The boat: 1972 22' Sportcraft deep-V runabout, guessing 3500lbs, with GM 250 I6 and Mercruiser 165 outdrive. Coastal use, northern California.
The currently installed 2bbl Rochester carb is in need of servicing, and was probably not quite properly installed when the motor was rebuilt 900hrs ago. Aside from the linkage not permitting the idle adjustment screw to do it's thing, the throttle has been sticky for some time and doesn't return to a consistent idle (this is not linkage related, that I can tell), and I'm pretty sure I recall there's no mechanical choke mechanism installed, though the plate is there and just works passively.
Rather than tackle this unit, I bought two replacement Rochesters off eBay. They are almost identical externally, but internally there are some differences, and I was hoping someone could help me pick the better one to use for my application.
One is a NOS unit with a tag marked 1351-4871A1, the other a 2010 Mallory (per the seller) reman. The bore in the NOS is slightly larger, and marked 1 1/4, the reman is marked 1 3/32. The venturi clusters are also a bit different in design. I've taken a couple of pics - the reman is the cleaner looking unit.
The jets in the reman unit are pretty clearly stamped 57. Those in the NOS carb are not clearly stamped. On one I can make out what looks like a 7, preceded by a partial stamp that appears to be the lower circle of a number, like 6 maybe. That circle appears to be fully closed, not like it was part of the number 5, but who can be sure. So *maybe* it's stamped 67. I can't find a rod that's an exact fit, but as best I can tell, the NOS jets are a bit larger in diameter than those marked 57. The conical taper in the top of the NOS jets is longer, such that the overall bore in those jets is a bit shorter than the 57 jets. Don't know if that makes a difference, but it makes the wiggle test with a slightly undersized pin harder to interpret. Pretty sure the NOS jet bore is bigger though.
The last difference I can see is that the NOS unit has some sort of bypass in the bottom of the accelerator pump tube, with a small plastic or ceramic ball, and a short passageway that leads over to the bowl. The bowl opening of that passageway has a cylindrical screen over it. The reman unit doesn't have this feature (bypass).
My thought was to go with the NOS unit, though it requires some clean up, largely because the accelerator pump works more smoothly, and because it's never been used. The pump on the reman sticks a bit, as best I can tell because the hole in the casing that the actuator arm passes through is slightly oversize for the arm diameter, and it binds under load. Can't 100% be sure that's why, but it's not as smooth. I've got a current rebuild kit, so the pump and needle valve in the NOS unit can be changed out for modern fuel compatible versions.
Is there a reason to prefer one over the other for the bore sizing, or venturi design, or any other reasons? I'd love to end up with better mid-range power, if this could be made to serve that end. I've got fairly low prop pitch, 17 if I recall, which works fine for most use, but when loaded and/or in heavy seas, I get pretty bad pre-ignition at anything above about 14 knots. I've adjusted the dwell and timing, but this has always been an issue with this boat. I never run above 3200 RPM or 22 knots, so top end is not something I care about.
The currently installed 2bbl Rochester carb is in need of servicing, and was probably not quite properly installed when the motor was rebuilt 900hrs ago. Aside from the linkage not permitting the idle adjustment screw to do it's thing, the throttle has been sticky for some time and doesn't return to a consistent idle (this is not linkage related, that I can tell), and I'm pretty sure I recall there's no mechanical choke mechanism installed, though the plate is there and just works passively.
Rather than tackle this unit, I bought two replacement Rochesters off eBay. They are almost identical externally, but internally there are some differences, and I was hoping someone could help me pick the better one to use for my application.
One is a NOS unit with a tag marked 1351-4871A1, the other a 2010 Mallory (per the seller) reman. The bore in the NOS is slightly larger, and marked 1 1/4, the reman is marked 1 3/32. The venturi clusters are also a bit different in design. I've taken a couple of pics - the reman is the cleaner looking unit.
The jets in the reman unit are pretty clearly stamped 57. Those in the NOS carb are not clearly stamped. On one I can make out what looks like a 7, preceded by a partial stamp that appears to be the lower circle of a number, like 6 maybe. That circle appears to be fully closed, not like it was part of the number 5, but who can be sure. So *maybe* it's stamped 67. I can't find a rod that's an exact fit, but as best I can tell, the NOS jets are a bit larger in diameter than those marked 57. The conical taper in the top of the NOS jets is longer, such that the overall bore in those jets is a bit shorter than the 57 jets. Don't know if that makes a difference, but it makes the wiggle test with a slightly undersized pin harder to interpret. Pretty sure the NOS jet bore is bigger though.
The last difference I can see is that the NOS unit has some sort of bypass in the bottom of the accelerator pump tube, with a small plastic or ceramic ball, and a short passageway that leads over to the bowl. The bowl opening of that passageway has a cylindrical screen over it. The reman unit doesn't have this feature (bypass).
My thought was to go with the NOS unit, though it requires some clean up, largely because the accelerator pump works more smoothly, and because it's never been used. The pump on the reman sticks a bit, as best I can tell because the hole in the casing that the actuator arm passes through is slightly oversize for the arm diameter, and it binds under load. Can't 100% be sure that's why, but it's not as smooth. I've got a current rebuild kit, so the pump and needle valve in the NOS unit can be changed out for modern fuel compatible versions.
Is there a reason to prefer one over the other for the bore sizing, or venturi design, or any other reasons? I'd love to end up with better mid-range power, if this could be made to serve that end. I've got fairly low prop pitch, 17 if I recall, which works fine for most use, but when loaded and/or in heavy seas, I get pretty bad pre-ignition at anything above about 14 knots. I've adjusted the dwell and timing, but this has always been an issue with this boat. I never run above 3200 RPM or 22 knots, so top end is not something I care about.
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