Am looking for clues as to why a ZRT800 would only be getting 4miles per gallon. When running the sled hard it runs like mad, it's very fast and has done well in local grass drags. Changed the pilots for winter and it drinks gas like an Irishman does beer on St Patricks Day.

Sled has the heads cut, mild cylinder porting, overbored carbs and V-Force reeds. Jetting is 450 mains and 50 pilots.

This actually started a while ago when I was screwing around with putting UFO's in the carbs. I never could get it to run right and fuel milage sucked. I yanked the UFO's and filled the holes in the slides. I assumed normal mileage would return but it didn't.

The motor was rebuilt due to major piston scuffing after tossing a couple picks through the heat exchanger. Can't remember if the UFO's were added right after the rebuild.

The 2 suggestions I was given were (1) bad fuel pump / torn diaphram or (2) the motor is blown / hole in piston.

The holes you filled in your slides, were those the ones for the 3 screws which held the UFO's in place? What did you use to fill the holes in with? It is well sealed there, not allowing air to pass by, or worse yet, cannot come off and get sucked into engine?
Also when I installed the UFO's in a couple of machines I had to grind the carb slide somewhat to the same profile as the UFO's. Did you do this? If you did and removed the UFO's your slide cutout angles would be TOTALLY different from stock! That will create a different venturi effect-change suction on your carbs. Carb slides will have a number stamped on them, 2.0, 2.5, 3.0 etc to identify the slide angle.

Get a new set of slide's,I can almost guarantee that's the problem.I tried to reuse the slide's on my 98 Mach after I took the UFO's out and ran in to the symptom's you speak of.Like Bobber said you have to grind the edge of the slide to make it work with the UFO.I put new one's in my machine at the time and problem solved.

it may not be your slides. a 3.5 did not require the slides to be ground and the zrt 800 and the tcats came with the 3.5. did you drill the hole in the needle jet? i suppose a bad fuel pump could do it but it would show up on 1 cylinder being really rich (the one where the port comes out of the case) as for a blown piston ,you would know if you had one. what are the reeds like do they seal, its ok for a small amount of light to be seen where they close but you dont want a 1/16 or 1/8 inch under the petal. have you checked your piston wash and plug colour at different throttle settings to see where it is using too much fuel? pull your exhaust off and look at the pistons to make sure there not chewed up a bit from some lean spots caused by the ufos (trust me on this one, i just did it) what is your needle position at and what needle jet are you running? 450 mains and 50 pilots sounds a little generous. im running 35 pilots and 410 mains in my tcat with k&ns q4 nj and needle in the middle .motor is stock though no porting or pipes