Or... you can tune a carburetor, but you can’t tuna fish (with apologies to REO Speedwagon)
When we swapped the tired 1963 144 for a 1978 200 in our Bird, it was immediately apparent that the 1978 carburetor wouldn’t work without extensive linkage modifications. Since we had a good 1963 Autolite 1100 that we’d already been using, we decided to convert it from Spark Control to ported vacuum and use it on our 200.
I say we had a “good” carburetor. “Good” is a relative term. The carb was “good” in that it started and ran the car, didn’t leak, and hadn’t burned the car to the ground. What more could you ask for? The carb was a Frankenstein in that the power valve on our rebuilt 170 1100 died within a year of our purchasing it, so I swapped the carb top from a 144 carburetor we had laying around, just to get a good power valve. When I swapped the top from the 144 carb, I reused the jet that was originally on the 170 carb top.
So to summarize: we had a 170 carb bottom with a 144 carb top, and we’re going to use it on a 200. I'm betting the calibration is off by a contry mile. Any takers?
Old meets new: our new-tech Air Fuel ratio rests its backside comfortably on vintage upholstery.
Wideband O2 meters (WBO2), such as the Innovate Motorsports LM-1, are a relatively new tool that give us weekend hacks the ability to do what only the pros could do a decade or two ago: continuously sniff the exhaust and get a readout of the air/fuel ratio (AFR). A ratio of 14.7:1 – stoichiometric – is erroneously considered the holy grail by some. First, a ratio richer than stoich is necessary under heavy load. Second, you can sometimes get away leaner ratios under light load, such a cruising. Also keep in mind that for gasoline with a 10% Ethanol mix – like that forced upon us here in
Just like with people, a proctological probe will reveal the health of our Bird.
We’d already warmed the engine fully (imperative before tuning begins) by going for a 20 minute drive, and we connected the LM-1’s sensor to the Falcon’s tailpipe and fired it up. We’d already adjusted the idle using the old fashioned vacuum gauge method, and even with a WBO2 it is still best to use this method. At idle, the engine wants what the engine wants, and the method used since time immemorial is still best. For the record, this method yielded an AFR at idle of about 13.5.
This exhaust clamp allows you to quickly get set up to monitor your Air Fuel Ratio. The O2 sensor screws into the top, as viewed here.
Going for a ride, things didn’t look so terribly bad… at first. We maneuvered around in slow city traffic, watching the readout on the LM-1. The Falcon was cruising at 13.0 – 13.2 (a bit rich) and at wide open throttle, we were recording about 11.0 – also a tad rich but not bad for a start. A trip down the highway in front of our shop provided a rerun of the previous observation: the Bird was cruising in the low 13's. That's too rich... improvements can be made.
Before you change anything, get a notebook. Write down every baseline measurement, write down what you changed, and write down the result. This is a must-do, in case you lose your way and need to get back to a known good baseline. Also, change only one thing at a time; it's too easy to get confused from multiple changes.
The main jet wasn’t marked, but measured .063” (a # 52 drill bit fit it snugly). At this point, I did something that isn’t recommended: I drilled carburetor jets because I didn’t have the ones I needed, but had an assortment that were too small. Drilling jets is a bad idea for a couple of reasons. First, the manufacturers don’t mark them until after they flow test them. Two jets with similar-sized orifices might flow different rates, and thus get marked differently. Second, drill bits generally don’t come in .001” increments, but jets do. There is sometimes a big jump in diameter between the different sized drill bits, making it impossible to get the jet size you need. My semi-acceptable plan was to get close with drilled jets, then buy a small assortment of "real" jets for fine tuning once I knew approximately what size I needed. As it turned out, I got close enough with a drilled-out jet… for now.
I installed a main jet with a diameter of .055” (a #54 drill bit) and it was obvious that I’d gone too lean. The engine had an obvious lean miss at cruise, and the LM-1 confirmed it: the AFR was dancing between 17.0 and 20.0, and the engine was NOT happy!
Only one thing left to do: drill another jet with a #53 bit (.059”) and see what happens. Going for a ride, it didn’t take long to realize that this was better… much better. The highway speed steady cruise AFR was in the 14.5 – 15.0 neighborhood. Oh yeah, this was good! But then I came to a hill, and saw that I still had work to do. As I increased the throttle opening to maintain speed up the hill, the AFR stayed in the 15.0 area, and then increased to the high 15’s as load continued to increase. I also began to hear light detonation. This is not what I wanted to see… I wanted to see the AFR drop into the 13’s under moderate load, but everything was upside down, and continued operation could damage the engine.
