Dave Stribling
October 27, 2014
Contributers: Dave Stribling

Pressurized Mach

I have a ’71 Mach 1 with 351 4V. The problem I have is excessive buildup of pressure in the fuel tank. I checked the gas cap for vent obstructions, but everything seems to be OK. I was hoping you could help me with some suggestions to solve this problem. Thanks, I read your column every month.

Bob Schenk
Milan, OH

Assuming you actually tried a new gas cap and you still have pressure buildup, the next thing to check would be your evaporative charcoal canister system. In 1971 Ford started pulling the excess vapors from the gas tank and running them into the intake to burn rather than just vent into the air. The system uses intake vacuum to draw the vapors from the tank. The charcoal canister has a valve in it to prevent pressure from returning back to the tank. If the system is clogged or not hooked up correctly you may not be drawing enough of the vapors out of the tank, and you may in fact be adding pressure to the tank through the feed line. There are two ports on the vapor canister—the big one goes into the side of the original air breather and the small one goes back to the fuel tank. If the vapor canister has been blocked off, clogged, or improperly hooked up, that may be the cause of your excess pressure in the tank. I’d start there.


Driveshaft Resto

I am restoring a ’67 Mustang GT fastback with a 390 4V and four-speed transmission. I am restoring the driveshaft now. I noticed there are rings painted on it. From the rear of the driveshaft there are two painted on it, but I cannot establish the color. The distance from the back is 11½ inches and 12⅜ inches. The width of these two rings appears to be ¼ inch. Further toward the front of the driveshaft is a ring at 20 inches that is ¾-inch wide and painted white. Next, there is a ring at 23 inches that is ¾-inch wide and painted blue. Lastly, at 25¼ inches is a ring painted red and ¾-inch wide. My question, is the color of the first two mentioned that are ¼-inch wide near the rear of the driveshaft? Why were these circles put on the driveshaft?

Gary Buckley
Greentown, IN

The driveshaft got several types of marks that are used for different things. Some are identification marks, some are alignment marks, and some are inspection marks. The three-color ¾-inch bands in the middle are identification marks. These three colors correspond with the car’s build sheet and told the assembly line worker which driveshaft to install in the car. The example build sheet here is from a ’69 428 big-block with an automatic, and its color ID is dark blue, tan, and white (DBTNWT). I am not 100 percent positive, but most of the ¼-inch bands I have seen have been brown or red (sometimes the colors get changed in the de-rusting process) and may be alignment marks (this end goes here). The photo shown has these lines at the front, not the back. (This one is assembled backward, whoops.) Finally, you will have yellow daubs at the back for inspection (maybe for the weight, that’s where I usually find them) and orange on the ends where the U-joint goes. You will probably find yellow on the yoke as well. Take lots of pictures so you know where to put them back on the ’shaft; and don’t worry about being too detailed either—the paint went on sloppy and usually ran to the bottom of the ’shaft.


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