Technical Tuning carb with a wide band air fuel ratio gauge

Happydaze,
Thanks for sharing your experience.


Phil

 

So my AEM fuel ratio gauge came last night and it looks like a good product, comes with complete directions, at least to install it. It said to install the sensor 18" from the exhaust port (i think it was) but did not say how far from the end of the pipe it should be. Like I said before, I'm running lake pipes and just have a short turn out at the end of the header. I was going to make up another pipe to replace the bolt on turn out to weld the bung into. Any idea how long to make it? How long after the sensor.

 

Any updates?


Phil

 

I installed one on my 59 Vette, running a 377" sbc. I could not seem to get the heavily modified Holley to perform just right, I thought this would help.
Well, 12 months later, it only made my tuning issues worse. Finally I thought I was getting it dialed in, the gauge quit working. AEM says it's low voltage causing the issue, my fluke disagrees. It does work on occasion, and is telling me I got it pretty close, but I no longer trust it.
So, I went back to seat of the pants, chassis dyno and an 1/8 mile drag strip. She's dialed in now, running great, doing all the right things. The gauge is coming back out, my son wants to use it, I wish him luck, he has 3 webers to deal with!

 

I have found that to be the case for a lot of tuning articles and info, for example, CFM calculators - they always suggest plugging in the highest, some insane RPM, but if you're not drag racing a big ass carburetor coupled with a rumpety cam isn't likely going to idle very well or run as smooth around town. There is a "sweet spot" in terms of "tuning" the actual air velocity through carburetor/intake that you want to hit based on intended RPM range you're likely to see or spend time at. Best fuel atomization, venturi effect blah blah. But you can tell. Cruising at 50 in high gear is totally different requirements than balls to the wall, and it also in turn affects the amount of manifold vacuum - and therefore, amount of vacuum advance - another important item for street driven cars that is usually omitted entirely from drag racing engines. There is definitely a synergy involved.

Another bit of wankery is how they suggest power valve # selection. Performance engines have very little manifold vacuum compared with stock, and they often use very low PV #s, or block them off and jet way up, etc.

Your friend probably knows how to read plugs from way back, and has all kinds of observations made over the years. He may have roasted a valve or two along the way! Plug reading is way, way more involved than just looking at the porcelain, and with modern gas pretty tricky. It doesn't color the same as it used to. The soot in the tailpipe is different too.

 

Sorry but no, between rain and yard work I have not been able to get the car out. I have stuff to do this weekend also so maybe next weekend.

 

Chris... Search Amazon.com for surgical tools, buy a needle holder (forcepts) to grab, lock, and hold the loop in the clip. Hard to lose a 4" pair of holders...

 

UPDATE so I installed the air -fuel gauge and I'm way rich through all ranges. so after adjusting my float level and idle mixture I got idle running in the 13.5:1 area. But with some throttle opening the mixture is still too rich pegs the gauge at the rich end at 10:1 ratio. At idle I have 11" vacuum and have a 3.5 power valve in the primary. When opening the throttle slowly so vacuum does not drop off the mixture still goes to 10:1 and vacuum is up over 14 or 15. Looking down the carb it does not look like the fuel is pulling over the main discharge so the excess fuel must be coming through the idle circuit, right? any ideas? Just so we are all on the same page, the carb is a 1850 holley vacuum secondaries that I have installed a metering block on the secondary side with a plugged power valve.

 

update added

 

What is your steady level cruise AFR at say 50 or 60? Don't worry about WOT until after that's dialed in.

Have you seen this page? It's pretty comprehensive.

http://members.tccoa.com/392bird/tuning.htm

"The goal of main jetting is to go as lean as possible without misfiring or surging. This will provide the best cruise economy. Any additional fuel required for maximum power will be delivered, upon demand. by the power valve. Some sources recommend examining spark plugs to determine jetting but with the advent of unleaded fuels this has become much more difficult to do. A single jet size change is only a 4 1/2% flow difference, so jump two sizes at a time until you surge or misfire, then drop back by single sizes. Be aware that ignition timing changes can alter the the optimal jetting so you may have to iterate. If you must go up or down 8-10 jet sizes then you have a problem either with the fuel delivery system or the carburetor is wrong for your application. It's handy to have a means of changing jets without having to disassemble the carb. Quickfuel provides for this by offering fuel bowls (P/N xxxx) with external plugs that can be removed to provide access to the jets. Note that these quick change bowls are only compatible with center hung floats. There also aftermarket kits (Percys and Willys) that achieve the same end."

 

You should be 2 points higher then you are at all speeds and idle. Lean out a couple jets sizes, readjust the idle screws, tinker with the timing to see if it has any effect on idle speed, and check to see where the vacuum advance is hooked up. My 250 inline runs down the road at 15.5 to 16.5 and idles in the high 14's. When you get the A/F right, the vacuum will fall into place then wide open throttle can be corrected with power valve changes.

