Need Carb Jet Guidance, Weber 40 IDFs Options

[nditiz1]

While I cannot disagree with Phil as his knowledge on all things far surpasses mine. Got more years on me Phil, but im still learning. Anyhow, I can't understand how the air corrector has any effect on the idle circuit. You can drive around with the main stacks gone. There would be no air corrector.

[bbrock]

To my surprise, the idle leaned out. I don't understand how, but suspect sorcery (IMG:style_emoticons/default/smile.gif)

Think of it as like a straw. You pull liquid up the straw by putting a partial vacuum above the fluid.

Small straw - very little air need to be evacuated from the straw to pull the fluid up say 1" in the straw. Now think 4" PVC pipe -- same amount of vacuum will provide same fluid lift but you'll need a good set of lungs to move enough air to establish that partial vacuum and to to lift fluid an inch in that PVC pipe that would be no problem in a small straw.

Now think of your jets. Bigger fuel jet and bigger air correction jet are more like the 4" PVC pipe. Your engine can only displace so much air per revolution -- so you get less lift of the fuel when flowing the same amount of air.

The "trick" here is that both jet and the air correction jet need to change (get larger) together for that analogy to hold. Otherwise, you're back to a singe variable and the way you're thinking of it (i.e larger fuel jet = richer & larger air correction jet = leaner). The two of them at the same time yields the 4" PVC analogy.

The main problem with carb tuning is that there are so many variables and they aren't all related in a linear manner and they don't always abide by the Superpostion Principle.

So in other words... it IS sorcery! (IMG:style_emoticons/default/av-943.gif)

[Superhawk996]

While I cannot disagree with Phil as his knowledge on all things far surpasses mine. Got more years on me Phil, but im still learning. Anyhow, I can't understand how the air corrector has any effect on the idle circuit. You can drive around with the main stacks gone. There would be no air corrector.

I don't want to claim any sort of mastery over carb tuning. Nothing is 100% clear in any of the theory of operation that I've looked at over the years. Everyone seems to do things a little bit differently.

So here goes:

At first glance the air correction jets (on top of emulsion tubes) don't seem to be related in any way.

But look at the fuel flow diagram:



You can see the idle circuit actually pulls fuel from the base of the emulsion tube.

Printed Weber theory of operation also states this. Fuel drawn though the main jet and though emulsion tube wells on its way to the idle jet.



From there on, it shows idle jet and the calibrated idle orifice (fixed - pressed into carb body) as controlling the idle mixture (along with needles).

My analogy is that by increasing the size of the main air correction jet (even at idle) you're affecting how much (or how easily) gas is lifted to the idle circuit for a given vacuum input. I'm just thinking of the pressure balance above and below the fuel level in the emulsion tube well at idle.

Who's to say I'm not smoking dope. (IMG:style_emoticons/default/smoke.gif) No one ever says that the main air correction is related to the idle circuit per se. I'm just thinking that at idle, if I completely sealed the main air correction closed, it would be easier for the idle circuit to pick up the fuel to a point higher than the emulsion tube well. As the air correction gets bigger, and more open to atmosphere, it gets slightly harder to make that lift (IMG:style_emoticons/default/confused24.gif)

The only thing I've really learned over the years with carbs is that there really are no independent variables and that all things like idle jets, main jets, air correction jets, and even the emulsion tube hole sizes and placements are all inter related. We all know how hard it can be to get a smooth transition from one circuit to another. What governs the mixture in that gray area where you're not quite off the idle circuit but not fully onto the mains. Only God knows for sure.

I don't think there is a perfect theory of operation that ever explains it all. I'm not convinced anyone knows other than what has been learned by 100+ years of experimentation.

[Superhawk996]

So in other words... it IS sorcery! (IMG:style_emoticons/default/av-943.gif)

Gotta agree to a degree (IMG:style_emoticons/default/laugh.gif)

[bbrock]

The only thing I've really learned over the years with carbs is that there really are no independent variables and that all things like idle jets, main jets, air correction jets, and even the emulsion tube hole sizes and placements are all inter related. We all know how hard it can be to get a smooth transition from one circuit to another. What governs the mixture in that gray area where you're not quite off the idle circuit but not fully onto the mains. Only God knows for sure.

I don't think there is a perfect theory of operation that ever explains it all. I'm not convinced anyone knows other than what has been learned by 100+ years of experimentation.

