1969 Mercury 350 lacks grunt.....

[Fouled Plug]

My brother recently aquired a Merc 350, serial #2571393. Believe it to be a 1969 model. The motor has 120/125# compression, good spark, cleaned carb. It still seems to push as though it is on one cylinder, barely moving a very light 14' Fiberform runabout. Has an intermittant miss as though second cylinder wants to join in the fun. Plugs look identical. Tried pumping primer bulb, but it stays firm. How do you adjust the mixture on the carb? Any thoughts would be appreciated. Thanks!

[Chinewalker]

Hi FP, Couple things to look at... More than likely you've got an ignition issue. Have the condensers been replaced recently? Making spark in the open air is one thing - under compression is quite another. I would go through the magneto - clean and gap the points at .020", replace the condensers with new, and try it again. You say both plugs look the same - same how? Are they carboned or clean? Try pulling and grounding out one of the plug wires. If it starts up and sounds like it did before, that's the "good" one, if it won't start at all - that's the "bad" one. If it still acts like it want's to "kick in" try squirting some mixed fuel into the carb while it's running - this will tell you in a hurry whether or not it's starving for fuel. If it doesn't make a difference, might be flooding out - try pulling the plugs immediately after shutting down and flick the open end of the plug towards the water - if you see some little drops of rainbow, then it may be fouled out.- Scott

[Fouled Plug]

Thanks for the tips, Chinewalker. We plan to monkey with the motor again this weekend. Do you have any info on carb adjustments? It wasn't obvious when looking at it where the high/low speed adjustments are. (Wish everyone ran knobs on the cowling for tweaking the mixture!) Both plugs were lightly carboned, no excessive crud nor were they 'steam-cleaned'. Used grounded test light to probe plug boots while running, each cylinder dropped mucho RPM's when grounded. I will start with your ideas, and go from there. Thanks again!

[Chinewalker]

The high-speed jet is fixed - located behind the brass plug in the lower front of the carb body. The low-speed is the slotted, recessed screw at about the 1:30 position when looking at the carb throat. Back it out about 1.5 turns from closed. It's possible you might have a clogged high speed jet - sometimes it mimmicks an electronics skip by dropping a cylinder. I would disassemble and go through the carb and clean EVERYTHING. Compressed air and carb cleaner can work wonders! Make sure the carb level is set as per spec - the bottom float lever should be dead-on level when the carb top is inverted.- Scott

[lifeandliberty]

don't know if this will help with your problem but it is part fo timing and synchronizing a 1969 merc model 350 from my repair manual.remove the flywheel. shift engine into fwd gear and install flywheel nut onto the bare crankshaft. tightin nut only until the crankshaft begins to rotate against th compression in the cylinders. remove spark plug and install dial indicator in #1 cylinder. place an ohm meter on the breaker point terminal. rotate the crankshaft clockwise until #1 piston is at TDC. adjust dial indicator to 0. rotate crankshaft counter clockwise until you reach .300btdc. at this point slowly advance the magneto againt the magneto stop. the breaker points should open(no cont. on ohm meter). if they did not open or opened before contacting the magneto stop. you need to adjust the linkage until the points open as the magneto contacts the stop. while the throttle is at full advance loosen th two screws on the carburetor pickup and adjust the plate until the carb is at full throttle(butterfly is wide open) check for 0.010-0.015" "play" in the carb shaft. this will prevent strain on the linkage. I know this is alot of info but I found it helpful on my 1969 merc 350. good luck!