Technical Carburetor Jetting The Old School Way

I have ordered an O2 bung and plug, gonna get that welded in ahead of time and then eventually pick up an LM-2 wide band analyzer. But I want to see how close I can tune using traditional methods.

It's the stock MC2100 that w/ 1.02" venturies, and have it dialed in pretty well. 48 jets are stock for 0-5000 and it runs great. I tried 46 jets and it doesn't "feel" right, but then heavy acceleration is handled by the power valve.

It just seemed to run a little "ragged" at steady cruise with 46 jets. Pulling the choke seemed to help, so it may be that 47 jets might be OK. It's my understanding that carbs used to run a little fat, but gasoline isn't what it used to be, and I'm running lots more ignition advance than stock, and dual exhaust. All of that tends to lean out the mixture.

Miles per gallon isn't really a concern except that maximum fuel economy means it's tuned about as well as it can be, by definition. And the more range on a tank of juice, the better. I like to use my truck for roadtrips and camping way out in the sticks. It's a longbed F100 stock 292 Y Block w/ 3.03 column shift manual and 3.73 gears. Around town stop and go w/ some cruising I'm consistently getting 12.9, so I'm figuring maybe 14 highway? Can this combo do better...

Any tricks to this, plug reading is next to useless these days. Engine vacuum is 19" at 550 RPM, what I need to do is see what the minimum cruise vacuum is and maybe bump the power valve up a bit?? It's the stock 7.5"

 

I'll be watching this,,,to see how my 7500# '57 F 600 compares...

 

Well years ago I bolted on a 500 CFM Holley, the plugs ran wet, black as coal and mileage was bad. I jetted it down about 10 steps and fooled around with the power valves and shooters and got it to run pretty well but the mileage was never there. Putting the right size carb on it made all the difference and almost doubled the mileage.

Reading some of the Jeep forums (2100 is a popular off-road carb) they talk about acceleration and bumping up jet size. But it's my understanding jets are basically in play for steady cruise conditions only, acceleration is handled by the accelerator pump shot, and the power valve. So jet size doesn't make any sense. Too small of jets can hurt economy as well, that makes no sense to me, but that's what I'm reading. A little fat is OK, a little lean can nuke a piston or burn valves.

 

You will wast a lot of time trying to tune modern pump gas , yr plugs will lie to you,
Install O2

 

I can see that. It's also true that weather and temperature plays a big role. It's running super good, but it's fun to wring things out.

 

I've had to up jet sizes on a few carbs lately to compensate for high alcohol content (noticeable lean surge at cruise). The pumps all say max 10%, but, that is a government term for 15% when we test alcohol content, over 10% is not uncommon today. Lean mixtures cause lower fuel economy because they burn too slow to be useful in the combustion cycle and they do not burn complete.

 

Plugs do not lie to you.
This will help a lot :
http://www.dragstuff.com/techarticles/how-to-read-plugs.html

 

Right, the idea here as I said earlier is to see how close I can get (or how far off I was) using traditional means compared with the O2 wideband. I know a lot of folks swear they won't tune a carb without one.

This plug looks too lean to me, but the flash washed out a lot of the tan color. The specks on the porcelin bother me too, could just be carbon or something bad? Too much timing? 48 jets.

 

This is another reason I want to try this, Want to understand what I'm doing and observe the results. The carb was a commercial rebuild, it does have the correct booster cluster as near as I can tell from the numbers. In other words it's not a basket case or frankencarb. But it did have 43 jets installed, way too small for sea level, in fact 44 jets are indicated for 10 to 15,000 feet. I drove it for a while. It was anemic, but it didn't run bad as such, and I never noticed surging at steady cruise. Here's a pic with 43 jets and Champion plugs, and one with Autolite 45 and 48 jets, looks a lot better. Some suggested in an earlier thread going to 50 jets - and maybe mileage would improve? Maybe that's what the O2 sensor will show??

 

Sure they do. Modern fuels do not leave the same deposits that old school gasoline, or race fuel did/does.

 

Add more advance, done.

 

All of my plugs look like that, on my closed-loop EFI vehicles.

 

Yes they will !!
modern pump gas with ethanol or not
Allways look Rich
And if you know what your are looking @
You can set all yr timming off ground strap of plugs , the o2 need to be Wide band and of good quality not a $59.99. More like $200 up
There more then jet's ,power vale ,idle richment screw's to tuneing a cab & a engine if your look for a EFI tune from a Carb ,
Need good Quality RPM gauge , & the biggest Vac gauge you can finde , A go pro helps also
AND WILLING to put time effort into it ,,
I have over 40 hrs on a set of 2 cabs on my Flatty,
Mine Vac is a 5 or 6 inch that reads 1/4 increments 0-30

 

Lots of carbs have been tuned without wide-bands.

Don't really know what your truck with the 292 and 3.73 gears should do. My 390 truck does a LOT better, but not running 3.73 gears, and neither of the carbs was made by Holley. And I don't have a wide-band.

Jon.

 

They look good to me but I'd do something with your ignition, its on the weak side and you might be interpreting that as your carb problem.
Sometimes specks are just specks.

 

The carb is working great as far as I can tell. What are the indications on the plugs that show an ignition problem? I appreciate the help!

