850 mercury running rich?

[matt88]

Finally got back on the water today after a month of grinding and glassing getting the transom and floor replaced. (still itching) Everything went better than expected with the test run today but the engine seems to be running a little rich. It is a 1973 850 2 stroke mercury. It runs great at wot but at low rpms it will run a little rough. To give you guys some background on this motor it has had both of the carbs completely tore down soaked and rebuild kits installed about one month ago and it also got a new water pump impeller. According to the manual I am supposed to seat the idle screws at 3/4 turn and tune from there accordingly. I done that and had to turn the idle screws on both carbs to about 1/2 turn from the seated position and the engine still seems to run too rich. If I try to turn the idle screws in any more it will start to cut out like its not getting enough fuel. I had a 9.9 johnson and it acted the same way until I got the plastic air box installed in front of the carb. Should my merc have one of these airflow boxes or air restrictors whatever there called!lol

 

[Moody Blue]

Re: 850 mercury running rich?

Early Mercs are finicky to tune. You really need to do a link and sync procedure to get it to run right. The timing and synchronizing of the carb linkages is critical. There is a great description on this forum detailing how to do it.

As a first stab at it, I'd say you have the idle adjustments a little too lean. Try backing the idle adjusting screws out 1 1/2 turns from lightly seated.