Weber questions for basic tuning

[rbogh901]

Guys, hi. Simple question but it might save me some hassles if someone can direct me.

I'm new to carb work but have moved to 5000ft. To complicate things it's becoming clear that whoever worked on my car previously has done things that need to be corrected.

At this point I've changed my idle jets and main jets and it has made a significant improvement. Synching and crude idle circuit adjustments have also been significant. After each adjustment I test drive to evaluate in order to see which makes a difference and which does not.

So next I would like to change E-tubes and air correction jets. How do you access/ remove them?

[Indescikov]

Go to Paul Abbott's "Performance Oriented" Web site for information on engine / carb jetting recommendations. Check out his technical pages.
He can make your Webers right.
He is the master.

Performance Oriented

[Steve@Rennsport]

Originally Posted by rbogh901

Guys, hi. Simple question but it might save me some hassles if someone can direct me.

So next I would like to change E-tubes and air correction jets. How do you access/ remove them?

Air correction jets are found at the top of the carbs; 3 per side. E-tubes are right underneath and easily fished out using a wooden toothpick right down the middle.

Remember though, emulsion tubes are matched to the camshaft profile so you shouldn't need to change that unless you change cams. Idle, main, and air correctors are the main tools for altitude compensation.

[rbogh901]

[QUOTE=Steve@Rennsport;8827137]Air correction jets are found at the top of the carbs; 3 per side. E-tubes are right underneath and easily fished out using a wooden toothpick right down the middle.
QUOTE]

That's what I see on the exploded diagrams. So, remove air cleaners, velocity stacks and plates to get to them?

I've got to get to a basic starting tune as it's clear they were set quite differently from most recommendations. The main jets were mismatched 115 in 1,2,3,5 and 6 with 120 in 4. Don't know how to tell if the 115's were reamed to match but they are all undersized by most reckoning for a 2.7 with 10:1 and S cams. 55 idles seem ok but I found a bit of debris in #2. Cleaning it was obviously good and then installing 135 mains really helped. I need to check the air correctors. Probably aiming for 180-190s but need to see what I have first.

[1QuickS]

You can remove without removing air cleaner boats but some special tools are VERY helpful. I show (see image below) what I have for these tasks:

• top tool is 3/32" dia TIG rod with a little "bump" near the end; the bump is adjusted to slip inside the E-tube & wedge, coat hanger wire will work well
• the next tool is something I bought when building plastic models as a grade schooler; outside diameter is 1/4" which allows insertion so the loosened main air correction jet can be grasped & extracted
• for installation of the main air corrector I use this special tool, it has a screw driver end with a split, the split allows the tool to grasp the slot which helps to install & start the jet. Alternately (like Steve suggests) a bamboo skewer can be used to hold the jet.
• last item is a special screwdriver that allows almost vertical alignment with the axis of the emulsion tube for tightening & loosening the main air corrector

If you provide main venturi size then I am sure Steve can recommend jetting as I can do; typical 2.7S jetting at sea level would be:
34mm main vennturis; 150 mains; F3 emulsions; 180 air corrections & 55 idles; this assumes IDA Webers (IDTP jetting would be different). I would leave idle jet alone and reduce main jet to 145 & try that. Perhaps Steve would differ.

[rbogh901]

Originally Posted by 1QuickS

You can remove without removing air cleaner boats but some special tools are VERY helpful.

If you provide main venturi size then I am sure Steve can recommend jetting as I can do; typical 2.7S jetting at sea level would be:
34mm main vennturis; 150 mains; F3 emulsions; 180 air corrections & 55 idles; this assumes IDA Webers (IDTP jetting would be different). I would leave idle jet alone and reduce main jet to 145 & try that. Perhaps Steve would differ.

Very useful Paul. I imagine it can be done with screwdrivers, needle nose and toothpicks but with more fiddling? Is the problem alignment or concern for dropping them? The TIG rod is soft enough to not worry about scoring the emulsion tube?