Which is why I don't quite follow the "carbs are simple and FI is complicated" logic. Sure, FI has a seemingly complicated array of sensors, but each is doing its discreet small part to measure factors influencing combustion and feeding them to a programmable processor that for the most part just opens and closes a collection of valves at the right time. Carbs have this strange alchemy of fluid dynamics of both liquid and gases swirling through a Rube Goldberg contraption for trying to meter a spray that is close enough to some ideal.

[930cabman]

The only thing I've really learned over the years with carbs is that there really are no independent variables and that all things like idle jets, main jets, air correction jets, and even the emulsion tube hole sizes and placements are all inter related. We all know how hard it can be to get a smooth transition from one circuit to another. What governs the mixture in that gray area where you're not quite off the idle circuit but not fully onto the mains. Only God knows for sure.

I don't think there is a perfect theory of operation that ever explains it all. I'm not convinced anyone knows other than what has been learned by 100+ years of experimentation.

Which is why I don't quite follow the "carbs are simple and FI is complicated" logic. Sure, FI has a seemingly complicated array of sensors, but each is doing its discreet small part to measure factors influencing combustion and feeding them to a programmable processor that for the most part just opens and closes a collection of valves at the right time. Carbs have this strange alchemy of fluid dynamics of both liquid and gases swirling through a Rube Goldberg contraption for trying to meter a spray that is close enough to some ideal.

I can somewhat explain my personal logic " FI is a system of electronic sensors, ECU(s), and complicated objects" Carbs being archaic "mystical" are more simple add gas, fool with a few things, bend this, drill that and you are going.
There can be little/no debate of which is more precise

[nditiz1]

Whoa!

Sure carbs are finicky to tune but no one will tell me the fastest way to get an engine running from dead is to work on the FI. Fuel, spark, and air. I digress...

[bbrock]

Whoa!

Sure carbs are finicky to tune but no one will tell me the fastest way to get an engine running from dead is to work on the FI. Fuel, spark, and air. I digress...

He he. I thought that might hit some nerves (IMG:style_emoticons/default/bye1.gif)

[GBX0073]

Great to see this thread on Carb tuning and the 914 world collective at work
Making some changes on my engine while it is out I have a snail
Planned on getting a wide band and placing the gauge in the center gauge clock slot
I was thinking of this one
Innovate Motorsports 39350 Innovate MTX-OL Plus Digital Air/Fuel Ratio Gauge Kits | Summit Racing
Or do I hold off and just try with the color tunes ?

[GBX0073]

Went Ahead and ordered
Innovate Motorsports 39350 Innovate MTX-OL Plus Digital Air/Fuel Ratio Gauge Kit
maybe a little overkill on the display. also picked up a gauge and sensor to pull head temp for Cylinder #3

[bbrock]

Well, I'm not sure if I took a step backward or forward tonight. I drove to town and bought new set of spark plugs. Drove to a nearby park and put them in. Then I readjusted the idle mixture screws but had a helluva time dialing them in. Car was stuttering worse than ever and farting and popping. Mix was lean not matter what I did. Really lean, like 15s. Stopped one more time before leaving town to readjust and back the mixture screws out enough to richen the mix. Drove home with the car sputtering and popping and got home in a REALLY bad mood. Thought I'd go back to basics and check the timing again. Went to plug the vacuum ports and found the retard port open. Yep, I must have snagged it and pulled it off changing the plugs! (IMG:style_emoticons/default/headbang.gif) At least it was a simple fix. Oh, and the timing was spot on.

With that out of the way, I was able to put a really good tune on the carbs with fresh plugs. Running smooth and sounding good again, I plugged in the computer to read the AFR. Idled at 10, driving 20-30 mph up to 3000 RPM, steady low 10s. AFR is also much more stable than before. No more jumping around. So at least my world makes sense again. .50 idle jets are too rich, .55s are really too rich, and .45s are too lean. Seems obvious where I need to go next. My hypothesis is that the fouled plugs were causing enough stumble to spoof the wideband into reading more lean that it actually was. That's the best I can figure anyway.

[Jake Raby]

Most carb issues are really ignition issues. If 50 idles are too rich and 45s are too lean, try 50s with more ignition advance. All my engines have wanted more timing at elevation, and modern fuel exacerbates this.
ALL the old carb jetting guidelines are superseded today, you can't use anything that's been written in stone for the last 50 years. Today's fuel likes timing, and more jetting.

