I have a basically stock 351 with a rebuilt 2150 carb.


After taking it out wheeling at elevation this weekend I'm definitely running too rich. While that's easy enough to adjust, I do have some problems with lean misfire if I lean it out much.


The challenge is finding a happy medium between running around town at 1000 feet elevation, and being able to go up to 7000 feet and still have a truck that runs sorta OK. At elevation now, you can smell that it's running way rich, and my MPG in the mid single digits confirms it (granted, this is running trails).


So...


When I replaced the carb, I could get 18+" of vacuum despite the vac leaks with the old one. On the new one the most I can tune is just over 15" of vacuum, which seems low, and maybe that's why it wants to run so rich...because it's not pulling in enough air.


At one point when I had the old carb that leaked vacuum through the throttle shafts, the truck ran fine on +20 degrees timing - no dieseling, etc.). I backed it off to about 14 degrees when I put on the new carb thinking that 20 was just too much, but I'm considering advancing it again. Here's the questions:


Will that by itself affect fuel consumption good or bad?
Do I run the risk of worsening a lean misfire with timing advanced further if I were to lean the mixture some?




I have all new tuneup components - OEM Motorcraft stuff - Plugs, wires, cap, rotor), so all that is good to go, and I'm reasonably certain I have no vac leaks anywhere, as everything has been gone through.


Input appreciated.

 

You did not say when it runs rich or lean but you can only adjust one easily and that is idle.
You also said the MPG went down so this would be on the high speed side and you can not change this easily as you have to open the carb to change the high speed jets.

Changing timing will change the vacuum reading and if setting the timing by vac gauge you set for highest reading. As for making it run better or worst for the lean miss I cant say as I have never come across this.

I would say adjust the timing for highest reading as long as you don't get pinging. You may need to adjust this when either up high land or down low land because of pinging.

As for rich lean - because you can not change jets easily and you know what jets work high & low and where you spend most of my time at, I would go in the middle with the jet. It is not the best for either one but will work better than good at one area and bad at the other.
Now if going high land 1 or 2 times a year I would jet for low land and maybe just before I go up swap in the middle of the road jets till I got back low land and swap the others back in.


Also have you looked in to jets for the MC 2150 carb? As far as I know it does not take Holley jets and you SHOULD NOT drill out jets.
I wonder if Holley still makes the "dial a jet"? If so a Holley 2300 v2 500 CFM carb would bolt on to your intake, add the dial a jet and think you would be good.

Other thing would be to go EFI as it would adjust for the conditions.
That is my .02
Dave ----

 

X2! Advancing the timing should add a little power, which should allow you to not press the throttle as much - which should add a little MPG.

Not a bad way to go. That's what I would do to mine if I could get around to it...

 

I would love to go EFI, but given everything I've sunk into this truck recently, it's just not in my budget right now.


that said, are there good soup to nuts kits out there that enable a total conversion that a guy could do in the driveway?

 

Over in the motor area for the 300 six, that's what I have in my 81 F100, there is a EFI thread going.
A post I saw today had a link to FiTech's EFI bolt on kit think it was #31003but I don't see it on their site?


The link had a write up of the kit on a chevy 262 L6/six but being the throttle body is a v4 it will bolt in place of a Holley carb.


The write up said it was under $1000 but don't know when it was written and is a self contained & learning setup.
Dave ----

 

There are a couple threads going now on the FiTech EFI. Heliguy has two of them in his trucks. They seem good and reliable. The base cost is about $1000, and about $1500 with the electric pump setup and various plumbing bits.

No real change in horsepower, but no more problems with cold start or altitude changes

 

Get thee a wideband O2 sensor, you'll get the carb dialed in better than you thought possible. With the right gearing you should be pulling 18 to 20 mpg, say. This will take some tuning, but it's worth taking the time.

Ignition curve or engine timing and carburetor tuning is how both power and economy and efficiency is made. A lean fuel mixture needs more timing, not less, believe it or not. The goal for timing is to give it as much lead or advance as it will stand at all times under all conditions, just short of pinging or engine knock.

