Single 44 idf weber on chevy inline six

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Single 44 idf weber on chevy inline six

Post by chevyfreak »

Hi all.
Been wondering. I'm trying/playing with intake and carbs on my 266ci inline 6. Chevy 250 bored to 4inch.
Planning on giving a 44 idf weber downdraft a try, Buzy in building an intake to use. My dilema is jet and emulsion tube sizes.
I know that it is used in brazil on the opala 250. Single, double and tripple carb setups.
But cant find any detailed info on jetting as a starting point.
What i have in it now is..
Main jet 135
Air jet 175
Emul f11
Idle 52
Pump jet 50
Venturi is 28mm but i have a set of 36mm as well which i am planning to use.

Any recommendations?
Thanks.
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Re: Single 44 idf weber on chevy inline six

Post by enigma57 »

How is your 250 set up? Cam specs? Comp. ratio? Planned use and redline RPMs? Single IDF on log type open plenum intake? Are you going to do the lump port mod to head? What size aux. venturi (booster) is in your carb now? It should be marked. You may need a magnifying glass to read the number, though.

I can give you a rough baseline, as I have been working with several combinations of Webers for my 292. The thing to remember is the IDF is designed for use on an IR intake and has no power valve. So when you reconfigure it for use on a plenum style intake, choke size (and corresponding throttle bore size) will be smaller and main jet, idle jet and pump jet will all be larger.

Do you already have the 44 IDF? Reason I ask...... The first thing is to decide what choke (main venturi) size you need. In this instance, you will need choke size from 28mm to 29mm. And there are recommended sizes of choke tubes relative to throttle bore size. Weber recommends ideally a 1.25 ratio as being optimal and you can generally go up or down 2mm from that whilst maintaining good fuel metering. But if you must choke a too large carburettor down and it exceeds those parameters by much...... A 44 IDF probably would not run as well as a 40 IDF if both were fitted with chokes that small.

Regarding emulsion tubes, begin with your present F-11 tubes. Holes and diameter look close to what you will need. If a change is necessary, you will find that out as you fine tune your carb under varying engine speeds and loading using an oxygen sensor.

Best regards,

Harry
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

Block has been bored to 4inch giving me 266ci. Stroke is std 250.
Cam specs?
Hyd Flat Tappet, 260/260-212/212-.489/.489-110

Comp. ratio? Calculated at 9.4 to 1
Planned use and redline RPMs? Redline 5000 to 5500. Basically an all round use but occasional redline.

Single IDF on log type open plenum intake? Yes. Single carb and log type intake.

Are you going to do the lump port mod to head?
No lump port. Mildly ported and center tube has been removed.

What size aux. venturi (booster) is in your carb now?
No idea, i'm still waiting for carb to be delivered. Should be 4.5.
Specs i have is only what the company gave me over the phone. Waiting on delivery to verify the smaller details.
Does have 28mm venturis in now. And i have a set of 36mm.

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Re: Single 44 idf weber on chevy inline six

Post by enigma57 »

My engine specs are not all that different from yours. If you can do one of the better bolt-in lump port kits, it will help. The 3 sets of siamesed intake ports on these engines result in more than adequate flow volume when the head bolt boss is cut out but its a lazy port with less port velocity than optimal. The lump port will at least increase port velocity a bit and redirect the direction of flow into the valve pocket so that its less of an abrupt turn.

As with any carburettor (especially Webers)...... Before you do anything else...... Pull top cover, make sure no dirt nor debris in bowl nor under needle seat and measure float level and drop. Adjust if necessary.

Also...... Just as with the old Stromberg 97 and other American carbs designed in the 1930s...... Webers require 3.0 to 3.5 PSI. When the needle seats are new, some will take 4.5 PSI for a while, but eventually, they will leak past the needle valve, so you will need a good fuel pressure regulator. If you are setting it up to act as a relief valve and route excess pressure back to the tank via return line, that will prevent the regulator from restricting flow volume as it would if positioned inline between fuel pump and carb needle seat. My '57 Chevy has no return line, so I will add one and connect it to a 5/16" nipple brazed into the side of the fuel tank filler tube. That way, I do not have to put a torch on the tank itself, as the filler tube is removable without disturbing the tank.

