2005 mercury 115 elpto (4 cyl, carb) has HIGH IDLE

[Louie]

I have a 2005 nitro with a 115 on it, boats ran great for me purchased it used a over a year ago. I havent been fishing much, but always top off the fuel or just run around to keep gas fresh in it also add marine stabil. The boat recently developed a high idle 1300-1500, now seems to be staying at 1500. In the past I had replaced the fuel lines and primer because
they had come apart. I rebuild the diaprahm fuel pump and cleaned the filter also. Boat ran fine after all that. Just recently this started for no reason. I had a friend go though the carbs, perform sync + link, check timing, end stops, idle mixture screws, etc... Even pulled the carbs twice and soaked them once. Reed valves look good, and its not sucking air in any where it should be. We are stumped. Kind of leaning towards somthing electrical... Battery might be kind of old, could it of fried somthing that wouldnt be that obovious? Side notes : tachometer has always bounced around like a faulty tach. Depth finder hds-5 wouldnt power up and kept cutting off in and around the same time I ran the boat and this started... any info much appericated!

[SilverFox579]

Please provide your serial number - http://www.bbcboards.net/mercury-mot...al-number.html

[EuropeanAM]

+1... please provide Engine Serial Number when posting.

BTW... final step of Sync-n-Link procedure is PROPERLY setting the engine idle speed (if that wasn't done, or can't be set, start over with the Sync-n-Link procedure, performing EVERY step exactly as outlined in the Mercury Service Manual, under "Engine Synchronization and Adjustments").

[Louie]

happen to have it on my phone here it is 1B068755 2005 ELPTO

assume that you have done sync n link, cleaned carbs, set idle on the top two, all your timing is in, and throttle cables are set right. (all this has been checked) and it still idles high like 1500-1800, you can back out some of the timing and get it to come down a little

[EuropeanAM]

There is a center (idle timing) adjustment screw on the throttle arm- it is what is used to adjust IDLE SPEED (via manipulation of the idle timing).

The BOTTOM (throttle stop) screw should remain set for proper alignment of the cam/roller (as per the manual).

[Louie]

yea both are set correctly and the timing has been set via the sticker on the cowling

taking it to a repair center tomorrow, were drawing blanks

[Louie]

this was messaged to me, i figured id share it here

The engine has to be getting fuel and air from somewhere to idle high. This is not an electrical problem. I would first check to make sure all 4 carbs are shut at idle. They probably are since the problem just started on its own and you have done a sync and link procedure.

Assuming this checks out, I would check the accelerator pump circuit. This engine is called a 2+2 engine. It only idles on the top 2 carbs and the bottom 2 kick in around 1800 rpm. To prevent hesitation on acceleration, there is an accelerator pump just like on a car that pumps fuel into the intake manifold behind the reeds. To keep the accelerator pump primed, there is always fuel flowing through the pump and back around to the fuel inlet via a small orifice. There are also some check valves at the injection point behind the reeds. If the check valves fail, fuel will be continually pumped into the cylinders. You can temporarily isolate the accelerator pump circuit by pulling a hose off of the accelerator pump and plugging it. If this cures the problem, then you know there is a problem with one of the check valves.

[EuropeanAM]

Also- in addition to the above:

Look at the four (4) carbs, on the PORT or LEFT side of the engine.

Confirm that ONLY THE TOP TWO carbs have IDLE AIR MIXTURE SCREWS (adjustable). The bottom two should have fixed jets/orifices (NON-ADJUSTABLE).

I did run across one (once) where someone had added needles to the lower two carbs- which over-rode the 2+2 feature at idle (high idle speed, and bogged on acceleration)...