The Stovebolt Site Forums The Shop Area The Electrical Bay Timing question.

I have mentioned this a while back after I got back from KC. My mpg dropped by 5 mpg on average on my 54. The only thing that was done since my trip to HQ in Match this year was oil change to 20w50 and the distributor was rebuilt and I was not happy with the slight ping on acceleration so I had it adjusted so it did not do it. Now the question, would any one or a combination of those cause a 25% drop in MPG. Jerry suggested the power valve was stuck. Suggestions. I have the kits to completely overhaul the carbs but if there is a better idea I am open to it.

I have not checked mileage in 35 years on any of my vehicles. People used to check it to brag about it. If truck runs good, I don't see a problem to be solved. I do see a problem to get initiated by screwing around with things. The terrain of each trip, the weather (temp/humidity), the altitude, the gas, the filters, air cleaner, spark plugs, your foot, your math, tire pressure, drive train drag, brake drag, Mother Nature and Father Time may be the cause. As well as the carb and yes, oil viscosity.

Ron, a 5 mpg drop in MPG is fairly significant if it happened on a round trip that you have done several times before.

I know that the weight of the oil can have a small effect on fuel economy, along with timing, tire pressure and all the stuff that Bartamos mentioned.

I would probably take a look at the timing first, as that is an easy adjustment, then maybe use a liquid "tuneup" to see if that might clean up the carbs a bit.

The 20 in 20w 50 is how it flows at cold temps. "w" = weather. The 50 is how well it flows at operating temps. The oil you are using is too high in both areas. Studies show gas mileage loss from 1% to as high as %12. Also, your oil being high viscous (thick) does not flow in cold or hot temps. This may be good for an older motor but not good for mileage. You can't have both unless maybe synthetic.
Oil companies say use the truck manufacturers recommended oil. That may not be true for our older trucks built in an era before oil was perfected. Before dual viscosity. Way before modern bearings and gears and such. The newer trucks recommend around 5w30/5w40.

I have gotten as high as 26 on a long trip. The book I keep is accurate. I have made 8 trips to KC and 2 to Maryland and one to South Carolina, the milage has been consistently in the mid-20s average for all those trips. The around town is off also about 2 miles to the gallon. I am going to reset the timing first. The unfortunate thing is I don't have a long distance trip planned until spring.

Good plan, good records, good math, good grief!

Bart, he could keep your books for you..............
are you still running 2 carbs ??

That's true. I need someone to figure out where all my money goes.

if it dropped 2mpg around town and more while travelling down the freeway (higher speeds, assuming more vacuum advance when cruising) I would either look at vacuum advance not working correctly (that will kill gas mileage when cruising down the freeway) or like Jerry said, power valve in carb stuck. I would think either one of those would be more noticeable at hiway speeds.

It does not sound so bad, 5mpg but I am talking about going from 26 mpg down to 15 and makes the overall average 15 so it is dropping 11 mpg. It won't be driven much until spring. It will get sorted out.

If all other tune-up settings and, ignition parts) are good, the carburetors (as suggested by others above) would be my guess.

Forgive my needing to ask (and, forgive my stupid questions), Ron, but what carburetors do you have on the 1954 235 engine (looks like it was overhauled in 2007)?

"Offenhauser intake and Fenton headers."

Have you measured compression?

Rochester b. 2

26 MPG? Excuse me for being skeptical, but are you sure that it wasn't 26 KPH? I do recall your earlier posts of crazy good mileage, and I was surprised yet happy for you.
Now I am wondering if there may be something up with your prior calculation of gas mileage, and that your current calculations are normal for your truck.
Carl

"It now has a Pacer rear end with 2.73 gears and a T-5 transmission." gallery information

It is a Nova rear end. I have made 8 trips to KC and 2 to Maryland and you can make fun of my bookkeeping but it is all there to see. Yes, there have been some issues but I have not been the only person to drive on the trips. On some of them, JDL was with me and drove out and back and observed the gas I put in and distance we traveled. This has been such a drastic change that it has to be figured out and not to dismissed because I chose to keep books on my travels. Carl, I don't think they are wrong but there is that possibility. You should read what Sherlock Homes sad about mysteries. when you have eliminated the impossible, whatever remains, however improbable, must be the truth?

No one has made fun of your record keeping. If you think that, you read it wrong. Record keeping is a great habit.

Another observation. I just went out to move the truck out of the garage to run some errands. It would not start but with the application of carb cleaner it did. Should the distributor rotate with increased rpm??? It has an HEI. Btw it has been sitting for about 2 weeks maybe 3. If the vacuum is not working could that affect the fuel mileage??

Yes.

Does it have a real HEI, or the Pertronlx junk? The GM HEI distributor housing does not move. A Pertronix-equipped original distributor housing will move just like one with points when the vacuum advance operates. Yes, an inop vacuum advance on either type of distributor will drop the gas mileage by a noticeable amount. It's easy to test either one- - - -use a long piece of vacuum hose attached to the unit and pretend it's a straw to a thick milk shake. On the GM HEI you'll need to remove the distributor cap to see the advance canister linkage moving the pickup coil.
Jerry

It is a GM from Langdon's. I remember the old ones moved but were not sure about this one I just had it tested and it is ok and also reset the timing, now I will have to find someplace to go out of town to test the reset. Still, have not checked the power valve, I am leaning toward that. Unfortunately, to check it I have to go somewhere long distance.

A 2.73 rear end gear with 0.73 overdrive should get you a serious improvement in gas mileage on the highway. Shoots down my clumsy guess.

Might that be too high?

Ed

That ratio combination is the best way I know of to get about 8 MPG. It keeps the power valve open all the time due to low manifold vacuum!
Jerry