I could have jetted richer to fix the lean condition under load, but then we’d be cruising too rich, and kill our gas mileage. Or, I could leave it the way it was and get improved gas mileage during low load conditions… until we killed the engine by running it lean under high load.
What to do, what to do?
Before we can understand what to do next, we must understand what the power valve does. Under load (low vacuum) it opens, allowing more fuel into the engine. The fuel enters “in parallel” to the main jet… think of it as the main jet getting bigger under heavy engine load. It works because engine manifold vacuum pulls the piston, and must do so against the force of a spring. This seats a ball, essentially closing the power valve. Under heavy load (when the vacuum is low) spring pressure is greater than the force of the vacuum, and the spring pushes piston, allowing the ball to unseat, which opens a channel to the flow of fuel. This additional fuel richens the mixture.
OK, enough technoid… how can I use this theory to make my car run better?
Study the photo for a moment. In the photo, the power valve is open… the spring has pushed the piston, the ball is unseated, and if there was fuel present, it would be free to flow past the ball into the tube below it. But where would it go then?
It goes here! The Power Valve Access Plug (it looks like a tiny freeze plug) has been removed, and inside you can see the brass Power Valve Restriction. This orifice meters the additional fuel when the power valve is open, and ours was too small. WAY too small!
Remove the Power Valve Access Plug by finding a drill bit that almost fits inside it and begin drilling it. The drill bit will dig into the sides of the plug and twist it right out. Then you can see the Power Valve Restriction. I measured ours with numbered drill bits, using the go/no go method. Ours was approximately .031” – a #69 drill bit fit snugly into it. At that point, I drilled it to the next larger size – a #68 drill (.032”).
Before buttoning up the carb, I had to seal the Power Valve Access hole. I didn’t want to put the same plug back in, because it is a press fit plug, and removing it multiple times (which we're likely to do) will cause it to be sloppy and not seal properly. So, I drilled the access hole with a #29 drill bit, tapped the hole to 8-32 (don’t tap it all the way through!) and inserted an 8-32 set screw into the hole. Now we had easy, repeatable access to the Power Valve Restriction.
Reassembling the carb and road testing it revealed a barely detectible improvement. So the carb came apart again, and the Power Valve Restriction was dilled again, this time with a #66 drill (.0325”). Again, there was an improvement, but we weren’t there yet. Two more times did the trick: a #64 drill bit (.036”) was the sweet spot. The AFR was dropping to the mid 13’s when we put the engine under load at highway speed! Things were looking much better.
By the way, an increase from .031" to .036" is a whopping 36% increase in metering area! More on that later.
It is at this point that we should take a look at the spark plugs for a sanity check. Just as expected, our inline 6 was running the outboard cylinders leaner than the inboards. Remember, the WBO2 is reading a combination of all 6 cylinders at the tailpipe – individual cylinders could be rich or lean and you’d never know it unless you read the plugs or had some other means of checking each cylinder’s AFR.
It is also at this point, if we drove the car on a regular basis, that we would purchase a few assorted jets, some larger and some smaller than we were now using, and experiment with swapping jets (one size at a time) to see how it affects fuel mileage. Remember, especially after jetting leaner, to recheck the AFR under load and make sure it isn’t too lean. This is also a good reason not to enlarge the Power Valve Restriction too much at first… if you later change to richer jets, making the Power Valve Restriction smaller would be a challenge.
There you go. Our Bird cruises without missing a beat, and if I hadn’t dialed in our carb, we’d have left some throttle response and fuel economy on the table.
Remember Junior High math class? Remember how you said you'd never use that stuff for anything fun? I'm about to change your mind!
While we were asleep in math class, the teacher explained that the area of a circle can be calculated by this formula:
pi * R^2 (say "Pie Are Squared")
Where pi = 22/7 (or 3.141592653589793) and
R^2 = the radius (half the diameter) of the circle times the radius of the circle... the radius squared.
When we change the size of orifices, we need to keep this formula in mind so we'll have a good idea, percentage-wise, how much change we made. For example, when we enlarge an orifice from .030" to .032", we've actually changed the area by a whopping 14%, not the 6% you might think from calculating the difference in diameters. When were're talking about metering orifices, it's the area that we care about.
Yeah, I hate doing math, too! Here's a handy online calculator that will do the heavy lifting for you. You're welcome!