My 86 f150 5.8 Auto with 200k miles starts, idles, and runs great until you put it under a load. an example, in drive with foot on brake it gets up to about 1500-2000 rpm (no tach) then it bogs. It continues to run but does not increase rpm. I've replaced Plugs, wires, cap, and rotor. Changed out coil and ignition module. Done electrical tests on ignition wiring and stator. Set timing and confirmed mechanical advance. Rebuilt carburetor (twice), rebuilt 2nd carb (motorcraft 4v). Changed fuel pump, confirmed pressure and flow and ran a fuel line into a can of gas. Did tests for vacuum leaks. Vacuum holds steady at 19 in at idle. Disconnected exhaust at manifolds. Compression reads a low of 130 and high of 145. Changed the egr valve. None of these have made a difference except it idles better after the tune up. Any ideas?

 

Try this. Attach a timing light (remember those?) and watch the marks as you rev the engine to make sure the timing is advancing. Otherwise, No.

 

There is a limit on where the RPM will go when doing a stall test.
When you try to engage the passing gear, downshift,on the road does it downshift as it should or refuse to downshift and give the result of "no guts" ?

 

First of all thanks for the input. With the vacuum advanced disconnected and pluged I've set timing at 12 deg btc (spec say 10) as I rev the engine it advances to about 34 deg. Timing mark holds steady (doesn't wander). When I put it in gear (timing still at 12) and rev the engine (foot on brake) it doesn't advance (not enough rpm to engage mech adv?) The passing gear (downshift) works fine with plenty of rpm (55+ mph in 2nd)

 

If you want to;
when you are power braking it, have someone close the choke blade a little bit and see if that corrects the problem. Just be careful and take every precaution so no one gets hurt. Let us know what result you get.

 

closing the choke made the engine sound different but it didn't seem to pull ant better. It still won't pull it's self of the parking brake

 

I know you stated you got mechanical advance.
Were you able to look under the dist plate and see if the advance weights are really free and not worn where they can hang up? Some had nylon bushings that wore and made for sloppy advance curves and if rusted enough , slow advance if any.

 

I did look under the stator plate and the weights were perfectly clean and free. I did notice that one of the springs was smaller than the other. That distributor is 4-5 years old and has done fine up until earlier this year. I have driven it with the vacuum adv disconnected and it ran fine. It acts like it's running out of fuel but when I look down the carb while brake torquing there is gas coming out of the venturis. I raised the float level and just got grey smoke. I'm starting to pull hair out

 

What does it do when brake torquing it and the vacuum advance is disconnected?

 

with the vac adv disconnected in park at idle timing is set at 12 deg (spec is 10) it's steady (doesn't wander)when I rev engine it moves to about 34. In drive at idle it's still at 12 but when I rev engine (with foot on brake) it doesn't advance. I don't know if it's turning enough rpm to engage mech adv, I don't have a tach to compare the 2