chickenman
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I would check fuel line routing, if RamJet is similar to traditional fuel injection with A loop system fuel return to the tank which I believe it is, than the lower your fuel level gets the hotter the fuel becomes. Heat transfer from the pump and close proximity to headers etc. could easily explain what you describe because the level of fuel in the tank would have A direct effect on heat dissipation... High fuel level keeps things cooler but can only do so much. My 2¢...
As for tuning... there is always room for improvement but with the controller that you have and the fact that G.M. has put A great deal of time in tuning for readily available pump gas I wouldn't sweet it if she runs good. You might Eek out A little more power but suffer with detonation issues in tuning for power because traditionally ignition timing is on the conservative side to avoid this issue...
a simple explanation would be that there is a void or air opening on the lower part of the fuel pump or pick up...when gas is low the suction goes to the path of least resistance which would be to draw air vs. fuel...mebbe too simple...if you've got a controller you'd most likely not get all that much outta tweaking or dyno tuning but there will be gains, worth the effort or not...as phoenix mentioned, look for a return line to the tank, it may have never been installed...
thanks guys good advice, you know your stuff. there is a return line and we have insulated muffler and fuel line helped a lot but I really think we need to completely re route all fuel lines way away from exhaust, even with insulation I still get issues just not as bad;
Also probably would help over all performance and mileage with cooler fuel...
thanks....CM...
Also finally got front end properly aligned went thru ball joints and a set of tires in less than 10,000 miles...
Dummies used original 54 chevy specs instead of 71 78 mustang 2 specs...Sure handles nice.....