[bbrock]

Most carb issues are really ignition issues. If 50 idles are too rich and 45s are too lean, try 50s with more ignition advance. All my engines have wanted more timing at elevation, and modern fuel exacerbates this.
ALL the old carb jetting guidelines are superseded today, you can't use anything that's been written in stone for the last 50 years. Today's fuel likes timing, and more jetting.

Thanks for the tip! That would be wonderful it if turned out to be a timing issue. I'm chicken to advance the timing much beyond stock spec without guidance from someone in the know.

It also occurs to me that the readings I took with the 45s would have been when the old plugs were beginning to foul. Wondering if it would be worth throwing them back in to see if they still read lean.

[Jake Raby]

Most carb issues are really ignition issues. If 50 idles are too rich and 45s are too lean, try 50s with more ignition advance. All my engines have wanted more timing at elevation, and modern fuel exacerbates this.
ALL the old carb jetting guidelines are superseded today, you can't use anything that's been written in stone for the last 50 years. Today's fuel likes timing, and more jetting.

Thanks for the tip! That would be wonderful it if turned out to be a timing issue. I'm chicken to advance the timing much beyond stock spec without guidance from someone in the know.

It also occurs to me that the readings I took with the 45s would have been when the old plugs were beginning to foul. Wondering if it would be worth throwing them back in to see if they still read lean.

You have to make the engine happy. That means throwing everything at it that you can, and note the results of each. Nothing runs worse than a rich engine that's running retarded. This will also run hotter, since the combustion process is occurring late, and the mix is still burning once the exhaust valve opens.

In other words you have to try everything. You have to try the stuff that you don't believe is the issue, or what you don't correlate to being a player in the issue.

Mr. Mistake is my teacher, and he has been for 3 decades. He's always right.

[930cabman]

Where are your float levels?

[bbrock]

Where are your float levels?

I've checked them 3 times. They are good. 10mm closed, 32mm open.

[bbrock]

Well I think I can finally declare victory. There has been a lot of fiddling since the last post and what's more fun that endless fiddling farting with carbs? [sarcasm dial set to 11].

First thing was to take Jake's advice and try the 50 idle jets with more advance. Adjusting to about 34 degrees total advance did wake up the engine and lean the AFR numbers a little. It was still running rich but I decided to call it good enough and run with it for the summer.

I had a 600 mile work trip to the Blackfeet Nation a couple weeks ago so decided to take the 914 on its first long trip since the restoration. It ran like a dream on the way up and was a blast to drive. I calculated the mileage and I got 28 mpg on a tank that included 50 miles of start/stop town driving and short stretches of highway that were more foot down acceleration than cruising. I thought that was pretty good. However, about 150 miles from home on the return trip, I started to notice a stutter and the tach would jump. It got worse the closer I got to home and I was pretty sure the plugs were fouling again.

I pulled the #1 and #2 plugs and sure enough. There was a carbon track forming on one side of #1 and deposits forming on #2. I decided to order a set of 47 idle jets rather than risk having to replace plugs every thousand miles. This morning I dialed in the new idle jets with new plugs. I noticed the #1 idle screw wasn't as responsive as the rest but I was still able to find best lean idle. The AFR at idle was about 11.3 which still seemed rich but I took it for a drive. I was very happy to see low speed (20-30 mph) numbers in the mid 12s to low 13s on the way out. I was really happy until I got it up to speed on the main circuit and it went way rich (10s and 11s) at 70 mph. Then on the way back home, it stayed rich even on the idle circuit. Something was definitely amiss.

Thinking of the unresponsive mixture screw, I decided to pull that carb to recheck floats, make sure it was free of blockages, etc. I found the o-ring was pinched on the #1 idle jet. Mystery apparently solved but I went through the whole carb anyway after replacing the O-ring. I also decided to switch the AC jets back to the original 2.0s to see if that leaned the main circuit. It didn't make much sense to me but I've given up on understanding all the possible combinations of how these carb changes affect AFR.

