Re: [AMC-list] AMC-list Digest, Vol 15, Issue 9

[Jesse <j2sax@xxxxxxxxx>]

Yesiree, Frank, I read the whole thing and, once again, I found it hard to believe that your mileage fell so abysmally once the "aerodynamics" kicked in (which actually starts to happen s little over 50, but does grow rather exponentially as speed increases).  I believe such a drastic drop in mileage is being contributed to by another element.  The below suggestion to tune the carb is a great place to start.  

I have had more than one 6 cylinder FSJ and, though I don't think I ever cruised at 80, at 75 the mileage drop was not almost 50% of my optimal.  I think something else is going on to make your 258 get as bad or worse mileage at 70-75 than any of my 360's.  Not trying to be contrary or arguementative, I just think you may be missing an opportunity to better your already cool FSJ!   If a fuel dumping condition does exist, it could shorten the life of your rig with unnecessary oil contamination and generally gum up the works, from Valves to exhaust... even worse if you have to run a cat where you live.  

Just sayin', that's all!  Either way, hope you have a GREAT Weekend, 

Jesse

>>>>>>>>>>>>>>>>Jesse, did you read the whole message? What I said was:
"It will cruise at 75-80, but gas mileage is cut nearly in half doing. Did it once, checked mileage -- 10 mpg. It normally gets around 17-18 in all around driving."
As long as I stay at or under that 65 mph threshold where wind resistance starts to really take a toll the truck gets 17-18 mpg. Only when pushing it and "standing in the four barrel" does gas mileage plummet. I have a vac secondary carb, but I've got it rigged so that the secondaries open at about 2/3 throttle whether there's enoug h vacuum to pull them open or not. The engine has over 100K on it and I don't think it produces enough vacuum sometimes to pull the secondaries open. I may have had them open on that high speed run more than needed.

17, 18 sounds about right, but as you increase MPH/increase wind load etc,
the mileage shouldn't plummet, just worsen.

This is a very fine-grained suggestion, as it sounds like you have it close
to right, but drive with a vacuum gauge in the cab, run to 75mph steady, o
rwhatever you'd like to cruise at, and see what the reading is. Then pull
the power valve from the carb, and make sure it's rating is lower than that!
If your cruise vacuum drops below the P.V. rating it opens and dumps a
crapload of fuel.

The vacuum secondaries are opened by a spring, and closed by vacuum, I think
you just misspoke there.<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
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