The carbs are Italian 40 IDA with 34 venturis. 1.5" od headers to Bursch muffler. Rough tuning of the idle circuit to blue/moderate yellow streaks with my Colortune has my mixture screws @ 1 3/4 turns out with 55 idle jets. Does that sound reasonable? It has made a real difference in low to mid range. I think the recommendations are for larger idle jets if you need more than 2 or 3 turns out at idle, in my case 900-1000 rpms.

I have 145 and 150 mains to try if you think they aren't too much but am working my way up. I can't figure out why there were 115s in there.

[1QuickS]

The inside of the emulsion tube is not critical, besides, you only need a slight interference to do the job.

Idle jets sound fine, Colortune is good for idle mixture tweeking, these engine do like a bit richer idle mixture to help take throttle from idle, driving is the test, be not afraid of a little larger idle jet opening. The reason why the number of turns open is suggested to be 3 or less is that three turns will effectively unscrew the mixture screw from the orifice so that it is no longer metering.

The problem with these items is they are deep in a hole that is hard to see into. If the main air corrector gets sideways (easy to do) then it would be difficult to extract without the grippers. You might consider removal of the top cover of the Weber but this is a lot of fiddling for a jet change. Perhaps a shop vacuum with a small diameter hose taped to the end would be a good way to suck up the main air corrector.

145 mains are pretty typical in 34mm venturis, I'd start there as a safe place to begin for your altitude and work around that.

[rbogh901]

Originally Posted by 1QuickS

The problem with these items is they are deep in a hole that is hard to see into. If the main air corrector gets sideways (easy to do) then it would be difficult to extract without the grippers. You might consider removal of the top cover of the Weber but this is a lot of fiddling for a jet change.

Thanks Paul.


So most people might be removing the top cover to get to them I suppose. Maybe this is why you rarely hear of people describing the process. Steve, or anyone, do you have an alternative technique or recommendation here? Does that mean replacing gaskets when re-assembling?


Based on the wonky jetting mentioned above I feel I need to document my emulsion tubes and air correction jets. The response to the very primitive tuning I have done so far has been very rewarding and a sign that things may have further room for improvement.

[profblue]

very interesting read. thanks

@Paul - how would your recommendations change for a 40 IDTP?
currently adapting one for my 2.4s with a mod-S cam (DC40) at sea level and was planning to go for:
34mm venturi
145 mains
55 idle
185 corrector
F3 emulsion

[1QuickS]

IDTPs are difficult. I recommend 170 air corrector jets vs 185s. Also, drill a 1/16" dia hole through edge of throttle plates to help keep them as closed as possible at idle, locate holes on edge opposite from where the progression holes are and about 1/8" inboard from the edge of the throttle plate. The IDTP progression holes actually have the first hole partially exposed at idle which will have the mixture screws closed off to achieve Lean Best idling mixture. Then, during progression, the fuel that is needed from the mixture screws is restricted. Also, install tuneable idle air correction jets and start with a 100 size jet, the OEM idle air correction jets are 140s which lean out the progression. Also, I like to replace IDTP mixture screws with IDA type, the IDTP screws are particularly sensitive to idle mixture adjustments.

[rbogh901]

Correct me if wrong. Looks like this is the home of the air corrector jets/emulsion tubes.

[1QuickS]

Yes, main air corrector screws into top of main throttle body and secures emulsion tube in its well. The picture you offer is not OEM but my creation to make my life a little easier.

[rbogh901]

Yes, I poached the photo from your thread. It's one of the few I've seen that actually show the location of the emulsion tubes.

As far as jetting goes, are there significant differences when tuning Webers vs PMOs or can anyone who has converted give their findings? That is to say, are the jets likely to be equivalent for a given engine or does one tend to go larger/smaller with PMOs?

[1QuickS]

PMO carbs use jets from two barrel Weber IDA carbs. Main jets are pretty much sized to the main venturi size, the ratio is approximately 4.3:1. In other words a 34mm main venturi would need a 4.3x34 or 146 main jet; basic starting point. Typical air correction jet would be 170 to 180. The PMO emulsion tubes have numbering that does not correlate to the triple Weber versions.

