Posted by Joe Divito (IP: 207.239.0.99) on January 09, 2000 at 09:51:09:
In Reply to: 75 Maverick and Tuneup Info 250 eng posted by Bill Pfister on January 08, 2000 at 05:49:27:
Six deg of timing is the factory spec for a '75 with the 250 six. If it runs worse, there's something else (sounds likely fuel related in your situation) wrong somewhere.
I believe turning the mixture adjustment screw in will lean out the mixture. Be VERY careful, though, because bottoming the screw with too much force will damage the carb, and you won't get a proper calibration anymore. In your case, it sounds like the carb is possibly defective, especially if you can't lean the mixture enough. Alternatively, there could be other internal engine wear--I've not seen it first hand, but I've heard of engines with closed crankcase ventilation systems (such as everything from the '70s to today) having enough blow-by that you could run the engine even with the mixture screws completely closed.
I'd recommend as a starting point doing a compression test to make sure there isn't internal engine wear. Do two tests (both after the engine is at its normal operating temperature). Get readings from each cylinder, and note the PSI. Then do a second, "wet" test by dropping a tablespoon full of motor oil into each cylinder being tested (do all the cylinders again this way). If the PSI increases in the "wet" test, then you likely have worn rings or cylinders. Generally you'll want something like 120-150 PSI dry, with no increase wet. You'll also want about the same PSI between all cylinders (there'll always be a slight variation).
If the engine is ok, then look at the carb. If you have minimal experience working with carbs, I'd recommend finding a good mechanic to either rebuild it, or locate a good quality rebuilt unit (from an auto parts store, or someplace like AutoKrafters).
Also, you can try to CAREFULLY advance the timing more than six degrees. I say carefully, since you don't want too much advance and damage the engine. Excessive CO is usually a sign of incomplete combustion (I assume you meant to say CO, not CO2). Advancing (by no more than a couple of degrees) will fire the cylinder sooner, and hopefully give enough extra burn time in the cylinder to get you over. Just make sure you back off slightly if the engine starts to ping or knock. You'll definitely damage the engine if it knocks. A slight ping (I mean barely audible) on steep hills or heavly engine load is ok, though.
Good luck,
Joe