Well the weber is on the deluxe finally. I am having a problem getting the basic set up right, so was throwing the question out there:

When increasing the engine speed slowly (not utilising the pump circuit) the carb 'pops' a lot. like a small carbie backfire. if accelerating quickly, no worries.

I have tuned in the idle mixture using a colortune, so have hopes that is close.

Any ideas what causes popping in weber carbs?

cheers
Matt

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68 Deluxe.... it's shiny..... I like shiny!!

Sounds like it's lean just above idle.
The idle circuit also supplies fuel via the progression holes until it gets onto the main jets.
I'd try richer idle jets, and screw the idle mixture screws in a bit so idle mixture is still OK.
I'm a Dellorto man though... but Webers are real similar animals.

Could also be the wrong emulsion tubes, maybe.

Post all your jet numbers here and someone can help.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

From my book on webers;
"An excessively lean overall mixture will see the engine missing and backfiring through the carburetor chokes under load. Continue to increase the main jet size as long as the power keeps going up."

So I'm guessing your idle jets are right but your mains are to lean. So maybe try a size or two up...

I hope that this is right and helps you out

How long did you run the car for?
I used to race datsuns with twin 45mm webers. They weren't choked, and used to pop and fart until the car warmed up. This was a normal part of the start-up procedure for the car. Once warm, the carbies were fine. Warm-up took up to couple of minutes typically.
KB

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hi Guys.
I have a 45 DCOE with the following jets :

145 mains
155 airs
36 mm chokes
4.5 aux venturi
F16 emulsion
50f9 idles
45 pump jets

I have some borrowed f2 emulsions and a couple of other things i can swap in.

I just wanted to get a good grasp on what is happening before i went trying to fix it

Cheers
Matt

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68 Deluxe.... it's shiny..... I like shiny!!

I think your air and main jets are both too small for 36mm chokes.

My Des Hammill 1275 book is pretty right in its Dellorto settings, here's what he rcommends for a 45DCOE, with 36mm chokes, on a 1275:

4.5 aux venturis
F2 (or maybe F16) emulsion tubes
160 main jets
180 air correctors
45 pump jets
45F9 idle jets
50 accelerator pump valve
accelerator pump arm height 14.5mm
15mm full droop float setting
7.5mm float level height
3/4 turn out on idle mixture screws

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Matt
from your decription, the idle jets maybe a little small. How does it drive? Does it cough just off idle? I would start there before looking at the mains and airs. As Kevin says, the mains maybe a little small but the airs being the size they are will even it out. I run 165 mains in my race engine with 38 chokes. Steve

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Steve

You may find this useful.

A GUIDE TO TUNING
WEBER CARBURETTORS
A BORG-WARNER EDUCATIONAL PUBLICATION
2nd EDITION 1976

TABLE No. 2
Size per cylinder in cc's Idle Jet Fuel Bleed Hole Size
in mm.

200 .35 or .40
250 .40 or .45
300-350 .45 or .50
400 .50
450-500-550 .50 or .55
600 .55 or .60
650 .60 or .65
700 .65 or .70
750-800-850 .70 or .75


TABLE No. 3
'F' Number One Hole Type Two Hole Type
Hole Size in mm Hole Size in mm
F1 1.35
F2 1.30
F 3 1.60
F 4 1.30
F5 1.60
F6 0.70
F 7 1.20
F8 1.20
F9 1.00
F10 No hole for use in I DA carburettors where air correction is in
idle jet carrier or holder.
F11 1.20
F12 0.90
F13 0.90
F14 1.20

TABLE No. 4
Idle jet air correction or bleed holes arranged from rich to lean
Rich F6
F12
F9
F8-F 11--F 14
F13
F2-F4
F5
F7
F1
Lean F3


PART 2

IDLE JET
Both the DCOE and IDA carburettors have an idle jet assembly which meters both fuel and air into the idle circuit.
At idling speed the idle mixture adjustment can be set to control the volume of mixed or emulsified fuel and air provided by the idle jet assembly and if a correct jet has been selected the setting of the idle mixture screw should be between a half and one full turn open.
As the throttle is opened from the idling position the throttle disc crosses a series of holes which are referred to as the secondary idle bleed cir-cuit or progression ports. These are fixed holes having no adjustment and are also fed by the idle jet assembly. Naturally it is important that a controlled mixture is fed through them so that smooth acceleration takes place from idle until the main jet assembly comes into operation. The control of this mixture is very closely associated with the idle jet air bleed (the 'F' number in DCOE units or the idle jet carrier in IDA models).
SELECTION
To determine these jet hole sizes, tables have been prepared, table
number 2 deals with the fuel bleed hole designated by the numbers 35, 40, 45, 50, 55 etc. and the sizes are given in mm. against the capacity of each cylinder. Where an engine has Siamese inlet ports it may be necessary to go one size larger than quoted.
Table number 3 gives the size of the idle jet air correction or bleed hole or holes against each 'F' number in mm's. It will be seen that the 'F' numbers do not run in sequence, but in table number 4 they have been arranged in their order from rich to lean.

