Merc 115 tune-up

[smike26]

My 79 Merc 115 Inline 6 runs great at full throttle but does not want to idle. As soon as I put it in neutral it dies. I'm going to perform a tune-up, but part of the timing procedure calls for the throttle to be wide open in forward gear. How is this done? Will a big plastic trash can work as a test tank? Also, if anyone else has had this problem please share what worked for you. By the way, it idled just fine before I changed plugs and I've been through two sets of brand new plugs since. Thanks to all...

 

[alcan]

Re: Merc 115 tune-up

Hi Smike<br /> High speed testing in a tank requires a special test wheel. This wheel replaces the prop. They provide a load but minimal thrust. You will not be able to do this in a trash can. I use a boat ramp in the off hours. Comman sence must prevail here if you decide to do the same. First safety, then courtesy. The ramp I use is in an industral area (noise) its hardend (enviromental)I use it at night so nobodys waiting. Just me and an occaisional cop wondering what the hell I'm doing.

 

[smike26]

Re: Merc 115 tune-up

Thanks alcan. Makes sense I guess, that way I can check my work on the lake.

 

[alcan]

Re: Merc 115 tune-up

Just be careful and be cool. This is not the safest thing to do.

 

[what200]

Re: Merc 115 tune-up

back the boat in the water but leave it strapped on the trailer. you only need to set the max advance so you don't need to actually WOT to set that since the timing is almost totally advanced before the throttles pickup.

 

[Yepblaze]

Re: Merc 115 tune-up

It's good you want to check the timing but.....<br /><br />Start your tune up with a compression check followed by removing all the carbs and cleaning out any and all passageways. Make note before removing the idle mixture screws as to what their setting was to get an idea where you will initially set them upon re-assembly.

 

[smike26]

Re: Merc 115 tune-up

Thanks yepblaze and what200. I'll follow your advice and post how it turns out.

 

[Gold Bear]

Re: Merc 115 tune-up

smike26 <br /><br />Unlike many outboards, yours does not have to be under a load to accurately set the WOT timing. the easiest way is ---<br /><br />1) Hook up a water hose.<br />2) Run the engine out of gas / disconnect fuel supply.<br />3) Connect a remote starter switch. If you have someone to help you they can hit the starter when you need it.<br />4) Connect a timing light.<br />5) Move the throttle to WOT.<br /><br />With water going to the lower unit, spin the engine over while the timing light is connected. Quickly look at the timing marks. You should be able to look at the marks in a split second. Adjust the timing advance screw then check it again until right on the mark. You do not want to spin the engine over for a long time because the fuel (that you ran out) is also the lubricant. But it will not hurt anything to do this just long enough to set the total timing.<br /><br />As far as idle goes, the throttle cable and idle stop screw should be set so the butterflys are closed (when you are in neutral). The idle screw makes sure the butterflys do not jam shut. The carb idle mixture screws should be 2-1/2 to 3 turns open from lightly seated.<br /><br />What kind of plugs are you using, and why have you changed them three times already?<br /><br />Good Luck,<br /><br />Gold Bear

 

[smike26]

Re: Merc 115 tune-up

Gold Bear,<br />Thanks for your reply. I changed spark plugs the first time after I de-carbed per someone's suggestion. Immediately after that is when my idle problem started so I went back to the NGKs (in place of the Champions). I'm going to rebuild the carbs and check the fuel pump diaphram and idle mixture (after a compression test and spark test)then I'll go to the timing. Please let me know if I'm on the right track. Thanks again.

 

[Gold Bear]

Re: Merc 115 tune-up

smike26 <br /><br />Your statement "Idle fine before I changed the plugs" probably gives use the best clue.<br /><br />A couple of things to check.<br /><br />Make sure that the spark plug wires are numbered correctly, and installed on the correct plug.<br /><br />Your engine should have surface gap plugs. When they become slightly carbon fouled, you can pull them, spray a bit of carb cleaner on the ends while gently wire brushing them. (I have a small wire brush about the size of a large tooth brush that works well.) When installing the plugs use a small dab of anit-seize on the threads, and a bit of dielectric grease on the tip of the terminal. I would clean and re-install the last set of plugs that worked well.<br /><br />In the dark, start you engine on a water hose, and while idling, spray the plug wires with some Windex. If there are any shorts to ground, you will find them that way.<br /><br />Good Luck,<br /><br />Gold Bear

 

[mellowyellow]

Re: Merc 115 tune-up

windex eh?... learnt somthin' new today <br />v.good tip Bear,<br />M.Y.