 

Well, I have not gone out on the road yet, this is just done in the garage, I know I will have to hit the road to get it right but am I wrong to think that just holding the throttle just off of idle with no load I could get a reasonable ratio?

I'm not even close to start looking at main jet size until I get the idle and transition circuits closer... right?

 

So you say I should be 2 points leaner at speed, ok thanks. and the vacuum is hooked to the standard Holley port on the side of metering block, it pulls from manifold.

 

It seems to me that I need to reduce the size of the idle speed jet in the primary metering block. (at this point I must say the carburetor is a swap meet item and I don't know what parts have been changed) I figure I have to change the idle speed jet because that is what controls the idle and transition flow, this or the air correctors up top. (idle bleeds) I'm thinking this because fuel is not flowing from the main discharge at the point that the mixture goes rich AND I get the 13.5:1 ratio at idle with the adjustment screws out only 1/4 turn.
I figure this idle jet has not been changed in the metering block as it is a press in thing and most likely not done, but the metering block may not be for this carb and not for my size motor. Just in case I did not say at the top of this post, the car in question is a '30 Model A sedan with a 292 Y block, mild cam and auto trans.

 

Interesting read.
I bought a wide band AFR gauge a while ago to use with a McCulloch supercharger setup on some future project. I have since sold the supercharger but still have the gauge.
I have been thinking it might be useful in fine tuning the 324 in my roadster.
It's running four "genuine" Stromberg 97s and a magneto.
It doesn't seem to have any real issues, other than uneven plug color, front plugs run cleaner than rear, I think it's likely due to the Cragar manifold design.

Anybody have real life experience tuning four Strombergs on a Cragar intake with the AFR gauge?

 

I read the whole topic so far, and yes, you will indeed need to be on the road and under full load for each condition you tune for. While I haven't tuned automotive carbs for quite a while I've tuned 2 stroke hipo sleds every season up to about 4 years ago. Wanna sweat carb tuning? Try that. Go the wrong way and you need pistons, rings, full gasket sets, and in the worst case a $300 cylinder!
The replies above are right there with regard to tuning and potential goals. And as was stated above also, YES, it can be info overload. Carbs are a giant FUEL/AIR COMPENSATION DEVICE. Short of Weber/Dellorto set ups the engine has to "share" fuel delivery from a single source. Get it close and fun, done. Perfect is just short of a pipe dream.

No, that's not a slap in the face to good tuning, it's just a touch of context to help.

 

Just read the whole page, thanks

 

The idle AFR isn't super super critical. It does affect AFR in the other circuits but overall it's like pissing alongside a fire hose. Set it where it idles smooth, see that the AFR is in the "recommended" range but don't obsess over the idle mixture numbers too much. The main thing is an overly rich idle mixture fouls plugs. After a minute or two idling the plugs will tend to "load up" and idle quality suffers, don't want that.

Actually yes, you want to get the cruise AFR jetting done next. Read and study the link I provided on carb tuning, it explains way better than I can.

Jet size up or down affects the wide open throttle AFR, even though it's technically a different circuit. So you can't really figure out what needs to happen in the power and acceleration circuits, until the steady, level ground cruise circuit is dialed in first. Drag racers jet up and down as required, but they don't care about cruise AFR, cuz they only run flat out.

Remember you want to tune each carburetor circuit more or less completely independently of the others, in isolation, and in a specific order. This ultimately saves time. Yes it's a pita, but you'll thank yourself everytime you drive it. What seems to happen is that as jet size goes down, so does the WOT AFR.

Measure your cruise manifold vacuum. Make certain power valve # is a couple points lower than this, and not opening at highway cruise on level ground, regardless of speed. This is very much dependent in your local altitude as well as camshaft. If it's too close to the tip in point you'll be sucking gas like no tomorrow, in even a slight headwind. And it will interfere with proper jet size.

Jet down until you reach your target AFR. Don't be afraid of 15 or 16 .., that's where you make your range or fuel economy. Be mindful of "lean surge" or misfire.

Under low load conditions (level ground) lean fuel mixtures won't hurt anything. Supposedly it takes less than 50 horsepower to cruise down the highway once you get going. At the same time, this is when vacuum advance really pulls in, with timing at 40°- 50° BTDC. Lean mixtures need lots of ignition advance, to get the fire lit on time. Your ignition has to be good and hot spark. This is how you can get an old V8 to pull 20+ mpg highway. But it takes some tuning.

Under acceleration, WOT, anytime under load, you cannot go lean AFR under any circumstances. Your tuning strategies are mostly centered around these two opposite concepts. On the highway lean it out till it squeeks, but anytime the go pedal is depressed you want plenty of fuel. Usually around 12.5 or so.

 

That is the plan, just wanted to get closer than pegging the gauge out on the rich side before hitting the road. After reading the page that Truck64 posted, I'm going to look at the idle transfer slot and the idle bleeds/corrector and see what is going on.

 

That seems like low vacuum reading for a mild cam.

