Topping out at 62mph?

Finally got the old k100 under an 80000lb load, and I can't get it to go the speed limit outside of Chicago (not that traffic ever found its way up to 45mph anyway).

I've tried both 8th gear direct, and up into overdrive, as well as 7th direct and overdrive, and I keep getting stuck between 52 (2000rpm in 8th direct) and 62mph (2000rpm in 8th OD) on level ground like I'm lugging it, then dropping 2-3 gears on the mole hills up in Wisconsin/Minnesota. I was tracking my speeds with GPS since my speedometer cable has been binding, so these speeds are accurate.

The only thing I noticed out of the ordinary is that the pickup tank is nearly dry (pouring out vapor, too), and the driver side tank looks like it still has about 80 gallons left in it (fuel was probably under 150*F) after 500 miles today. I'll drop about 40 gallons in the pickup tank tomorrow and see if it helps. (2 rear tanks are closed if anyone is wondering)

The only thing that puzzles me is that every once in a while it'll just take off to 70mph (had the pedal to the floor all day) until I hit another hill, then its back to low speeds. Also, if I pick up speed on a downhill, I can maintain 70mph with the throttle, again, until I hit another uphill, and the 4 way flashers go back on. Also made sure I didn't have the engine fan on (its labeled backwards, so I made that mistake earlier today and lost about 5mph to the fan alone).

Inspected the throttle rod, and it had free travel as far as I could see behind the doghouse skirt.

Any ideas? Or is this just standard gutless behavior from the VT 903 set at the factory 290hp?

**EDIT
Also made sure my interaxle lock isn't engaged.
Top speed bobtail and with just the trailer was 75mph.

 

The fuel lines are probably as ancient as they can be before swelling shut. The crossover line bulges a bit going into the hard shell between tanks, so that was first on my list. I was looking to get all neoprene lines before I get a fuel pump rebuild as well. I'll give the lines to and from the main tank a good looking at while I'm getting unloaded, and can run just that tank if need be for a while with some new lines once I get some fuel in there.

Is there a good way to check the breakover arm on the throttle shaft with the truck running? My batteries (possibly the alternator as well) are toast, so the trucks running another day until I can get this load off and swap them out at a shop. Then I'll probably try to find a nice light load back home.

 

Alright. Doesn't sound too costly for throwing parts at the problem. I'll see if Sioux Falls KW can squeeze me in for that today.

 

I know its a slow truck, but 62 was the BEST I was getting out of it. I'd get stuck at 45-55mph for up to 10 miles before I hit the right grade to get up to a sustained 62.

I've got it in the shop, and they've already gotten the throttle up to full throttle (which it wasn't doing), and they mentioned that its only running at 30% of normal boost pressure.

I do my own work on it when I can, but how am I supposed to be both in and under the truck to watch the throttle travel? How am I supposed to pressure test the fuel system with only a set of wrenches on me, since I've had to take planes and busses to get to the truck? Yesterday I installed new batteries, fixed a radiator and oil leak, adjusted the slack adjusters, and noticed a brake chamber was coming loose, and locked it down myself, all in a loves parking lot waiting to get into the shop. I do my own work, but I have limits.

I'd like to reliably be able to do 65 on flat ground, since 45 in a 75 zone is pretty #### near ticketable. I'm not expecting a race truck, but I was planning on adding propane and turning up the fuel pump once I get the kinks worked out, and it starts making a bit of money just to help it up the hills a little. It runs up to 2500 and does 75mph just fine bobtail or deadhead, and I'm not expecting to do that loaded.

I've been downshifting at 1700, so that might be where I'm going wrong. Should I downshift 8OD to 8 direct to 7 direct, or 8OD to 8 direct to 7OD to 7 direct?

I chose this truck because I can do most of the work myself, and once I know everything is working right, I'll be more prone to make a call that its working wrong, and get into it myself. Until then, I'm going to ask questions, and get help when I need it.