Need Carb Jet Guidance, Weber 40 IDFs Options

[bbrock]

Plans to Megasquirt my car got delayed over the winter so my Webers will stay on for another summer. So, I finally hooked up my wideband to see if any adjustments are needed to keep the engine happy and safe until I can do the conversion.

The only real "problem" area is idle and low speed driving on the progression circuit where it is running too rich. It idles smooth and steady at 700 rpm with AFR of 12.3. That with idle mix screws ~ 2 turns off their seats. That's the leanest I can adjust and keep the engine happy. Just off idle at 20-40 mph @3000 rpm it goes even more rich to around 11.0 +/- 0.5. I don't think that's alarmingly rich but wasteful and I do get a bit of annoying surging/bucking driving below 30 mph.

Current jetting is as follows:

Venturi - 28
Emulsion tubes - F11
Main jet - 115
Correction Jet - 200
Idle jet - 50

I'm thinking I should reduce the idle jet but not sure by how much. I can get .047, .045, or .040. Any suggestions?

I could also use some advice on mains and/or air correction jets. If I didn't plan to swap the carbs to FI, I'd try for a little leaner highway cruising mix for better economy and maybe flexibility. In limited testing running in good ranges for performance at the expense of economy - WOT ~12.5, Highway cruise ~13.5 but rarely goes above 14.0 even on downhill and decel. Weather is crappy here this weekend so I've only done a short drive in hilly conditions. I need to get it down in the valley for some long level runs to see where it settles. I think I can live with the numbers I'm seeing now, but do wonder what it will do when I attempt to cross a 10,000 ft. pass. I'd rather it not die of oxygen starvation if I attempt it. Any thoughts?

BTW, I live at 6,000 ft. elevation so I'm not surprised it is running on the rich side with out of the box jetting.

[Superhawk996]

Warning: you’re measuring using an electron microscope and trying to adjust with a sledge hammer.

Chasing stoichiometric mixture of 14.7 (100% gas) is folly with carbs.

Stoichiometric is a moving target. It is about 14.0:1 for E10 since the ethanol brings along its own oxygenation. Do you know what is in your tank at any given time?

Also know that even with FI, there are plenty of times when fuel is being thrown at the engine to control temperature off the engine, address detonation, or to cool the catalytic converter.

Engines rarely make best power at Stoich. This chart may help.



Ultimately you need to feed more fuel to the leanest cylinder. But, I don’t think you’re monitoring AFR on a cylinder by cylinder basis so trying to get to 14.7:1 at the tailpipe could leave you very lean at one cylinder.

You are better off tuning by plugs, power, and cylinder head temps than chasing Stoich that may very well leave you running lean on high load grades, at elevation where you have less oxygen.

What I can tell you is that back in the day, carbs tended to be run rich both for power and to accommodate the need to drive coast to coast without putting holes in pistons.

[Superhawk996]

https://www.youtube.com/watch?v=lG1OHoqeQTU


I think think you’ll be fine on carbs at 10,000 feet. Try to keep up with the Rambler (IMG:style_emoticons/default/av-943.gif)

[rfinegan]

if you are at 2 turns, you are running on the lean side for your idle circuit and need a larger jet

[914Toy]

What size is your engine? Have you adjusted/balanced/equalized the airflow of all six throttles at idle?

[NARP74]

My car was adjusted at a much lower altitude then brought to 6000 ft. Here is what an old Porsche mechanic set mine to and it runs pretty well with Dual W40s at altitude.
I'll put them in your order for easier comparison, the book lists them differently.
Venturi 28mm
Emulsion F11
Main 1.15
Correction 2.10
Idle .55

[bbrock]

Thanks all. The engine is stock 2L (1971cc) with european pistons getting 8.2 compression. The carbs are balanced for air flow (4 throats). The car is running great except the slight bucking at low speeds when running on the progression circuit.