After re-dialing the carbs the AFR was still around 11.3 at idle, but as soon as it was under light load, it leaned out to mid 12s to low 13s. Drove around on the idle circuit for quite awhile and it stayed consistent. Taking it up to speed was very satisfying. Cruising at 60 mph is low to mid 12s and cruising at 70-80 mph is mid 12s to mid 13s. I like those numbers! There was some stumble on transition between circuits, but a bit of fiddling with the accelerator pump adjustment smoothed those out. I really appreciated how being able to read AFR during acceleration helped to dial those in. I think I could get it even better, but I'm pretty happy with where it is.

So glad to have this finally done. Monday I have to head back to Blackfeet Nation so will be repeating the trip for a good shake down. Here is my final setup:

Altitude: 6,000 ft.

Venturi - 28
Emulsion tubes - F11
Main jet - 115
Correction Jet - 200
Idle jet - 47

[930cabman]

Well I think I can finally declare victory. There has been a lot of fiddling since the last post and what's more fun that endless fiddling farting with carbs? [sarcasm dial set to 11].

First thing was to take Jake's advice and try the 50 idle jets with more advance. Adjusting to about 34 degrees total advance did wake up the engine and lean the AFR numbers a little. It was still running rich but I decided to call it good enough and run with it for the summer.

I had a 600 mile work trip to the Blackfeet Nation a couple weeks ago so decided to take the 914 on its first long trip since the restoration. It ran like a dream on the way up and was a blast to drive. I calculated the mileage and I got 28 mpg on a tank that included 50 miles of start/stop town driving and short stretches of highway that were more foot down acceleration than cruising. I thought that was pretty good. However, about 150 miles from home on the return trip, I started to notice a stutter and the tach would jump. It got worse the closer I got to home and I was pretty sure the plugs were fouling again.

I pulled the #1 and #2 plugs and sure enough. There was a carbon track forming on one side of #1 and deposits forming on #2. I decided to order a set of 47 idle jets rather than risk having to replace plugs every thousand miles. This morning I dialed in the new idle jets with new plugs. I noticed the #1 idle screw wasn't as responsive as the rest but I was still able to find best lean idle. The AFR at idle was about 11.3 which still seemed rich but I took it for a drive. I was very happy to see low speed (20-30 mph) numbers in the mid 12s to low 13s on the way out. I was really happy until I got it up to speed on the main circuit and it went way rich (10s and 11s) at 70 mph. Then on the way back home, it stayed rich even on the idle circuit. Something was definitely amiss.

Thinking of the unresponsive mixture screw, I decided to pull that carb to recheck floats, make sure it was free of blockages, etc. I found the o-ring was pinched on the #1 idle jet. Mystery apparently solved but I went through the whole carb anyway after replacing the O-ring. I also decided to switch the AC jets back to the original 2.0s to see if that leaned the main circuit. It didn't make much sense to me but I've given up on understanding all the possible combinations of how these carb changes affect AFR.

After re-dialing the carbs the AFR was still around 11.3 at idle, but as soon as it was under light load, it leaned out to mid 12s to low 13s. Drove around on the idle circuit for quite awhile and it stayed consistent. Taking it up to speed was very satisfying. Cruising at 60 mph is low to mid 12s and cruising at 70-80 mph is mid 12s to mid 13s. I like those numbers! There was some stumble on transition between circuits, but a bit of fiddling with the accelerator pump adjustment smoothed those out. I really appreciated how being able to read AFR during acceleration helped to dial those in. I think I could get it even better, but I'm pretty happy with where it is.

So glad to have this finally done. Monday I have to head back to Blackfeet Nation so will be repeating the trip for a good shake down. Here is my final setup:

Altitude: 6,000 ft.

Venturi - 28
Emulsion tubes - F11
Main jet - 115
Correction Jet - 200
Idle jet - 47

I was unaware of an idle jet 47? (correction, Pierce has them in 1/2 sizes) either way victories are always good. Would it be safe to say the Webers will stay and the $$ you save will go into another project?

[bbrock]

I was unaware of an idle jet 47? (correction, Pierce has them in 1/2 sizes) either way victories are always good. Would it be safe to say the Webers will stay and the $$ you save will go into another project?

Nope. Drove the 914 almost 500 miles today over several high mountain passes (it choked). Carbs are running better and I got great mileage, but are still nothing like a good FI. The carbs and the 1950s era ignition have to go. There is no way I could trust this car on a long trip, and no, 500 miles is not a long trip.