I have experimented with the 2 bbl Motorcraft carbs a lot, Not for what you don't want to hear. They are extremely hard to make them get good gas milage, In part due to their design. You cant really get to the idle feed restrictors for one, And these make more difference than most think. Second is the small venturi, This causes air speed to be high coming thru the carb, Which in turn pulls more fuel. Next is jet availability., You can find some smaller jets and i have drilled some (used solder to close present hole and redrill). But it would be less costly for you to put a 600 cfm 4 barrell on and probably double your present mileage. And as i always recommend, Get a wideband. Even if your not a tuner you can watch the wideband and alter your driving style to get the better mileage. Also there is a number stamped on the outside of the throats, It will be like a 1.25 or something of the like. This is the throat or venturi size, If you must keep the 2 barrell find one with the highest number possible. I think they actually made one with a 2 inch venturi. This will help slow the air speed thru the carb. In turn it will run a little leaner with the same jets you have now.
I have a 4bbl Holley waiting for an intake, I bought it in better days in hopes that I would be able to do the 4bbl swap. Eventually I'll score an intake I can afford(nearly free) and I'll need sealant and gaskets. Right now even $5 is a big deal. I am laid off and yadda yadda. I really appreciate the information though. I think I'll start with the rebuild kit to stop the leaking accelerator pump.
I got an easy 20 mpg with my old 302 and 4 bbl edelbrock on a single plane. If I kept my foot out of it. But, I could also suck a gallon per mile, if I put my foot IN to it. On the new 750 carb, I drove 250 miles on about 3/4 tank, and it was running pretty rich out of the box, at 13-13.5 A/F. about 14 mpg driving mostly granny-style. I am now running in the 14-14.5 range and need to do another test. BTW, I use a battery operated GPS. I can reset the odometer, then use it as an accurate speedometer, and when I refill my tank, it tells me how far I drove since the last fillup.
Probably need to replace the balancer. They have a tendency to slip on the I6's when they get older, rendering the the timing mark inaccurate. My 200 gets 22-25 mpg hwy. depending on how hard I run it. I've got MSD ignition on it, with a header and a Rochester BC carb. Along with the Holley, I'd also recommend a full tune up and converting it to electronic ignition. If you can keep your foot out of it after those upgrades, I guarantee your gas mileage will double.
I have had terrible luck with Holley carbs. Edelbrock is my carb of choice. I have 4 of them on vehicles and plan to put them on the others. I have gotten much better mileage with 4 bbl's than I got with the 2bbl's. If it were me, I wouldn't even fool with the 2bbl and go straight to 4bbl. You might also consider changing over from points to Pertronix with an Ignitor coil. Jim
I agree with Coutangman, to a point. Edelbrocks are GREAT on stock engines, but once you start adding heads, headers, etc. to them, they lean out quidkly. That is why I used an Edelbrock 1406 (600cfm) for years., But each time i added a performance part to the engine, it ran leaner, and I had to rich it up, unti I ran out of adjustments. I ended up running it at the richest rod/jet settings I could before swapping it out. Then I wen to to a Holley double pumper model. 750 HP Streeet. Now I have to lean it out a little to tune it in.
Do you still have POINTS in the disti? The first set in my six cyl Comet were good for a year (when I bought it) but after a tune up with points, plugs, filters, disti cap & rotor, I started to have chronic misfires, bogging, and "worse" mileage (not as bad as yours, but not as good as the little six could be. 3 sets of points later and I could not keep them "set" accurately for more than a few weeks... probably a combination of slack and sticking in the breaker plates and - I think - an incorrect condenser rating. When I went back to the original condenser the car ran better... maybe from excessive heat during summer driving. Timing and dwell always seemed to be drifting by a few degrees and that caused bogging. Ultimately I put in a Pertronix elec. ignition - timing is perfect - no misfiring, car runs dead smooth, mileage is better, and no more pinging on reg. gas. If you'r spending money on a tune up before looking at leaks or rebuilding the carb, the Pertronix seems well worth the money. I rebuilt a 2bbl from the 302 with a $15.00 kit - the instructions are pretty good - not a complicated carb. I thought it would be more difficult than it was. Take your time with the little pieces. It's a fun project if you have good lighting, a big kitchen table, and some patience and small pliers. Make sure the 2 parts to the choke aren't stuck - the coil spring inside can come off and the rebuild kit includes new gaskets. And - uh - if your idle dropped or it's running rough - vaccum advance hose; is it still connected and not leaking? WHiskeee
Me again!... I am the PERTRONIX babbler... apparently stock points will misfire up to 10% of the time on an old engine - that equates to one misfire for every 2 engine revolutions. My timing used to "bounce" constantly at idle so I 'averaged it' to set the timing around 6 BTDC - all the car could run without pinging. Frequently the timing flash would run off the crank marker alltogether - a misfire. The elec. ignition is dead smooth. My plugs and wires were good. Now I run at 10 BTDC and no pinging.
You really can't get an accurate mileage number unless you fill the tank twice and know the miles driven between fill-ups. Factory fuel gauges rely on info from a float that is a close cousin to what is used in a toilet. I also highly recommend a Pertronix ... really changed the personality of my 351. It has a fairly rowdy cam in it and the choke has been milled off of the carb. Since changing to the Pertronix, it starts in half the number of attempts. Runs much better now too. Mine is years old now ... the Pertronix II looks like a nice improvement ... multiple sparks at lower rpms. Ditch those points ... they are perfect only when you install them, and simply degrade from there.
When I switched to pertronix, it starts EVERY time, with no choke. It may not stay running on cold days, but it fires right up immediately after I turn the key.
Pertronix is only $60-70 or so. How much is a duraspark setup? By the way, what was the original question?
As far as I know my engine is mostly original...1973 parts. At least it doesn't look like much has been changed. The distributor seems like an OEM type part so I'm sure it uses whatever they came with.
