Quote:
Originally Posted by findog78
Thanks for all the input. I agree the first step should be a tune. I rebuilt the carb and it actually runs like crap now anyway. I do remember a little loss of torque like 68stepbed describes when I took the pipes that exit out the rear of the tire off and just went straight out mufflers on the old exhaust. Id really prefer the low end to the top end but was set on having them exit in front of tires this time. I also think this is a classic case of bigger is not always better.
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Engines are just air pumps. Anything that changes the flow through the engine affects the entire system. By going to a 2.5 in exhaust you have changed the flow characteristics and that means the carb is no longer set up correctly. The drop in backpressure has increased the air flow on the intake side and that means mixture is no longer correct. Adjusting the idle screws changes the mixture at idle but does nothing for any other throttle position. If you can get an air/fuel ratio meter that will save you from having to 'read' your plugs but I'm betting that carb needs a jet change because of the increased flow.
Here is a link to a PDF (
http://www.crankshaftcoalition.com/w...rafbtuning.pdf) that will help you set your carb up correctly. I can't tell you how because I won't use anything but a Holley. it's a Ford Chevy thing not an issue with performance. The AFB is a good carb, but I can't get them to run correctly.
Set Initial timing, if you have an HEI then start on that carb but if you are running points, adjust the dwell and recheck timing BEFORE digging into the carb. Once that is done check advance at 4000 rpm to see where the max is. Somewhere around 36 degrees total is a good starting point. Gap and install new plugs and put 10-15 miles on the truck. Remove the plugs and take a close look. Sooty black means you are rich. Tan means you are on the lean side. 14:1 is about the right air to fuel ratio if you have a meter. If you are running rich drop a jet size. Lean go to a larger jet. Here in Tucson I am at 3200 ft above sea level but in winter, density altitude drops that to sea level and I have to change to a richer jet. In mid summer, density altitude is 7000 ft so I change to a smaller jet to lean out the mixture. Good Luck!