Why is my top speed 70 mph?

-Going back to post#46, re; restricted exhaust.
-Over the years Ive come across I think three restricted exhausts, so its not that common I suppose. I remember a mid 2000s truck that came in,with those symptoms. I drilled a hole in the exhaust pipe just ahead of the catalitic convertor and plugged a pressure guage in there. I found 4psi in there at idle, and climbing with load, on a road test. Not knowing what it should be, and wanting to be sure of my diagnoses,I checked it after the cat,where it was as good as zero.So I sent him off to the muffler shop, where the problem was cured. (Im not fond of exhaust work, and they were practically next door.) Afterwards I found less than 2psi under acceleration in 2nd gear, and then the wee holes were welded up
-I know you likely dont have cats, but you do have a muffler.Same story.
-I used to see that all the time on 2-stroke dirt bikes in the 70s and 80s. In fact I made it part of my tune-ups to burn out the carbon from those plugged baffles.
-Certain 4-strokes used to come in with loose baffles that would rotate and choke the exhaust too.
-When I was young,about 17or so, I was pulled over for a noisy exhaust and given a week to show up at the station house with it quieted down. So I rolled up some cardboard from a big box(the corregated kind), to make plugs, rammed them up the pipes, and locked them in there with nails.When I showed up for inspection I was pretty sure that lazy cop wouldnt bend over and look. Phew, was I relieved when he just asked me to rap it a couple of times. I passed, went around the block, and pulled the nails out. I hit the gas and those home made baffles shot out of there like from a cannon. Do you imagine I went and picked them up? NO, I was 17. The point Im getting to is this: while those cardboard plugs were in there, that 340 was a bit of a dog, too. Actually, very doggie.I hammered that plug in there with a wooden stick. Now Im not saying that your exhaust is plugged, just that its something to consider and very easy to check. Carb type fuel pressure testers, and/or old style vacuum guages are callibrated just right.
-Happy hunting