If your engine uses 34mm main venturis then both Weber & PMO carbs would have 34mm venturis for that engine.

My web site has a carburetor sizing discussion with selection methods for main venturi sizing.

[rbogh901]

Originally Posted by 1QuickS

PMO carbs use jets from two barrel Weber IDA carbs. Main jets are pretty much sized to the main venturi size, the ratio is approximately 4.3:1. In other words a 34mm main venturi would need a 4.3x34 or 146 main jet; basic starting point. Typical air correction jet would be 170 to 180.

If your engine uses 34mm main venturis then both Weber & PMO carbs would have 34mm venturis for that engine.

.

And so if a 150 main jet was appropriate in an IDA it would be the same in a PMO? It's an assumption and would seem intuitive. In other words, jets in IDA's would be the same in PMOs with similar venturis?

[1QuickS]

Good enough for a starting point, each engine needs to be run to check for proper jetting.

However, 150 main jets are all 1.50mm diameter for the fuel delivery orifice. 34mm main venturis are all 34mm in diameter. Fuel delivery through the main circuit is based upon air volume and an appropriate fuel supply through the main jet. If the main air correction jet and main jet are the same and the venturi size is the same then engine will receive the same fuel delivery regardless if PMO, Weber, Solex or Zenith. Emulsion tubes tailor the fuel delivery profile from initiation of main circuit through redline. Need to test after jetting.

[Mark Henry]

Resurrecting an older thread because I found it very informative and I have a question on tuning along the same lines.

My car is a 914 with a 3.0 big ports, twin plug Patrick dizzy and MSD, 120/104 web cams, mahle rsr style pistons, 10.5 cr, with fresh 40 webers and a 1-1/2" MSDS headers.
I know I'm leaving power on the table and should get 46mm carbs and a bigger exhaust, but honestly on a narrow body 914 with 205 tires I can already light up the tires in 2nd and 3rd.
Just looking for some jetting advice to optimize the current set-up.

I have 34mm vents, 140 main jets, F3 emulsion tubes, 180 air correction, 55 idle, ported stock manifolds, custom phenolic spacers, K&N filters and rain hats. I have twin O2 meters on it and I'm running 30 degrees total advance. Floats are set and I have 3.5-4 lbs of pressure.

Right now it's running fantastic, no transition issues, almost like FI, but at 3000rpm cruise and idle i'm getting about 12.5 AFR and if I kick it down or pull a steep hill at 3-5K I'm a hair over 14.7. I've been slowly reaming the main jets and I'm up to about 140 now. I think I'd like to get it to around 13.5 AFR.

Just keep reaming the mains?

[juanbenae]

it sounds like the lean mixture of 14.7 remains consistent throughout the main circuit and not just a momentary spike when you get on throttle? in my limited experience if this is the case id say yes, a larger main jet might be needed.


if you are reaming the jets at some point during tuning you want to go back down a jet size you will have to solder and ream again. jets are not that expensive I might suggest getting a set of 155s if you are still lean after continuing to ream the 140s. when tuning its convenient to have multiple options on hand you can quickly move back and forth from.


just as a point of reference I too have a webered big port 3.oL, same cam grind, 9.5/1 single plug motor. my 40's have 36mm venturi, 60 idles and 160 mains so you should have plenty of room for the mains in the 155-160 neighborhood I would expect..

[Mark Henry]

I reamed my main jets I believe they are now 145 (IIRC .053") and I have a pretty steady 12.5:1 AFR across the board, seemed like they were smaller than the 140 (.051"). Still a bit rich at idle at 12:1
I may have took it a bit too far as I was aiming for 13:1 AFR, Although I'm confident with my reaming I'm going to buy some correct main jets and a set over /under.

Thanks.

[1QuickS]

Idle mixtures will be rich, not much can be done about it since leaning idle mixture out will result in a progression that is too lean to drive.