The Stovebolt Site Forums The Shop Area General Truck Talk why am i only getting 6 mpg?

i've got the 53 gmc with the 228 6cyl with a rochester 1bbl and the 4:10 4speed combo and i just figured my mpg yesterday and it's pretty bad.
i rebuilt the carb a few weeks back and it works great. i've put plugs, points, wires, distributor cap and rotor and coil on it and it only burns the slightest bit of oil.
1. would a zenith carb be better? maybe one professionally rebuilt?
2. would the vacuum advance not working cause bad mpg? i don't see it advance when i manually increase the idle speed, but i don't know if that's a good test or not.
3. lastly, would a better investment be the 3:55 patricks gears for the rear end? i don't go over 45 mph anyway, but even at that speed, the rpm's might be eating the mpg's.
Help?

I guess the first question is: how bad is "pretty bad?"

Edit: never mind...tired and forgot the title of the thread "why am I only getting 6 mpg?" Most of the rest applies though. Don't overlook dragging brakes, or similar mechanically induced resistance.

Having asked that, allow me a brief moment on the soapbox, whereupon I opine that due to the way gasoline is made, as well as the way the ugly companies who manufacture gasoline are made, there is only so much gas mileage that you can get out of a gallon of gas...unlike back in the day. In other words, either due to the makeup of petroleum distillates, as well as the additives therein, or due to the corporations themselves, we're only going to get less than optimal gas mileage while the corpor-rats get more than optimal gas profits.
--end of rant--

Do you know any old mechanics in your area who have access to a 2- or 4-gas analyzer? A 2- or 4-gas analyzer can tell pretty quick what's coming out of the tail pipe, unburned, or partially-burned Hydrocarbons wise. Especially since you don't have a catalytic convertor to cover up potential problems in your engine.

There is a possibility that your carb may be dumping fuel into the intake manifold, but may not be showing up as rough running. Dumping will show up on the analyzer as high CO, whereas partially burned fuel (generally from a misfire of some kind) will show up more as high HC.

Let's play "let's pretend." Let's pretend that you have an intake manifold leak. Normally, that'd cause a misfire and the engine would run rough, particularly at idle. Now, let's say your carb was dumping fuel at the same time. Since older cars with higher mileage generally tended to run smoother with a richer-than normal air/fuel ratio, the dumping fuel (high float, leaking gasket, accelerator pump leaking), could actually mask a lean condition like a leaking intake manifold, causing you to believe that according to the way it's running, all is well. Edit: well, except for the old 6 mpg thing.

The vacuum advance could contribute to poor fuel economy, though I wouldn't expect to see hugely bad numbers just out of a bad advance.

Your test is a good test for the manual part of the advance test...are the flyweights mechanically advancing the timing is what it checks. It's also a good test at a high rpm, when your manifold vacuum comes back, but not from off idle, where your manifold vacuum goes away. Not sure if your vehicle controls vacuum via the manifold or venturi on the carb, but I'm guessing on the manifold.

Anyway, if you don't see any advance when you rev the engine, you've got a problem with your distributor, and it will need to be serviced with a kit, even overhauled, or replaced, if need be. You should see (guessing here) 4 to 6 degrees of advance through the flyweights alone. If you have venturi (carb) vacuum, you will see several more degrees of advance off idle, whereas manifold vacuum only advances the timing at idle and under cruise conditions (when the throttle plate is more closed than open).

Having said all that (minus the rant), it absolutely could be a combination of things, but I wouldn't think your gear ratio would contribute that much to the problem, again, depending upon how bad "pretty bad" is.

Edit: per my remarks at the beginning, I see how bad "pretty bad" is.

for #2, need to goose the throttle to see the dist move, not just up idle - it changes w/ strong change in vacuum, and returns as vacuum normalizes - you could try setting the timing w/ a vacuum gauge, might be more accurate for mileage

that's suprisingly low, but you may not be able to get anymore than 12-13mpg, and a 228 may just be a gas hog - never checked the one I had cuz I only used about 2 tanks of gas a year anyway!

the gear change could help, tho you might eat the gas gettin up speed instead - the way you drive is a lot of what mileage you get - shift to keep rpm's in the low/medium range, don't mash the pedal - and of course you need to make sure your mileage is true - speedo and odometer may not be accurate