@Superhawk996 I'm not chasing stoic at all. In fact, I'm completely ignoring stoic. AFR 12.5 at WOT and 13.5 accelerating or hill climbing seems good and I'm happy with that. What surprises me is that it doesn't lean out when the throttles close on decel or downhill. I would also expect a leaner mix at steady highway cruise although I need to collect more data on that. That makes me wonder if it is starving for air which is what I would expect at my elevation. Interesting that @NARP74 is running a larger correction jet than mine. That said, I'm happy with these numbers so long as the car doesn't choke climbing Beartooth Pass. I'm skeptical though. I remember the days of trying to drive from Denver to the Eisenhower tunnel in a car jetted for low altitude (not fun). And not being able to start a VW bus parked at 11,000 ft. at a remote trailhead after a week of backpacking. Call me paranoid (IMG:style_emoticons/default/smile.gif)

The real issue that I would like to address is the rich mix a low speed. @rfinegan , if I tune the idle mix by ear, I'm about 2-1/2 turns out on the screws which is right where Bob Tomlinsons' Weber Manual says is typical. However, the AFR at that setting is in the low 10s which seems way too rich for idle so I turned them back in a half turn where I still get smooth idle but a little more reasonable 12.3 AFR which is certainly not lean. Seems like a larger idle jet would be in the wrong direction. Also, there is the bucking I'm getting when driving at low speed on the idle/progression circuit. Tomlinsons says that is usually from too large of an idle jet so I'm a bit confused.

[nditiz1]

I'm interested if they are still available.

When the bucking happens does the AFR shoot to 20s? That would be leaning out. How does it feel if you pull the main stacks and just drive around on the idle jets?

[bbrock]

I'm interested if they are still available.

When the bucking happens does the AFR shoot to 20s? That would be leaning out. How does it feel if you pull the main stacks and just drive around on the idle jets?

Will have to wait for better weather before any more road testing. Pouring rain for the last 3 days and our gravel road is like driving through wet cement. However, I have six miles of data driving fairly level at low speed (20 mph) while the bucking was happening. AFR ranges from 10.8 to 12.3 with 90% of readings in the 11s.

Did not know you could drive without the main stacks in place. Will have to try that when the road dries out a bit (IMG:style_emoticons/default/biggrin.gif)

This morning I started from scratch readjusting the idle air mixture. Tuning by ear settles on the same 2 turns off seat as before. I readjusted the idle speed just a tad higher as I had it right on the margin before. Rebalanced the carbs. I'm now getting a steady idle AFR of 11.1 and the airflow reads just under 4 on the snail.
When I rev the car parked in neutral, AFR dips to low 10s as the accelerator is pressed and peaks at 15 when I let off before settling back to idle at 11.1. Again, I'd expect the AFR to peak somewhere closer to 17 when I come off idle.

BTW, someone will ask - the floats are adjusted properly.

[rhodyguy]

Slight bucking at low speeds? How low and @ what RPM? What do your plugs look like? Drop down a gear or speed up and stay on the main circuit. It will hurt your MPG a bit but so what? That's the hand 100+ year old technology deals you.

[Superhawk996]

What surprises me is that it doesn't lean out when the throttles close on decel or downhill.

It won't, and you shouldn't be too surprised. @bbrock

At decel (especially down grade) you have high vacuum below the throttle plate and you're pulling plenty of fuel via the idle circuit yet the throttle plates are fully closed and the only air is via cylinder equalization bypass and what little bleeds past the throttle plate edges.

No decel fuel shutoff like modern fuel injection.

As noted by Nditiz1 -- you can pull an awful lot of fuel via the idle circuit. Most people don't realize that they are running on the transition ports more often than the mains unless accelerating hard or under high load.

Don't know if you've ever stumbled across Performance Oriented website but wealth of information on carbs, jetting, etc. Primarily aimed at sixes but the Weber theory of operation is largely the same for IDF's.

http://www.performanceoriented.com/performance-tuning-2

[bbrock]

Slight bucking at low speeds? How low and @ what RPM? What do your plugs look like? Drop down a gear or speed up and stay on the main circuit. It will hurt your MPG a bit but so what? That's the hand 100+ year old technology deals you.

I've said bucking but I think a slight surging is a better description. As long as I drive a steady 20-30 mph, you can feel the power hunt maybe a second or two between oscillations. I try not to lug the engine and keep it above 3000 rpm. Standard residential speed limit in our town is 25 mph and I'll go as low as 2500 rpm cruising those, but no lower. MPG is more important to me than HP. Fuel economy on this engine has been less than ideal, especially when I do much city driving. Of course, the ultimate solution will be to ditch the carbs. The more I use them, the more I hate them. I know some people love their carbs. I'm not one of those people.

What surprises me is that it doesn't lean out when the throttles close on decel or downhill.

It won't, and you shouldn't be too surprised. @bbrock

At decel (especially down grade) you have high vacuum below the throttle plate and you're pulling plenty of fuel via the idle circuit yet the throttle plates are fully closed and the only air is via cylinder equalization bypass and what little bleeds past the throttle plate edges.

