry

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Re: Note on Weber carbs by Mr. Passini

Post by Caprimaniac » Sun Oct 06, 2019 7:39 am

Alot of good input here.

Never thought of doing changes to oritentation of the carbs for them to line up; check placement of studs (redo studholes or use offset studs...) and have precicion spacers (nylon or other hard non-conducting material instead of the two layers of cardboard gaskets currently used.)

I am not after "Nice idle". I'm after drivabilty from 1000 - 7000 RPM, and a STEADY idle. Remember the engine stalling, coughing and stop at the redlight earlier; most probably due to the linkage not working properly. Faulty mechanic....

I do fully agree that a difference in vacuum at higher RPM has a great downside. Unfortunately; if the blades are not synced good enough due to a flimsy linkage, then the system is not good enough, and need to be redesigned. What I find the MOST reliable linkage for the V8 4x downdraught setup is: the two rows of carbs need to be faced in the same direction. Then you can bolt the rows together With a single brace that will pull in parallell (given height of the two rows are identical). In most (all?) cases, this mean the manifold need to get a makeover for a New bolt- pattern. The linkage is what it is and will stay like that, however adjusted as good as I can, taking slack and bind out.

Yes, gtr 1969: I think Your 3. point is very legit; I have seen the same thing

Is it possibly correct to state this: If Your multicarb IR- engine is up and running and idles at what you figure are synchronized carbs AND you find a deviation in vacuum at one barrell, you should bring the engine up in RPM ( say 3000) and recheck... Then adjust, if needed, the throttle shaft for that carb. (On a sidenote; will you be able to adjust the vacuum by using the air bypass at this rpm?). And then If, one barrell STILL deviates from the others. Should you set this carb shaft so one barrell is higher, the other a bit lower...... Middle of the road, so to speak? OR should you be happy as long as ONE barrel reads the same as the others, and one still lower?

Other options could be: check shaft play, check butterfly and screws..... repair. Or get a New carb.

Harry/ Enigma- You'll figure out a linkage for Your DCNF's, sure. I did it for my 3.0 Essex many years ago, but that was easy as they are inline.
Maybe you'll find some good guidlines in a DCOE- linkage?
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Re: Note on Weber carbs by Mr. Passini

Post by grandsport51 » Sun Oct 06, 2019 1:07 pm

OP Caprimaniac,
What model DCOEs or other ?
When I was learning to fool with DCOEs an older Italian gent told me to always use the spring loaded rubber mounts and gaskets because of vibrations putting foam in the fuel
It’s 40 years later but I’m sure it still applies.
Dave B.
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Re: Note on Weber carbs by Mr. Passini

Post by grandsport51 » Sun Oct 06, 2019 1:31 pm

PS Maybe applies more to the RVP in today’s Fuel?
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Re: Note on Weber carbs by Mr. Passini

Post by modok » Sun Oct 06, 2019 8:29 pm

I think it has to do with how much you shake the carburetors, and also what direction you shake them.
Along the axis of the flat pivot should be least sensitive, because that won't upset the needle, and the fuel is in narrow gap that way, usually.
90 degrees to it, then the fuel can more easily surge side to side.
Up and down of course, definitely not good.
Side-drafts on a straight four would have a lot more up/down shake than on a six

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Re: Note on Weber carbs by Mr. Passini

Post by enigma57 » Mon Oct 07, 2019 1:28 am

Caprimaniac wrote:
Sun Oct 06, 2019 7:39 am
Harry/ Enigma- You'll figure out a linkage for Your DCNF's, sure. I did it for my 3.0 Essex many years ago, but that was easy as they are inline.
Maybe you'll find some good guidlines in a DCOE- linkage?
:D Right you are, Caprimaniac! Once I've got a baseline tune on the 3 DCNF carbs and can trial fit them on the intake, I'll be better able to visualize how the throttle linkage must go.

The challenge in fabricating throttle linkage in this instance is for the most part a function of space constraints versus carburettor orientation. You see, with the DCNF downdraught carbs, orienting them with fuel bowls outboard on the inline 6 would allow a straight pull outboard on linkage when opening the throttle blades. Then redirecting direction of pull for each of them 90 degrees to align with a straight pull back towards firewall. Pretty straight forward.

However...... The mixture and adjusting screws are on the opposite side from fuel bowls, so mounting them with fuel bowls outboard would place the adjusting screws inboard, jammed between carb body and rocker cover, making adjustments a real PITA. That's why I'd rather have the adjusting screws outboard and accessible. Even though it will require an added 180 degree change of direction in addition to the aforementioned 90 degree change in direction of pull for the throttle linkage.

I have seen the type linkage the DCOE sidedraught setups use on the inline 6. Not sure whether I may be able to adapt a part of it in this instance, but once I have the carbs trial fitted, I'll eyeball it and a solution will come to me.

Best regards,

Harry
Last edited by enigma57 on Mon Oct 07, 2019 1:41 am, edited 1 time in total.

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Re: Note on Weber carbs by Mr. Passini

Post by Geoff2 » Mon Oct 07, 2019 1:40 am

Weber linkages.

One of the 'problems' with Webers [ some models ] on multi carb setups is the throttle linkage design. The throttle arm fits a slot on the shaft; there is often an arm on the other end of the shaft, a WOT stop. The WOT arm contacts a projection on the carb to limit throttle blade travel @ WOT; the arms 'move' on the shaft because there is some clearance in the flats of the shaft & arms. The synch has changed. Result: after a WOT burst, carb do not return to idle. I hope I have explained this OK. I have had the problem with IDFs & DCOEs. I fixed the problem by having only one arm per carb. The WOT stop is on the main linkage, not individual carbs.

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Re: Note on Weber carbs by Mr. Passini

Post by modok » Mon Oct 07, 2019 1:57 am

Put the WOT stop in the linkage, and don't assume they will make the peak power at 90 degrees open, because.....a lot of times they don't anyway, often a bit more or less is better, or makes no difference. Synch is most important around 1/4 open

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Re: Note on Weber carbs by Mr. Passini

Post by enigma57 » Mon Oct 07, 2019 2:02 am

Geoff and modok, the DCNF carbs have only a single throttle arm and have an adjustable idle speed screw built into carb body that contacts this arm. There is a place on the arm that is designed to hit a raised portion on throttle body at WOT, but no adjustment there. I will rework these arms so they contact only the idle speed adjusting screw and will make my throttle linkage so that the linkage itself has an adjustable stop at WOT.

Thanks for the reminder. Much appreciated.

Best regards,

Harry

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Re: Note on Weber carbs by Mr. Passini

Post by modok » Mon Oct 07, 2019 2:13 am

Brilliant!

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