Re: 2+2 motors
Anytime the carbs are removed for any reason, they MUST BE linked and sync'd again. Cleaning carbs is one of THE most important maintenance items in keeping your motor happy and performing well, once done and all adjustments are made to perfection, your motor should run fine for many years as long as your fuel quality is good, you have a water separating fuel filter, and you keep using your boat.
Service manuals are available on-line thru links found in the 'sticky' posts at the top of this forum, check them out, get a copy of your manual and read it carefully, a couple of times.
After the carbs are cleaned, follow the detailed directions to sync the carbs, and link them to the timing lever.
Pay particular attention to the idle mixture on the top two carbs, the throttle pick-up roller adjustment as well as the accelerator pump adjustment. These are the three things that make the greatest difference in how well your motor makes the transition from 2 to 4 cyls. Once dialed in you will feel like you have a totally different motor installed.
Guys,
This was the subject matter of another thread on iboats forum. As mentioned elsewhere, the way the 115 and 125 4-cylinder 2-stroke Mercs were set up was the following: only the top two carbs have idle jets; the lower two cylinders just manage to suck in a bit of fuel at idle, through the main jets, in order to remain lubricated. But they don't really get any power on idle. However, don't be mistaken. Other than idle, there is actually no transition from 2 to 4-cylinder operation. What happens is that as you move forward the throttle lever, initially you only get spark advance, and the engine gains revs with whatever fuel it can get from the top two cylinders (the ones with idle jets). The four throttle valves remain firmly closed. When you push the lever to approx. 2000 RPM, hell breaks loose because all four carbs open simultaneously. In addition, there is plastic snail cam on the throttle linkage that presses a button on the accelerator pump. This little pump is connected via plastic tubes to the lower two cylinders, and gives them a squirt of juice to help them gain RPM. If there wasn't this extra little squirt, the engine would stumble and cough a bit until the lower two cylinders manage to get enough "rich" fuel mixture. In practice, this is an engine that you cannot keep running at anywhere between 2000 and 2700 RPM. It's either more, or it slowly falls back to below 2000.
Having said that, once you get used to it, this really one great motor. Mine is upwards of fifteen years old, has a thousand + hours on it (and in salt water, too) and runs absolutely like a top. Regarding carbs and synching and all that, if it ain't broken, don't fix it. I have never taken off my carbs, and never had a problem. However, I do make sure to empty the carbs at the end of each season (there's a drain plug on each float bowl), I pump out the fuel tank bone-dry and I don't use ethanol-laced gas (and by the way, I don't use stablizer either 'cause there ain't no fuel left in my boat during the winter). The piston heads are clean as whistles, no carbon build-up. The only thing that needs a very occasional tuning (once every three or four seasons, no more) are the two idle screws and idle. It's fairly straightforward for those who know the tricks. You can get 900 RPM in neutral, with a comfortable rumble and no stalling (make sure you have clean spark plugs, well gapped and the right ones: NGK BPZ8 HN10). The trick is that you MUST tune these idle screws with the LU in the water (don't drop that screwdriver in the water!). Doing it on muffs is a waste of time; there isn't enough exhaust back-pressure to do a decent job.
BTW, for those who have the official Mercury manual: I have it, but it does not give the slightest word of explanation on the strange 2+2 carburetor setup. Looks as if nobody at Mercury wanted to take ownership of such a bizarre design... Regardless, if you maintain this motor properly, it should work forever. More than what a lot of people would say of newer 4-Strokes...
Happy sailing!