No decel fuel shutoff like modern fuel injection.

As noted by Nditiz1 -- you can pull an awful lot of fuel via the idle circuit. Most people don't realize that they are running on the transition ports more often than the mains unless accelerating hard or under high load.

Okay, that makes sense. It does bring me back to the idle jets though. Larger jets would let it suck more fuel (wasted) on decel - no? I assume most city driving is happening on idle and progression ports and that's where I'm running rich. I feel like a better number for those slow speeds running on idle jets and progression would be 13s or even low 14s since the load is light.

[Superhawk996]

I assume most city driving is happening on idle and progression ports and that's where I'm running rich. I feel like a better number for those slow speeds running on idle jets and progression would be 13s or even low 14s since the load is light.

Yes, would agree under light load city driving, you're definitely on the transition ports.

I might be inclined to try a larger air correction jet.

Stealing directly from Performance Oriented w.r.t. air correction jet:

Larger orifice diameters shorten the duration and leans the fuel mixture delivered through the idle jet.

Looking at how NARP is calibrated at altitude, he's larger on fuel jet and larger on air correction. It very well be that both need to change to get what the engine wants.

But, if it were me I'd stay where you are on the idle jet and go larger on air correction trying to lean out the idle circuit a little and maybe move you to the main jet a little sooner. However, I think the problem you'll have is that if the air correction leans it out too much - you're going to end up backing out the idle needles more. The more you move them out the less effect they have as you get the needle taper further and further out of the needle bore orifice. You're already two turns out so I'm not sure you want to go much more. That may very well be why NARP ended up with a larger fuel jet AND a larger air correction jet.

Having said all that . . . you're not going to be happy until you have FI given your OCD nature ( (IMG:style_emoticons/default/grouphug.gif) ) and what you're trying to achieve (Fuel Economy). Per my 1st post, you're using a highly sensitive modern tool (Wideband AFR) to try to tweak an ancient technology (carbs) for Fuel Economy. You're bound to be left wanting what you can't have some where in the operating range.

@bbrock

[bbrock]

I assume most city driving is happening on idle and progression ports and that's where I'm running rich. I feel like a better number for those slow speeds running on idle jets and progression would be 13s or even low 14s since the load is light.

Yes, would agree under light load city driving, you're definitely on the transition ports.

I might be inclined to try a larger air correction jet.

Stealing directly from Performance Oriented w.r.t. air correction jet:

Larger orifice diameters shorten the duration and leans the fuel mixture delivered through the idle jet.

Looking at how NARP is calibrated at altitude, he's larger on fuel jet and larger on air correction. It very well be that both need to change to get what the engine wants.

But, if it were me I'd stay where you are on the idle jet and go larger on air correction trying to lean out the idle circuit a little and maybe move you to the main jet a little sooner. However, I think the problem you'll have is that if the air correction leans it out too much - you're going to end up backing out the idle needles more. The more you move them out the less effect they have as you get the needle taper further and further out of the needle bore orifice. You're already two turns out so I'm not sure you want to go much more. That may very well be why NARP ended up with a larger fuel jet AND a larger air correction jet.

Having said all that . . . you're not going to be happy until you have FI given your OCD nature ( (IMG:style_emoticons/default/grouphug.gif) ) and what you're trying to achieve (Fuel Economy). Per my 1st post, you're using a highly sensitive modern tool (Wideband AFR) to try to tweak an ancient technology (carbs) for Fuel Economy. You're bound to be left wanting what you can't have some where in the operating range.

@bbrock

Thanks. NARP said his carbs were tuned for lower elevation and then moved to 6000 ft. I assume he's happy with the tune though or wouldn't have posted.

I'll try larger correction jets for sure. CB Performance sells jets pretty cheap so I might go ahead and buy a set of .45 and .55 idles to play with.

You are dead on that I won't be happy until I get the FI on (IMG:style_emoticons/default/smile.gif) However, I'm not looking for perfection with these carbs, but would like to get a little better than low 20s mpg while I drive this summer and smooth out that low speed. Even though it is minor, that kind of (IMG:style_emoticons/default/stromberg.gif) drives me crazy. Performance-wise, the car runs like a champ. There is a bit to be gained in driveability though.

I'm off to collect more data.