 

There are so many out of tuch , you don't have to leave the O2 gauge to look cool, you uses as a tunning tool then you can remove !!
a lot of you don't understand just because you think your Ride runs good doesn't mean it's running correctly or Efficient !!
That why so many are disappointed and ashamed when they put there Ride on the stopwatch !!!

 

Truck64, did you change the timing between the Autolite and Champion plugs???
Notice the coloring on the ground electrode is not the same. It should be up near the bend.
Also you may try to "J" cut your electrodes to gain some even burning. Take a Dremel and cut it back to the center of the Positive electrode to expose it.

 

True, but modern fuel blends are making it hard to read plugs, and that's just getting worse.

If I can give you your vehicle back, 3-full- seconds faster in the quarter, and with almost double the freeway mileage, after just one day in the shop, are you going to say no?

 

If you double my fuel mileage, I am going to have to start selling fuel!!!

Jon.

 

No !!
I would change it back ,
not traditional tuning !!

Some need to understand some modern methods inprove HotRod's by makeing them more enjoyable ,for those that are less knowledgeable

 

As a matter of fact yes. I curved the distributor, it's running 17° initial with mechanical limited to 20° and around 10° somewhere with vacuum advance, light springs all-in by 3000 RPM. It's a stock Y block with low compression, maybe 8 to 1 so it will tolerate lots of advance even on pump gas.

You're talking about side-gapping the plugs? I've read about that. Did Index the plugs just for grins. This did seem to change the exhaust note, for the better.

 

http://www.wallaceracing.com/plug-reading-lm.html

 

Eddy and Gimpy - I am not arguing against the use of the wide-band; and certainly am not sufficiently arrogant to state that I can tune exactly as well as an EXPERT with a wide-band. They are wonderful tools, but many can tune sufficiently well to get by without them for the street. If I were doing racing of any type, I would use one. And somewhere around here, I have an older exhaust gas analyzer that I did use a few times, but saw no improvement in my tuning.

At least 95 percent of all carburetor issues, including tuning, end up being something else. Rather than spending the money, and the necessary learning curve time, for the wide-band, I would suggest many would do better with a used oscilloscope, to properly diagnose ignition issue. One could also argue that the oscilloscope has a learning curve, but there exist some picture books that even a total novice can understand by comparing the picture in the book with the pattern on the 'scope.

Bottom line - our hobby has survived many years without electronics of any type (my oscilloscope is a 1955 model).

But for those that enjoy the electronics, go for it.

Jon.

 

Only you are you Jon. I suspect that you have a skill level that far exceeds the average reader.

I use a wideband O2 sensor for speed, more than anything else, well that, and I get cars that other people have already "worked on", and are usually pretty screwed up from chasing ignition issues, via the carburetor.

Only my vintage motorcycle still has carburetors. All of my stuff is either diesel, or EFI, including the vintage stuff. I have 10,225-feet of elevation change to deal with out here (-282 to +9,943).

 

Quote Gimpy:

"Only my vintage motorcycle still has carburetors. All of my stuff is either diesel, or EFI, including the vintage stuff. I have 10,225-feet of elevation change to deal with out here (-282 to +9,943)."

WOW - probably the best argument I have heard for efi! That kind of elevation differential would certainly be challenging for any carburetion setup!

We do sell a lot of metering rods, most of which I have to make, for Carter carburetors for elevation issues. For that kind of differential, would require three sets.

Jon.

 

First I learned tuning the old school way.
Then I learned tuning with a wide band 02.

Then I met guys who first learned tuning with wide band and never learned the old school way.

Then I met guys who've never been able to tune anything successfully who think it's because of their lacking a wide band 02.

If you're a great tuner it doesn't matter who's got a wide band
If you're a good tuner, a wide band CAN make you a better tuner.
Then again if you're not a tuner, youre not.

It's kinda fun to compare your efforts to what a guage says. But the seat of the pants feel beats the guage discrepancy.

 

One of the reasons I'm going to get a wideband sensor is (I read a LOT when I'm interested to learn) all of the discussions have one common theme, and that is something along the lines of how far out of tune a carburetor can be and still run good. They didn't realize it till they had hard data. Somebody here in one of the archived posts said they went from 12 mpg, to 16 mpg on a road trip w/ a 350 and wideband tuning. That's in a car, I don't suppose a pickup can do as well, I dunno but that's significant and extends the range quite a bit.

 

The difference is knowing when a carburetor if running good and when one is running right.

 

Truck - the wide band can be a very useful tool; but if you do not already have one, acquire a circuit manual on your specific carburetor type (also the standard specifications if you can find them). The wide band will present the tuner with data, but the tuner must understand the function of the circuitry of the given carburetor to know what to do with the data.

Two other comments on tuning:

(1) The easiest carburetor tuning is always done PRIOR to the purchase of the carburetor (see the second line in my signature).
(2) Probably the "special carburetor tool" that I have used to solve more "carburetor problems" than any other .......is a good set of jumper cables! Simply connecting one cable from the ground post of the battery to a head bolt has made a lot of rich running carburetors suddenly perform much better! Probably not true with those that frequent the H.A.M.B., but certainly true of the general public!

Jon.