My Weber carbs are DCNF, so choke size and jetting will work out close, but your IDF emulsion tube is larger in both length and diameter and the well it sits in is deeper and larger diameter, as well. So emulsion tube number is the main thing that won't necessarily be a direct interchange. I would suggest beginning with your present F-11 e-tubes, as the placement of holes is similar to mine. Your 28mm chokes and 4.5 or 4.0 aux. venturi should be fine, as well.

This is our baseline setup and it should be close enough to get your engine running and you can fine tune from there. A friend and I have run extensive tests on the road and using an O2 sensor on his car over the past 2 years, so this should get you close......

29mm choke tubes (main venturi)...... Your 28mm chokes should be fine
4.0 aux. venturi (booster)............... If yours has 4.5 aux. venturi, should be fine
65 idle (slow running) jets............... You may want to start with 70 idles to be safe and drop to 65 if 70s are too large. See note below.
180 main jets
185 air correctors
70 accelerator pump jet

You will need to drill (1) hole in each throttle blade, as these Weber carbs designed for IR intake...... When reconfigured for open plenum type intake...... The relationship of throttle blade to progression holes at idle (tickover) is such that you cannot easily get them to idle at less than 1,000 RPMs without uncovering 1st progression hole. We experimented with hole size quite a bit. This is the smallest size that will do the job and making it larger did not help. We used a 2.4mm drill bit, but you can also use a 3/32" bit (these are 2.38mm).

We are presently running as an experiment, adjustable idle jets just now that were NOS retrofit items for small 1970s Eastern European cars with Fiat engines and Weber IMB carbs before the fall of the Soviet Union......

https://www.ebay.com/itm/WEBER-Adjustab ... i0&vxp=mtr

However priour to that, we ran either 65 and 70 idles, depending upon whether the e-tubes we were experimenting with were lean or rich running.

The adjustable idles work very well and hold their adjustment setting surprisingly well. These happen to fit your IDF carb and our DCNF carbs as well. If you want to pick up a pair to play with, they are relatively inexpensive at $8.00 US each. They do take a week or 2 to arrive (disregard mention of Ireland in ad, these are sourced from somewhere in Eastern Europe and ours went through the Chzech Republic and Germany before arriving). There is a rubber O-ring inside (easily replaced) and ours were coated in grease since the 1980s to protect them. Just soak them overnight in WD-40 to dissolve the grease and lube the O-rings before using and you are good to go. An old toothbrush works well for this.

Also...... Run a good hot ignition and rework your distributor to limit mechanical advance when at upper limit so you can advance initial timing whilst keeping total timing around 34 - 35 degrees total. These old tractor heads do not have the most recent (efficient) combustion chamber shapes and you don't want to get into detonation.

Hope this helps,

Harry
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Re: Single 44 idf weber on chevy inline six

Post by Geoff2 »

Main jet will probably need to be larger because the carb is feeding multiple cylinders. For more reliable fuel level control, you might want to switch to Viton n/seats, rather than the steel ones supplied by Weber. I believe RaceTep in Calif sell them. You should probably get a larger orifice size if feeding multiple cyls.
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

Geoff2 wrote: Thu Nov 07, 2019 4:28 am Main jet will probably need to be larger because the carb is feeding multiple cylinders. For more reliable fuel level control, you might want to switch to Viton n/seats, rather than the steel ones supplied by Weber. I believe RaceTep in Calif sell them. You should probably get a larger orifice size if feeding multiple cyls.
I only use the viton tips on all my webers. They last longer as well with low presure from pumps. Have a selection of sizes i can use.
I'm mainly run dcd and dgv/dgas webers. And then monojet and quadrajet.
So idf is new to me and use different jets and emulsion tubes from my other webers. So looking to get a balpark and go from there.