[NARP74]

They were originally tuned at a much lower altitude, ran like crap at 6k ft. I had them retuned at 6k ft and those are the numbers I posted. Not sure what they were when I got it here. It did run poorly before the retune, spitting and popping at idle and low cruise out of the neighborhood. Ran a little better after a warm up, but still not great.

[bbrock]

They were originally tuned at a much lower altitude, ran like crap at 6k ft. I had them retuned at 6k ft and those are the numbers I posted. Not sure what they were when I got it here. It did run poorly before the retune, spitting and popping at idle and low cruise out of the neighborhood. Ran a little better after a warm up, but still not great.

Thanks for the clarification. Super helpful! (IMG:style_emoticons/default/beer.gif)

I have some better data now. I drove the car down to the valley (5,000 ft. elevation) where I could find long stretches of quiet road relatively level. It's clear the idle circuit is running rich. Consistently rich () through all speeds @~3000 rpm up to 50 mph.

0-50 mph: ~10.5 - 11.5 but mostly right at 11.0
Cruising level 50 mph: 11.5-12.1
Cruising level 60 mph: 12.5
Cruising level 70 mph: 13.5
Cruising level 80 mph: 13.5 - 14.0
WOT: ~12.5

Even at high cruising speeds the AFR dips down to low 11s/high 10s if the throttle is backed off to just barely past idle.

It seems AFR is pretty good whenever it's pulling from the mains, but as soon as the throttles close to idle/progression, it pulls too much fuel.

I just ordered 2.10 correction jets and a set each of .45 and .55 idle jets. That way I should have a combo that will work better than the current setup.

[nditiz1]

Another thing to look at is the accelerator pump. If they are coming in too soon that could account for the low end richness. 47.5 - 50 have usually been good for the idle jets on these 2 liter engines. Your mains look really good.

[bbrock]

Another thing to look at is the accelerator pump. If they are coming in too soon that could account for the low end richness. 47.5 - 50 have usually been good for the idle jets on these 2 liter engines. Your mains look really good.

Thanks. I wondered about that. IIRC I have the pumps adjusted fairly conservatively but will check. It is running rich even when holding the throttle steady though so won't rule out the accelerator pumps as potentially contributing, but don't think they could be all of the problem. Will definitely revisit the adjustment though.

[Superhawk996]

(IMG:style_emoticons/default/popcorn[1].gif)

Will be watching to see how this turns out with new jets. The data and your logic seems sound.

[bbrock]

Nothing new to add but I did some reading (yes, dangerous) about the air bypass screws that led to some philosophizing. I'm a curious guy and need to know how things work. I was curious whether the bypass screws could be used to lean out the idle mix (as opposed to the mixture screw which only adjusts the volume of the idle mix introduced at idle). The quick answer is No, but it took me into a controversy over the bypass screws. The CB (Tomlinsons) Weber manual suggests turning the bypass screws 1/2 turn out in pretune settings which is what I followed. Most people say the bypasses should be closed except as needed to balance the idle air flow of barrels within a carb. Some predict dire effects if they are opened except to balance a barrel. Well, maybe, but probably not.

Thinking through it, the idle adjustment screws simply open or close the butterflies to get a desired idle. Open a bypass screw will raise the idle without changing the position of the butterfly. So the effect of starting with bypass screws 1/2 turn open is that the desired idle will be set with the butterflies more closed than if the bypass screws are closed. In theory, opening the bypasses could drop the butterflies at idle farther below the progression ports than if they were closed. That could create hesitation on acceleration. I don't know if the opposite could be true where having bypasses closed could result in butterflies engaging progression ports at idle which would be bad.

I'm not noticing hesitation on acceleration, but will make sure the bypasses are closed to see if it changes idle AFR at all. Unlikely since if anything, having them open a little should ensure I'm not drawing mix through progression ports at idle.

This all led to a bit of an epiphany about the Tomlinsons manual that so many of us depend on to learn how to set up carbs. It was written prior to the days of O2 sensors. Tuning was by ear and by feel. Maybe occasionally a dyno was used but even then I'll bet tuning for max power was emphasized and economy was mostly an afterthought. As a result, recommendations in that book tend to create rich conditions that will make an engine run well, but not necessarily efficiently. We could probably use an updated manual that walks through tuning carbs with a wide band. Then again, I've come to the conclusion that other than for nostalgia or originality, going FI is the better, and ironically IMO, the less complicated path. Still, I'm glad I tried the Webers. It has been an interesting journey.