(cross post from another board in case there's a different audience here...)
I'm having a lack of power issue in our '77 Eagle 05 with an 8v71 and HT740.
Here's some more background, and I hope it is either relevant in pointing out an issue or helping rule out something that isn't:
Last summer I removed the high idle solenoid and put in a Jake buffer switch (the old Jake switch never worked right and just sensed the throttle position). Changed the oil, oil filter, air filter, and fuel filters. I replaced all of the fuel lines from just before the primary filter all the way through to the heads and added a priming pump.
In the photo the line from the tank goes to an AN fitting AN-10 hose and loops around to the top of a (3/8?) brass tee. That connects down to a Detroit check valve, another tee, and the out another AN fitting and AN-8 hose to the inlet of the primary fuel filter. The tees connect to an Airtex E8153 pump for priming. That pump is a 10-14psi pump, and met the specs of what I could find as being ok to use and not damage the engine fuel pump. The primary filter connects to the inlet of the engine pump with AN-8 hose and fittings. Out of the engine pump, to the secondary filter, and to the engine are all AN-6 fittings and hose.
This first started on a return trip camping last fall. The drive out was uneventful and had no issues. Three days later, about an hour into the 1.5 hour return drive I felt the power drop and had trouble accelerating. Fluid levels were normal, plenty of fuel, no white or black exhaust. When accelerating I felt a vibration after shifting into third gear and a more when in fourth gear. I drove it the rest of the way home at 30-40 mph.
Long winter, didn't do much with the bus... This weekend I went over the connections and checked that nothing was leaking, oil and coolant levels were good (and hadn't changed since the last trip).
Today I took the bus out for a quick trip to get fuel (it had about 100 gallons in it) and see if the problem was still there. Engine starts normally, no issues. Sounds normal. Driving when cold is ok but seems like it doesn't have as much power as I recall, but nothing really obvious and I hadn't driven in a while. After about twenty minutes of driving I started to experience a lack of power - when accelerating I start to feel a slight vibration after shifting into third gear and a little more when in fourth gear. A few times when starting from a full stop there wasn't much power at all (barely started moving) and then after two or three seconds I would feel more power and accelerate. Back home, I used a temp gun on the exhaust manifold and all cylinders seemed to be within 20 degrees of each other (I checked just after I shut the engine off, if that makes a difference). I checked the primary filter for water/sediment (the Baldwin filter has a drain) and got out what would have been a half-inch or so of really dirty water and then just fuel.
I usually point at the things that changed (buffer switch, fuel lines) when a problem crops up, but the initial run last year was good, so I'm wondering if something unrelated happened and is causing this problem.
I don't have a pressure gauge to test the output of the fuel pump and see if that is working ok. The vibration/shuddering makes me think of fuel starvation. The AN fuel lines should be sufficient for the pressure and volume needed, and I would hope the brass tees and check valve aren't too much of a restriction to cause a problem.
I find it interesting that the problem gets worse when he engine gets warmed up.
Any thoughts on where to start, or probable causes? I was going to try again tomorrow and drive with the priming pump on and see if that helped at all - good idea or bad?
I appreciate everyone's suggestions or comments.