Bill

thanks for the info bluenwhitegocart. i'm getting 6 mpg's so i'm definitely trying to find out what's wrong here.
if i understand your post correctly, it could be a leaking intake manifold or bad distributor.
there are certainly plenty of opportunities for a intake manifold leak as there are takeoffs for a pcv valve that runs to the valve cover and also the wipers are still vacuum operated, along with what i believe to be a non functioning disto advance.
i get a carb spit when i floor the engine if it's cold, but i get about 22lbs of pressure on my best idle mixture setting of the carb so i assumed it's just a cold engine as i don't use the manual choke since i'm in socal and it's usually over 80 degrees when i drive.
i do however, have an exhaust manifold leak on one of the cylinders, which actually doesn't sound too bad, sorta like headers, but don't know about the intake manifold.
also, the iron valve in the exhuast stream that is supposed to open and close, (whose name i can't remember now) is rusted shut, so i assumed that might also add to the cold characteristics of the motor.
if you're sure the distributor should move some off idle, then i've got a problem cause it don't!
can you point me to an article on distributor operatoion so i can understand this more? i wasn't aware anything (weights) would move it except the vacuum advance pot.
thanks for any and all info.
tony

I'm not saying it is for sure a leaking manifold, though anything's possible. I actually came back to this thread on account of I thought of another thing that could make your mileage drop, and I see that you have detailed a related issue in this post, namely your exhaust.

I had a 72 Chevy half ton that I bought in San Diego some years back, and the simple act of replacing everything in the exhaust system from the headers back made an AMAZING difference in the old 350 cid, and it didn't run all that bad previous. Exhaust leaks, collapsed inner exhaust pipes (the reason i was back on this thread), exhaust heat stoves like you describe stuck shut, can all contribute to poor mileage, and/or driveability problems. It's strange how well we think our old beasts run until we make some improvement, and have it run hugely better. Of course, as I talked about, sometimes changing one thing that was covering up something else could make it run worse until you work all or most of the issues out, so just be prepared for that.

What part of So. CA are you in?

I wouldn't get too worked up about the distributor just yet...it sounds like you have more immediate issues to take care of first...though, as I said, in combination, it could be contributing to the overall mpg problem.

sounds like i need to break down and try redoing the intake/exhaust manifolds and associated pipes. the bolts look like they will break if i look at them sideways so i didn't want to do much if it was something easier like a vacuum advance or correct zenith carb replacement that would help fix this problem.
i'm in moreno valley btw, close to riverside california, ~ 60 miles east of los angeles, out in the desert!
thanks for the info.

red58, you might be onto something with the speedo. i put a slightly taller tire (215/85.16) on it lately and that of course adds weight and makes the speedo under-register a bit. maybe it all just adds up or subtracts, whichever way you want to look at it!
thanks much for everyone's help.
tony

Taller tire does mean you are traveling farther than the odometer shows.

Has anyone asked if you have a fuel leak? If its leaving fuel when parked, or pumping it out as you drive, that could seriously reduce your mileage!

You have some really good points here to check on. Pull out a spark plug or 2 & see if there is black soot on the porceline or the edges of the shell. If it is you are getting to much fuel which I suspect being that poor of mileage. Pull the top off the carb & check the float level & make shure no gas has leaked into the float. Check the power piston goes up & down. See what size jet is in it. usually 053-055. Sometimes a PO may have changed it or drilled it out. The vacuum advance should move when you speed up the engine. You are either not getting vacuum to it or the diaphram is bad. The weights they are refering to are separate from the vacuum & are called mechanical advance. They are down inside the dist. The dist rotor should turn a little if they are not stuck. The higher gears are great for these old trucks but you need to find out the problem first. You can verify the speedo by comparing the odometer to the mile markers on the interstate.

wrenchbender,
yes, there is some soot on the plugs.
i rebuilt the carb myself (it went well and i didn't have any parts leftover :-))using a good kit, and i did check the float and it's not compromised. also, the power piston went up and down easily and i get that spray of fuel when you punch the pedal. lastly, the jet was replaced with the one from the kit.
the advance don't move a lick when sittig still with me manually blipping the gas pedal. i can advance the disto by hand though, so it's not stuck. i have good vacuum going to the advance cause if i remove it the thing idols badly.
would/could bad/stuck disto weights and/or bad/blown vacuum advance cause such poor mileage in your opinion?
i'm sure the valves could use adjusting but they don't chatter.
i have about 22lbs of pressure at the intake manifold at idle so i assume the valves aren't too bad burnt although there is a slight, distinctive burnt valve miss at the tailpipe.
forgive me because this is my first project and i don't know much about disto theory, but i'm leaning towards this, the vacuum advance and the incorrect rottenchester carb as possible problems.
certainly the 1" taller tires cut down the mileage registration accuracy, but if I give a couple extra mpg and i'm still at 8 mpg!
p.s. as for checking mile markers on the interstate, can't do that as i can't go over 40 mph with the tall gears in it now.

