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Single 44 idf weber on chevy inline six

General engine tech -- Drag Racing to Circle Track

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chevyfreak
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Single 44 idf weber on chevy inline six

Post by chevyfreak » Wed Nov 06, 2019 12:00 pm

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.
Chevyfreak.
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enigma57
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Re: Single 44 idf weber on chevy inline six

Post by enigma57 » Wed Nov 06, 2019 7:13 pm

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

chevyfreak
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak » Wed Nov 06, 2019 10:38 pm

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 » Thu Nov 07, 2019 2:50 am

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 » 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.

chevyfreak
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak » Thu Nov 07, 2019 5:01 am

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.

Chevyfreak
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Re: Single 44 idf weber on chevy inline six

Post by chevyfreak » Thu Nov 07, 2019 5:10 am

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.

Chevyfreak.
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