Merc 1150 and carb jet size

[bbitter]

I own a 1978 115 hp Merc in-line with back drag carbs. The previous owner had increased the main jet size (unknown size). The factory jet is .072 and I have considered changing back to the factory jet because the engine tends to "bog" out of the hole if given wide open trottle. I just read (in this forum) that increased jet size may be a good thing to prolong engine life. Can anyone explain this and the relationship to gasoline octane?

 

[Chris1956]

Re: Merc 1150 and carb jet size

bbitter, I have a '77 150 HP merc. The merc mechanic always told me those motors liked the bigger main jets. Bogging upon start up is usually due to improper idle jet adjustment, dirty idle passage ect, not larget main jets. You can easily find out what the standard mains are and replace as necessary.<br /><br />On the octane thing. I also have been told that the remedy for poor quality gasoline was larger jets. However, I think the answer with these old motors is to make sure the maximum spark advance is at or below factory spec, listen for pinging or preignition and watch for overheating.<br /><br />You could also run "Plus" gasoline.

 

[bbitter]

Re: Merc 1150 and carb jet size

Chris 1956 - Thanks for the info

 

[Gold Bear]

Re: Merc 1150 and carb jet size

bbitter <br /><br />Open the idle adjustment screws 1/4 at a time until the bog goes away. My 1972 model 115 likes 2 turns open from lightly seated.<br /><br />Good Luck,<br /><br />Gold Bear