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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

Harry.
Thanks for the info.
For now this head runs open. Buzy with another spare head that i'm doing seat inserts and new valves and springs and planning the lump insert as well. That will be used at a later stage,
All my inline 6s runs hei spark and checked mech advance setup already and some spring changes made.
The normal line of webers i know, so always use a filter before the carb and i run it at 3 psi mostly with return lines. Learned the hard way :D .

As soon as i receive the carb i can dig in more and get more details. Just getting on the planning stage to see where i have to start and what extra i need to start tuning.

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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

enigma57 wrote: Thu Nov 07, 2019 2:50 am

29mm choke tubes (main venturi)...... Your 28mm chokes should be fine

You will need to drill (1) hole in each throttle blade, as these Weber carbs designed for IR intake...... When reconfigured for open plenum type intake

Harry
Hi harry.
I received the carb. Still waiting for my jets. Coming from a different supplier.
However, the main venturi in the barrels is 36 and not 28.
So will first have to get other venturis before bolting it on.
Did notice however that they have adjustable idle air bypass. Will try it first before drilling a hole in the blades.

Also been wondering if 2x 44idf carbs would be better.
Sadly my mind never stops pondering. Always thinking about something.
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Re: Single 44 idf weber on chevy inline six

Post by Geoff2 »

36mm chokes in a 44 mm bore sound about right. 28mm too small, better in a 40mm bore.
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

Geoff2 wrote: Fri Nov 29, 2019 2:48 am 36mm chokes in a 44 mm bore sound about right. 28mm too small, better in a 40mm bore.
I dont mind trying them. Just waiting for the jet kit i ordered, should have that by end next week. Then the playing starts, again. :lol:

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Re: Single 44 idf weber on chevy inline six

Post by BCjohnny »

Rule of thumb I've always used is choke/venturi needs to be around 80% of throttle bore for most things apart from full on horsepower build, when low/mid eighties might work better ....... 28mm chokes will put you around 64% which IMO is too small.

I'm not saying 28mm won't run but might throw off jets/settings as to what would be 'normal' and a 44 IDF will probably end up flowing less than a 40 IDF with the same chokes.

Strangling a big carb with little venturis doesn't make sense ...... I'd stick with the 36s unless there's something peculiar about your set up that would dictate otherwise.
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

BCjohnny wrote: Fri Nov 29, 2019 5:51 am Rule of thumb I've always used is choke/venturi needs to be around 80% of throttle bore for most things apart all out race, when mid eighties might work better ....... 28mm chokes will put you around 64% which IMO is too small.
Thanks for sharing. Will add this info to my archives. Any info is assisting me on where to begin and then tuning thereafter.
Idf carbs is new to me so dont have all the experience with them. Pretty simple carb in design.
Got a fairly good deal on one so giving it a go.
Im used to the dgv and dcd series webers and then some rochesters.

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Re: Single 44 idf weber on chevy inline six

Post by enigma57 »

My apologies for taking so long to respond, chevyfreak. Have been having issues with server here timing out and haven't been able to get on the forum as often as I would like. Managed to get on a couple days ago and was attempting to post this to you whenI got bounced off and it never got posted......

I agree with Geoff and Johnny. As I noted in my earlier post......
enigma57 wrote: Wed Nov 06, 2019 7:13 pm...... Do you already have the 44 IDF? Reason I ask...... The first thing is to decide what choke (main venturi) size you need. In this instance, you will need choke size from 28mm to 29mm. And there are recommended sizes of choke tubes relative to throttle bore size. Weber recommends ideally a 1.25 ratio as being optimal and you can generally go up or down 2mm from that whilst maintaining good fuel metering. But if you must choke a too large carburettor down and it exceeds those parameters by much...... A 44 IDF probably would not run as well as a 40 IDF if both were fitted with chokes that small......
Regarding a 250 inline 6 engine in mild state of tune such as yours and fitted with a single twin throat carburettor on open plenum log type intake...... A single carb such as for instance, a 38 DGAS with 27mm chokes is sufficient with respect to flow. The issue is efficiency. On the one hand, such a carburettor will have good throttle response. But due to the length of the intake and the headport layout having 3 sets of paired (siamesed) intake runners...... You will not have even fuel distribution with a single, centrally positioned carb.

