So last Saturday, I took my '76 4x4 highboy with a 390ci and automatic trans to the county fair to see how it will pull. I first experimented by chaining it to a tree. Same thing that happened at the pull, happened when chained to the tree. In 4-Lo & tow gear #1, the truck would just bog down when I stomped on the accelerator, not spin the tires in the dirt. Like the trans just wasn't geared to pull such heavy weight. Only when I put it in drive and 4-Hi, do I get a little bit of spinnage from the wheels. The motor is healthy and, running the truck through the mud or down the country roads, it screams. Yet at a truck pull, it's a total failure, only pulled 71 ft while the others of it's kind pulled 220+ft. Is it that I have an automatic and they have manual transmissions allowing them to rev up the rpms before they let 'er rip? Seems that way to me. My truck has 33" tires. I just got the truck running a couple of months ago after it was abandoned in a barn for 25 yrs. Some people at the pull questioned if my trans was blown. Sure doesn't seem like it from running it through all gears while testing it out down the dirt roads. I think that the original automatic trans just wasn't geared for such heavy towing. What do you think? If nothing else besides a miserable loss, at least I didn't break anything. Thanks for your input.

 

If it spins the tires in high range, and will not in low, you do not have any problems in the transmission.
Actually sounds as though you have a timing issue. Sounds like the advance is not working properly. When rpms ar not high enough the truck is not making any power. Since the same problem exist in high range the truck is just struggling to make some low end power. How was the cam installed? Do you recall if you advanced it or left it straight up? How does the truck do while in two wheel drive? Will it do a burn out in 2wd?

 

use 4-low next time and shift it manauly

 

The motor is a stock mercury '68 390ci. The cam is original. I didn't advance the timing at all, left it at 6deg btdc. I didn't try 2wd when I tied it to a tree. I guess I'll try that next weekend. But out on the road, it will spin the rear tires in 2wd. It correctly shifts through the gears, automatically or if I'm shifting from low 1 into low 2 and then into drive, whether I'm in 4wd or 2wd. The truck has the stock '76 Duro-something electronic ignition system. I thought that it would take care of any timing advance issues when under load but hell, what do I know? What's your best guess? Should the truck be able to spin the wheels when in 4wd hi or lo when tied to a tree? Should all four wheels spin at the same time or will they take turns?

Thanks again for your responses, guys. I really appreciate the input.