Automania/Repair & Maintenance

AUTO QUESTIONS AND ANSWERS FOR WEEK 22

by Bob Hagin

Q. I have a 1979 Ford F100 truck with a 302 cubic inch engine and an automatic transmission. I pulled the engine and had it rebuilt as a long block and reinstalled it myself. It runs great but it now has a slight vibration. At slow speeds you can just feel it and from 50 to 60 MPH it makes the ashtray rattle. It then smooths out to a slight vibration again. I was told that it was in the driveline so I had two different mechanics check it out. Both of them said that the driveline and the universal joints were OK but couldn't come up with an answer as to why it vibrates. I thought that I had marked the flywheel and torque converter and replaced them the way that they came out. I have been running it for about a year now and it hasn't gotten any better nor any worse. W.M. Merced, CA

A. If the engine in your truck didn't vibrate like that before you went through it, obviously somebody did something wrong. As a test, you could remove the driveshaft and rotate it 90, 180 or 270 degrees but I don't think it's a driveline problem. When I go through an engine of any design, I always have the reciprocating and rotating parts balanced individually and as a unit before I put it together. I learned that lesson early on when an engine I did vibrated so bad that it would give me a headache if I drove it very far. I also experienced that kind of a vibration when I inadvertently used mismatched bolts to attach a clutch pressure plate. The ultimate cure is to dismantle the engine, balance it out and reassemble it paying close attention to even the smallest detail.

Q. Lately I've become interested in owning a true sports car, one that goes good, handles well and is old enough that I can afford to pay for it with cash. I've about settled on the old-style Mazda RX-7 because it fits all the above criteria but I've heard so horror stories about its rotary engine. Do you think that I should look for something else? M.F. New Orleans, LA

A. All the bad things you've heard about the old Mazda rotary engines is true. They're fragile if mistreated, fuel-hungry and hard to keep from becoming a polluter. On the other hand, they put out a lot of power for their size and have the potential for being pumped up to very high power ratings. The older RX-7 is a true two-placed sports car in that performance outweighs carrying capacity. There's lots of speed equipment available for it but its not cheap. Road & Track did a series on going through an '83 RX-7 for performance in its November and December '95 and January '96 issues. Find copies and your decision will be obvious.