Can you run a single 44 IDF with 36mm chokes on this engine with an open plenum intake? Yes, but it will be a bit large and throttle response will not be as crisp as it would if you had smaller chokes.

Will 2 carbs have better fuel distribution? Yes, if positioned at mid points between siamesed end ports and siamesed center port. However, 2 twin throat carbs having 36mm chokes will be much too large for this engine with the type intake you describe and you will find driving the car at engine speeds you will encounter in a typical road car to be quite challenging. A better choice would be the smaller 40 IDF with 28mm or 29mm chokes in my view. And yes, you could run 2 of them if you chose to.

Good luck with the idle air bypass screws. Its worth a try. I tried that with my DCNF carbs and was surprised to find that the internal passages in body of carb are too small to provide sufficient volume of air to equal drilling 2.4mm holes in each throttle blade.

Best regards,

Harry
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak »

enigma57 wrote: Fri Nov 29, 2019 5:11 pm My apologies for taking so long to respond, chevyfreak. Have been having issues with server here timing out and haven't been able to get on the forum as often as I would like. Managed to get on a couple days ago and was attempting to post this to you whenI got bounced off and it never got posted......
Harry
Hi harry. No worries. Over here i struggle to get good signal to the mobile phone. So understand your struggle.
enigma57 wrote: Fri Nov 29, 2019 5:11 pm
Regarding a 250 inline 6 engine in mild state of tune such as yours and fitted with a single twin throat carburettor on open plenum log type intake...... A single carb such as for instance, a 38 DGAS with 27mm chokes is sufficient with respect to flow. The issue is efficiency. On the one hand, such a carburettor will have good throttle response. But due to the length of the intake and the headport layout having 3 sets of paired (siamesed) intake runners...... You will not have even fuel distribution with a single, centrally positioned carb.

Can you run a single 44 IDF with 36mm chokes on this engine with an open plenum intake? Yes, but it will be a bit large and throttle response will not be as crisp as it would if you had smaller chokes.

Will 2 carbs have better fuel distribution? Yes, if positioned at mid points between siamesed end ports and siamesed center port. However, 2 twin throat carbs having 36mm chokes will be much too large for this engine with the type intake you describe and you will find driving the car at engine speeds you will encounter in a typical road car to be quite challenging. A better choice would be the smaller 40 IDF with 28mm or 29mm chokes in my view. And yes, you could run 2 of them if you chose to.

Good luck with the idle air bypass screws. Its worth a try. I tried that with my DCNF carbs and was surprised to find that the internal passages in body of carb are too small to provide sufficient volume of air to equal drilling 2.4mm holes in each throttle blade.

Best regards,

Harry
1. On this carb the idle air bypass channels is about 1/8 and screws about 5mm in size( 3/16) and a short taper so will see how it goes. Could just work out. If need be maybe smaller hole in the blade and fine tuning with the screws.
2. At that time i asked about it i already ordered the 44idf. Got a very good deal. Paid in your money $65 for it and shipping was another $25.
3. I tried a 38dgv. Runs good low down but motor feels strangled from 3500rpm.
Had a old 40dfa cleaned it gave it a go, ran good up to 4500rpm and had some other issues, carb body is not good , warped in many places so needs quite alot of attention. Older version so no power enrichment system.


In prosess of making an intake for the idf so by the time i get my new jets i can give it a go.
Wil see how it responds. Nice thing on the idf is the replaceable venturis so can change them as needed. Makes the playing a bit more easier.

Regards.
Adriaan.
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Re: Single 44 idf weber on chevy inline six

Post by Geoff2 »

I doubt you can get 28mm chokes for the 44 IDF. 32 is the smallest in the W catalog. 28mm in a 44 bore is going to hurt pressure recovery. There is a good reason that all choke style carbs have their choke [ main venturi ] sized within a certain range of bore sizes.

Also agree with Harry that the bypass screws will probably not supply sufficient air for your purpose.
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