 

Chris - Snap-On has a wonderful tool with a spring loaded clip that you can clip on to your hairpin, remove the hairpin with the tool, and the hairpin will be retained by the tool. The same tool is also quite useful in installing the hairpins.

Most NAPA parts houses have a large assortment of carburetor clips. Sold individually.

Truck64 - your posts are better than many of the carburetor books I have read.

Highlander - agree about the one source (if one only has one carburetor) and the GENERAL cylinder imbalance. However: if one really gets anal about the tuning, one can use multiple bungs (or use a hand-held infrared temperature sensor) so that one can actually "see" the cylinder imbalance, and calibrate accordingly. The carburetor (except with an IR intake) does NOT feed the cylinders, rather the carburetor feeds the plenum, and the cylinders draw from the plenum. Cylinder pulsing to the plenum will result in the various bores of the carburetor seeing different signals.

Sometime take a look at the jetting Chrysler and Carter used on the street hemi with dual Carter AFB's. Each of the 8 carburetor ports is calibrated differently which leads to a plenum saturation imbalance, which in turn minimizes cylinder imbalance, as each cylinder draws from a different area of the plenum(s). The Chrysler engineers REALLY did their homework!

The wide-band IS a wonderful tool, but it will NOT convert a novice into a tuner. Understanding the function of each carburetor circuit and their relationships is probably more important than the wide-band. The wide-band provides data. What the tuner can do with the data depends on an understanding of the specific carburetor.

Jon.

 

I know you're right Jon. I didn't want to take the message too far is all.

I ran my racer with a self-tuned 1050 Dominator on a Dart intake. Ran a cpl seasons getting not only better but learning when to quit. So one night a guy from our "gang" wants to bolt on his ultra-uber-double throw down-whazoo-mega $$$$ Dominator and show me how much faster and better my car would run with it. It started as an 1150 and was wet flow checked and metered and boostered and, well, you get the idea.

So imagine my satisfaction and his disappointment with the results as it gave me 1 MPH avg and about .05 less ET avg over 3 full runs. Late season good air, honest results. He paid well over a grand for this thing, I bought mine used for $300 and already had a multitude of Holley tuning parts. This where I fully agree with truck64, only interested in WFO results. I staged foot to the floor, never lifted til the stripe, and a 1/2 mile of better mid range and throttle response on the return road wasn't worth $7 to me, let alone another $700. Just to note, he ran a 509, me a 477. Both roller cam high compression bracket racers. He ran a 'Glide, I ran a 400.


Of course road manners and economy matter to all of our carbbed "toys" and I apply my best to the stock stuff for these Packards and whatnot. The idea of an AFR gauge on a classic never occurred to me, but y'all got me thinking now, dammit....

 

I want to thank all you guys for pitching in on this discussion. I'm running an Edelbrock 1407 on my 383 stroker with an AFR hooked up and I was reading on straight and level cruise control Highway a steady 13.2 to 13.4 AFR. Downloaded and printed the correction charts from Edelbrock picked an alternate jet size and Rod size and strolling down to my local performance parts store here in Austin got the right parts stuck them in yesterday went out for a little Cruise and now I'm at 15.5 on the average! I was getting about 10 miles a gallon hopefully I can get up to 12 or 13 haha

 

Don't know what body you have this in, but believe with a little work you can do better than 10 MPG. The last carbed vehicle I owned that didn't do better than 10 was a 454 27' Winnebago!

Keep after it!

Jon.

 

Yea, it is a cool gizmo, I have to get out on the road and see where my cruise ratio is at now. Rain all day yesterday, maybe next weekend.

 

Just ran across this thread, and sat here and read it all as I'm sure my roadster and my coupe are both on the fat side. Roadster is '31A with 355 SBC with Edel air gap 2X4, with simultaneous hook up on Edel carbs 1803&1804 500cfm w/air valve secondaries and a fairly radical cam. Coupe is '40 Ford SBF 302 with 600 Holley and a mild cam.
Reading this thread, I'm wondering if I need to put bungs on both sides (dual exhaust)? Shorty headers on the coupe, and Speedway "roadster" headrs and side pipes on the roadster.
Since both manifolds have plenums and are 180* would only one side work as long as I did the same thing to both sides of the carb? Or in the case of the 2X4s the same to both sides of both carbs?
Jon, I'm gonna have to keep an eye out for the Snap On truck for that gadget to grab the hairpins, as they give me fits!

 

I'm only on the one side. I have lake pipes so I guess after I get it to where I want it I could put my test pipe (the pipe with a bung that bolts to the header) on the other side and see how close they are.

 

Dave - picture:

http://www.thecarburetorshop.com/Carburetor_clip_tool.jpg

The picture doesn't show everything, but there is a plunger attached to the spring loaded-top that goes through the tube. Depressing the top will cause the end of the plunger to appear, and the end has a clip on it. Hook the clip into the carburetor clip and release the top. The plunger will then retract and hold the clip. Actually MORE useful for installing clips in really tight places. I have at least two of the tools so I can misplace one and still find one when I need it

Jon.

 

Thx Jon.