EXAMPLE
Take a four cylinder engine of 1275 c.c., divide by 4, this is 319 c.c., per cylinder. From TABLE No. 2 it shows that both 300 and 350 c.c., can use, as a starting point, either a 45 or 50 idle fuel bleed hole size, but as the example has siamesed ports, 50 is the jet to start with.
To get a reading for the correct selection of the idle air bleed or 'F' number hole size, it is recommended that a midway choice, say F8 (1.20MM) be used which will give an idle jet assembly number of 50F8.
To check this selection, start engine and bring it up to normal operating temperature; leaving chokes, secondary venturi, mains, emulsions, air cor-rections, etc., as fitted. Carefully set the idle mixture screws (diagrams 1 & 2) to obtain the most even idling. This is done in conjunction with the idle speed screw (diagrams 1 & 2). Take time to allow engine to settle down after each adjustment-due allowance should be made if a competition camshaft is fitted as this generally produces rough idling.
PROGRESSION PORT CHECK. Having correctly set the idle speed mixture and rate, increase engine speed, by turning idle speed screw, to a point just below that at which mixture is seen to come from the secondary venturi discharge nozzle. To check for correct progression port mixture, turn idle mixture in and out.
If mixture is correct, turning screw either way will drop engine speed. Should speed increase when screw is turned in, it means the mixture is too rich. Conversely, if speed increases when screw is turned out, the mixture is too lean.
Correction of lean mixture may be accomplished by increasing the idle fuel jet diameter or reducing the idle air jet diameter. Determination for either of these can be found by reference to the number of turns out the the idle mixture screw had to be turned to achieve a smooth idle. Optimum set-ting on an idle mixture screw should be 1/2-1 turn out; if it was necessary to go beyond this setting and progression stage as outlined above is weak in-crease fuel jet diameter. If idle mixture screw setting was acceptable and progression stage is lean-decrease the idle air jet diameter. Should progres-sion port mixture be rich, obviously the reverse procedure would apply.
This setting is very critical to ensure a smooth changeover from idle to main jet operation.

I don't know how to rearrange the tables so I hope its not confusing.
Hope this helps
RonR

Many thanks Ron!

I haven't yet had a chance to digest all that you have said yet, but i intend to read over your answer many times!!

Thank you!!

I have now put in a pair of 60F8 idles and F2 emulsion tubes and that seems to have helped get rid of the problem. So far. Will test drive tomorrow and get back to you all

Cheers
Matt

One other thought I had, the carb i have is the 45DCOE_152 model. It has two additional adjustment screws, one on either side of the carb body, about 1.5" from the throat of the carb, each with a lock nut. How should these be set?

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68 Deluxe.... it's shiny..... I like shiny!!

Could they be venturi locking screws?

Nope. The locking screws for the venturi are under the barrels, these are nearly directly on top of each barrel (about 15 degrees off centre) and seem to have a passageway into the carb throat.

I have been told they could form part of an emissions compliance system but don't really understand them.

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68 Deluxe.... it's shiny..... I like shiny!!

Well I am out of ideas. (I only had one!)

Matty they`re from the later series of webers for polution control stuff,,, the 2 extra screws are basically extra idle air bleed adjustment,,, similar to what the dellortos have std

What you have described & the way you have described the problem does sound more like an idle/air problem, so changing main jets & main air correctors or emulsion tubes isn`t going to fix that,,, but those new idle jets you`ve fitted sound way rich to me, maybe 50F9 will be better

edit,,,OH !!!! & i`d try to find a pair of 3.5 Aux venturies for it,,, minis tend to like 3.5s better than any others,,, i only fit the bigger ones on the really big motha race donks with big motha holes,,,, hope that helps

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Hey matt, thanks for the tips. How should i use the extra idle air adjustment screws?

Those idle jets were the only ones i had spare that were any richer. I fitted a pair of 45F9 s but they just made it worse.

Might try a pair of 34 mm chokes too, if i can find a reasonably priced set.

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68 Deluxe.... it's shiny..... I like shiny!!

Hey matt, thanks for the tips. How should i use the extra idle air adjustment screws?

Those idle jets were the only ones i had spare that were any richer. I fitted a pair of 45F9 s but they just made it worse.

Might try a pair of 34 mm chokes too, if i can find a reasonably priced set.

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68 Deluxe.... it's shiny..... I like shiny!!