I have a 1963 Mercury 850 inline six ( 90 cu. in. ) which I have modified to make it like a Merc 1000. I installed the exhaust baffle out of an old Merc 1000 and upgraded the carbs to the KC1A carbs also from an old Merc 1000. My question is this. Are the factory carburetor jetting tables set on the rich side? I have heard in the past that some manufactures set the carb rich to avoid a lean condition and possible engine meltdown. Most of my boating is at 5000 ft. in elevation or higher and I am trying to get the carb jetting dialed in. I currently am running (.069) jets which the factory spec book lists for sea level to 4000 ft. The factory spec book lists (.067) jets for 4000-7000 ft. Which would be about right for my elevation. I don't have the (.067) jets but I do have a set of (.064) jets. I figure if the tables are set overly rich anyway, how risky would it be to run the (.064) jets? You old Merc Guru's have any thoughts on this?
Hi Doug,
How do your plugs look now after a hard run? If they're running really black, you could definitely go a bit leaner. Are the .064s the ones out of your original 850 carbs? If the plugs are running a chocolate brown now, then going that much leaner would probably be too much. If the motor were mine and the plugs were running real dark, I might try it for a quick run witha clean set of plugs and then read the plugs again. Try it at your own risk...
- Scott
Use the hi-altitude jets meant for a 100 HP. You're moving a lot more air thru the motor with the 100HP carbs than with the old 85HP items.
Wouldn't be a bad idea as suggested to read the plugs just for G.P., after rejetting.
Keep in mind that regardless of jetting you'll produce less power at higher elevations due to the reduced air density. But rejetting will at least allow it to run cleanly.
If you find you're not getting up to the rated rpm at the higher elevations, you'll probably need to change to a prop 2" less in pitch.
I appreciate your responses. I will try a high speed run with the (.064) jets and a clean set of new plugs. She runs really good now, but I am always looking for ways to make it run even better. 5200 RPM on a 80 degree day at 5200 ft. of elevation with a brass 2-bladed prop 13 3/4" X 19 pitch with a light load. Will turn 5500 RPM and 40 MPH in the early morning when the air is in the 30's. The (.064) jets will flow about 86% of what the (.069) jets will flow, and air density decreases about 3-3.5% per thousand feet above sea level, so at 5000 ft. I figured the (.064) jets would be just about right. 82.5 -85% of sea-level air density.
Old Mercury(1963) inline six carb question??
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