What is up with this gas mileage???

[Bubb]

So a little history:

Bought my 68 D100 about 8 years ago. Came with a Rochester carb on it. Me, not knowing any better at the time, replaced it with a rebuilt Rochester carb. I had nothing but problems with this carb the whole time. I had to have the idle set to 1200 rpm's just to stay running, and I STILL had to drive with one foot on the gas and one on the brake so it wouldnt die at stop signs. Also had a major issue with the throttle linkage not aligning properly, and the choke not working, too much else to list. With this carb dialed in as best as I could get it, I would average 12-13 mpg. Not bad for a 318, but could be better considering the idle rpm issues.

So... I decided to take the Stromberg WW of my 65 D100. Rebuilt it, and put it on. Got it dialed in good. Idle set at 700 rpm, and now only have to use one foot to drive! However, I'm now averaging 6-7 mpg! My driving is mostly in town, some highway. Im not crazy with the gas pedal. I checked my spark plugs and they are as clean as they were when I installed them 3000 miles ago. Less than 1000 miles on my fuel filter, fuel is clean 87 oct. Truck runs better than it ever did. I have no problems accelerating, no pinging or coughing. Am I missing something?

So... is this the norm, or is there anything else that might be wrong or I can check? I'm just having a hard time believing that I correct my carb issues and go down in mpg's.

Brad

[Jims68]

That does sound strange and very POOR mpg... anyone stealing your gas? These trucks are pretty easy to siphon gas out of.

Is this how you determine your MPG? (I don't mean anything by this)

I determine MPG by this method:

Fill gas tank.

Drive X amount of miles

Fill gas tank again

Divide gallons used into miles driven.

Example: 127 miles driven, and 10.5 gallons of gas to fill. 127 divided by 10.5 = 12.09 MPG

Jim

[Bubb]

That is exactly how I am computing the mpg's. And I have a locking gas cap with no evidence of tampering. No signs of fuel leaking anywhere either. Unless it is leaking only when I'm driving? I dont know, but I'm sure there would be signs. Also checked my oil for the hell of it for fuel dilution. Nope, not that either.
Scratching my head.

[Bubb]

Just got done reading some internet articles about it. Some things I will check, tire pressure, air filter, and I know my battery is on its way out. Could it be that my engine is working harder to charge a dying battery? Also, I went from an all open K&N air cleaner to a stock snorkle one. Could this be a cause as well?

[ufo]

Did the station you buy fuel from change the ethanol content?
Just a shot in the dark with that idea.

[Bubb]

I dont think so, but I'll look into it just in case.

[Marc]

Also, how accurate is your speedo? I have to use my GPS on my phone every time I drive my truck because my rear-end is not stock and nor is my tire size. Thus, me traveling 160 miles in real life, my speedometer will say I have gone 102miles. Could make my MPG seem like it was horrible but my speedometers is just horribly wrong.
might try to verify your speedometer somehow. Either with a GPS or mile markers or a buddy following you with a accurate speedo. :)

-Marc

[Bubb]

Everything I'm running is exactly the same as when I had the Rochester carb on it. The only change I made to my configuration was the carb and air cleaner, which is when my mpg's went down. That's why I'm thinking it has to be something related to that.

[Dood]

Hey, the ethanol idea is interesting. I'd love to hear a practical assessment of ethanol fraction on fuel economy for these engines, e.g. /6, SB, RB carb Chryslers. I don't know any place around here (PNW) that sells more or less than 10% ethanol, but I've heard that 15% is in the works as part of recent federal legislative changes.

I don't know much about 2 barrels Rochesters, which was it, out of curiosity?

[Bubb]

Which was what? What model Rochester? I'm not sure, I'd have to dig it out of my tool box. I just know it was a 2bbl with a little chevy bowtie on the top (probably why it ran like crap). lol

[soopernaut]

Some things I will check, tire pressure, air filter, and I know my battery is on its way out. Could it be that my engine is working harder to charge a dying battery? Also, I went from an all open K&N air cleaner to a stock snorkle one. Could this be a cause as well?

None of these things will make that much difference. E10 or E15 wouldn't make that much difference either. Something isn't working correctly.

[runcharger]

A few things. First of all it sounds like the Rottenchester was crazy lean and had a vacuum leak.
Other thing is, how did you hook up the vacuum advance for the distributor? It makes a big contribution to fuel mileage.

Sheldon

[EGroscup]

Bubb, when we had "oxygenated" gasoline shoved on us here all of my vehicles with a carb dropped 15% of the fuel economy. I believe that Sheldon is also on track with the vacuum advance. No vacuum at idle and it should increase as you throttle up. Let us know what you find and good luck.

[Bubb]

You know, that makes a lot of sense with the fuel. I just switched the carb over when I moved down here from WA. I'll take a look at the vacuum advance. But I hooked it up to the diaphram on the carb, same place the old one was. Thanks for the inputs guys. I'll be in touch when I get it figured out.

Brad

[Bubb]

Correction on my last statement. My vacuum advance line is connected to a straight line in the back of the carb. Are you saying im not supposed to have vacuum there at idle? Cause I do. I pulled off the line with the truck running in park and had a strong vacuum.

[surfnuke9]

Real world data on my 68 Pontiac showed that when the winter blend (it has 10% ethanol) is stocked at my local gas station, my avwerage mileage dropped from 15.2 mpg to 14.1 mpg.

I dont think ehtanol could be ALL of your issues, but the fuel in San Diego likely is differnt than the fuel in WA state. Curse you CA Air Resources Board !!

[sweatybetty]

Correction on my last statement. My vacuum advance line is connected to a straight line in the back of the carb. Are you saying im not supposed to have vacuum there at idle? Cause I do. I pulled off the line with the truck running in park and had a strong vacuum.

you shouldnt have any vacuum at idle. the advance needs to be hooked to a port that has more vacuum as the rpms go up. none at idle.

[Bubb]

Ok. That makes sense. I'll try that and see what happens.

[Dood]

FWIW, after using the vacuum gauge to do some timing, carb adjustment and other debugging, I mounted it in the dash and used it to 'learn to drive more efficiently.'

[lbman]

I have found , up here in Whatcom County. that Union 76 Stations are the only ones where you can find Regular GAS.
it sure is nicer on small carbs than Methanol. I get 4 MPG better in my old gold wing. more power also.
I am fighting carb problems also..had a worn out carter 2290 I put on the 2245 from the -3 motor i picked up. i have no signal to vac on Idle. this is causing me to play with vac advance springs to get the off Idle mid range not to surge. one difference is the carb base gasket. the old gasket was oblong the new one fits the bottom of carb,
probably why I had vac on idle before.
I am still having problems with off idle acceleration. especially if i try to nail it..cough's through carb..pump is working, .timing is perfect static wise.
I will check jet size versus other carb with # drill bits. seems to lean too me..
LB