Weber head scracher?

[Old H2S]

I was messing around with my idle jets and measured the 2 lower flow holes at the bottom and they were exactly 1mm each. I enlarged the holes by .001" and my AFR went up by 1 point? I did not change the jet main hole on the tip that is the main metering for the circuit. Do you know why it made the idle circuit leaner?

[75 911s]

The idle jet is part of the slow speed circuit. It's drawing air in through the Air corrector on top and fuel through a hidden galley from the float bowl. Each Idle jet size come in half mm increments. You effectively may have moved to the next size idle jet by modifying them. What was your idle jet size to start with? 55? Air is drawn into the air corrector (little brass inserts in the top) and passes through the idle jet. So you can see how your AFR would increase because now you have a bigger hole allowing more air to enter the slow speed circuit.

Lots of info at Performance Oriented

[Old H2S]

Look at the cut away diagram. What you described is the standard description from Paul's site. My conundrum is the idel jet flows fuel from the tip backwards to the 2 holes, then get mixed with air after leaving the jet holder, mixing does not happen inside the jet but going down the well. Nothing but fuel goes through the tip, and tip size has not been reduced. The 2 back holes only let the fuel out, not air in.
Increasing the out flow hole size would reduce fuel flow speed at smaller jet sizes increasing hang time in the well, more emulsication = leaner mix? But the idea mix screw stays the same?
Flame on gents..

[stownsen914]

Making the idle jets .001" shouldn't cause much difference, and certainly shouldn't cause it to go leaner. An AFR change of 1 point is significant but not huge. Perhaps something was disrupted during the disassembly/reassembly process? Or was the weather or temp significantly different on your before/after days? Carbs can be sensitive to such changes.

[Old H2S]

Ok, what has me messed up is the factory drawings are not matching the physical description of what is going on inside the idel jet. Even though the fuel is flowing out of the 2 little holes in one direction air is getting in the other. Adjusting the out flow holes is not a documented way of enrichment but it is way more forgiving than the tip size jetting hole that must be opened in sub-thousands accuracy. The total set up would work better if the out flow holes are indexed to top and bottom and is easy to do since they rotate in the holder and will be the next thing I try

[75 911s]

Did you lube up those Idle jet holder O-rings? Maybe some air is getting sucked in there. They look dry in your photo.

[Old H2S]

No I did not lube the O rings.. Bad on me, I got lots of spares and just replace them but I was thinking about them leaking last week because the screws were not real tight and I mess with this car every day.

[snbush67]

I think that your larger secondary holes are letting more air in to mix in the idle jet, while the unchanged main hole lets in the same amount of fuel.

Read down further on Paul’s description of detailed operation. Both during idle and progression air and fuel mix in the idle jet.

{The fuel passes up a dedicated fuel gallery located between the emulsion tube well and the fuel gallery running down the outside of the throttle body. It then passes through the idle jet where atmospheric air from the idle air bleed jet mixes with and emulsifies it before continuing down the external fuel gallery.}

{The idle and progression circuit achieves the same result using a different method; air from the idle air bleed jet is mixed with the raw fuel inside the body of the idle jet.}

[Old H2S]

All true..And as you pointed out the larger jets have 1.2mm holes so, the idle jet is now a mini emulsion tube mounted side ways. The why, buy mixing in air the surface tension goes down and faster response reaction time to carb inputs lowering AFR at idle. So with 6 jets mounted in random directions should the holes be mounted vertically for easy air injection or sideways for less mixing?
This is some BS minutia for your Monday morning coffee brake, the OCD guys have a new problem to worry about..

[Old H2S]

OMG it works, I must make a YouTube channel.. 30 extra HP, car revs to 9000, 35MPG swear.
Now the next question, should the bottom pointing hole be larger that the top?

[Steam Driver]

Wow! With numbers like those you could sell at J.C. Whitney! Alongside the “Mini-Supercharger” things.

[snbush67]

I guess that to experiment you could clock the holders and index the jets so the holes line up. Fuel goes in the big hole then gets sprayed out the small holes. It seems that to get the most emulsion then they should be horizontal.

[snbush67]

Tom, What are your light cruise AFR’s to get that MPG? Don’t want to go too lean at top end.

Fun fact, I was getting great numbers everywhere but was going a wee bit lean at light throttle tip in, I had tried every configuration of jets and tubes, etc. to get good performance. I reamed the main hole in the idle jets from .55 to .56.5 and that was rich enough to get to perfection. Smoothed out the rough spot, idled at 13.5, 12.8 light cruise (25-27 MPG HWY) and when I nailed full throttle it went to 10.5 and finished at 13.2 redline.

[Old H2S]

Mine is at 57
Idle is 13.2
When to F7 emulsion tube to get the transition right.
Did 400 miles this morning at 20 MPG
Did 400 miles last Friday with the 55 idle jet but that was too lean but MPG was 24.9 and I did not like the lean tip in.
Yesterdays tune change I started with idle air correctors 3 turns out and shut down the idle speed screw to 850 RPM and then started to balance side to side and the engine is much happier with the blades closed down keeping the transition holes closed.
AND 2 PSI on the fuel pressure cured a lots of problems.. Thank you Shane!

[porschetub]

Originally Posted by Old H2S

Mine is at 57
Idle is 13.2
When to F7 emulsion tube to get the transition right.
Did 400 miles this morning at 20 MPG
Did 400 miles last Friday with the 55 idle jet but that was too lean but MPG was 24.9 and I did not like the lean tip in.
Yesterdays tune change I started with idle air correctors 3 turns out and shut down the idle speed screw to 850 RPM and then started to balance side to side and the engine is much happier with the blades closed down keeping the transition holes closed.
AND 2 PSI on the fuel pressure cured a lots of problems.. Thank you Shane!

Good result, well done, however you may want up the fuel pressure ,don't have Webers but run my Zenith carbs regulated to 3 psi and no issues ,think Weber spec is 3.5 to 4.0 from memory .
Cheers.

[snbush67]

Yes, the Weber spec is 3.5 to 4 and the 2 lbs that Tom is running is lower than normal, but if it’s working and the bowls aren’t running dry then lower pressure is ideal. I have always ran the lowest pressure I can while maintaining the right amount of fuel in the float bowls, the PMO porthole windows make it easy to check.

Overflow is dangerous, too much pressure can cause overflow, and a lot of carburetors will weep with too much pressure, and the filters can get soaked with fuel.

A test to make sure your fuel pressure is good is to do a longest hardest high rpm pull you’re going to ever do, and if you don’t run dry you should be all right. With PMO’s you can shut down and check the port holes and see where you’re at. I like the float levels on the upper edge of the dot in the porthole.

[Old H2S]

I was getting weeping on a new set of Webers at 3.5PSI.
Shane told me to try 2.5 and problems are gone.
I was running hard for 12 hours with blasts to 100MPH, no stuttering..

[porschetub]

Originally Posted by Old H2S

I was getting weeping on a new set of Webers at 3.5PSI.
Shane told me to try 2.5 and problems are gone.
I was running hard for 12 hours with blasts to 100MPH, no stuttering..

Interesting I had spill over @ 3.2 psi dropped back to 3.0 psi and problem solved ,new float valves and levels checked with PMO level gauge also .
Thanks for the reply ,and again well done,cheers .