The idea with carb tuning generally is to lean it out as far as possible by jetting down when cruising (at any speed) on level ground and lightly loaded, but provide plenty of juice when the go pedal is mashed or under load, going up hills, etc. It's OK to run lean while steady cruising, but never under load. To get the "best of both", you'll have to drill out the PVCRs after jetting down, "power valve channel restrictors". This is where the wideband O2 sensor comes in. They make carb tuning easy.

Jetting makes a difference when going significantly higher in elevation, but it's only a couple sizes or so. When you get it dialed in though, you may not want to bother. Jet changes on the Autolite carbs is pretty easy though in the field.

More likely though, what you're seeing is the effect of the wrong power valve. Engine manifold vacuum keeps it closed except when power enrichment is required. At 7k feet, it may be open constantly as engine manifold vacuum is probably now below the opening level of the power valve and would explain the mileage issue. They are sized or stock value is for Sea level. When us kids were on vacation at Pikes Peak, CO with mom and dad, I remember watching a steady stream of fuel drizzling out of the tailpipe on the cars as we started getting up there in elevation. Now I know why. Power valve sizes are based on altitude, in the same way that jet size is selected. Often overlooked.

 

Thanks Tedster. Your post helped me affirm what I'm doing next. I had timing backed off about 6 degrees from where it used to be, and I have the idle settings set very rich to potentially compensate. I'm going to start over....bump that timing back up where it originally was with the old carb and lean this thing out as much as it'll stand and see how she runs.


Great MPG in this truck with 4.10s, 35s, and big lift isn't going to be possible, but if I could get in the 11-13 range I'd be tickled.

 

Not to hijack the thread, but if somebody with experience in using/reading/monitoring a wideband O2 sensor could explain the process, provide a link or some pictures to equipment they use/like - that'd be great!

 

I bought an Innovate LM-2, it's pretty slick. What you want is a wideband type sensor, that's the key. The narrow band type are basically useless for serious carburetor tuning. Get an O2 sensor bung installed (welded) in the tailpipe while you're shopping and you'll be able to tune right away when it arrives.

The instructions are pretty sparse. They describe how it works, how to set it up, but they don't really describe what actual AFR numbers to shoot for. Had to dig around for those. Keep in mind that a carbureted engine can't run the extremely lean mixtures that a fuel injected engine might. But somewhere around 16 to 1 is optimal for our old school trucks, cruising down the highway. This is where fuel economy is made.

The takeaway is though, one can't just jet down till she squeaks and call it good. It's more complicated than that. Another consideration is if you have to pass smog in your state, and whether the truck came with a catalytic converter. A "lean burn" strategy isn't practical with a catalyst, as it will meltdown, and it won't pass NOX emissions. But this tuning strategy will increase mileage or range. I went from around 9 mpg to 12 after installing a stock sized carburetor, and then from 12 to 16+ mpg using the wideband. I think I can probably improve that by a mpg or two, just roughed it in. It all adds up, getting the ignition curve and vac advance optimized, good plug wires and ignition and hot coil, valve lash, everything. A good ignition is required to ignite lean mixtures.

A vacuum gauge particularly and a tach is also your friend here. The power valve is often the culprit for poor performance and poor mileage. A 6.5" size is often stock, but depending on altitude and engine tune this needs to be changed out to a different value. Measuring the lowest vacuum at cruise is a big help here for selecting a power valve. Only want it open when necessary, and again - when the main jetting is leaned out for good mileage, the power valve circuit needs to be wallered out a smidge to increase the fuel mixture ratio under load.

Going lean under acceleration - under load - is Not Good

That's where the wideband pays for itself. There's no guessing. You can have both respectable fuel economy on the highway and around town, and light the tires off when required. It takes a good understanding of the different carb circuits - idle, transition, cruise, acceleration, power valve, etc. I'll dig up some links.

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