You could always make your own marks on another stretch of road using another vehicles odometer. Or have someone follow you with walkie talkies and compare odometer readings over a 5 or 10 mile distance and see if there is a discrepancy.

As my long gone father in-law used to always say, "there's more than one way to skin a cat".

This is a really interesting thread, as I am afraid to check the mileage that I am getting with my '59 viking. I am quite sure that it is not very good.

John

The lack of vacuum advance will definately hurt gas mileage. Thats exactly what they were put on for.The diapram must be ruptured. You can check it with a hand vacuum pump or suck on it. Be careful you don,t know what might be inside of it. How about the initial timing? did you set it with a timing light.

set the ignition timing as far advanced as possible.just short of spark knock and hard starting.then make sure the advance mechanism is working.i got anther 2 mpg by running my 250 about 4 degrees advanced from book specs.

what about the 'R' and 'A' settings on the distributor that are supposed to be set to match the gas you're using? should i be in the middle of the marks using regular gas or in the 'A', which i assume is advanced?

I would not pay much attention to the marks. Just set using a vacuum gage and road test. As mentioned above, just shy of pinging and hard starting is probably the best.

Also, a common problem with older carbs is the power enrichment circuit. If the spring is not the optimum tension it will richen the mixture under lighter loads than needed just killing mileage.

From your description of the plugs being rich I would bet on this or a float level problem.

Did you replace the manifold to carb gasket? The Rochester has a vacuum passage on the base that goes to both the power piston and the vacuum advance. If you use the wrong base gasket (it has notches) or are missing the heat insulator spacer (it has notches too), you cover up the vacuum passage in the rochester and get exactly the behaviour you describe. It does not affect the idle mixture/vacuum, only light throttle/cruising mixture.

um...i think i did. i remember looking at the gasket and seeing the hole and saw that there was a corresponding hole in the bottom of the carb so i got that part right. but, i didn't check to see if the hole in the intake manifold was clean or obstructed. if it's the latter, then it is gonna be just as bad as if i put the gasket on backwards. as for the spacer, i don't remember that coming off when i redid the carb. i didn't get a new one in the kit either. probably messed that one up, but it's my first rebuild. i was glad it started! :-)

There is no hole in the intake manifold, the carb relies on the notch in the spacer to provide the passage.

Also, I have noticed on more than one Rochester that the casting that holds the power valve ball that the power piston contacts tends to warp, keeping the power valve from retracting reliably.

The other thing that you will want to check is the float level. If the float level is too high and the gasket on the inside of the air horn is leaky (a common problem with Rochesters) you will get fuel dumping down the inside of the carb.

thanks much for everyone's help on this. in addition to a new vacuum advance, i think i'm gonna invest in the correct carb, a zenith 1bbl for this girl. who's got a good source for this carb and does it have to be specifically for the 53 year / 228 cu engine? the manual shows slightly different model numbers for different models in the same year so i wasn't sure if one carb would fit everything from a 101-22 up through a 400 for the 47-53 year timeframe.

thanks much for everyone's help on this. in addition to a new vacuum advance, i think i'm gonna invest in the correct carb, a zenith 1bbl for this girl. who's got a good source for this carb and does it have to be specifically for the 53 year / 228 cu engine? the manual shows slightly different model numbers for different models in the same year so i wasn't sure if one carb would fit everything from a 101-22 up through a 400 for the 47-53 year timeframe.

Check your valve adjustment, timing, point gap, tire inflation, choke butterfly ( make sure that it's opening fully ) condenser, and coil. If you aren't smelling gas then your carb is probably not the problem but the needle valve could be stuck. Just take the carb off and do a quick rebuild. It probably needs it anyway.

After you get the vac advance working. If you happen to keep the rochester, carefully take off the top of the carb. If the fuel level is near the top, adjust the float so it shuts off sooner about 1/8 inch lower. Too high will give you sooty plugs. Check the new jet you put in from the good kit, maybe too big. Put the old one back in. Get the engine hot and shut off and set the solid lifters~valves to specks. Valves set too tight will give you poor gas mileage such as you are experiencing. Friend of mine has a 50 1/2 ton GMC 228 like yours. We put in a Nova 3:08 rear and a 59 chevy 3 speed, bolted right up and he is now getting 18 mpg. Perfect gear for Calif and the desert.